Fission product yield
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Yield can be broken down by:
- Individual isotope
- Chemical element spanning several isotopes of different mass number but same atomic number.
- Nuclei of a given mass number regardless of atomic number. Known as "chain yield" because it represents a decay chain of beta decay.
Isotope and element yields will change as the fission products undergo beta decay, while chain yields do not change after completion of neutron emission by a few neutron-rich initial fission products (delayed neutrons), with halflife measured in seconds.
A few isotopes can be produced directly by fission, but not by beta decay because the would-be precursor with atomic number one greater is stable and does not decay. Chain yields do not account for these "shadowed" isotopes; however, they have very low yields (less than a millionth as much as common fission products) because they are far less neutron-rich than the original heavy nuclei.
Yield is usually stated as percentage per fission, so that the total yield percentages sum to 200%. Less often, it is stated as percentage of all fission products, so that the percentages sum to 100%. Ternary fission, about 0.2% to 0.4% of fissions, also produces a third light nucleus such as helium-4 (90%) or tritium (7%).
Mass vs. yield curve
If a graph of the mass or mole yield of fission products against the atomic number of the fragments is drawn then it has two peaks, one in the area zirconium through to palladium and one at xenon through to neodymium. This is because the fission event causes the nucleus to split in an asymmetric manner. Yield vs. Z - This is a typical distribution for the fission of uranium. Note that in the calculations used to make this graph the activation of fission products was ignored and the fission was assumed to occur in a single moment rather than a length of time. In this bar chart results are shown for different cooling times (time after fission).
In general, the higher the energy of the state that undergoes nuclear fission, the more likely a symmetric fission is, hence as the neutron energy increases and/or the energy of the fissile atom increases, the valley between the two peaks becomes more shallow; for instance, the curve of yield against mass for Pu-239 has a more shallow valley than that observed for U-235, when the neutrons are thermal neutrons. The curves for the fission of the later actinides tend to make even more shallow valleys. In extreme cases such as 259Fm, only one peak is seen.
Yield is usually expressed relative to number of fissioning nuclei, not the number of fission product nuclei, that is, yields should sum to 200%.
The table in the next section gives yields for notable radioactive (with halflife greater than one year, plus iodine-131) fission products, and (the few most absorptive) neutron poison fission products, from thermal neutron fission of U-235 (typical of nuclear power reactors), computed from .
The yields in the table sum to only 45.5522%, including 34.8401% which have halflife greater than one year:
|t½ in years||yield|
|1 to 5||2.7252%|
|10 to 100||12.5340%|
|2 to 300,000||6.1251%|
|1.5 to 16 million||13.4494%|
The remainder and the unlisted 54.4478% decay with halflife less than one year into nonradioactive nuclei.
This is before accounting for the effects of any subsequent neutron capture, e.g.:
- 135Xe capturing a neutron and becoming nonradioactive 136Xe, rather than decaying to 135Cs which is radioactive with a halflife of 2.3 million years
- Nonradioactive 133Cs capturing a neutron and becoming 134Cs which is radioactive with a halflife of 2 years
- Many of the fission products with mass 147 or greater such as Promethium-147, Samarium-149, Samarium-151, Europium-155 have significant cross sections for neutron capture, so that one heavy fission product atom can undergo multiple successive neutron captures.
Besides fission products, the other types of radioactive products are
- plutonium containing 238Pu, 239Pu, 240Pu, 241Pu, and 242Pu,
- minor actinides including 237Np, 241Am, 243Am, curium isotopes, and perhaps californium
- reprocessed uranium containing 236U and other isotopes
- activation products of neutron capture by the reactor or bomb structure or the environment
Ordered by yield (thermal neutron fission of U-235)
|6.7896%||Caesium||caesium-133 133Cs → 134Cs||2.065 y||neutron capture (29 barns) slowly converts stable 133Cs to 134Cs, which itself is low-yield because beta decay stops at 134Xe; can be further converted (140 barns) to 135Cs|
|6.3333%||Iodine, Xenon||iodine1-35 135I → 135Xe||6.57 h||most important neutron poison; neutron capture converts 10%–50% of 135Xe to 136Xe; remainder decays (9.14h) to 135Cs (2.3My)|
|6.2956%||Zirconium||zirconium 93Zr||1.53 My|
|6.1%||Molybdenum||molybdenum 99Mo||65.94 h||Its daughter nuclide 99mTc is important in medical diagnosing.|
|6.0899%||Caesium||caesium-137 137Cs||30.17 y|
|6.0507%||Technetium||technetium 99Tc||211 ky||Candidate for disposal by nuclear transmutation|
|5.7518%||Strontium||strontium 90Sr||28.9 y|
|2.8336%||Iodine||iodine131 131I||8.02 d|
|2.2713%||Promethium||promethium 147Pm||2.62 y|
|1.0888%||Samarium||samarium-149 149Sm||virtually stable||2nd most significant neutron poison|
|0.9%||Iodine||iodine-129 129I||15.7 My||Candidate for disposal by nuclear transmutation|
|0.4203%||Samarium||samarium-151 151Sm||90 y||neutron poison; most will be converted to stable 152Sm|
|0.3912%||Ruthenium||ruthenium 106Ru||373.6 d|
|0.2717%||Krypton||krypton 85Kr||10.78 y|
|0.1629%||Palladium||palladium 107Pd||6.5 My|
|0.0508%||Selenium||selenium 79Se||327 ky|
|0.0330%||Europium, Gadolinium||europium 155Eu → 155Gd||4.76 y||both neutron poisons, most will be destroyed while fuel still in use|
|0.0297%||Antimony||antimony 125Sb||2.76 y|
|0.0236%||Tin||tin 126Sn||230 ky|
|0.0065%||Gadolinium||gadolinium 157Gd||stable||neutron poison|
|0.0003%||Cadmium||cadmium 113mCd||14.1 y||neutron poison, most will be destroyed while fuel still in use|
Ordered by mass number
|6.3333%||iodine-135 →||xenon-135 →||caesium-135|
Ordered by halflife
|2.8336%||131I||8.02d||Important in nuclear explosions and accidents but not in cooled spent nuclear fuel|
|6.7896%||133Cs → 134Cs||2.065y||neutron capture converts a few percent of nonradioactive 133Cs to 134Cs, which has low direct yield because beta decay stops at 134Xe|
|<0.0330%||155Eu → 155Gd||4.76y||both neutron poisons, most will be destroyed by neutron capture while still in reactor|
|0.2717%||85Kr||10.78y||Current nuclear reprocessing releases it to atmosphere|
|<0.0003%||113mCd||14.1y||most will be destroyed by neutron capture while still in reactor|
|5.7518%||90Sr||28.9y||One of two principal medium-term radiation and heat sources|
|6.0899%||137Cs||30.17y||One of two principal medium-term radiation and heat sources|
|<0.4203%||151Sm||90y||Most will be destroyed by neutron capture while still in reactor|
|6.0507%||99Tc||211ky||Dominant radiation source among FP in period about ×104 to ×106 years; mobile in environment; candidate for disposal by nuclear transmutation|
|0.6576%||129I||15.7My||Mobile in environment; candidate for disposal by nuclear transmutation|
Ordered by thermal neutron neutron absorption cross section
|2,650,000||6.3333%||iodine-135 135I → 135Xe||6.57 h||Most important neutron poison; neutron capture rapidly converts 135Xe to 136Xe; remainder decays (9.14 h) to 135Cs (2.3 My)|
|254,000||0.0065%||gadolinium 157Gd||∞||neutron poison, but low yield|
|40,140||1.0888%||samarium-149 149Sm||∞||2nd most important neutron poison|
|20,600||0.0003%||cadmium 113mCd||14.1 y||most will be destroyed by neutron capture|
|15,200||0.4203%||samarium-151 151Sm||90 y||most will be destroyed by neutron capture|
|0.0330%||europium 155Eu → 155Gd||4.76 y||both neutron poisons|
|96||2.2713%||promethium 147Pm||2.62 y|
|80||2.8336%||iodine-131 131I||8.02 d|
|6.7896%||caesium-133 133Cs → 134Cs||∞
|neutron capture converts a few percent of nonradioactive 133Cs to 134Cs, which has very low direct yield because beta decay stops at 134Xe; further capture will add to long-lived 135Cs|
|20||6.0507%||technetium 99Tc||211 ky||candidate for disposal by nuclear transmutation|
|18||0.6576%||iodine-129 129I||15.7 My||candidate for disposal by nuclear transmutation|
|2.7||6.2956%||zirconium 93Zr||1.53 My||transmutation impractical|
|1.8||0.1629%||palladium 107Pd||6.5 My|
|1.66||0.2717%||krypton 85Kr||10.78 y|
|0.90||5.7518%||strontium 90Sr||28.9 y|
|0.15||0.3912%||ruthenium 106Ru||373.6 d|
|0.11||6.0899%||caesium-137 137Cs||30.17 y|
|0.0297%||antimony 125Sb||2.76 y|
|0.0236%||tin 126Sn||230 ky|
|0.0508%||selenium 79Se||327 ky|
- Purkayastha, B. C., and G. R. Martin. "The yields of 129I in natural and in neutron-induced fission of uranium." Canadian Journal of Chemistry 34.3 (1956): 293-300.