Chapter 8
Nuclear Fission
energy for war and peace
Soon after the neutron induced radioactivity was discovered, several groups of researchers
bombarded uranium by neutrons and analyzed the radioactive products formed thereof.
However, results were not what they expected, and they misinterpreted of their results due to
their expectations. The German group eventually interpreted their results in terms of fission,
and fission was discovered in the midst of World War II.
Nuclear fission is a process, by which a heavy nuclide splits into two or more pieces. From
masses of nuclides, scientists knew that such a process would release a lot of energy. Thus,
research on nuclear fission became the top secret in Germany, England, France, the former
U.S.S.R., and the United States. It was obvious that a weapon employing the nuclear fission
would be so powerful that whoever had developed it would win. Such a weapon would destroy
the enemy in such a way that even the winner will be scared.
Soon after the discovery of nuclear fission,
the technology was employed to make
bombs. The desire to win the war sped up
the development of atomic bombs. Two
such bombs were used in the war. The
destruction was so massive and horrified
that no additional nuclear bomb has been
deployed in wars.
.... For in the development of this organization (the
United Nations) rests the only true alternative to war,
and war appears no longer as a rational alternative.
Unconditional war can no longer lead to unconditional
victory. It can no longer serve to settle disputes. It can
no longer be of concern to great powers alone. For a
nuclear disaster, spread by winds and waters and fear,
could well engulf the great and the small, the rich and
the poor, the committed and the uncommitted alike.
Mankind must put an end to war or war will put an
end to mankind.
Political will and public pressure have
diverted human effort to develop peaceful
applications of nuclear energy. As a result,
nuclear power reactors have supplied
energy for our need for decades and they
continue to do so. Yet, reactor accidents
have changed the public opinion and about
nuclear energy. The future of nuclear
reactors became uncertain. However,
energy demand is always on the increase,
and at some point, the public opinion may shift again.
225
John F. Kennedy
September 25, 1961, Address to the United Nations.
Nuclear Fission Reactions
Soon after their discovery, neutrons were used to bombard all kinds of material to induce
radioactivity. Neutron bombardment of uranium produced complicated radioactivity. Because
researchers were looking for heavier nuclides, they took a long time to discover that neutron
bombardment actually split uranium into two light nuclides, and such a process is called
nuclear fission.
The Discovery of Fission Reactions
No doubt, you have heard of the term nuclear
fission because of the infamous atomic bombs
and power producing nuclear reactors. Because
a new concept is required to recognize fission,
the story leading to the discovery of fission is
particularly interesting.

Why did researchers bombarding uranium
with neutrons?
What were researchers looking for?
What did the radioactivity indicate?
How was nuclear fission discovered?
Why did neutron bombardment produce
many products, and what are the products?
Which isotope undergoes fission?
Are neutron released in the fission process?
Neutrons
?
?
Uranium
?
-radioactivity
After their discovery, neutrons were used to bombard all elements including uranium. At that
time, three prominent research groups separately led by Enrico Fermi (1901-1954) in Rome, F.
Joliot (1900-1958) and I. Curie in Paris, and Otto Hahn (1879-1968), Lise Meitner, and Fritz
Strassmann in Berlin diligently bombarded uranium with neutrons, hoping to produce
transuranium elements (elements heavier than uranium).
After bombardment by neutrons, uranium samples became radioactive. Beta particle emission
was particularly noticeable. The three groups competed with one another to identify the
products in these experiments. Since they worked in three different countries, the competition
was an international race for the first to identify the nuclide(s) produced. They expected
neutron capture reactions followed by  emission to produce elements 93 (E93) and 94 (E94),
which were unknown at that time. They expected these reactions:
U (n, ) 239U92 (, ) 239E93 (, ) 239E94.
238

Otto Hahn (1879-1968) and Fritz Strassmann (1902-1980) were credited with the discovery of nuclear fission received the
Nobel Prize for Chemistry in 1944, due to the fact that Hahn and Strassmann published the results without Meitner's name.
The Enrico Fermi Award in 1966 went to Hahn, Strassmann, and and Lise Meitner (1878-1968).
226
They published papers offering various interpretations and attacked each other for
misinterpretations. The arguments went on for some time while the war intensified in Europe.
The French group precipitated a  emitter with half-life 3.5 d together with lanthanum, and
they interpreted it as an isotope of actinium (Z = 89), which drew criticism from Hahn who
argued that  and proton emissions were not possible.
Hahn, a chemist, Strassmann, an analytical chemist, and Meitner, a physicist, worked together
to make transuranium elements. Meitner was very excited when she detected a great increase in
radioactivity after uranium was irradiated with neutrons. Strassmann, applied analytical
chemistry skills to precipitate the radioactive products. He understood that hydrogen sulfide
(H2S) would not precipitate uranium radioactive decay daughters, but after neutron irradiation,
he precipitated most of the radioactive products using H2S. The half-life measurements
indicated to them that not one but many elements were produced (Shea, 1983).
How could one reaction give rise to so many different products? The three groups struggled
over this question for many years, and eventually Hahn's group, including Meitner on exile,
came to the conclusion that the neutron bombardment caused 235U to split into small
fragments whose mass numbers are slightly more or less than half the mass number of
uranium. Products, which they earlier thought to be the radioactive as isotopes eka-rhenium,
eka-osmium, eka-iridium and eka-platinum, co-precipitated with rhenium, osmium, irridium
etc, were later identified as fission products not transuranium elements. The nucleons in a
nucleus form a liquid drop, and the fission process is very much like splitting a drop into two,
often unevenly.
The Official History of the Manhattan Project (US,
The Liquid Drop Model and Fission
1977) gives the following story. The discovery of
neutron induced fission was first brought to
Copenhagen by Dr. Meitner, who, as a nonAryan, was exiled from Germany in 1938. She
used barium ions, Ba2+, as a carrier and
precipitated the radioactive products from the
neutron bombardment. None of the chemical
methods could separate the radioactive ingredient
from the barium ion, and she concluded that the
isotope 238U must have split into two fragments.
The atomic weight of barium, 137, is a little more
than half of 238. When she arrived at
Copenhagen, she communicated her thought to
Dr. Frisch, who communicated the information
to his friend N. Bohr, who was in the United
States at the time. Bohr conveyed Meitner’s insight to Fermi who immediately changed his
strategy of research. In March, 1940, Fermi's group (working in Columbia University) found
that only the less abundant (0.7%) isotope 235U underwent fission.
227
When Fermi (1901-1954), Szilard (1898-1964), and F. Joliot (1900-1958) learned of the fission
of uranium, they were anxious to find out which isotope underwent fission and if neutrons
were released in the fission process. If neutrons are released, they had envisioned a chain fission
reaction because the newly released neutrons induce more fission reactions. Almost at the same
time and independently, researchers in various groups discovered that only 235U underwent
fission and neutrons were indeed released. The release of neutron made the fission reactions
candidates for a very powerful weapon known as the atomic bomb, which was eventually built
and used.
During the lecture, a movie on the life of Lise Meitner called "The Missing Link" will be
shown. The title was derived from her isolation of the isotope protactinium, 231Pa91, an element
unknown until that time. Elements thorium, Th90, and uranium U92 were well known, but the
element between them will missing. The movie is a very good video essay that addresses
several issues including politics and power.
Skill Building Questions
1. What is the lesson learned from the story leading to the discovery of neutron induced fission? (New
concepts are required to recognize new phenomena. Concepts are important tools in research.
New phenomena create new concepts, and new concepts give new interpretation to old
phenomena. Many concepts became theory, and we take some of them for granted.)
2. Why are reactions induced by neutron bombardment of uranium so difficult to explain? How can uranium
fission products be identified and confirmed? (The neutron bombardment caused 235U to split into
small fragments whose mass numbers are slightly more or less than half the mass number of
uranium. Thus, many products were produced. We will discuss this further in the next few
sections.)
The Fission Nuclear Energy
The discoveries that one of the uranium isotopes 235U underwent nuclear fission reactions, and
that neutrons were released opened a new frontier for research and development. The
discoveries were so important that they were treated as top secret because of the its potential
applications in war. Furthermore, these discoveries were made at a time when the entire world
was at war, and the war sped up the research and development.

How much energy is released per fission reaction?
How can the amount be estimated or calculated?
What are the applications of fission energy in war and peace?
A strategist asks many questions about a new discovery, and a philosopher plans for the future.
The questions and plans are seeds for further research and development.
We concentrate on the fission process and the energy aspect in this section. A spontaneous
reaction releases energy. Neutron induced nuclear fission reactions are spontaneous reactions
and they release energy. This quantity is important, because it affects fission research and
228
development. Both theoretical considerations and practical measurements have been carried
out to give estimates of the amount of energy released in fission reactions. Some examples
showing how estimates can be made are given here.
You have learned that when nucleons bind together to form a nuclide, energy is released. The
energy so released is called binding energy. The average binding energy is the largest for
nuclides with mass number around 56. Thus, splitting up a heavy nuclide such as uranium to
give nuclides with mass number about 117 releases energy. A rough estimate is to consider an
even split of 235U to give two nuclides of mass numbers 117 and 118. A search of stable
nuclides with mass numbers 117 and 118 are 117Sn50, and 118Sn50, their masses being 116.902956
and 117.901609 amu respectively. The mass of 235U92 is 235.043924 amu. The difference in
mass
235.043924 - (116.902956 + 117.901609)
= 0.2394 amu (931.5 MeV/1 amu)
= 223 MeV.
E = m c2
In reality, a fission reaction usually gives two unequal fragments, plus 2 to 3 neutrons. These
neutron-rich fragments are beta () emitters. As a more realistic example to calculate the
energy of a fission reaction, let the two fragments be isotopes of rubidium and cesium plus
three neutrons. The reaction can be represented by
U+n
235
142
Cs55 + 90Rb35 + 4n + Q.
where Q is the mass equivalence of energy released. The neutron-rich cesium and rubidium
isotopes are not stable, and they undergo radioactive decays:
Cs  142Ba +  (~1 m)
90
Ba  142La +  (11 m)
90
La  142Ce +  (58 m)
90
142
142
142
Rb  90Sr +  (half-life, 15.4 m)
Sr  90Y +  (27.7 y)
Y 
Zr (stable) +  (64 h)
90
Ce  142Pr +  (51015 y)
142
Pr  142Nd (stable) +  (19 h)
142
The total energy can be calculated from the measured masses of 235U, 90Zr, 142Nd, and neutron
but some of the energy will not be released during the operation of the reactor due to the very
long half-lives (in this case of 90Sr, 142Ce). However, a calculation to estimate the energy may
proceed in the following way:
Reaction
Mass /amu

U92
 90Rb37 + 142Cs55
+ 3n
+ Q.
235.04924 = 89.904703 + 141.907719 + 3 x 1.008665 + Q
235
The radioactive nuclide 90Sr is present in the fallout of nuclear explosions. It threatens animal and human health,
because its properties are similar to calcium. Ingested strontium ions reside in the bone structure.
229
Q = (235.043924 - 89.904703 - 141.907719 - 3 x 1.008665)(931.4812 MeV/1 amu)
= 191.4 MeV per fission
The energy of 191.4 MeV is equivalent to 0.0000307 J or 307 erg, which is released per fission
of 235U nucleus. Fission of one kilogram (1000 g) of uranium-235 will release 7.861019 J
1000g U 
1mol U 6.023  10 23 U nuclei 191.4MeV 1.602  10 13 J



235g U
1mol U
1U nucleus
1MeV
= 7.861013 J
This amount of energy is equivalent to 2.2×1010 kilowatt-hour, 22000 megawatt-hour, or
22 giga-watt-hour. This
amount of energy keeps a
100-watt light bulb lit for
Energy (MeV) distribution in fission reactions
25,000 years.
Kinetic energy of fission fragments
167 MeV
In the fission process, the
8
Prompt (< 10–6 s) gamma () ray energy
fragments and neutrons
Kinetic energy of fission neutrons
8
move away at high speed
carrying with them large
7
Gamma () ray energy from fission products
amounts of kinetic energy.
The neutrons released during
7
Beta () decay energy of fission products
the fission process are called
Energy as antineutrinos (ve)
7
fast neutrons because of
their high speed. Neutrons
and fission fragments fly
apart instantaneously in a
fission process. No delayed liberation of neutrons was ever observed. Gamma rays (photons)
equivalent to 8 MeV of energy are released within a microsecond of fission. As mentioned
earlier, the two fragments are beta emitters. Recall that beta decays are accompanied by
antineutrino emissions, and the two types of particles carry away approximately equal amounts
of energy. Beta decays often leave the nuclei at excited states, and gamma emission follows.
Estimated average values of various energies are given in a table here.
Skill Building Questions
1. Give an example to show how the amount of energy released in a nuclear fission can be estimated.
2. Assume 235U splits into two fragments with masses 100 and 132 and three neutrons. Find the masses of
stable nuclides with these masses. What is the fission energy in this cases?
3. How is the fission energy distributed among the various forms?
4. Calculate the speed of a neutron which has kinetic energies of 1 and 2 MeV respectively.
230
The Cyclotron and Fission Research
The machine built by Cockroft and Walton accelerated protons, which smashed 7Li nuclei.
Any machine that speeds up the velocity of particles are called particle accelerators. Particles
from accelerators induced many nuclear reactions, and the value of accelerators in the study of
nuclear reactions was soon realized.

How can particles be accelerated?
How to build particle accelerators?
What are the purposes of particle
accelerators?
How can particle accelerators be used to
study fission reactions?
A Sketch of the Cyclotron
Ions, originated from the center of the cyclotron,
accelerated by alternate voltages between the
Dee’s follow a spiral path acquiring high energy
and exit from a window.
Various types of particle accelerators have
been built, using electric potentials or
electromagnetic forces. Linear particle
accelerators made particles moving faster
along a straight line; whereas cyclotrons
accelerated them as they travel along circular
path. The cyclotrons built by Ernest O.
Lawrence in Berkeley, California belong to
the latter type, and they have given useful
results.
High
voltage
Experiment
station
A Dee
A cyclotron has two hollow D-shaped (Dee)
sections assembled together with a small space in between. A magnetic field deflects the
particles into spiral motion. By applying alternated voltages between the Dees, the cyclotron
accelerates charged particles to desirable energies. By changing the strength of the magnetic
field, particles of various energies are made available. The first such cyclotron has a diameter of
only 13 cm, and it accelerated protons to a maximum energy of 13 keV. Cyclotrons built later
with larger diameter accelerated particles to energies between 10 and 100 MeV.
Accelerated particles are used to induce nuclear reactions as discussed in the last Chapter.
Reactions between accelerated charged particles from cyclotrons and light nuclides produced
neutrons of variable energy. The following are some of the reactions:
7
Li (p, n) 7Be
T (p, n) 3He
1
H (t, n) 3He
2
D (d, n) 3He
2
D (t, n) 4He
3
T (d, n) 4He.
3

Ernest O. Lawrence (1901-1958) received the Nobel Prize for physics in 1939 for the invention of cyclotron, the first
particle accelerator to accelerate particles to high energy.
231
Reactions between tritium (3T or t) and deuterium (2D or d) are particularly important, because
these are fusion reactions. Furthermore, they release neutrons of various energies.
Cyclotron induced nuclear reactions provide neutrons of controlled energy for the study of
fission. For example, some reactions of protons with medium-weight nuclides are listed below
together with their threshold energy and neutron energy range.
Reaction
Threshold* Energy range (keV)
energy (MeV) of narrow-energy neutron
51
V (p, n) 51Cr
2.909
5.6-52
45
45
Sc (p, n) Ti
1.564
2.36-786
57
57
Fe (p, n) Co
1.648
2-1425
__________________________________
* The threshold energy is the minimum energy of proton required for the reaction.
Energy from the neutron source 27Al (, 1n) 30P mentioned earlier can not be varied. Neutron
sources from the cyclotron have an advantage over neutron sources induced by natural
radiation, because neutron energy can be varied. This enables the study of energy dependence
of neutron induced fission reactions. The variation of cross sections for neutron-induced
fission as a function of neutron energy is a vital piece of information for nuclear reactor
design. The study showed that fast neutrons (energy ranges from 10 MeV to 10 KeV) are not
effective to induce fission, but slow neutrons (0.03 to 0.001 eV) are very effective. Slow
neutrons are also called thermal neutrons, because their energy corresponds to room
temperatures.
Skill Building Questions
1. What are particle accelerators? What are cyclotrons? How do they accelerate charged particles?
2. What are the applications of particle accelerators?
3. What are the advantages of neutron sources from nuclear reactions induced by particle accelerators? (They
provide neutron sources of variable but definite energy for experiments.)
232
The Synthesis of Plutonium
The intention to synthesize transuranium elements
by neutron bombardment of uranium split 235U
nuclei.

Can transuranium elements be made by
neutron capture reactions of uranium?
If so, why are tranuranium elements not
detected?
How can plutonium be dynthesized?
What are the properties of plutonium?
Why is plutonium a strategic material?
Pluto
Neptune
Uranus
Experiments to produce elements 93 and 94 by the (n, ) reaction are sound, but so much
fission products were produced that they impaired the detection of transuranium elements.
The cyclotron, however, provided high intensity neutrons of definite energy, and it gave a
chance for success.
The cyclotron provided neutrons for E. M. McMillan (1907-) and P. H. Abelson (1913-), to
bombard uranium. In the summer of 1940, they confirmed one product as element 93, and
named it neptunium, Np, after the planet Neptune. They inferred that Np would decay by
emitting a  particle converting itself into element 94, named plutonium after the planet Pluto.
These reactions are summarized bellow:
238
U + n  239U + 
239
U (half life 23.5 m)  239Np + 
239
Np (half life 2.35 d)  239Pu + 
or in short notations:
238
U (n, ) 239U ( , ) 239Np ( , ) 239Pu
or
238
U (n, 2) 239Pu
Actually, the neutrons with high kinetic energies are used to produce transuranium elements.
The fission theory by Bohr and Wheeler suggested that 239Pu would undergo fission. Thus, the
cyclotron in Berkeley was put to work to produce enough plutonium for experiments. By mid1941, the fission characteristic of plutonium was well established.
Plutonium was first detected (1940) by Glenn T. Seaborg, Joseph W. Kennedy, and Arthur C.
Wahl, from the reaction 238U (d, b) 240Np using deuterium from the 60-inch cyclotron at
Berkeley, California. The most important isotope is 239Pu, because it has a long half-life (24,400

Edwin M. McMillan (1907-1991) shared the 1951 Nobel Prize with G. T. Seaborg (1912-1999). He discovered neptunium
from the decay reaction 239U (, ) 239Np. In 1940, he and Philip H. Abelson (1913-) isolated Np and discovered the reaction
239Np (, b) 239Pu, and they were credited for the discovery and synthesis of plutonium.
233
y) and it is a fission fuel. This isotope can easily be produced using a breeder reactor, which
shall be described later in this Chapter.
Plutonium has a very high electrical resistivity and a density of 19.84 g cm-3. It is chemically
reactive, dissolving in acids and forming various ions of characteristic colour in water: Pu3+,
blue-lavender; Pu4+, yellow-brown; and PuO+2, pink. Many compounds of plutonium have
been prepared, often starting from the dioxide, PuO2), the first compound of any synthetic
element to be separated in pure form and in weighable amounts (1942). Isotope 244Pu gives a
melting point of 912 K, boiling point 3,508 K. The concept of critical mass will be introduced
later, and you may be interested in knowing that the critical mass of 239Pu is only 300 grams.
Skill Building Questions
1. Describe the neutron capture reaction of uranium leading to the formation of neptunium and plutonium.
2. What is the significance of the synthesis of neptunium and plutonium?
3. Why is 239Pu the most important isotope of plutonium?
Uniting Political and Nuclear Powers
Neutron induced fission reactions release energy and neutrons. The
amount of energy in fission reactions is alarmingly large and the
liberation of neutrons in the fission process gives the possibility of an
explosive chain reaction, which releases a tremendous amount of
energy. At that time, the scientists already foresaw the danger of
nuclear power, especially if the technology falls in the wrong hand.
Since securing the presidency of Germany in 1933, Hitler became a
dictator, and many top scientists in Austria, Hungary, Italy and
Germany felt uncomfortable. Scientists with ethnic backgrounds
other than German felt threatened. Many European scientists had escaped the Hitler regime
and come to the United States. At the time fission was discovered, Hitler invaded Poland,
Hungary, Slovak and other European countries. Many scientists were concerned that Hitler
would make use of fission to build bombs. Such a move is a threat to the entire world.

Why scientists feared the danger of nuclear power?
What was the political situation when fission was discovered?
Why were so many scientists felt threatened?
Who took the initiative to bring the issue to the most powerful political leader and why?
What did they do individually and collectively and why?
What would you do if you were afraid of the nuclear power falling into the wrong hand?
What is an effective strategy to prevent a disaster from happening?
234
Among the concerned people were three Hungarian refugee scientists Leo Szilard, Eugene
Wigner, and Edward Teller, who thought the time has come to unite political force with
nuclear power. They thought Hitler had the potential and possibility of developing atomic
bombs. This matter should be brought to the immediate attention of the President of the
United States, Roosevelt. To achieve this, they needed someone with a reputation. They
convinced Albert Einstein that such an action was a necessity. Szilar, Wigner and Teller
composed a letter for Einstein, and Einstein signed the letter* as the sender. They took the
matter so serious that they convinced the economist Alexander Sachs to personally deliver the
letter to the White House. (Use Einstein and letter and Roosevelt as keys to search the Internet will
get many sites containing this letter)
Refugee scientists from Hungary, Germany and Italy (L. Fermi, 1955) have worked under a
totalitarian political system, in which totalitarian leaders controlled everything including
universities and researchers. The governments knew whatever went on in university
laboratories. They were fearful of Germany being the first to develop the atomic bomb. Thus,
they took the initiative to bring the issue to the president of the United States. Most American
scientists at that time were usually unfamiliar with this type of political control, and they felt
less threatened.
The likelihood that Germany might develop an atomic bomb caused President Roosevelt to
act and he decided immediately to create an Advisory Committee on Uranium that would
give financial assistance to universities engaged in uranium research. The sum of $300,000
(remember these are 1940’s dollars) was immediately allocated to Columbia, Princeton, MIT,
Chicago, California, Virginia etc. Research on uranium and fission is complicated, and
information on many aspects of uranium and of fission is required. Each group worked on
one or more aspects of fission, and nuclear research was intensely carried out by many young
and old scientists.
The Chicago group worked on uranium, and the California group worked on plutonium. Soon
they realized that enriched 235U or pure 239Pu would be required for the construction of an
atomic bomb. For building an atomic bomb, production of enough fissionable material is the
most important task. However, other information such as identification of fission products,
accurate cross section for neutrons as functions of energies, moderating neutron motion, and
percentages of neutrons that induce fission are all required.
Review Questions
1. What is power? What do high political positions, reputation, money, science and knowledge have in
common? (Each of these represents a form of power. Agree or not agree, elaborate your view.)

*
Szilard (1898-1964), Wigner (1902-1995) and Teller (1908-) all played important roles in nuclear physics. Read about them.
Wigner shared the Physics Nobel Prize for 1963 with two other physisists, and Teller is known as the father of the hydrogen
bomb.
For a copy of Einstein’s letter, consult http://www.anl.gov/OPA/frontiers96/aetofdr.html
235
2. Comment on the action taken by Szilard, Wigner, and Teller. (Political and economical forces do
influence scientific development. The objective of this question is to raise the awareness of the
impact of science on politics and vice versa.)
3. Comment on the reaction of President Roosevelt.
Thermal Neutrons
Fermi's group irradiated uranium samples with neutrons. They surrounded the samples with
different materials at various times and found samples surrounded by water, wood, and
paraffin more radioactive.

Why surrounding the samples with water, wood, and paraffin increased their radioactivity?
What happens to neutrons when they collide with atoms and molecules in a medium?
Which one is more effective in slowing neutrons, heavy nuclides or light ones? Why?
What are the energies of neutrons after they have scattered many times with atoms?
How do these energies depend on the temperature of the medium?
After Fermi's group has learned of nuclear fission, they attributed the fission radioactivity
increase to the moderation (slowing down) of neutrons by hydrogen and light elements in
water, wood, and paraffin. They thought that neutrons are slowed quickly by collision with
protons, because the two particles have comparable mass. Neutrons can transfer almost all
their kinetic energy to proton in a collision.
Materials used to slow down fast neutrons are called moderators. On the average, 20
collisions with protons are sufficient for neutrons to reach an equilibrium state that further
collisions will no longer change their average kinetic energy. The average kinetic energy of
neutrons depends on the temperature of the
A Maxwellian Distribution of Kinetic
medium, in which they are in thermal equilibrium
Energy (Ek) of Molecules in a Material.
with. These neutrons are called thermal neutrons.
In the following paragraphs, we further describe
No. of molecules with
kinetic energy Ek
how energy of the molecules depends on the
temperature.
Molecules in a medium are constantly in motion:
298 K
373 K
vibration, rotation, and translation. The average
kinetic energy of molecules is directly proportional
to the temperature in K. At room temperature (293
K) the average kinetic energy of all molecules is
Ek
0.025 eV. Of course, some molecules have higher
and some have lower kinetic energies than 0.025 eV.
In fact, the kinetic energies of the molecules have a Maxwellian distribution. This skewed
distribution is depicted here, and it is different from the normal or bell shaped distribution. As
the temperature changes, the skewed distribution shifts slightly to give a higher average kinetic
energy.
236
The neutrons collide with molecules and atoms in the medium constantly, and their energies
have the same distribution as those of the molecules.
Neutrons are often classified as fast, thermal, and cold neutrons according to their kinetic
energies. Fast neutrons have a kinetic energy exceeding some threshold, typically 0.1 or MeV.
Neutrons just released from the fission reactions are fast neutrons. After some collisions with
atoms in the medium, they become thermal neutrons and their typical average kinetic energy
is 0.025 eV. The average kinetic energy of cold neutrons is less than 0.01 eV. Slightly different
boundaries of division may be given in other literature due to differences in view points or
definition of room temperatures, but these are typical values. Cold neutrons are either from
super cold hydrogen moderated experimental reactors or selected by diffraction from crystals.
There are some special applications for this type of neutrons.
Skill Developing Questions
1. Explain the following terms: moderator, fast neutrons, thermal neutrons and cold neutrons.
2. Sketch a distribution for kinetic energies of thermal neutrons.
3. Estimate the velocities of a neutron whose kinetic energies are 0.1 MeV, 0.025 eV, and 0.01 eV
respectively.
Thermal Neutron Cross Sections
Cross section () is a measure of the probability of a given reaction, as we have discussed
elsewhere. Cross sections are further classified according to types and reactions. Since thermal
neutrons are readily available thermal neutron cross sections (c), are important nuclear
data. They are usually given for each nuclide to indicate its probability of thermal neutron
capture. For possible fission material, the thermal neutron cross section for fission (f) is
also given.

How do you look for a suitable fission material for nuclear reactor from a nuclear data
source?
What parameters indicate the suitability of fission material?
What materials are suitable for the construction of fission reactors and bombs?
What are the thermal neutron cross sections (c) of some key elements that are useful for
the construction of atomic bombs and for nuclear reactors?
Thermal neutrons are much better than fast neutrons at inducing nuclear fission. Thus,
thermal neutron cross sections for all nuclides have been studied, because many materials are
required for fission device (bombs and reactors) constructions.
Around 1940, the Uranium Research Program measured thermal neutron cross sections for
various reactions of almost all nuclides. In the following list, thermal neutron (capture) cross
sections () and thermal neutron fission cross sections (f) are given for some key nuclides.
237
Half-lives (t1/2) of the radioactive nuclides are also given, because they are important properties
of the nuclides regarding fission device.
1
2
H
 c /b 0.33
f /b
t1/2 /y
12
14
C
N
0.0034 1.82
H
0.00052
16
O
0.0002
113
Cd
19,820
233
235
U
U
46
98
530
580
5
1.6×10 7×108
238
U
2.7
2.7×10-6
4.5×109
Note the large difference in cross sections between hydrogen, 1H, and deuterium, 2H, given
above. The difference warrants the extraction of heavy water (2H2O or D2O) from natural
water for fission device applications. Since the cross section for deuterium is small, heavy
water, D2O, does not absorb many neutrons, and using it as moderator for reactors gives
sufficient neutrons for using natural uranium as a fuel. If pure water is used as a moderator,
hydrogen atoms absorb to many neutrons and 235U enriched uranium is required as fuel.
Carbon and oxygen have very small thermal neutron cross sections compared to nitrogen.
When Fermi built the first nuclear reactor, he used carbon (graphite) as the moderator, and he
put the graphite (moderator) in cans to reduce nitrogen in reactor.
The extremely large thermal neutron cross section of 113Cd makes cadmium a good neutron
absorber or eliminator. The element cadmium contains many isotopes. The abundance (in %)
and thermal neutron cross sections (b) are listed below:
106
c / b
Abundance /%
Cd
1
1.25
108
Cd
1
0.89
110
111
Cd
0.1
12.45
Cd
24
12.80
112
Cd
2.2
24.13
113
Cd
19,820
12.22
114
Cd
0.3
28.37
The abundance of 113Cd is moderate but adequate. Furthermore, the neutron-capture reaction
113
Cd (n, ) 114Cd leads to a stable isotope. These properties made cadmium a very desirable
material for the nuclear technology industry.
The thermal neutron cross section of fission of 235U is 160,000 times larger that that of 238U.
Fission of 238U is negligible. This difference made it necessary to enrich 235U for nuclear energy
and atomic bomb material. Research in the 1940s revealed another important fissionable
isotope of plutonium 239Pu. Even though other isotopes of plutonium had higher cross
sections than 239Pu, their half-lives are very short. The half lives and thermal fission cross
sections of plutonium isotopes are given below for your reference:
236
f
t1/2
Pu
150
2.9 y
237
Pu
2100
45 d
238
239
Pu
Pu
17
742
88 y 24131 y
240
Pu
0.08
6570 y
241
Pu
1010
14 y
242
Pu
0.2
3.8×105y
Other factors in nuclear energy considerations were methods and costs of production. All
these factors led to the conclusion that only the production of 235U and 239Pu are feasible and
practical. Production of 233U was not worth considering.
238
Review Questions
1. What is the significance of thermal neutron cross sections and thermal neutron cross section for fission?
2. Compare the difference in thermal neutron cross section for hydrogen and deuterium. Describe the
implication of the difference.
3. What are the thermal neutron cross sections for isotopes of the following elements: boron, zirconium, and
cadmium? What are the products of neutron capture reaction for the stable cadmium isotopes? What are the
consequences of the capture reaction?
4. What are the abundances of uranium isotopes in natural occurring uranium? From the thermal neutron
cross sections and abundance, discuss work required for using uranium as a fuel for nuclear energy
generation. (Consult a hand book for the required data)
Fission Products
Fission products are nuclides produced in fission reactions. As suggested earlier, rubidium
and cesium as two of the possible fission products. Finding out fission products is certainly a
strategic project the fission research.

What are the fragments produced in nuclear fission?
What rays are emitted from the fission product, and why?
How does radioactivity of fission products vary over time after fission?
What is the distribution of the nuclides in terms of mass numbers?
What is the impact of fission products on the applications of fission reactions?
After capturing neutrons, the compound nuclei 236U are at excited states with excess energy.
The 236U nuclei undergo fission;  or  emission. The half-life for fission is much shorter (10-14
s) than those of  and  emissions (half-life for  decay is 2.3107 y). Fission is the preferred
process.
Since many nuclides are produced in the fission process, the study of fission products requires
the separation, identification, and quantitative determination of various elements and isotopes.
Since heavy nuclides contain more percentages of neutrons than light nuclides do, fission
products from the fission of heavy nuclides are too rich in neutrons. Thus, fission products
emit  particles until they are stable. This aspect has been illustrated when we estimated the
energy of fission reactions.
Since the nuclei usually split into two pieces of different masses, the mass numbers of fission
products range between 40 and 170. In terms of elements, they range from potassium, to
tungsten, nearly all the elements in the 4th, 5th, and 6th periods, including the lanthanides*. They
include alkali metals (K, Rb, Cs), alkaline earth metals (Ca, Sr, Ba), all the transition metals
*
We shall discuss the fission products lanthanides again in the section Natural Reactors.
239
from scandium to tungsten, metalloids (Ge, Sb, Te, Se, etc) halogens (Br, I) and inert gases (Kr
and Xe). Thus, separation of fission products into various elements is a complicated operation.
Some (2 to 4) neutrons are released per
fission reaction. The atomic numbers of
Sketch of Slow-neutron Fission Yields from
fission products are difficult to
235
U as a Function of Mass Number.
determine, because they rapidly undergo
 decay. Studies have revealed that most
log(Fission yield)
fission events are asymmetrical, with
|
heavy and light fragments, rather than
|
...
...
symmetric (with two equal fragments).
|
.
.
.
.
Relative amounts (in percentage of total
|
nuclides produced) of nuclides formed
|
.
.
.
.
are called fission yields. The plot of
|
.
fission yields from 235U against mass
|
.
.
number gives two peaks, one between
|
mass 80 and 110 and the other between
|
.
.
120 and 160. Between the two peaks is a
|
low yield region, the center of which
|
.
.
corresponds to a mass number 113. A
|
symmetric fission produces two
|___.__________________________.
60
90
110
140
170
fragments of mass number 113 if no
Mass
no.
neutron is emitted. The yield distribution
depends on the kinetic energy of the
neutrons, but all plots have the general
feature of two peaks in similar area. The
two peaks have slightly different shapes when kinetic energies of the neutrons are different.
Atomic bombs and nuclear reactors are two types of fission application. Fission-product data
and their behavior are of fundamental importance, because they have a great impact on the
environment and society. Fission products are left following bomb explosions and reactor
accidents. For example, some typical long-lived fission products such as 90Sr and 129I are used
for monitoring nuclear explosions and accidents. These data are also essential for reprocessing
used nuclear fuels and nuclear waste management.
Management of used or irradiated fuels also depends on radioactivity of fission products. Most
fission nuclides have very short half lives. After a decade, few nuclides remain radioactive. A
very low yield nuclide 85K has a half life of 10.7 years, and two other nuclides, 90Sr and 137Cs
have half lives of 29 and 30 years respectively. There are no fission nuclides whose half-life lies
between 30 and 105 years. Fission products with half lives greater than 100 years with yields
greater than 10–4 are 126Sn (1105 y), 126Tc (2.1105 y), 91Tc (1.9106 y), 135Cs (3106 y),
107
Pd (6.5106 y), and 129Tc (1.6107 y).
Fission products affect the operation of reactors in many ways, one of which is the absorption
of neutrons by fission products. The high-yield fission product 135Xe has a c of 2,640,000 b,
and a half-life of 9.2 hours. The presence of this product lowers the level of fission, and this
240
effect is often referred to as xenon poisoning. The chain reaction of the atomic pile in
Hanford suddenly stopped in July, 1944. John Wheeler, the poisoning expert, was consulted.
After checking the control parameters of the reactor before the interruption, he concluded that
it was the xenon. A few hours later, the reactor resumed function, and this is consistent with
the half-life of 135Xe.
Skill Building Questions
1. Assume two neutrons and 133Xe are produced in a fission reaction, what is the other fragment? Work out
the decay scheme and show the half lives of the fission products. (Consult a handbook for required
data).
2. Both 129I and 131I nuclides are produced in nuclear fission. Suggest a method for their isolation. What are
the half-lives of these nuclides? What are the daughter nuclides in the decays of these fission nuclides?
The First Fission Nuclear Reactor
Research on uranium has been divided into several tasks. With strong financial support from
President Roosevelt, some facts are well known to the inner circle of researchers involved with
the uranium project. Neutrons are released in nuclear fission of 235U. Thermal neutron cross
sections for many elements have been measured.

Will uranium undergo a chain fission reaction?
Will the chain reactions lead to a runaway explosion?
Can a chain fission reaction be controlled?
How to control a chain reaction to sustain for a long period of time?
Since neutrons are released, uranium undergoes a chain fission reaction, when the neutrons are
moderated, and sufficient number of them will cause the next generations of reaction. Firmly
believed in this, Fermi’s group assembled uranium into an atomic pile to test the feasibility of
a sustained chain fission reaction. They used natural uranium with graphite as moderator,
cadmium in the control rod, and boron in the neutron detector. These are the key
requirements for nuclear reactors.
Because this was the first atomic pile, only the trial and error method was available to them.
They experimented with various materials as they assembled the atomic pile. They used water
as the moderator at the beginning, and it did not work. They thought water absorbed too
many neutrons. They switched to graphite, still not working. They attributed the failure to the
impurity in graphite, so they purified graphite, and made it into bricks. Due to high thermal
neutron cross section for nitrogen (1.82 b), they put graphite and uranium into cans and
removed the air from them. Step by step, they identified and solved many problems. They
placed alternate layers of graphite bricks and pieces of natural uranium and constructed an
atomic pile in a racquet court at Stagg Field at the University of Chicago.
241
Another major problem for the first nuclear reactor was the size of the atomic pile. Various
calculations have given an estimate of the amount of required uranium, but experiments give
the ultimate test. Fermi’s group built up the pile, and tested the operation as the size grew.
After years of effort, the atomic pile had a sustained chain reaction of a fission nuclear reactor
on December 2, 1942 (Fermi, 1955). This was the beginning of the controlled fission reaction.
Its success not only provide the pile as a tool for other research, the reactor became a research
tool for future reactor design. Its operation provides data for the construction of larger and
more sophisticated reactors. It was indeed a great event.
The way they built the first reactor was risky and dangerous in today’s standards. For example,
control rods were manually handled. When the reactor was powered up for testing, the
emergency measures were solutions of boron and cadmium compounds ready to be poured on
to the pile by people standing on guard. On the other hand, every step was handled carefully,
and the reactor operation did not have any major problems.
In 1946 the first controlled nuclear chain reaction in Russia was achieved at the Kurchatov
Institute, four years following Fermi's in Chicago.
As of 17 August 1995 there were 425 nuclear power reactors in operation worldwide. At that
time, the U.S.A. had 107 nuclear reactors in operation, generating the most nuclear power,
more than twice that of France, the world second largest. According to the Uranium Institute
information, (www.uilondon.org), Belgium, France, Lithuania, and Sweden, had more than
50% power supplied by nuclear reactors in 1996, whereas the U.S.A., Canada and Japan had
22, 16, and 33% respectively.
Skill Building Questions
1. What is a chain reaction? In the fission reactions, what is the chain carrier? What is the principle of a
fission reactor? How can a chain reaction be controlled from a runaway explosion?
2. What method was used to build and test the first nuclear reactor? (Trial and error is a powerful
method for problem solving).
3. Why was water not a suitable moderator for Fermi’s first nuclear reactor? What are desirable properties for
a material used as moderator? (light mass, low thermal neutron cross section, low cost of
production, and desirable engineering properties)
242
The Atomic Bomb Project
The US government was under pressure to complete building the atom bomb because of the
threat that Germany’s ability under Hitler to build it first. In 1942 the United States put the
"Manhattan Project" into effect to speed up the construction of the atom bomb.
In December 1994, one of the most unusual events in postal stamp history occurred. A
planned U.S. Atomic Bomb Stamp was canceled less than twenty days after it was disclosed.
The stamp was a full color portrait of the atomic bomb's mushroom cloud with the caption
"Atomic bombs hasten war's end, Aug 1945". It was one of a set of ten commemorative issues
planned for the 50th anniversary of World War II in 1995.
Thus, issues related to the atomic bombs are always controversial. A summary of events in the
creation of the atomic bomb is given here because there is a lot to learn. However, you are the
judge for what is valuable.
The Manhattan Project
The responsibility of constructing the atomic bomb was first given to the Atomic Committee
of the Office of Scientific Research and Development (OSRD). In May 1942, committee
members E. Lawrence, A.H. Compton, H. Urey (all three were Nobel laureates), L. Briggs, E.
Murphree met with J.B. Conant, the director of OSRD.
On September 23, 1942, the Uranium Committee met with Secretary of War Henry L.
Stimson, Chief of Staff General George C. Marshall and other top military officers including
Major General Leslie R. Groves, who was named (assigned) the Executive Officer to carry out
the policy. The building of the atomic bomb was placed under the control of the military, with
a head office located in New York City at the Manhattan Engineering District. Thus, it is
known as the Manhattan project.

What is the Manhattan project?
What are the goals of the project?
How did it achieve its goals?
What are some of the management skills used in the Manhattan Project?
How did the project involve Canada and the U.K.?
The Manhattan Project was carried out under secrecy. Workers on the project were not
allowed to communicate their information to each other unless they were authorized to do so.
This process was called compartmentalization, and as a result some top scientists working
on the project did not know the ultimate goal was to build atomic bombs.
Once the Manhattan project was underway, discussion between Grove and J. Chadwick
brought cooperation between Britain and America. Both governments decided to relocate the
work to a place near Chicago from Cambridge, England. A joint venture between the Britain
and Canada started, and a large research establishment was set up in Montreal under the
243
general direction of the National Research Council (NRC) of Canada. Practically, the whole
Cambridge group under Dr. Halban moved to Montreal. In 1944, he was succeeded by J.D.
Cockroft. The joint British-Canadian-American enterprise selected Petawawa, Ontario on the
Ottawa River for a heavy-water nuclear reactor to produce both 239Pu and 233U, with heavy
water and uranium supplied by the United States. An experimental pile was constructed near
Chalk River, and a residential village of the workers was named Deep River. The first zeroenergy exponential pile (ZEEP) started operation on September 7, 1945. This was the first
nuclear reactor outside U.S.A.
General Grove controlled scientists working in the Manhattan project. The military mentality
was and still is different from scientists, and there were constant conflict between the
management and the scientists. The scientists thought building one, at most two bombs would
finish the war, but Grove followed the military principle: "once you have a bomb, its constant
supply must be maintained".
The history of the Manhattan project is interesting, and every worker has a unique version of
his or her own. To preserve the history, many top scientists were interviewed, and the records
preserved. Actually, many of the workers were Nobel Prize winners, some of whom were
awarded after World War II. This results in the publication of many books, and there is plenty
of information available from the Internet maintained by U.S. government libraries. There are
some official versions of the history, but personal reflections are more interesting to read.
Many aspects are associated with the Manhattan project. These aspects range from chemistry
to chemists, from physics to physicists, from engineering to engineers, from civilian to military,
from war to peace, from victory to defeat and from history to technology. Whatever your
interests are, you can always find something in this great human endeavour which started with
a simple objective of building a powerful bomb. However, once started, it has developed into
perhaps the most important project of human history. Many sections in this Chapter is part of
this project, and you may also find a lot to read on the Internet as well as in the Library.
Skill Developing Questions
1. What are the purposes of the Manhattan project? (problem solving).
2. How did the Manhattan Project affect the nuclear technology on Canada? What role did Canada play in
the Manhattan Project?
3. What type of person was General L. Groves? He was not a scientist, and the building of the atomic bomb
was a scientific challenge. How did Groves manage his difficulty? Who are his advisors?
244
Producing Materials for Atomic Bombs
The Manhattan Project was established to apply
nuclear fission for the construction of atomic bombs.
Extensive research has established that 235U and 239Pu
were practical fissionable nuclides. These were the
most desirable commodities for the atomic bomb
construction. Furthermore, they were required for
testing related to the design, and eventually the making
of the bomb.
235
239

How can pure 235U and 239Pu be produced?
What are the methods for their preparation or
production?
What was the principle used for each method?
U
Pu
At the start Several methods for the production of fission material were proposed, but at that
stage, none was a guaranteed success.
For uranium, the tasks were to separate* 235U from natural uranium which consists of 0.7%
235
U and 99.3% of 238U. The production of 239Pu involved synthesis of an entirely new element.
Urey's research group had extensive experience with gas diffusion. They knew that gas
molecules with different molecular mass could be separated by a diffusion method. They also
learned all properties about the compound UF6. Thus, they suggested a separation of 235UF6
from 238UF6 using a gas diffusion method. Lighter molecules pass through membranes
containing pin holes faster than heavier molecules. The molecular weight of 235UF6 is 389
compared to 392 for 238UF6. A gas of UF6 made from natural uranium contains both 235UF6
and 238UF6. When this gas passes through membranes and long tubes, the gas that first comes
through contains a little more 235UF6. After many stages of concentrations, the gas is more
concentrated with 235UF6. This is known as the gas diffusion method. Since the molecular
weights differ so little the industrial operation is a long and laborious process.
Another method suggested by Urey’s group is called the centrifuge method. The group
wanted to create a strong field of gravity by high-speed centrifugation for the separation of the
lighter gas. Like the gas diffusion method, the centrifuge method feeds UF6 gas into a series of
vacuum tubes 1 to 2 meters long and 15-20 cm diameter, each containing a rotor. When the
rotors are spun rapidly, at 50,000 to 70,000 rpm, the heavier molecules 238UF6 increase in
concentration towards the cylinder's outer edge. There is a corresponding increase in
concentration of 235UF6 molecules near the center. Enhanced concentration is further achieved
by inducing an axial circulation within the cylinder. The enriched gas is drawn off and goes
forward to further stages while the depleted UF6 goes back to the previous stage.
*
For uranium enrichment, consult http://www.uic.com.au/nip33.htm and
http://www.fas.org/nuke/guide/usa/facility/portsmouth_oh.htm, and http://www.npp.hu/uran/3diff-e.htm
245
To obtain efficient separation of the two isotopes, centrifuges rotate at very high speeds, with
the outer wall of the spinning cylinder moving at between 400 and 500 meters per second to
give a gravitation field a million times that due to the Earth.
Lawrence's group in California has built
machines to accelerate particles, and they knew
well how to manipulate the behavior of particles.
The group suggested an electromagnetic
method to first ionize the UF6 molecules,
accelerate them by electric field, and then bend
them by a magnetic field. Due to the mass
difference, 235UF6+ and 238UF6+ would bend along
different curvatures in an electromagnetic field.
Uranium Isotope Enrichment by the
Electromagnetic Method.
Another method called thermal diffusion to
enrich 235U developed in the Naval Research
238
From a
UF6
Laboratory was by Philip Abelson, who explored
235
particle
collector
UF6
nuclear energy for submarine propulsion. This
accelerator collector
project was led by Ross Gunn. The method also
makes use of UF6. Liquid UF6 is placed in the
middle layer of three long vertical concentric
pipes. Hot steam at 550 K is sent through the inner tube and warm water at 340 K is sent
through the outer layer. The liquid UF6 in the middle layer is sandwiched between two
different temperatures, and this causes the lighter 235UF6 to rise to the top and be collected.
Because hot steam is required, this method consumes too much energy, and the top portion is
only slightly enriched in 235U. The naval research lab was not part of the Manhattan project.
But the slightly enriched material sped up the enrichment process using the electromagnetic
method. Despite the strained relationship between Ross Gunn and the Atomic Committee, the
thermal diffusion technique was also adopted in Oak Ridge for the mass production of 235U.
Plutonium can be produced by bombarding 238U with fast neutrons, from particle accelerators
and from nuclear reactors using either graphite or heavy water as moderators. After a full day
of discussion, the Atomic Committee could not decide whether to eliminate even one
technique. They recommended to President Roosevelt to support all methods for the
production of fissionable material. Facing a national emergency, Roosevelt authorized an all
out effort for the production of 235U and 239Pu.
Skill Developing Questions
1. What methods are available for the enrichment of 235U? Describe how each of the method works.
2. How can 239Pu be produced? (Review Nuclear Reactions).
246
Critical Masses
Neutrons outside a nucleus decay with a half-life of 12 m. They are also absorbed by other
materials than nuclei of 235U or 239Pu. Only fractions of neutrons from fission are captured by
235
U nuclei leading to another fission reaction. In a small quantity of fissionable material, so
few neutrons induce fission reactions, and a chain reaction will not be sustained. The
minimum quantity for a sustained chain reaction to take place is called the critical mass or
critical size. Of course, the critical mass depends on the moderator, chemical and physical
states, shape etc. In general, the term refers to the minimum quantity for an explosion.

What are the critical smasses of 235U and 239Pu?
How can they be determined?
How to design and build devices for their investigation?
There are many factors to consider in order to estimate the critical masses for 239Pu and 235U:
the cross sections of all materials involved, the volume, the half life of neutrons, path of
neutrons etc. Thus, estimates must be verified by experiments. These experiments were
extremely dangerous, because an accidental assembly of a critical mass would lead to an
explosion. The experiment to determine the critical mass was called tickling the dragon's
tail. It was done at night in the remote Omega Canyon.
By 1944, enough 239Pu was produced. A fearless
32-year old Canadian from Winnipeg called
Louis Slotin was one of the workers who
performed these experiments. They operated a
strange machine called the "Guillotine". Pieces
of fission material dropped from the top of the
Guillotine. The neutron intensity was monitored
as the piece dropped and past an almost critical
assembly. As larger pieces were used, the
neutron activities increased. Using the
Guillotine, the two pieces were in contact for a
very short period, and they had hoped the short
contact time would limit the chance of a nuclear
explosion.
The Idea of a Guillotine for Critical Mass
Determination
Neutron
monitoring
devices
Releasing
mechanism
235
239
U or
Pu
Today the critical mass is known to be 300 g
(2/3 lb) and 900 g (2 lb) respectively for pure 239Pu and 235U. The critical mass depends on the
shape and purity of the sample.
Shortly after the war, L. Slotin, died in an accident due to overdose of neutron radiation, when
he, on May 21, 1946, separated an explosive assembly to protect his colleagues in the room
with his bare hands.
As mentioned at the start, the critical mass depends on the shape and other factors. To further
reduce the critical mass for atomic bombs, the Manhattan Project used explosion around
chunks of fission material to reduce the volume to increase the efficiency of neutron capture.
247
Atomic bombs are sophisticated devices, their design and construction are secretive. Nuclear
weapons include warheads for strategic ballistic missiles, smaller tactical nuclear weapons,
artillery projectiles, demolition munitions (land mines), antisubmarine depth bombs,
torpedoes, and short-range ballistic and cruise missiles. The U.S. stockpile of nuclear weapons
reached its peak in 1967 with more than 32,000 warheads of 30 different types; the Soviet
stockpile reached its peak of about 33,000 warheads in 1988.
Skill Developing Questions
1. Why is there a minimum mass requirement for a sustained chain reaction? What is the meaning of critical
mass?
2. How would you design an experiment for the testing of a critical mass for a nuclear reactor design or bomb
design?
3. Why are the critical masses of 235U and 239Pu different?
Nuclear Bomb Explosions
Under the management of Leslie Grove and the leadership of many top scientists such as Luis
Alvarez, Herbert Anderson, Hans Bethe, Niels Bohr (code name Nicholas Baker), Lyman J.
Brigs, James Chadwick, Arthur H. Compton, James B. Conant, Enrico Fermi, Richard
Feymann, Otto Frisch, James Franck, Clarence Johnson, Ernest Lawrence, Willard Libby,
John Marshal, Robert Mulliken, Edgar V. Murphree, Robert J. Openheimer, I. I. Rabi, Glenn
T. Seaborg, Emilio Segrè, Leo Szilard, Edward Teller, Robert L. Thornton, Harold C. Urey,
Eugene Wigner, Walter H. Zinn* , etc., Large quantities of 239Pu and 235U have been produced.

What kind of and how many workers were involved in building the atomic bomb?
How was the work divided in the Manhattan Project?
When and under what circumstances were the first three atomic bombs exploded?
What are the dangerous effects of atomic explosions?
The business of building the bomb started with research in the basic sciences of mathematics,
chemistry and physics. Scientific results were applied by engineering design, manufacture, and
processing through management. Jobs involved research, design, engineering, construction,
operation and management. Enormous financial resources (forms of power) have been
allocated to the Manhattan project, which recruited a large number of workers.
Two major sites of fission material production facilities were the 59,000-acre Oak Ridge and
the 450,000-acre Hanford Engineer Work. The total work force in the two areas was
125,000, and many of them had advanced degrees. Openheimer was the director for the design
and construction of the atomic bomb in a third site called Project Y (Los Alamos Laboratory)
within the Manhattan Project. Enormous amounts of work were done outside these sites.
*
Walter H. Zinn was a native of the Kitchener-Waterloo area.
248
Almost the whole of United State, Canada, and the U.K. were engaged in making the atomic
bombs.
A nuclear explosion is an uncontrolled nuclear
fission. When a critical mass is assembled,
neutrons from the natural fission process
initiate a chain reaction. The number of nuclei
undergoing fission reactions increases rapidly
leading to an explosion. The energy released in
fission reactions blow the fission material apart,
and at some point, the chain reaction stops. In
order to get a high efficiency, the construction
of a bomb is more sophisticated. Using
chemical explosives around the fission material
causes an implosion that compresses the fission
material into a high-density assembly. This
construction not only reduces the critical mass,
it confines the material from flying apart until
most of the fission material was used up.
The Implosion Arrangement
Ignition
points
Chemical
explosive
239
Pu
Fission material is surrounded by chemical explosive
which is ignited at many points simultaneously. The
explosion forces pieces of 239Pu together and even
reduces the volume to reduce the critical mass.
On July 16, 1945, a plutonium (Fat Man) bomb was tested in the desert area called Jornada del
Muerto (Journey of Death), part of the Alamogordo bombing range in southern New Mexico*.
Two hemispheres of plutonium, attached to a 30-meter steel tower, was forced to fuse
together by implosion so that they formed a critical mass for a rapid chain reaction (Rhodes,
1986). This is known as the Trinity Test, which was estimated to have cost (1945)
US$2,000,000,000.
Although the resignation and arrest of Mussolini on July 25, 1944, and suicide of Hitler on
April 30, 1945, ended the war in Europe, Japan was still engaged in fierce fighting with the
U.S. in the Philippines. Besides, Japan had occupied a large part of China, Vietnam, Malay
Peninsula, all of Korea, Hong Kong. Thus, work on the atomic bomb did not slow down.
Three weeks after the Trinity Test at 8:15 am local time on August 6, 1945, the atomic bomb
Little Boy was dropped on Hiroshima by a modified (to carry the A-bomb) B-29 bomber. The
235
U-fueled bomb exploded 660 m above the city to eliminate local fallout. Two pieces of 235U
were loaded into a special gun, one at the muzzle, and the other in the barrel. When fired, the
two pieces with a combined weight of 60 kg contacted, a critical mass was reached, and
explosion followed. The explosive energy was equivalent to 20,000 tons of TNT. The 235U
bomb has never been tested before. Note that pieces were brought together to reach the
critical size in different fashion, due to the property difference between uranium and
plutonium.
Three days later on the 9th, a 239Pu-fueled bomb exploded over Nagasaki, destroying half of
the entire city. This bomb is the same type as the Fat Man used in the Trinity Test. The heat
*
For more on The Trinity Site, New Mexico consult http://www.viva.com/sw.trinity.html
249
and light emitted when the bombs exploded were so intense that everything within 1000 m
vaporized. The explosion was seen as far as 300 km. Following the light and energy release
is the explosion shock wave that destroyed buildings and deafened ears. Secondary fires
caused further destruction. Earthquake detection instruments around the world measured the
shock wave. The fallout and radioactive fission products caused short-term and long-term
health problems for an indefinite period of time. A total of 130,000 died in the two explosions.
Many of the injured were left in such an awful condition that they envied the dead.
In Germany, the War Office received
enthusiastic letters in April 1939 for military
Sixteen hours ago an American airplane dropped one
application of nuclear fission. Experiments
bomb on Hiroshima, Japan, and destroyed its
of chain reactions using uranium and
usefulness to the enemy. That bomb had more power
graphite were planned. Werner Heisenberg
than 20,000 tons of T.N.T. ....
had erroneous results about the graphite
Harry S. Truman
1
experiment, and recommended that heavy
water be used as moderator. The German
team had enriched uranium to contain 1%
235
U for the project. Several times, Allies’ bombing destroyed the prototypes. The project was
still at the experimental stage when Germany was defeated. The German bombs, moderated
by heavy water, had such enormous sizes that they would have had to be delivered by huge
trucks or ships.
The Joliot-Curie team in France and the British weapon project also started work on the
possibility of building an atomic bomb, but neither had developed to the stage of actually
building the bomb at the end of WW II.
Skill Building Questions
1. How do the masses come together to reach critical masses in atomic weapons?
2. How do atomic bombs destroy and what are the differences between atomic bombs and chemical explosives?
What are the four stages or means of destruction by nuclear weapons? ( radiation and heat, shock
wave, secondary fires, fallout and radioactivity)
The Nuclear Arms Race
During World War II, the Soviet Union under the leadership of Stalin competed with the U.S.
and Britain for military superiority, even though the Soviets fought Hitler together with the
Allies. The cooperation between the United States and Britain (Canada was part of Britain at
that time) in the Manhattan Project made the competition fierce. The U.S. was, and still is, the
leader of the North Atlantic Treaty Organization (NATO), and the competition between the
communist block and the capitalist block is known as the cold war. The period lasted more
than 40 years until the communist block disintegrated due to economical failure of the
communist system, which did not give incentive for hard working.
250
Since the beginning of the Uranium project in the U.S., information and results on uranium
research were treated as top secret. Spy wars between the two blocks not only heated up the
cold war, but they raised much psychological tension in everyone’s mind living during that
time.

Why are results of scientific research treated as top secret?
What information is secret?
Why nations spy on each other?
How did the former Soviet Union carry out the atomic and hydrogen bomb projects?
The secret of science is for everyone to discover, but research is costly in terms of time and
manpower. Thus, nuclear data and principles are considered top secret. Spy activity for atomic
data and information is practiced.
In the Soviet Union, Igor Vasilyevich Kurchatov (1903-1960), first observed the fission
reaction in 1934 and he wrote a book called Splitting the Atomic Nucleus in 1935*. He built
cyclotrons for the study of radioactivity. In 1939, Kurchatov and his coworkers published
studies of nuclear chain reactions. The Soviet scientists also learned the German discovery of
fission reactions. However, their work on uranium was suspended when Germany invaded
Russia. The Soviet government needed scientific and technical strength for weapons of more
immediate value (Britannica, 1973).
After Germany surrendered at Stalingrad on February 2, 1943, Stalin heard of the American
effort to build atomic bombs. He ordered the program to be re-established in 1943 under the
control of Kurchatov. Soon, other physicists joined the panel.
By late 1940, Igor Kurchatov and others in Leningrad started studies on nuclear reactors after
having learned of nuclear reactions. One of Kurchatov's students observed the gradual
disappearance of articles on fission in US journals, and he wrote an impassioned plea to several
outstanding USSR physicists and the State Defense Committee to continue research on the
chain fission reaction. The government and Soviet scientists reacted to it, and the Uranium
Institute was established in Moscow.
The most noted spy within the Manhattan project is Klaus E.J. Fuchs (1911-1988). He was a
German born physicist who joined the German Communist Party in 1932 while he was a
student at the University of Kiel. He became a British citizen in England. When Otto Frisch
told him to come to the U.S. to work on the atomic bomb program, he went to the Soviet
Embassy in London. During his time in the U.S. he was regularly contacted by the Soviet
intelligence chief Gaik Ovakimian via H. Gold. He supplied the Soviet with a treasure house of
information about the atomic bomb, including the design of the 235U and 239Pu bombs. As a
result, the Soviets were well informed of the Manhattan project. The secret material was codenamed the "candy", and Gold was the "candy man" (Brown and MacDonald, 1977).
*
If the year is accurate, Kurchatov discovered fission before the German group did.
251
The "candy" certainly helped the Soviet nuclear program. Four years after the explosion in
Hiroshima, the Soviet Union detonated her first atomic bomb on August 29, 1949, much to
the astonishment of the U.S. government, and after confirmation by U.S. scientists, president
Truman informed the public of the explosion on September 23. Americans were alarmed, and
this led to the speedy development of the hydrogen bomb under the leadership of Edward
Teller in the US. Two years after the U.S. tested a thermonuclear (hydrogen) bomb, the
Solviet's also tested one. The cold war heated up and the arms race became frantic. Both the
U.S. and the Soviet Union have developed so many fission- and fusion- bombs that they could
have destroyed the world had there been a war.
Bethe hypothesized that most of the energy from the stars, including the sun, is derived from
fusion reactions, and soon after the World War II, efforts were directed towards the creation
of a weapon of tremendous power by utilizing fusion. In 1952-1953, the so-called hydrogen
bomb became a reality.
Skill Developing Questions
1. Highlight some of the events of the Cold War? (Korea war, Vietnam war, Berlin wall, Cuba crisis
etc. You will get thousands of sites searching with the key word “cold war”.)
2. In your opinion, what event marks the end of the cold war? (Have an idea of your own, rather than
take others’ words for granted)
3. What are the worst threat during the cold war? How does the end of the cold war affect our lives?
4. Had the cold war not ended, what can and shall we do?
252
Nuclear Reactors
Having seen the devastation of atomic bombs, scientists, including those who worked on the
Manhattan project, campaigned against further nuclear weapon development. They wanted to
turn the giant industries associated with the Manhattan project into peaceful applications,
especially as an energy supply.
Classified by purpose, there are two types: research and power generation nuclear reactors.
Research reactors provide subatomic particles for experiments and for working out the
parameters for reactor design, but power reactors are used for converting nuclear energy into
electric energy. Either one may be a breeder, which produces more fissionable material than
it consumes. All reactors make use of fissionable heavy nuclides 235U or 239Pu.
Controlled Nuclear Fission Reactions
Unlike uncontrolled nuclear explosions, chain fission reactions are controlled to maintain
steady states in nuclear reactor technology. The energy released is diverted away to serve our
needs.

What are the key components of fission reactors?
How can we control chain fission reactions from explosions?
How can fission energy be diverted?
Designs of fission reactors are
complicated with much of the work done
by engineers, particularly nuclear
engineers. However, the basic principle
for nuclear reactor is simple. A reactor has
some basic components: a reactor core
holding fission material or fuel;
moderator, control rods holding neutron
absorbers, monitoring devices and
indicators of operation, and an energy
transfer system.
Basic Elements of Fission Reactors
Control rods
moderator
Reactor
Core
Energy transfer system
Monitoring
Both natural uranium and 235U enriched
devices
uranium have been used in power
reactors. The melting point of uranium is
1403 K, but uranium metal undergoes a
phase transition at 933 K. The structure change in phase transition limits the operating
temperature to below 933 K. The melting point of UO2 is 3138 K. Thus, most modern
reactors employ uranium oxide pellets as fuel. The pellets are clad in either stainless steel or
Zircaloy. Zircaloy is zirconium (Zr) with 1% tin, and very small amounts of other metals. Its
usage was due to low thermal neutron absorption (about 1.5 b compared to 2.5 b for iron).
Zirconium is a hard, lustrous, silvery metal, very corrosion-resistant due to the formation of an
253
oxide layer coating, but will burn in air. It is unaffected by acids, except hydrogen fluoride
(HF), and alkalis. It has several isotopes with masses between 90 and 96, with an average
thermal neutron cross section s = 0.184 b. Stainless steel is preferable to Zircaloy for hightemperature, sodium-cooled reactors.
Since neutrons from fission reactions are fast or high energy neutrons, they have to be slowed
down in order to be captured by fissionable nuclei. Moderators are compounds containing
light nuclides such as 1H, 2D, 4He, 12C, 16O, 19F. The common substances suitable for
moderators are:
graphite,
H2O, D2O
He (100 Atm and 1273 K)
Be (high temperature liquid metal).
Na (773 to 873 K used in breeder reactor)
BeF2 + ZrF4 ( for GCR)
Combinations of moderators and coolants have been used for reactors.
Rates of fission reactions are proportional to the numbers of neutrons in the core area, and
these numbers are controlled by lengths of control rods in the core area. Control rods are
made of materials with high thermal neutron cross sections. These include:
Cadmium, Boron, Carbon, Cobalt, Silver, Hafnium, and Gadolinium
We have discussed the thermal neutron cross sections of cadmium earlier. Cadmium is the
most common material considered for control rods. However, the requirements for fission
reactors are different from nuclear bombs.
Another effective neutron absorber is gadolinium, which consisted of the following isotopes.
Their natural abundances and neutron capture cross sections are given:
152
Gd
Abundance /%
0.20
c / b
< 125
154
Gd
2.15
90
155
156
Gd
14.73
61,000
Gd
20.47
2
157
Gd
15.68
255,000
158
Gd
24.87
2.4
160
Gd
21.8
0.77
There are two isotopes of Gd that are particular potent for capturing neutron, both have odd
number of nucleons. Neutron capture reactions 155Gd (n, ) 156Gd and 157Gd (n, ) 158Gd lead to
stable isotopes of the same element.
The monitoring system consists of neutron, alpha, beta, and gamma detectors as well as
thermometers and liquid flow meters. Neutron intensities and radioactivity of some fission
products are monitored constantly, and all readings are displayed in control rooms. Careful
measurements before let to a comprehensive system for operation of the reactor.
The energy transfer system takes heat released from fission reaction away from the core by
pumping a cool fluid (gas or liquid) to the core.
Skill Building Questions
254
1. What are the key components of nuclear reactors? What materials are used in each of the components?
2. Why are uranium and plutonium oxides instead of metals used for the core of fission reactors? What are
the key requirements of nuclear reactors in terms of chain reactions?
3. What elements are good moderators?
4. What elements are commonly used for the construction of control rods and why?
5. What is the form of energy released in nuclear fission? How can the energy be diverted away? How is the
energy delivered to industry and home usage?
Types of Nuclear Reactors
After the war, the United States set up a Civilian Power Reactor Program (CPRP) to
coordinate the study of nuclear reactors for peaceful application of nuclear energy. The
Atomic Energy of Canada, Limited was and still is the Canadian body for nuclear technology
applications. Electric energy was seen as the basic utility to benefit the society the most.

How does one build an effective nuclear reactor for electric power generation?
Why are there so many types of nuclear reactors?
The first few nuclear reactors were built to study how nuclear reactors function, and perhaps
to produce plutonium for atomic bombs. After WW II, CPRP was set up to find out the most
economical and safe way to convert fission energy to electric energy. The Program decided to
investigate the following eight types of power reactors:
Fast Breeder Reactors (FBR)
Aqueous Homogeneous Reactors (AHR)
Heavy Water Moderated Reactors (HWR)
Pressurized Water Reactors (PWR)
Boiling Water Reactors (BWR)
Organic-Cooled Power Reactors (OCPR)
Sodium Graphite Reactors (SGR)
Gas-Cooled Reactors (GCR)
Although this is not an exhaustive list, it gives a good summary of various ways to construct
nuclear reactors. This list is a result of human ingenuity in turning scientific discoveries into
applications.
Fast breeder reactors are unique in that they are designed to produce fission material 239Pu or
233
U while generating electric energy. Large amounts of 238U capture fast neutrons and produce
239
Pu by the reaction 238U (n, 2) 239Pu. When 232Th is used instead of 238U, 233U is produced by
the reaction 232Th (n, ) 233U. Since neutrons at higher energy are required, water is not a
suitable moderator. Most FBRs use molten sodium as a coolant because it is economical, and
compatible with other structural materials. Sodium boils at 1153 K, and thus there is no need
to pressurize it. It is also a good thermal conductor, and the hot liquid from the reactor core
255
can be used for steam generation. FBRs require 20 to 30% fissionable material to produce
enough neutrons to reach critical operation. As a result, the cores are smaller (1.5 m diameter
by 0.9 m high) than the thermal reactors.
Aqueous homogeneous reactors (AHR) are also unique in that they have no cores.
Compounds of fission material are dissolved in a solution. The fission fuel and moderator are
in one homogeneous phase. Accidental explosions of large tanks of uranium solutions (passing
critical sizes) have been known. These tanks are uncontrolled homogeneous reactors.
Heavy water moderated reactors (HWR) are moderated by heavy water 2D2O. Typical
examples of HWR are the CANadian Deuterium Uranium (CANDU) Reactors. Using
heavy water as the moderator, natural uranium instead of 235U enriched uranium is used.
Producing heavy water is a simpler process than enriching 235U.
Boiling water reactors (BWR) operate at the boiling temperature of the water (373 K) in the
reactor core zone. Steam is generated and passed directly to the turbine for electric energy
generation. The disadvantage of this system is the radioactive steam in the turbine. The boiling
of water at the core made the water an ineffective moderator, resulting in operational
problems. Thus pressurized water reactors (PWR) operating under high pressure by multiple
pumps are more practical. The Westinghouse PWR operated at a pressure of 2500 psi (pound
per square inch), and pumped 93,000 gallon per minute. The coolant temperature ranges from
560 to 600 K (Masche, 1971). This coolant went through a heat exchanger to convert water
into steam for driving turbines of generators. The efficiencies of PWRs are about 30%.
Because of the success of pressurized water reactors, there has been talk about boiling heavy
water reactor (BHWR) or pressurized heavy water reactors (PHWR). In these reactors,
natural uranium can be used because of lower thermal neutron absorption of the heavy water.
Sodium graphite reactors (SGR) are moderated by graphite (12C) and cooled by molten
sodium metal. The design of these reactors is similar to that of fast breeder reactors. Natural
uranium is used as fuel when graphite is the moderator. These reactors produce 239Pu. When
the core is cooled by other gases, they are called advanced gas-cooled reactors (AGR). The
first AGR was built to produce plutonium for atomic bombs. The first AGR for electric power
generation was planned in the U.K., to take advantage of the experience already gained in the
plutonium production reactor. There are two AGRs in Hinkley Point and in Somerset, U.K.
They started power generation in 1976. The CO2 enters the core at 567 K and leaves at 918 K
to heat the water coil for steam generation. A few AGRs are in use in the U.K. and France, but
both countries have switched to light water reactors using enriched uranium. The AGRs can
reach a net efficiency of 40%.
Skill Developing Questions
1. Name five types of reactors and give a short description for each type.
256
2. Do fast breeder reactors create energy? Is the principle of energy conservation violated in the FBR
technology?
3. What are the advantages and disadvantages of the CANDU reactors?
Breeder Reactors
For fear of running out of supply in the future, scientists and engineers tried to build fast
breeder reactors (FBR) that produce more fuel than they consume while generating power.

Why are breeder reactors desirable?
What are some of the general characteristics of FBR?
What nuclear reactions can be used to breed fission material?
Which type of breeder reactor is the most economical?
How many breeder reactors have been built?
How many breeder reactors are in operation today?
According to the type of fuel bred, there are two types of breeder reactors. The 233U cycle (or
thorium cycle) makes use of the reactions, 232Th (n, ) 233U. Some related data regarding this
cycle are given below:
Th (half life, 1.4×1010 y) + n  233Pa + , (c = 7.4 b, f = 5.6×10-5 b)
232
Pa ( , ) 233U
233
(half lives, 27.0 d for 233Pa, and 1.6×105 y for 233U).
The thorium cycle breeder reactors require thermal neutrons. Molten metal and cold water are
used as the coolants.
The 239Pu cycle (or the uranium cycle) makes use of the reaction
U + n
238
239
U
(c = 2.7 b)
U ( , ) 239Np ( , ) 239Pu
239
The uranium cycle breeder reactors require fast neutrons. Liquid metal and steam may be used
as coolants for fast breeding.
257
The cores of breeder
reactors are small but
they contain highly
enriched fission fuel. The
enrichment reach 40 to
60% compared to 3% for
HWRs. The cores are
Boron
blanketed by layers of
238
232
graphite
U or Th to absorb
shield
the escaping neutrons.
Molten sodium or other
metal is used to transfer
the heat of fission from
the core to the heat
exchanger for power
generation. Basic parts of
a FBR is illustrated here.
Basic Elements of a Fast Breeder Reactor
Fuel
loading
machine
B
R
E
E
D
E
R
Core
Magnetic
pump
B
L
A
N
K
E
T
Heat
exchanger
The FBRs have been in
operation in the U.S., U.K., France, the former U.S.S.R., Japan, and perhaps in China.
In the U.S., Experimental Breeder Reactor No. 1 (EBR-1) began operation in December 1951.
The core had an approximate diameter of only 21 cm and a total volume of 6.2 liter. The core
power was 960 kw. During one of the critical experiments, the reactor had a melt down in
Nov. 1955. The Experimental Breeder Reactor No. 2 (EBR-2) had a power rating of 60 Mw.
A bigger Enrico Fermi Atomic Power Plant was designed to give 300 Mw.
There had been considerable interest in developing reactors for aviation and ships. Many
submarines are powered by nuclear reactors. The United State cooperated with France and
England in these ventures, and the former USSR had its own program. Fast breeder reactors
burn a large fraction of uranium, and the fuel costs were low. The heat produced in these
reactors can be used for power generation and other functions.
In Britain, the first FBR was built at Dounreay (Scotland), and it started operation in 1959.
This reactor core contained 46.5% 235U enriched uranium metal clad in niobium tubing. These
tubes formed a hexagonal array 0.54 m in diameter, and 0.54 m in height. The small reactor
core and the blanket produced 60 Mw thermal energy, and required a sophisticated cooling
system to handle. The electric power from this reactor was 14 Mw, giving an efficiency of
23%. The total mass of uranium was 220 kg. The breeder blancket contained 20 tons of
depleted uranium. The reactor was shut down in 1977. It served well, and it provided data for
the construction of bigger FBRs. A larger one built at the same location became operational in
1974.
Russia had a prototype called BN 350, which started operation in 1970. A French prototype
Phenix started in 1973. Phenix output an electric power of 250 Mw, and the next generation
Super Phenix, which generated 1200 Mw electric power, became operational at Creys Malville
258
in 1985. This is a pool type reactor with a core of 3.66 m in diameter by 1 m high and contains
32 t of uranium and plutonium oxide with plutonium concentration of 15% in the center
increasing to 18% in the outer elements. The coolant is 3200 t of molten metal.
A Washington Representative of The Federation of Electric Power Companies of Japan
recently wrote: ”To maximize these benefits on a long-term basis, the commercialization of fast breeder
reactor (FBR) technology remains the long-term goal in Japan's future energy program. To attain this goal, the
Power Reactor and Nuclear Fuel Development Corporation (PNC) has developed both an experimental and a
prototype FBR. The experimental reactor "JOYO" has been operational since 1977. In April 1994, the
prototype reactor "MONJU" reached criticality.” Thus, breeder reactors are actively being studied
despite the bad publicity of nuclear reactors due to accidents in recent years.
Skill Building Questions
1. What fission fuels are produced from breeder reactors, and by what reactions?
2. Describe some design features of a breeder reactor.
3. What is the future of breeder reactors?
The CANDU Reactors
The governmental body for nuclear technology in Canada is the Atomic Energy of Canada
Limited (AECL). It has played a key role in the development of nuclear technology in Canada,
including the development of the CANadian Deuterium Uranium (CANDU) Reactors.

Why are the CANDU reactors unique?
What are the unique features?
What are the advantages and disadvantages of CANDU reactors?
Most fission processes generate on the average between 2 to 3 neutrons per fission. The
neutron yields depend on the temperature of the moderator, and the following are some
typical average neutron yields for neutrons of various energies.
235
U + n(0.025 eV thermal)  fission + 2.42 n.
235
U + n(fast)  fission + 2.58 n.
233
U + n(thermal)  fission + 2.49 n.
233
U + n(fast)  fission + 2.59 n.
239
Pu + n(thermal)  fission + 2.88 n.
239
Pu + n(0.5 MeV fast)  fission + 3.0 n.
Some of these neutrons are captured by fissionable nuclides leading to another fission to
maintain the chain reaction, but most are wasted because they are absorbed by the moderator,
238
U, and the supporting structural material of the reactor. If each generation of neutrons fails
to produce at least as many fission reactions in the next generation, the chain reaction stops.
Natural uranium contains only 0.7% 235U. The combined absorption of 238U and 1H2O in the
moderator made natural uranium impractical as a fission fuel. The much smaller thermal
259
neutron cross section of 2D made heavy water D2O an attractive moderator for using natural
uranium as feeding stock for nuclear reactors.
The first heavy-water-moderated, light-water-cooled reactor, research reactor NRX was
commissioned in 1947. Natural uranium metal sheathed in aluminum was used in the core. Its
success led to the commission of NRU ten years later. NRU was a 200-Mw-research reactor,
moderated and cooled by heavy water. Later a nuclear power demonstration (NPD) station
was built at Rolphton, Ontario, Canada's first venture into power reactors and the first heavywater nuclear power plant in the world. NPD was a 20-Mw prototype, and it provided data for
the design and construction of CANDU reactors at Douglas Point Ontario, which was built
with the cooperation between AECL and Ontario Hydro.
The CANDU reactors are a successful venture in reactor technology. Some features of
CANDU reactors are described below.
(1) Today, 22 CANDU reactors supply 20% of electric power in Canada. AECL sold the first
CANDU reactor to Argentina in 1974. It and one sold to Republic of Korea become
operational in 1983. Some have been sold to India and China.
(2) The fuel tubes are tied together to form bundles, which are loaded into horizontal tubes of
the reactor cores for CANDU reactors. Fuel bundles can be replaced without having to shut
down the reactors. The Pickering Unit 7 sets a world record for continuous operation of 894
days without a shutdown.
(3) CANDU reactors use natural uranium oxide as fuel, eliminating expensive enrichment
process, but they require heavy water as moderator. Extracting heavy water is probably more
economical, but new methods for 235U enrichment may off set the cost in the future.
(4) Using natural uranium as fuel generates larger volumes of nuclear wastes in CANADU
reactors. More space is required to dispose CANDU nuclear wastes compared to reactors
using 235U enriched uranium as fuel. However, CANADU reactors also produce 239Pu, but the
costs for processing 239Pu is also higher than those from FBRs.
Recently, the web site (http://www.aecl.ca/) of AECL gave the following statements in its
overview: “Atomic Energy of Canada Limited (AECL) was established in 1952 as a Crown corporation
and reports to the Parliament of Canada through the Minister of Natural Resources. AECL's mandate has
naturally evolved over the decades from the development of peaceful applications of nuclear energy to a focus on its
flagship product, CANDU power reactors, and MAPLE research reactors.
AECL develops, markets, and manages the construction of CANDU power reactors, MAPLE research
reactors, performs associated R&D, carries out underlying reactor research, supplies CANDU and light water
reactor (LWR) support services, and offers radioactive waste management products and services.
AECL and its Canadian and international business partners have designed, engineered, supplied components
and managed the construction of CANDU units on four continents. The CANDU reactor is ideally suited to
providing the electricity requirements in the rapidly-growing economies of the Asia-Pacific region, and in other
countries around the world.
For more about CANDU reactors, visit the web site of AECL (aecl.ca). There are some
photographs about the nuclear core and reactors.
260
Skill Developing Questions
1. What are the unique features of CANDU reactors?
2. What are the advantages and disadvantages of CANDU reactors?
3. Ontario Hydro uses CANDO reactors for power generation. Find out the following: number of reactors in
operation, number of reactors in idle, amount of power generated by each reactor, total power consumption in
Ontario, percent of power supplied by nuclear reactors, other means of power generation and their share of
power generation, and power export or import in Ontario.
Nuclear Reactor Accidents
From scientific or engineering point of view, an accident is a series of undesirable events that
took place due to accumulated causes. A full disclosure of the background information and of
the details of the events helps to improve future design and operation. An accident is costly in
economic, cultural and social terms. Nuclear accidents attract attention, because of the
radioactivity associated with them.

How safe or unsafe are nuclear reactors?
What major nuclear accidents have happened in the past?
What are the major causes of these accidents?
For each case, what are the events taking place in sequence?
What can be learned from the accidents?
How can the accidents be prevented?
What are the consequences of accidents?
As early as 1952, a heavy-water-moderated light-water-cooled experimental reactor in Chalk
River had a partial melt down. Some radioactive materials were released, but in those days,
nuclear events were top secret. In 1958, a fuel element in another HWR was not cooled, but
radioactivity was contained.
Graphite fire occurred in a graphite-moderated gas-cooled reactor in Britain. About 20,000 Ci
of radioactive 131I were released. Steam explosion and graphite fire was blamed for the
Chernobyl reactor accident near Ukraine in 1986.
Sodium-cooled fast breeder reactors at Lagoona Beach (near Detroit) in the U.S. experienced a
cooling system blockage resulting in a partial meltdown in 1966. A secondary sodium leak
happened to a similar reactor in Japan in 1995. Some accidents have also happened for light
water reactors. The Three Mile Island reactor accident that happened in 1979 involved a PWR.
A BWR belonging to the U.S. army near Idaho Falls exploded in 1961.
Accidents do happen despite great effort made in their prevention. Those mentioned above
were some of the reported accidents, but there are other unreported nuclear reactor accidents.
To date, there are about 500 nuclear power reactors in the world, and the number is increasing.
261
Events in two well-known reactor accidents illustrate some problems encountered in nuclear
power technology. The accidents already have had their social and economical impacts due to
extensive reports in the media. These reports concentrate on the damages and effects, rather
than how the accidents have happened. Those contain technical details appeared only in
scientific and technical journals. Experts are more interested in the events leading to the
accidents than the phenomena of accidents, because they want to learn from the events to give
better designs, policies and guidelines to prevent or reduce accidents in the future.
Three Mile Island Accident: Reactor number 2 on Three Mile Island (TMI-2) was a
pressurized water reactor (PWR) designed to generate 3000 Mw heat (Thomas, 1980 and
Martin, 1980).
The reactor core consisted of 93 metric tons of UO2 made with enriched uranium. The UO2
powder was pressed, sintered into pellets of 9.4 mm in diameter, 45 cm length. The pellets
were stacked inside Zircaloy-4 cladding tubes of 10.9 mm diameter by 3.9 m in length. There
were 208 fuel rods in each fuel assembly, and 177 fuel assemblies in the core, which contained
three enrichment levels of 235U: 1.98%, 2.64%, and 2.96% by weight. Fuel assemblies were
protected with cladding covers from coolant corrosion.
Block Diagram of a Pressurized Water Reactor
Relief
valve
Reactor &
Pressurized
containment primary
building
cooling
loop
Heat
exchange
and steam
generator
Cooling
tower &
housing
Turbine & generator
Reactor
core
Pumps Auxiliary
pumps
&
valves
Secondary
cooling
loop
The reactor generated 2772 megawatts of heat, and the primary coolant was pressurized (15.1
megapascals or about 150 atm) water at 565 K, and heated up to 593 K at the outlet. A heat
exchanger and steam generator converted water in the secondary cooling loop to steam,
which rotated the turbine of the generator.
At almost exactly 4:00 am, March 28, 1979, a pump which supply the feed water to the steam
generator (secondary cooling loop) failed due to the trip of another pump, which was
connected to the same water supply system in series.
262
When the pump failed, operators tried to start the auxiliary pumps of the secondary cooling
loop to remove the heat from the primary coolant so that the reactor could be shut down to a
standby conditions. The valves of the auxiliary pumps had been closed for an earlier test
operation, but the valves were never turned back on. Since no feed water reached the steam
generator to remove heat from the primary coolant, the volume of the primary coolant
increased due to heating. The increased pressure of the system led to the opening of the relief
valves. All these happened within 9 seconds.
The relief valves did not close after the pressure fell to normal operating conditions, resulting
in loss of coolant. Part of the core was uncovered by water, and energy released from decays
of fission products heats up the core causing it to melt. The core damage was the worst of any
commercial power reactors till that time. At high temperature, the Zircaloy-4 was oxidized by
water, producing a large volume of hydrogen gas that eventually ignited. Extensive damage was
made to the core, the containment building, and the auxiliary building.
No one was killed or injured, and the leak
of radioactive material was limited.
However, the damage became the wellknown nuclear TMI-2 reactor accident. The
psychological pressure experienced by
residents near the reactors during and after
the accidents was immeasurable.
Long-life Fission Products in the Core after the
Accident at Three Mile Island.
Isotope
Activity /Ci
Half-life
Amount*
85
9.7104
10.7 y
4.71013
90
7.5105
28.8 y
9.81014
K
Sr
The decay energy of fission products
129
I
2.210–3
1.6107 y 1.61012
caused a melt down. The release of
131
radioactive fission products was a major
I
8.04 d
7.0107
7.01013
concern. From the fission yields, we have
133
Xe
5.25 d
some idea of their quantities. Some the
1.5108
9.81013
long-life fission products that remained in
137
Cs
30.2 y
8.4105
1.11015
core are tabulated by Voilleque (1980). The
* Amount = Activity x half-life (s)/0.693,
activity giving was called core-intensity, but
a number related to the number of nuclei, but
the details of measurements were not
due to uncertainty in the definition of Activity,
available. Thus, the total activity could not
Ithese value do not reflect the "real" amount.
be estimated. However, the activity enables
us to calculate the relative amount present.
Multiplying the activity by its half life gives a quantity which reflects the quantity of a nuclide.
The amounts for the nuclides in the table indicate higher quantities for 90Sr and 137Cs. These
are fragments of high yields, in agreement with the fission yield curve.
The Chernobyl Accident: The former USSR's nuclear power generation was derived from
reactor technology they have used to produce 239Pu. In the early 1970s, a team of scientists and
engineers embarked upon an ambitious power generation program. They coined the acronym
RBMK for the graphite-moderated, channel-tube-cooled reactors. Two pairs of 1,000 Mw
RBMK were commissioned at Leningrad in 1974, and later in Kurst and Chernobyl (1,500
Mw). Reactor 4 in Chernobyl had been in operation for three years prior to the accident, and it
had been running at 83% capacity in 1985.
263
Electric power is required to operate various controls for the shut down of a nuclear reactor.
Most power reactors have battery reserves in case of power failure. On April 26, 1986, Reactor
4 at Chernobyl was scheduled for a safety test in case of a sudden power failure. Ironically, the
safety test led to a catastrophic accident.
During a power failure, the pump would stop to send steam to drive the turbine of a
generator. The engineers wanted to find out if the power generated by the residual momentum
of the generator and turbine would be able to safely shut down the reactor. Since the electrical
system was tested, they ignored possible problems with the reactor. They paid little attention
to the safety of the reactor.
The test team shut down the emergency core cooling system (ECCS) before they started the
test. From 1.00 to 13:00 hr on April 26, the reactor power was gradually reduced to 50% as
planned and it was held at that level till 23:00 hr, at which time they further reduced the power.
When the power from the generator driven by the reactor become too low, the power from
the reactor can no longer safely operate the controls. The operators switched the controls to
power supplied by the grid (the nation wide network of regulated electric power). They forgot,
however, to send a signal to hold the power of the grid. Because the controls used so much
energy, the grid power was reduced to a low level resulting in unsafe operating condition.
Several undesirable conditions such as requiring excessive pumping for the steam remover,
over heating in the core area, and difficulty in reactor control were observed at this stage.
When reactor power fell to 30 Mw, they had to manually withdraw some of the control rods to
bring it back to 200 megawatt.
Between 23:30 to 1:20 hr, a series of operational and control difficulties were encountered due
to inadequate cooling. The operators should have shut the reactor down at this time, but they
increased power hoping to correct the problems. The reactor power became unstable, rose
rapidly at 1:23:40 hr, at which time the shift foreman ordered the shutdown of the reactor, but
it was too late. There was insufficient capacity left in the control rods which were in core, and
the others at the top of the core could not be inserted fast enough to overcome the power
increase caused by other factors. Power surged to 100 times the designed capacity generating
excessive steam, fragmented the reactor core, and eventually the catastrophic destruction of
the reactor building (Young, 1987).
The accident caused extensive damage. About 30% of the reactor core melted at temperatures
of 4000 to 5000 K. A quarter of the graphite block blew away from the core. Some 10% of the
graphite were burned in a fire; and the 1000-ton roof of the building housing the reactor blew
off, damaging all the pipes and electric connections. The chain fission reaction halted only
when the reactor disintegrated. After the explosion, emergency crews dropped sand, boron
and lead compounds to cover the core material to extinguish the graphite fire and to contain
the fission products from releasing.
The reactor core disintegrated and the buildings were destroyed. More than 7 million Ci of
radioactivity were released into the atmosphere. Thus, the Chernobyl accident affected a much
wider area than the TMI-2 accident. Due to the hot core and the graphite fire, nearly all the
264
inert gases in the fission products such as 133Xe, 85Kr, and 85mKr escape into the atmosphere on
the first day.
Much of the volatile fission products 131I, 132Te, 134Cs, and 137Cs had also escaped. Radioactive
iodine (131I) strontium (90Sr, 89Sr), etc., escaped on the first day into the atmosphere came down
with the rain in Scandinavia and other northern European countries (Clough, 1987).
Despite the containment effort, heat from radioactive decay (~15 Mw) caused the core
material to heat up a few days later, releasing more radioactive nuclides on the 7th, 8th and 9th
days. At that time, they pumped liquid nitrogen to cool the core material, and the fission
products release stopped on day 10. Different compositions of radioactive nuclides were
released in nine (9) days, and wind and rain carried them to various parts in Europe. Detailed
study of the compositions of radioactive nuclides in the atmosphere atop the damaged reactor,
and those that came down with the rain agreed with the idea that the wind was responsible for
the further distribution.
The health and social implications of reactor accidents will be discussed later after we have
considered the interactions of radiation with matter and the safety issues of ionized radiation.
Learning From Accidents: Nuclear reactor accidents influence not only the nuclear industry
but also everyone's life. Prior to these accidents, nuclear energy was considered one of the
safest and most economical, resulting in the construction of many nuclear power plants for
industrial applications and for raising living standards. Accidents have brought the issues
related to nuclear energy into the political arena. The public now wants to decide the future of
nuclear energy, so the hands of experts are no longer free. Information on nuclear technology
must be known to and understood by every educated citizen, and nuclear technology experts
must also bear social and environmental responsibilities.
Over-simplified descriptions have been given above for the reactor accidents. A full disclosure
of the details would require many volumes of documents to sort out all the lessons we and the
experts need to learn. In short, both design deficiencies and operational faults led to these
accidents. In both accidents, energy released by fission products contributed to the accidents.
It is easy to say that human errors are involved, but giving the same circumstance of operation
and mental state of the operators, another choice might not be possible. From the description,
one can get some idea about errors made during the course of the accidents, but there are
other reasons. Experts should be and are inspecting every piece of the records and they are
studying the damaged reactors for clues to improve future design and operation procedures.
There is much to learn from accidents.
Future of Fission Reactors:* Fission reactors have been used for research, power generation,
and breeding fissionable nuclides. Just after World War II, nuclear energy was considered
reliable, clean, economical, and plentiful. Thus, much development had been made for power
generators. As a result, all types of power reactors mentioned earlier had been developed and
are commercially available. Nearly 300,000 to 380,000 Mw were generated in 1960, and nuclear
*
For more information of civil nuclear industry, visit the web site of Uranium Institute: www.uilondon.org
265
power plant was in great demand in 1967 and 1968. Japan ordered three light water reactors
(LWR), and began operation in 1970. According to the report by Nagashima & Izumi (1968),
Japan would have reached 40,000 Mw by 1985, and 150,000 Mw by 2000, with a breeder
reactor planned for 1985. Many reactors had been ordered by developing countries, and the
energy definitely raises living standards of people in them.
In the 1970s, the public became aware of the danger of radiation, and problems with
radioactive waste compounded the predicament. Furthermore, the TMI-2 and the Chernobyl
Accidents had given a negative image to nuclear power generation, and they had slowed down
the development. Companies who invested heavily on nuclear power generation had not
anticipated these developments. Indeed, many difficulties are present in decommissioning
(orderly dismantling) a nuclear power plant. All factors such as construction, maintenance,
fuel, decommission, safety, solving social and environmental problems, and waste
management, have to be considered in the calculation of cost for nuclear energy.
The future of nuclear power generation very much depends on the costs of coal, gas and fossil
fuel. Economic conditions, energy demands and public perception will play important roles
about the future of nuclear energy. However, burning fossil fuel increases CO2, a green house
gas. Much scientific research is required to balance human demands of energy and their longterm survivor.
Skill Building Questions
1. Briefly describe the events resulting in the meltdown of the Three Miles Island reactor No. 2? What changes
in operation procedures will you suggest in view of the accidents?
2. Describe the Chernobyl's nuclear reactor accident.
3. Why are we more concerned with the radioactivity of the fission products, but not that of 235U and 238U in
nuclear reactor accidents?
Natural Fission Reactors
In 1972, a French chemist H. Bouziques analyzed the 235U abundance of uranium samples. He
found some contained 0.7171% 235U, which was low compared to normal values of 0.71950.7204%. He noticed that these low 235U samples came from Oklo, Gabon in West Africa.
Some samples from that area has 0.440% 235U, and this led to further exploration of Oklo.

What are the reasons for the low abundance of 235U in the samples?
Can it be due to natural fission reactors?
Could natural nuclear reactors exist? How did they start and end?
What evidences support the natural fission reactor site long ago?
266
One possible explanation of the low abundance is that 235U was depleted due to natural fission
long ago. The depletion of 235U may be due to natural fission reactors. When a large quantity of
uranium ore is concentrated in one location, a critical size can be reached. When moderated by
water, natural fission reactors are possible. However, additional supporting evidences are
required. Had there been a natural reactor, the fission products must be present.
Much of the fission products are rareearth elements, and the French group
studied isotope distributions of rare earth
elements in Oklo. In general, isotope
distributions of natural elements and
elements from fission products are
different. For example, the isotope
distribution of neodymium (Nd) is given
here. Apparently, the isotope distribution
of Nd from Oklo resembles that of fission
products. Isotope distributions of other
elements in Oklo ore showed further
evidence for a natural reactor, and they
estimated the existence of a natural
reactor 1.8 billion years ago, which was
dated by various radiometric dating
methods (Kuroda, 1982).
Isotope distribution of neodymium
mass
Natural
Fission
Oklo*
142
143
144
145
156
148
150
27.11
12.17
23.85
8.30
17.22
5.73
5.62
0
28.8
26.5
18.9
14.4
8.26
3.12
0
25.7
29.3
18.4
14.9
8.2
3.5
* Values after correction for decay and other
factors.
II
In fact, in Oklo, several natural fission reactor sites had been identified, one of which had a
110 m area containing 235U depleted uranium ore. The ore contains the rare earth elements:
lanthanum, cerium, praseodymium, neodymium, europium, samarium, and gadolinium.
Uranium ore from other areas lack these elements. Furthermore, other fission products such
as yttrium, zirconium, ruthenium, rhodium, palladium, niobium, and silver were found in
uranium ore of suspect natural reactor areas. There were iodine, krypton and xenon due to
fission in the natural reactor area as well. Fission products which were water soluble such as
rubidium, cesium, strontium, barium, and cadmium were not found, but zirconium, 90Zr,
resulting from decay of strontium, 90Sr, was found. These findings supported their
conclusions, and there had been many international studies on the Oklo phenomenon.
About 1.8 billion years ago, the 235U abundance would have been about 3%, like some slightly
enriched uranium today. It had been hypothesized that organisms in water increased oxygen
levels that oxidized the uranium making it soluble. The flow of water and die off of the
organisms concentrated uranium in certain areas, developing favorable conditions for natural
fission reactions. Water is a natural moderator, and the reactors were left to natural control.
For example, when the core temperature was too high, water would have boiled away.
Reduced moderation slowed fission rate. The presence of some fission products such as xenon
might have added to the slow down. When 235U was depleted, the chain reaction could no
longer be sustained, and the natural reactor died off (Cowan, 1976, Maurette, 1976).
267
Fast neutrons should have been present in
239
Pu

natural nuclear reactors, and they caused
nuclide mutations aside from fission.
239
Np


Reactions such as
235
235
236
238
239
U (n, ) 236U ( , ) 232Th,
U
U
U
U
and

238
U (n, ) 239Np ( , ) 239Np ( , ) 239Pu
should have taken place. The
232
Th

(n, )
transmutation of nuclides is shown here.
Note that 239Pu contributed to the fission
as well. The estimated neutron flux
Transmutation of nuclides in natural fission process
intensity was 1.5x1021 n/cm2. The French
director of the Oklo study group, R.
Naudet estimated an energy release of 15,000 Mw-years in the reactor zones, and 6 tons of
235
U consumed. The reactor life spanned for about 150,000 years.
Sites of natural fission reactors can provide valuable information such as the pattern of
distribution and migration of fission products. Data on the stability of geological structure
might also contribute to our needs in view of radioactive waste disposal (AECL, 1992).
After the discovery of a natural reactor, many hot spring sites were studied. The Cluff Lake
deposit in Saskatchewan, Canada, had similar conditions as those in Oklo (Libby, 1979).
However, there was no firm evidence pointing to a natural reactor in Cluff Lake.
Skill Building Questions
1. If the abundance of 235U is 0.72% today, what was its abundance 1.8 billion years ago?
2. What are the evidences for the existence of natural nuclear reactors in Oklo area?
3. How much energy had been released if 6 tons of
235
U were consumed?
Exercises
1. Who discovered nuclear fission? What was the social background and international
condition at the time? What was the implication of the discovery?
2. How was the element neptunium produced? Which isotope of neptunium has the longest
half-life? How was the element plutonium produced?
3. What particles should be used to bombard uranium to make transuranium elements?
Write the reaction equation and describe the cross section as a function of the energy of
the incident particle.
268
4. Assume the thermal-neutron-induced fission of 235U (mass = 235.0439) gives two
fragments of mass 140 and 93 (plus 3 neutrons). A check on the properties of nuclides
indicates that 140Ce (mass = 139.90539) and 93Nb (mass = 92.90638) are stable isotopes.
Estimate the total energy (including energies of beta-decay of the fission products)
available in this fission process. (Ans. 200 MeV).
5. In a fission reaction, the isotope 235U after capturing a neutron, splits into two fragments
plus on the average 2.5 neutrons. Assume the reaction produced two fragments, one
identified as 137Te (Z = 52), and two neutrons. What are the atomic and mass numbers of
the other fragment? What is the decay scheme for 137Te that would lead to a stable isotope
137
Ba (Z = 56)? Make up a simplified chart of nuclides to show the decay relationship.
6. In a nuclear fission reaction of 235U, three neutrons and a 147La nucleus are identified. What
is the third product X in this reaction? Hint: use the Periodic Table of Elements and
check the mass number from a handbook listing all nuclides.
7. Describe the four methods used for separating 235U from natural uranium. Can these
methods be used for separating isotopes of other elements? How was the isotope
deuterium separated from natural hydrogen?
8. Assume the abundance of 235U to be 5.0% when the earth was created. Calculate the age
of the earth from the distribution of 235U and 238U (now 0.72 % and 99.28). Note that the
assumption may not be valid.
9. Assume the percentage to be 0.72 % today. Calculate the percentage of 235U 1.8×109 years
ago. The half lives are: 235U, 7.038×108 y, 238U, 4.468×109 y. (Ans. 3.1%, see Natural
Fission Reactor in Text).
10. Assume that the average of all fission reactions is represented by the reaction.
235
U + n  136Te + 97Zr + 3 n.
And a check for stable isotopes with mass numbers 136 and 97 indicates that 136Ba56,
97
Mo42 are stable. The atomic masses are: 235U = 235.0439, 136Ba = 135.9044, 97Mo =
96.9058. Other required constants can be found elsewhere in the lecture notes.
(a) Calculate the energy released in the fission process, including energy from radioactive
decay of the fission products. (Ans. 201 MeV or 3.36 x 10-11 J per fission)
(b) A nuclear power plant is required to generate 100 megawatts, (108 J per second).
Calculate the 235U consumption in grams per second. (Ans. 1.2 microgram)
(c) If the efficiency of energy conversion is 60 percent, what is the consumption rate of
235
U?
11. What is the cross section of a nuclear reaction? Why are cross section data of various
nuclides required for the design and construction of atomic bombs and nuclear reactors?
12. What are moderators? What properties make some compounds good moderators? Why
are heavy water and graphite good moderators for nuclear reactors using natural uranium
as fuel, but normal distilled water not a suitable moderator?
269
13. From the atomic mass of He, 235U and 207Pb on the chart of the nuclides or Table of
Isotopes, calculate the total energy released for the (4n + 3) series of the four families of
radioactive series.
Further reading and work cited
Brown, A.C. and MacDonald, C.B. (Editors) (1977), The secret history of the atomic bomb, Dial
Press/James Wade.
Clough, P.N. (1987), The Chernobyl accident -- source terms and related characteristics, in The Three Mile
Island accident - diagnosis and prognosis, ACS Symposium Series 293, Edited by Toth etc.,
American Chem. Sociity Publication
Cowan, G.A. (1976), A natural fission reactor, Sci. Am. 235 (1), 36 (July)
Maurette, M, (1976), Fossil nuclear reactors, Ann. Rev. Nucl. Sci. 26, 319
Fermi, L., (1954), Atoms in the family, University of Chicago Press.
Libby, L.M. (1979), The uranium people, Crane Russak & Co., Charles Scribner's Sons, New York
Martin, D., (1980), Three mile island: prologue or epilogue?, Ballinger Publishing Co.
Shea, W.R. (1983) Otto Hahn and the rise of nuclear physics. Several essays edited by Reidel, 1983.
(QC773.078)
Thomas, G.K. (1980), Description of the accident in The Three Mile Island accident - diagnosis and
prognosis, ACS Symposium Series 293, Edited by Toth etc., American Chem. Sociity
Publication
Rhodes, R. (1986), The making of the atomic bomb, Simon and Schuster.
US (1977), Official history of the Manhattan project, Government Publication.
Voillequé, P.G. (1980) Fission product behavior, in The Three Mile Island accident - diagnosis and
prognosis, ACS Symposium Series 293, Editors: Toth, L.M. etc. American Chem. Society
Publication
Young, J.D. (1987), Chernobyl -- the accident sequence in Chernobyl, a technical appraisal, Proceedings,
British Nuclear Energy, London, October 3. pp. 27 - 42 (TK1362.S65.C479)
Hahn, O. (1966), Otto Hahn: a scientific autobiography. Translated by W. Ley, Charles Scribner's
Sons (QD22H2A313)
Groueff, S., (1967), Manhattan project, the untold story of the making of the atomic bomb, Little, Brown
& Company (QD22H2A313)
Izumi, K. and Izumi, T. (1968), Uranium requirement, long-term nuclear power generation program in
Japan, Symposium on the economics of nuclear fuels held by Internation Atomic Energy
Agency, Viena, 1968
Masche, G., (1971), Systems summary of a Westinghouse pressurized water reactor nuclear power plant,
Nagashima, Westinghouse Electric Corp
Wilson, D., (1983), Rutherford, simple genius. Hodder and Stoughton (QC16R8W5x)
270