Nuclear Fission

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Nuclear Fission

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Updates 20111029
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Nuclear Binding Energy
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Nuclear Binding Energy
1. Ni-62 (8.7946 Mev/nucleon)
2. Fe-58 (8.7925 Mev/nucleon)
3. Fe-56 (8.7906 Mev/nucleon)
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Uranium-235 Fission
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Uranium-235 Fission
A slow neutron can be captured by a uranium-235
nucleus, rendering it unstable toward nuclear
fission. A fast neutron will not be captured, so
neutrons must be slowed down by moderation to
increase their capture probability in fission
reactors.
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Uranium-235 Fission
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Fission Yield
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Fission Fragments
Average fragment mass is about 118. More probable
to break up into unequal fragments. The most
probable fragment masses are around mass 95 and
137. Most of these fission fragments are highly
unstable (radioactive), and some of them such as
Cs-137 and Sr-90 are extremely dangerous when
released to the environment.
From Wikipedia
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A Fission Fragment Example
A common pair of fragments from uranium-235
fission is Xe and Sr 235U n ? 236U ? 140Xe
94Sr 2n The xenon decays with a half-life of
14 seconds and finally produces the stable
isotope cerium-140. Strontium-94 decays with a
half-life of 75 seconds, finally producing the
stable isotope zirconium-94. These fragments are
not so dangerous as intermediate half-life
fragments such as cesium-137.
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Fission Products
Many of the early decays of short-lived nuclei
are high-energy, most of the long-halflife
fission product nuclides are of relatively low
energy and relatively less biological risk.
Exceptions are in red.
Long-lived fission products Long-lived fission products Long-lived fission products Long-lived fission products Long-lived fission products
t½ (Ma) Yield () Q (keV) ß?
99Tc 0.211 6.1385 294 ß
126Sn 0.230 0.1084 4050 ß?
79Se 0.295 0.0447 151 ß
93Zr 1.53 5.4575 91 ß?
135Cs 2.3 6.9110 269 ß
107Pd 6.5 1.2499 33 ß
129I 15.7 0.8410 194 ß?
Medium-lived fissionproducts Medium-lived fissionproducts Medium-lived fissionproducts Medium-lived fissionproducts Medium-lived fissionproducts
t½ (yr) Yield () Q (keV) ß?
85Kr 10.76 .2180 687 ß?
113mCd 14.1 .0008 316 ß
90Sr 28.9 4.505 2826 ß
137Cs 30.23 6.337 1176 ß?
121mSn 43.9 .00005 390 ß?
151Sm 90 .5314 77 ß
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U-235 Fission product isotope signatures
The isotope signatures of natural neodymium and
fission product neodymium from U-235 which had
been subjected to thermal neutrons. Note that the
Ce-142 (a long lived beta emitter) has not had
time to decay to Nd-142 over the time since the
reactors stopped working.
The isotope signatures of natural ruthenium and
fission product ruthenium from U-235 which had
been subjected to thermal neutrons. Note that the
Mo-100 (a long lived double beta emitter) has not
had time to decay to Ru-100 over the time since
the reactors stopped working.
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Dangerous Fission Fragments
Cs-137 Sr-90 I-131
30.17 y, ß-, ? 28.79 y, ß- 8.02 d, ß-
Similar to potassium and rubidium by living organisms and taken up as part of the fluid electrolytes. This means that it is passed on up the food chain and reconcentrated from the environment by that process. Sr-90 mimics the properties of calcium and is taken up by living organisms and made a part of their electrolytes as well as deposited in bones. As a part of the bones, it is not subsequently excreted like Cs-137 would be. It has the potential for causing cancer or damaging the rapidly reproducing bone marrow cells. Strontium-90 is not quite as likely as cesium-137 to be released as a part of a nuclear reactor accident because it is much less volatile, but is probably the most dangerous components of the radioactive fallout from a nuclear weapon. Iodine-131 may give a higher initial dose, but its short half-life of 8 days ensures that it will soon be gone.
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Typical Fission Reaction
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Typical Fission Reaction
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Energy Calculation
95Rb      ?  95Sr    e-    ?e  (beta minus
decay)
Average for a typical fission of U-235 2.4
neutrons 215 MeV(0.231 amu)
Energy (MeV) distribution in fission reactions Energy (MeV) distribution in fission reactions
Kinetic energy of fission fragments 167
Prompt (lt 10-6 s) gamma ray energy 8
Kinetic energy of fission neutrons 8
Gamma ray energy from fission products 7
Beta decay energy of fission products 7
Energy as antineutrinos (ve) 7
U-235 235.0439299
Rb-95 94.92930289
Cs-137 136.9070895
3n 3.025994747
Delta(amu) -0.181542809
E(MeV) -169.1060531
E(J) -1.6316E16
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Nuclear Power Plant
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Schematic Diagram of a Nuclear Reactor
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Typical Neutron Absorption Cross Section vs.
Neutron Energy(neutron speed)
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Good Neutron moderators

• large scattering cross section
• small absorption cross section
• large energy loss per collision

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Neutron absorber
The most prolific neutron absorbers are elements
that become stable by absorbing a neutron.
Boron, Cadmium and Gadolinium are all used as
neutron absorbers in commercial fission power
plants, while Xe-133 occurs naturally as a
byproduct of the fission process.
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(No Transcript)
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Fuels

• U-235
• Some reactors operate with natural uranium (0.7
  U-235), some with slightly enriched uranium (3
  U-235). Since weapons require about 90 U-235,
  the uranium used in reactors cannot be diverted
  to weapons use.
• Pu-239

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Arguments against nuclear energy

• Accidents
• Waste
• Expense

24
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Arguments for nuclear energy

• Political instability in oil-rich areas
• Does not contribute to global warming.
• Renewables Cannot supply all energy needs for
  several decades

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Spent U-235 Fuel

• Spent fuel from nuclear reactors still contains
  considerable amounts of 235U but now has
  generated significant 239Pu.
• After 3 years in a reactor, 1,000kg of
  3.3-enriched uranium (967kg 238U and 33 kg 235U)
  contain 8 kg of 235U and 8.9 kg of plutonium
  isotopes along with 943 kg of 238U and assorted
  fission products.
• Separating the 235U and 239Pu from the other
  components of spent fuel significantly addresses
  two major concerns. It greatly reduces the
  long-lived radioactivity of the residue and it
  allows purified 235U and 239Pu to be used as
  reactor fuel.
• (Courtesy of the Uranium Information Center)

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Nuclear Fission-Spent Fuel
Hazards of the radioactivities in spent fuel
compared to uranium ore
From Science, Society and Americas Nuclear
Waste, DOE/RW-0361 TG
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Fusion and Fission Yields
D-T Fusion
U-235 Fission