Chapter 6 – Nuclear Energy
Dr. Muhammad Yousaf
CHY 583
Copy right © Muhammad Yousaf, 2018, 2020, 2021.
Copyright
The copyright to this original work is held by Dr. Muhammad Yousaf and students
registered in course CHY583 can use this material for the purposes of this course but no
other use is permitted, and there can be no sale or transfer or use of the work for any other
purpose without explicit permission of Professor Muhammad Yousaf.
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Why Nuclear Energy?
Global warming, caused by the anthropogenic emission of carbon dioxide is a very serious threat
for our planet.
Nuclear power plants have the capability to produce a significant part of our electric power at
relatively low cost and without any carbon dioxide emissions.
The fuel for the nuclear power (Uranium) is found on Earth in abundance that it can produce a
huge amount of power.
Nuclear power fuel is ~ 8000 time more efficient than fossil fuels.
Nuclear power plants need 28 tonnes of fuel per year whereas the coal power plant takes about
2,000 tonnes of fuel per week where the fuel is a fossil fuel and it has very dangerous by-product
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Atomic Structure
Mass Number
# of protons = # of electrons
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Atomic Number
Isotopes
There are atoms that have the same number of protons and electrons, but different
numbers of neutrons. Such atoms are called isotopes i.e., the same atomic number but
different atomic mass.
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Radioactivity
In general, stable isotopes have approximately the same number of protons and neutrons. When
there is a significant imbalance in the numbers of protons and neutrons in the nucleus, the atom is
unstable and may undergo a spontaneous transformation to become the nucleus of another
element. This spontaneous transformation process is called radioactivity.
Examples: U-235, U-238, Pu-238
In general, it is a spontaneous decay process of unstable isotopes. The half life of an isotope is also
a commonly used parameter to characterize the decay process.
Examples: T1/2 of iodine-131 is 8 days whereas that of Sr is 30 years i.e., each elements has its
own T1/2.
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Discovery of Radioactivity
Henri Becquerel first discovered the emission
of rays from minerals.
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Marie Curie
In 1899, Marie Sklodowska Curie
applied the term radioactivity to
the spontaneous emission of
radiation by certain elements.
Marie Curie won two Nobel
Prizes—one in chemistry, the
other in physics—for her research
on radioactive elements.
1 Curie (Ci) = 3.7 × 1010 disintegrations/second
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Radioactivity is assessed by
counting the no. of disintegrations
of a sample (alpha, beta and
gamma emission) in a given time
period. Measured in the unit of
Curie, millicurie, microcurie,
nanocurie and picocuies.
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Radioactive Decay
Radioactive isotopes undergo
decay series until they reach a
stable species.
The radioactive decay of U-238
and Th-234 are first two steps of a
14-steps sequence.
All isotopes of all elements with
atomic number 84 (Po) and higher
are radioactive.
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Half-Life
A half-life, t1/2, is the time required for
the level of radioactivity to fall to onehalf of its initial value.
Each radioisotope has its own halflife. Some, like plutonium-239, take a
very long time (24,110 years),
whereas others, like plutonium-231
(8.5 minutes), decay very quickly.
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Half-Lives of Selected Radioisotopes
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Radioisotope
Half-life (t1/2)
Found in Used Fuel Rods of
Nuclear Reactors?
Uranium-238
4.5  109 years
Yes
Potassium-40
1.3  109 years
No
Uranium-235
7.0  108 years
Yes
Plutonium-239
24,110 years
Yes
Carbon-14
5,715 years
No
Cesium-137
30.2 years
Yes
Strontium-90
29.1 years
Yes
Thorium-234
24.1 days
Yes
Iodine-131
8.04 days
Yes
Radon-222
3.82 days
Yes
Plutonium-231
8.5 minutes
No
Polonium-214
0.00016 seconds
No
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Types of Nuclear Radiation
Radioactivity includes alpha, beta, and gamma rays:
Name
Symbol
Composition
Charge
Change to the Nucleus that Emits It
Alpha
4
2He or α
2 protons
2 neutrons
2+
Mass number decreases by 4
Atomic number decreases by 2
1 electron
1−
Mass number does not change
Atomic number increases by 1
photon
0
No change in either the mass number
or the atomic number
Beta
0
−1e
Gamma
0
0γ
or 𝛽
or γ
Gamma rays are a part of the electromagnetic spectrum, with more energy (shorter wavelength)
than X-rays.
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Radiation Effects
Nuclear radiations are hazardous because alpha, beta and gamma particles have sufficient
energy to ionize the molecules they strike. Example; H2O to H2O+ by beta particle.
Dose (Sv)
Dose (rem)
Likely Effect
0–0.25
0–25
No observable effect
0.25–0.50
25–50
White blood cell count decreases slightly
0.50–1.00
50–100
Significant drop in white blood cell count, lesions
1.00–2.00
100–200
Nausea, vomiting, loss of hair
2.00–5.00
200–500
Hemorrhaging, ulcers, possible death
5.00
>500
Death
rad = “radiation absorbed dose”: absorption of 0.01 J of radiant energy/kg tissue
rem = “roentgen equivalent man”: Q x number of rads, where Q is a relative biological
effectiveness factor (1 Sievert (Sv) = 100 rem)
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Nuclear Fission Reactions
In 1938 Hahn and Strassmann in Germany observed that barium-139 was produced when
uranium-235 interacted with a beam of neutrons. This was the first demonstration of a fission
process.
Fission occurs when neutrons are captured by heavy nuclei, such as U-235. The resulting nucleus
is unstable and soon splits into two large fragments. A few—typically two or three— free neutrons
are also released in the process.
1
0n
+
235
92U
→
236
92U
→
141
56Ba
92
+ 36
Kr + 3 10n
Three stage process.
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Chain Reactions
During the fission reactions, 1 neutron
produces various fission products and a
few additional neutrons as in the case of
U-235.
Each of these generated neutrons can
initiate the fission of U-235, which will
generate more neutrons, and so on.
This is known as a chain reaction, in
which one of the products becomes a
reactant, making it possible for the
reaction to become self-sustaining.
The critical mass of U-235 is 15 kg (33
lb), which is needed for spontaneous
fission reaction in the presence of a
neutron source.
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How Much Energy Is Released?
E = mc2
It is possible to obtain a tremendous
amount of energy from a very small
amount of matter.
For the fission of 1.0 kg of U-235, the
mass of products is 0.1% less than
original U-235.
Therefore, m = 1  10−3 kg
E = (1.0  10−3 kg)(3  108 m/s)2
E = (1.0  10−3 kg)(9.0  1016 m2/s2)
E = 9.0  1013 kg m2/s2 = 9.0  1013 J
(recall: 1 kg m2s−2 = 1 joule)
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Energy Production
Electricity generated by nuclear
and fossil fuels is identical.
The rate of energy production is
power; a common unit of power is
joule per second, J/s, or watt.
Figure: The Byron nuclear power plant in Illinois.
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Nuclear Power Plant
Nuclear power plants are similar to the conventional power plants. In nuclear power plants, the
heat is supplied to the cycle from the nuclear reactor. The reactor is a substitute for the boiler or
the combustion chamber.
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Nuclear Reactors
A nuclear reactor is a Chamber in which nuclear chain reactions are initiated, controlled, and
sustained at a steady rate (as opposed to a nuclear explosion, where the chain reaction occurs
in a split second).
1. Reactor fuel
2. Fuel moderator/coolant
3. Control systems/control rods
4. Safety devices
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The Uranium Fuel Source
The uranium fuel in the reactor core is
uranium(IV) oxide, UO2, comparable in
height to the diameter of a U.S. dime.
These pellets are placed end-to-end in
tubes composed of an alloy of zirconium
and other metals, which are grouped
into stainless steel-clad bundles.
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The Reactor Fuel
Uranium (92U235 and 92U238 with traces of the radioactive isotope 92U234) and thorium (90Th232)
are the only naturally occurring minerals that may be used as fuel in nuclear reactors.
Of these, 92U235 is the only fissile material and the main fuel of the
current generation of thermal nuclear reactors.
The other isotopes, 92U238 and 90Th232; are fertile isotopes. With neutron
bombardment these isotopes are converted to the fissile isotopes
239 and
233 respectively.
94Pu
92U
The fuel is usually slightly enriched uranium, which typically contains
2.0–3.5% of the isotope 92U235. Uranium oxide (UO2) is the most
commonly used reactor fuel materials.
The reactor fuel is in the form of small cylindrical pellets (diameter 1 cm
and height 1.5 cm), placed in the fuel elements/fuel rods (ID slightly
greater than 1 cm and the length of 5 m).
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The Fuel Moderator and Coolant
Moderator:
• Common water (H2O) has excellent moderating properties and is very inexpensive.
However, because the capture cross section of water is relatively high, water moderated
reactors typically require the use of slightly enriched uranium as their fuel, which adds to the
fuel cost.
• Heavy water (D2O) is an excellent moderator, but it is expensive to produce in large
quantities (0.0156% of deuterium vs 99.98% of hydrogen).
• An aqueous solution of boric acid (H3BO3); used in Byron nuclear reactor.
Coolant:
• Water (H2O) is commonly used as a reactor coolant as well as the moderator. Heavy water
(D2O) has better moderating characteristics, but its production cost is high.
• Carbon dioxide (CO2), helium (He) and argon (Ar) are the most commonly used gases in
Gas Cooled Reactors.
• Mercury (Hg), potassium (K), sodium (Na) or a mixture of the last two are the most
commonly used coolants in the Fast Breeder Reactors, where the rate of heat produced per
unit volume of the reactor is very high.
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The Control Systems/Rods
The elements boron, indium, cadmium and hafnium have the
ability to absorb neutrons and may be used in control
systems.
• A frequently used control rods are made of boron carbide,
BC.
The control rods may be completely or partly inserted in the reactor.
The system of control rods is suitable for emergency shutdown of the reactor.
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Nuclear Reactor Types & Power Plant
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The CANDU Reactor
The CANDU is a heavy water reactor (HWR) that was developed by the Canadian nuclear
program. It is essentially a PWR operating with D2O as coolant and moderator.
Light water (H2O) is cheap and well-suited to be both a reactor moderator as well as coolant.
However, it use slightly enriched uranium (2–4%) because of high capture cross-section (0.66
barns). (Natural uranium consists of 0.715% 92U235 and 99.285% 92U238)
On the contrary, heavy water (D2O) has an extremely low capture cross-section (0.001 barns)
and absorbs a very small number of neutrons. Reactors that are moderated and cooled with
D2O operate with natural uranium, which is significantly cheaper than enriched uranium.
However, D2O is expensive to produce, because the isotope must be separated from common
water, where it exists at a molecular ratio of 1:6,000.
Thus, the CANDU reactor has higher capital cost associated with the production of D2O, but
lower fuel cost, because it uses natural uranium.
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Cooling of Nuclear Reactors
The adequate and continuous cooling of the reactor core is the critical regardless of the type,
unlike other power plants, because of
• Residual neutron flux
• Radioactive decay
If the thermal power produced is not promptly removed, it can lead to the melt down on the
reactor.
Example: a typical nuclear reactor that normally produces 3,400 MW thermal power would
continue to produce heat at the following rates: 170 MW at 10 s after the shout-down, 68 MW
after 1000 s, 26 MW after 10 h, and 4 MW after 40 days.
• Nuclear power plants have a primary cooling circuit and one or two emergency cooling circuits
that are designed to operate continuously, even when the plant does not produce any electric
power.
• All the three nuclear accidents happened due loss of coolant.
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Accidents in Nuclear Power Plants: TMI-2
Three-Mile Island: accident happened in Unit 2 of
the Three-Mile Island power plant (TMI-2)
(Harrisburg, Pennsylvania) on March 30, 1979 due
to the failure of a condensate pump. The
emergency cooling system was not operational.
Because the heat generated by the reactor was not
removed at sufficiently high rate, the water level at
the top of the reactor was significantly lowered
(PWR reactor type) which resulted to a temperature
increase to 2,100 0C.
The fuel cladding (zirconium alloy) partly melted and also reacted chemically with the steam to
produce hydrogen.
In the case of TMI all the damage was contained within the reactor and the only environmental
consequence was the release of small amounts of radioactive steam.
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Accidents in Nuclear Power Plants: TMI-2
SCAM operation started at 8sec.
The release of vapors into the containment building, initiated the ECCS at 2min4sec.
One of the ECCS pumps was shut off by the operator at 4min38sec.
After 8min, operator realized that feed water valve was not working which was fixed. Pressure was
continuously dropping but was maintained at 1030 PSI between 20 min – 1h14 min.
• At that time, one coolant pump was shut off & the other was shut off at 1h40 min.
• At 2h18min the operator realized that the pressurizer valve was not functional which was fixed. In
the mean while, the damaged was done to the reactor.
• The produced H2 was bled off to the generator to react with O and more damage was avoided.
•
•
•
•
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Accidents in Nuclear Power Plants: Chernobyl
On 26 April 1986, reactor # 4 at the
Chornobyl Nuclear Power Station blew
up during a routine daily operation.
Nearly nine tons of radioactive material
were hurled into the sky.
• A large quantities of hydrogen produced (like in the Three-Mile Island accident) which caused
an explosion in the reactor. The explosion completed the destruction by lifting and permanently
tilting the biological shield of the reactor.
• Additionally, the control rods were made of graphite (unlike those used in the U.S.) which
caught on fire.
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Accidents in Nuclear Power Plants: Chernobyl
At 1:23:04 am, the experiment was initiated by shutting down the steam supply to the turbine
when the reactor operating at 200 MW and 18 rods inserted (a min limit of 28/211). They overrode the reactor automatic shut-down system at 1:23:10. Significant steam generation increased
the power. At 1:23:40 a vigorous boiling occurred. At 1:23:43 power increased to 530 MW and
300,000 MW at 1:23:48 am resulted first explosion. The H2 gas explosion occurred at 1:23:58 am,
shield of the reactor destroyed and the interior of the reactor exposed.
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Another Disaster: Fukushima
Caused by two natural disasters, an
earthquake followed by tsunami, which
resulted in the meltdown of three
reactors.
After earthquake, control rods were
inserted and emergency cooling
system was turned on. Flood waters
knocked out the electrical generators
needed for cooling water. This caused
overheating of the fuel, which then
started a chemical reaction that
generated hydrogen gas.
Most of the released radiations were contained in the four containment buildings. However, the
venting of the steam and the fires that followed the accident increased significantly the
radiation levels close to the reactor and in the entire Fukushima region.
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Breeder Reactors
Natural uranium consists of 0.715% 92U235 and 99.285% 92U238. The conventional nuclear
reactors only utilize the isotope 92U235 which is a fissile isotope.
The fertile isotope 92U238, can be converted to 94Pu239 (which is a fissile material) with the help
of a breeder reactor.
A fast breeder reactor has the dual purpose of heat/steam production and fissile material
production.
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The Future of Nuclear Energy!
The current, high grade natural uranium reserves in North America are estimated at 8,000
Quads (8,000×1015 Btu), sufficient to supply all the energy needs for the continent for
approximately 100 years, if only 92U235 is utilized.
However, the same high grade ore would be sufficient to supply the North American continent
with energy for 5,500 years if the 92U238 isotope, which is a fertile material, were to be
converted to 94Pu239 and used as fissile material.
In addition, if all the known reserves (high and low grade) of uranium were used in breeder
reactors, this amount would be sufficient to supply the energy needs of North America for more
than 30,000 years.
Conversion of 90Th232 to 92U233 (fissile isotope) can add to the above supply.
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The Future of Nuclear Energy: To Breed or Not to Breed?
The Fast Breeder Reactors may hold the answer to the solution of the energy challenge of the
human society.
Fast Breeder Reactors (FBRs) would solve the problem of 92U238 waste by converting it to the
fissile 94Pu239 and will consume the latter as a fuel.
Drawbacks:
FBRs produce 94Pu239, which is the fissile material used in nuclear bombs and one of the risks
associated with their operation is a small nuclear explosion.
The production of 94Pu239 contributes to global political and military instabilities and poses a
threat by itself.
The very high power density of FBR’s and the necessity to use Na or K as coolants has been
the cause of minor accidents.
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Top 10 Nuclear Generating Countries
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Worldwide Expansion of Nuclear Power
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Nuclear Waste and Environmental Effects
The transportation and storage of the waste materials from the nuclear power plants is a significant
global environmental threat.
At present, the nuclear waste is typically stored in temporary
facilities (water pool), where the nuclear waste is immersed.
The heat produced by nuclear disintegrations is convected to
the water of the pool, which is maintained at almost constant
temperature by evaporation.
Any accidental or intentional release of radioactive materials
from these sites may render whole regions uninhabitable.
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Treatment of Nuclear Waste
Initial Treatment of Nuclear Waste:
Vitrification (glassification) of the waste: The nuclear waste (after evaporation of volatiles) is
melted with glass and solidified in steel containers i.e., vitrified.
Vitrified materials are very stable. They are hard, water resistant, have very low erosion or chipping
and are believed that they are capable to last unaltered for thousands of years.
Concentration of the waste. Concentrated to a smaller volume usually with ferric hydroxide
(flocculation) removes highly radioactive metals from aqueous solutions.
Synrock is a complex chemical material of nuclear waste stabilization. Synrock consists of
hollandite (BaAl2Ti6O16), zirconolite (CaZrTi2O7) and perovskite (CaTiO3). The zirconolite and
perovskite become hosts and immobilize the actinide elements by trapping them.
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Treatment of Nuclear Waste
Long term disposal of Nuclear Waste:
Geologic disposal: Either in deep and stable formations on the earth or in the deep sea.
Transmutation: Transformation of radionuclides to other materials that are not radioactive.
Waste re-use: Accompanies by the concentration process, the produced high-radioactivity
materials may be re-used in a nuclear reactor for the production of additional power.
Space disposal: Given that it costs more than $25,000 to lift a kg of mass to the space, this is
extremely expensive and has not been proven to be a reliable way of nuclear waste storage.
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