Science A 52
Lecture 22 May 1, 2006
Nuclear Power
What is it?
What are its problems and
prospects?
Spring 2006
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Lecture 22, 1
Nuclear Fission
On of the most interesting accounts a fission and the
discovery of the release of two or more neutrons during
fission is on the WEB along with the voices of the major
participants.
You can almost feel the excitement in their voices as they
described their reactions as experimental results came in.
Let us listed to a few.
http://www.aip.org/history/mod/fission/fission1/02.html
Spring 2006
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Lecture 22, 2
History of Nuclear Fission
http://www.aip.org/history/mod/fission/fission1/02.html
Here is a summary from the above site.
It was only in 1938 that many scientists began to focus
their attention on uranium, the heaviest of known elements.
Leading the pack were two German chemists, Otto Hahn and
Fritz Strassmann. For over thirty years, Hahn had been
working with another talented scientist, Lise Meitner.
However, Meitner was of Jewish ancestry, and had to flee
Nazi Germany. Otto Hahn recalls...
Miss Meitner - Professor Meitner - had left our laboratory
on July 1938 on account of these Hitler regime things and
she had to go to Sweden. And Strassmann and myself,
we had to work alone again and in the autumn of '38 we
found strange results.© Harvard Science, A 52 FHA+MBM
Spring 2006
Lecture 22, 3
Fission History - continued
By 1938, Hahn and Strassmann were among a number of
scientists who were trying to find out what products are
formed if you shoot neutrons into heavy elements. They
hoped to find elements even heavier than uranium. Such
altogether new elements would surely have scientific interest,
and perhaps even practical uses. But the substances Hahn
and Strassmann produced looked like radium or barium, two
known and almost chemically identical elements...
HAHN: We made precipitations, Strassmann and myself,
where we could be absolutely sure that there could be
nothing else but either radium or barium.
Spring 2006
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Lecture 22, 4
Now Meitner and Frisch -her nephew -understood what had happened in Hahn and
Strassmann's experiment. The neutrons which they had shot into uranium had indeed
been captured by the uranium nucleus. But then the nucleus changed shape, vibrated,
and came apart entirely. This was not the usual slight transformation of a nucleus.
The picture did however fit neatly with a recent theory of Niels Bohr's. He believed
that a nucleus behaves like a liquid drop, and a liquid drop when hit hard enough,
might stretch until it broke in two. Now, if that happened to a nucleus, a lot of
energy would be released—atom for atom, far more energy than any process
seen till then.
This is what Meitner and Frisch thought happened to the nucleus
of uranium after it had captured a neutron
Spring 2006
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Lecture 22, 5
A cartoon of U235 neutron capture
http://www.atomicarchive.com/Fission/Images/fission.gif
The target 92 U235 nucleus is often unstable with the
additional neutron,and in ~ 10-14 seconds it splits into two
fragments of nearly equal mass that are also unstable and
in turn eject neutrons and γ rays. Not all captures of a
neutron of 92 U235 results in a fission. Some captures
produce 92 U236+ γ ray energy.
Spring 2006
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Lecture 22, 6
Let us listen to comments on
one who was there when the
first reactor went critical.
http://www.aip.org/history/mod/fission/fission1/09.html
Spring 2006
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Lecture 22, 7
Energy release during fission
To make the calculation of the energy released in fission
we need to make a mass balance of all of the particles
before and after fission - we will fine that the mass after
fission is less that that before. The “missing” mass has
been converted to energy.
Some of the energy will be in the kinetic energy of the
fission fragments, of the neutrons, and the gamma rays.
In particle physics energy is generally expressed in electron
volts.
1 eV = 1.6x10-19J
1eV is a very small unit of energy,but a proton has a very
small mass. Let us see how to use this unit of energy.
Spring 2006
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Lecture 22, 8
Charge on the electron
http://www.recipeland.com/facts/Oil-drop_experiment - The_apparatus
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Lecture 22, 9
Energy expressed in Electron Volts
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html#c2
Spring 2006
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Lecture 22, 10
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html#c1
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Lecture 22, 11
Fission
Calculation of mass conversion to energy
We can use Einstein’s mass-energy relation:
E = mc2
For nuclear calculations it is most useful to express the
energy in MeV ⇒million electron volts.
An electron volt is the amount of energy given to an
electron accelerating it through a voltage difference of
one volt.
We need the famous equation of Einstein cast in mass
units used in particle physics.
Spring 2006
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Lecture 22, 12
Fission
Calculation of mass conversion to energy
E(MeV) = m(amu) X 931
One atomic mass units (amu) is 1/16th
the mass of O 16 .
Using this scale the mass of the
proton and the neutron are nearly 1.
Mass of hydrogen atom = 1.00813 amu
Mass of the proton = 1.00758 amu
Mass of the neutron = 1.00897 amu
Spring 2006
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Lecture 22, 13
Fission
Calculation of mass conversion to energy
• We are now prepared to make a calculation of the
energy released in fission of 92U235. What we need
to do is to determine the products of fission when
this nucleus captures a neutron and fissions.
• The fission reaction is:
U235 +neutron →2 fission fragments +
ν neutrons +β- and γ-rays + energy
To proceed we need to know the fission fragments
and then their weight in amu. The fission yield of
fission fragments from U 235 are known.
Spring 2006
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Lecture 22, 14
Glasstone, Samuel and
Edlund,Milton;
Nuclear Reactor Theory, van
Nostrand,
1952, pg.67
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Lecture 22, 15
Fission
Calculation of mass conversion to energy
The isotopic weight of U235 = 235.124 amu, and the neutron
involved in fission has an amu =1.00897. Therefore the
reacting particles have a total weight of 236.133 amu and a
mass number of 235 + 1 = 236
The most probable type of fission, nearly 6.4% of the total,
gives products with mass numbers of 95 and 139, adding
gives a mass number of 234 - two mass numbers less that the
input, hence this fission must yield 2 neutrons. The fission
products are stable nuclides with masses of 94.945 and
138.955, respectively. Doing the mass balances
Before = After Fission
236.133 = mass converted to energy +(94.945+138.955) +
2x1.00897
∴mass converted to energy = 236.133 - 235.918 = 0.215 amu
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Lecture 22, 16
Fission
Calculation of mass conversion to energy
∴Energy released in this fission would be 931x0.215 =
198 MeV
The average values for all fissions of U235
Glasstone, Samuel, Nuclear Reactor Engineering,van Nostrand, 1955; pg.22
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Lecture 22, 17
Fission Neutrons per Capture
•Fission of 92 U235 leads to the possibility of a
chain reaction because an average of
2.5 neutrons are released in each fission of
235.
92 U
•Not every time a U235 atom captures a neutron
does fission occur, sometimes just this reaction
occurs:
U235 + neutron →U236 +γ-rays
•Making an allowance for non-fission capture,
there are only 2.1 neutrons released for each
time a neutron is captured in U235
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Lecture 22, 18
Heat released by 1 pound of
Fissionable Material
is 3.6 x 1010 BTU
or
fission of 1 gram of material
gives roughly 1 Mw day of
power
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Lecture 22, 19
Natural Uranium
• Isotopic Composition of Natural Uranium
Spring 2006
Mass
number
234
%
.006
235
0.712
238
99.282
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Lecture 22, 20
Making a Nuclear Reactor
• It is impossible to make a nuclear power reactor
using natural uranium since the concentration of
U235 is so low. Only 2.1 neutron are available for
the chain reaction.
• Fission neutrons are released with high KE, and
fission occurs most easily with thermal neutrons low neutron absorbing material is needed to slow
the neutrons down without capture. Steel tubes
and the like easily absorb thermal neutrons and
cannot be used in a reactor using natural U.
• The first reactors built used natural U and were
very large - very pure graphite moderators.
• Now slightly enriched U is used in power reactors.
Spring 2006
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Lecture 22, 21
Making a Nuclear Reactor
• The first reactors built at Oak Ridge and Hanford
were not for power but to create Plutonium239 a two
step decay product from neutron capture in U238.
• Pu239 can be used for bomb making and is
separated from U and fission products chemically.
• About 5% of the power of the reactor is in the
decay of the fission products (FP).
• This means that when the reactor is shut down,
heat continues to be released by the FP and
cooling must be provided.
• Disposal of the FP and the Pu produced remains a
serious problem.
Spring 2006
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Lecture 22, 22