nuclear fission
NUCLEAR FISSION Murray Ch 6
1
NUCLEAR FUEL Fission Fuel Energy Density: 8.2 x 1013 J/kg Fuel Consumed per 1000-MWe: 3.2 kg/day
San Onofre Nuclear Plant / California 3 units, total electric power ~2500 MWe
FOSSIL FUEL
Fossil Fuel (Coal) Energy Density: 2.9 x 107 J/kg Fuel Consumed by 1000-MWe Plant: 7,300,000 kg/day
2
Murray Fig 6.1
Simple illustration of fission. The nucleus acts somewhat as a water drop. When struck by neutron projectile, the compound nucleus oscillates, distorts, separates into fission fragments, additional neutrons and gamma radiation. A couple hundred MeV released. Almost any element with Z>90 can be fissioned with sufficiently high neutron energy. (note that high energy alphas and gammas can also induce fission) Typically, a fission process results in two FF. The entire periodic table of elements and isotopes can formed. (more than two FF can occur, called ternary FF(√), ~1/400 fissions. Often the third FF is small such as tritium, He, ) Many isotopes undergo spontaneous fission: Pu-240?, Cf-252, Even U-235 undergoes spontaneous fission, but w/ very long half-life (1017 years!) ν = average number of neutrons emitted per fission (=2.44 for U-235, =3 for Pu-241) Prompt fission neutron energies ~0.1 –10 MeV
3
A Few Terms: Chain Reaction Additional neutrons are produced to sustain the fission process Critical Fission reaction is exactly balanced in a steady-state condition. Neutron Production = Neutron Losses Fissile Material A material that can produce a self-sustaining chain reaction
U-233
U-235
Pu-239
Pu-241
4
Fertile Material A material that can be converted into a fissile material
Th-232
U-238
Pu-240
Fissionable Material A material that can contribute to the chain reaction but not be able to sustain a chain reaction by itself
Th-232
U-238
Pu-240
Z>90
5
TABLE 6.1
Murray
The energy released per fission of U-235 is ~ 200 MeV This table shows how the energy is distributed. The KE of the FF, released neutrons and primary radiations are available as heat (191 MeV). The energy carried away by the neutrinos from beta decay is lost.
6
Fig Knief 2.8 U-235 neutron energy distribution spectrum ~independent of incident neutron energy causing the fission Probability vs. neutron energy. Prompt fission neutron energies ~0.1 –10 MeV Area under the curve is one, or 100% of the emitted neutrons. The most probable energy is ~0.7 Mev and the average energy ~2MeV Note that within light water reactors we use thermal neutrons (~0.025 eV), thus the fission neutrons need to slowed or moderated.
7
BY-PRODUCTS OF FISSION: The source of high-level radioactive waste
8
Fission product decay, RE: Knief Fig 2-7 The entire periodic table of elements, and their isotopes are produced from fission. FF are radioactive. Fission is the source of high-level radioactive waste. This figure illustrates the decay chains for a couple of radioactive FF. Prompt emission of neutrons. ν = average number of neutrons emitted per fission (=2.44 for U-235, =3 for Pu-241) Prompt fission neutron energies ~0.1 –10 MeV Note the delayed emission of all kinds of radiations (beta, gamma, ..) Delayed neutrons (not shown) constitute ~1% of the n population in a critical reactor and are important in reactor control. Fission Product Distribution (Fission Yields/May West Curves) Similar to Fig 2.6 Knief, Plot of fission product yield vs. atomic mass number. Usually a ~1/3 – 2/3 distribution of masses. A ‘double-hump’ distribution. Sometimes called the May West Distribution. (May West was a popular, sexy movie starlet during the 1930’s) Higher percent yields with mass numbers ~90-100 (Sr-90, Nb, Mo, …) and ~130-150 (I-131, Xe-135, Cs-137, Sm-149…) The asymmetry becomes less with increasing neutron projectile energy.
9
10
May West 1933
“my little chickadee” W.C. Fields
11
12
Notes Distribution of isotopes is fission signature. Oklo natural reactor story. Oklo: map of Gabon Oklo Phenonomon Gabon, West Africa ~1.7 Gy (1.7 billion years) ago a uranium deposit became inundated with water’ Went critical, nuclear chain reaction sustained ~100,000 years U-235 enrichment was greater then, comparable to current LWRs ~3-5% U-235 half life ~ 7(10)8 y How do we know it went critical? Soil samples yielded May West Curves for Fission Product Distribution. Recent evidence of natural geysers / steam vents?
Decay Heat from 3,000 MWt LWR Reactor The radioactive decay of fission products produces heat DECAY HEAT. This is a plot of Knief Eqn 2.10, decay heat vs. time Important for spent fuel handling, waste management, reprocessing, & reactor safety.
13
14
Decay Heat from 3,000 MWt LWR Reactor 8 MW
P(t) ~ P(0)At-a
6
A = 0.066 a = 0.2 Murray p. 337
4
2
0 0
1
10
100
1000
10000
years
15
16
17
1
NUCLEAR FUEL Fission Fuel Energy Density: 8.2 x 1013 J/kg Fuel Consumed per 1000-MWe: 3.2 kg/day
San Onofre Nuclear Plant / California 3 units, total electric power ~2500 MWe
FOSSIL FUEL
Fossil Fuel (Coal) Energy Density: 2.9 x 107 J/kg Fuel Consumed by 1000-MWe Plant: 7,300,000 kg/day
2
Murray Fig 6.1
Simple illustration of fission. The nucleus acts somewhat as a water drop. When struck by neutron projectile, the compound nucleus oscillates, distorts, separates into fission fragments, additional neutrons and gamma radiation. A couple hundred MeV released. Almost any element with Z>90 can be fissioned with sufficiently high neutron energy. (note that high energy alphas and gammas can also induce fission) Typically, a fission process results in two FF. The entire periodic table of elements and isotopes can formed. (more than two FF can occur, called ternary FF(√), ~1/400 fissions. Often the third FF is small such as tritium, He, ) Many isotopes undergo spontaneous fission: Pu-240?, Cf-252, Even U-235 undergoes spontaneous fission, but w/ very long half-life (1017 years!) ν = average number of neutrons emitted per fission (=2.44 for U-235, =3 for Pu-241) Prompt fission neutron energies ~0.1 –10 MeV
3
A Few Terms: Chain Reaction Additional neutrons are produced to sustain the fission process Critical Fission reaction is exactly balanced in a steady-state condition. Neutron Production = Neutron Losses Fissile Material A material that can produce a self-sustaining chain reaction
U-233
U-235
Pu-239
Pu-241
4
Fertile Material A material that can be converted into a fissile material
Th-232
U-238
Pu-240
Fissionable Material A material that can contribute to the chain reaction but not be able to sustain a chain reaction by itself
Th-232
U-238
Pu-240
Z>90
5
TABLE 6.1
Murray
The energy released per fission of U-235 is ~ 200 MeV This table shows how the energy is distributed. The KE of the FF, released neutrons and primary radiations are available as heat (191 MeV). The energy carried away by the neutrinos from beta decay is lost.
6
Fig Knief 2.8 U-235 neutron energy distribution spectrum ~independent of incident neutron energy causing the fission Probability vs. neutron energy. Prompt fission neutron energies ~0.1 –10 MeV Area under the curve is one, or 100% of the emitted neutrons. The most probable energy is ~0.7 Mev and the average energy ~2MeV Note that within light water reactors we use thermal neutrons (~0.025 eV), thus the fission neutrons need to slowed or moderated.
7
BY-PRODUCTS OF FISSION: The source of high-level radioactive waste
8
Fission product decay, RE: Knief Fig 2-7 The entire periodic table of elements, and their isotopes are produced from fission. FF are radioactive. Fission is the source of high-level radioactive waste. This figure illustrates the decay chains for a couple of radioactive FF. Prompt emission of neutrons. ν = average number of neutrons emitted per fission (=2.44 for U-235, =3 for Pu-241) Prompt fission neutron energies ~0.1 –10 MeV Note the delayed emission of all kinds of radiations (beta, gamma, ..) Delayed neutrons (not shown) constitute ~1% of the n population in a critical reactor and are important in reactor control. Fission Product Distribution (Fission Yields/May West Curves) Similar to Fig 2.6 Knief, Plot of fission product yield vs. atomic mass number. Usually a ~1/3 – 2/3 distribution of masses. A ‘double-hump’ distribution. Sometimes called the May West Distribution. (May West was a popular, sexy movie starlet during the 1930’s) Higher percent yields with mass numbers ~90-100 (Sr-90, Nb, Mo, …) and ~130-150 (I-131, Xe-135, Cs-137, Sm-149…) The asymmetry becomes less with increasing neutron projectile energy.
9
10
May West 1933
“my little chickadee” W.C. Fields
11
12
Notes Distribution of isotopes is fission signature. Oklo natural reactor story. Oklo: map of Gabon Oklo Phenonomon Gabon, West Africa ~1.7 Gy (1.7 billion years) ago a uranium deposit became inundated with water’ Went critical, nuclear chain reaction sustained ~100,000 years U-235 enrichment was greater then, comparable to current LWRs ~3-5% U-235 half life ~ 7(10)8 y How do we know it went critical? Soil samples yielded May West Curves for Fission Product Distribution. Recent evidence of natural geysers / steam vents?
Decay Heat from 3,000 MWt LWR Reactor The radioactive decay of fission products produces heat DECAY HEAT. This is a plot of Knief Eqn 2.10, decay heat vs. time Important for spent fuel handling, waste management, reprocessing, & reactor safety.
13
14
Decay Heat from 3,000 MWt LWR Reactor 8 MW
P(t) ~ P(0)At-a
6
A = 0.066 a = 0.2 Murray p. 337
4
2
0 0
1
10
100
1000
10000
years
15
16
17
