Evolution of radioactivity in nuclear waste Long-lived fission product

1 evolution of radioactivity in nuclear waste

1.1 short-term
1.2 medium-lived fission products
1.3 actinides
1.4 long-lived fission products

evolution of radioactivity in nuclear waste

nuclear fission produces fission products, actinides nuclear fuel nuclei capture neutrons fail fission, , activation products neutron activation of reactor or environmental materials.

short-term

the high short-term radioactivity of spent nuclear fuel fission products short half-life. radioactivity in fission product mixture short-lived isotopes such , ba, after 4 months ce, zr/nb , sr take largest share, while after 2 or 3 years largest share taken ce/pr, ru/rh , pm. note in case of release of radioactivity power reactor or used fuel, elements released. result isotopic signature of radioactivity different open air nuclear detonation fission products dispersed.

medium-lived fission products

after several years of cooling, radioactivity fission products caesium-137 , strontium-90, each produced in 6% of fissions, , have half-lives of 30 years. other fission products similar half-lives have lower fission product yields, lower decay energy, , several (sm, eu, cd) destroyed neutron capture while still in reactor, not responsible more tiny fraction of radiation production @ time. therefore, in period several years several hundred years after use, radioactivity of spent fuel can modeled exponential decay of cs , sr. these known medium-lived fission products.

krypton-85, 3rd active mlfp, noble gas allowed escape during current nuclear reprocessing; however, inertness means not concentrate in environment, diffuses uniform low concentration in atmosphere. spent fuel in u.s. , other countries not reprocessed until decades after use, , time of kr have decayed.

actinides

after cs , sr have decayed low levels, bulk of radioactivity spent fuel come not fission products actinides, notably plutonium-239 (half-life 24 ka), plutonium-240 (6.56 ka), americium-241 (432 years), americium-243 (7.37 ka), curium-245 (8.50 ka), , curium-246 (4.73 ka). these can recovered nuclear reprocessing (either before or after cs , sr decay) , fissioned, offering possibility of reducing waste radioactivity in time scale of 10 10 years. pu usable fuel in existing thermal reactors, minor actinides am, non-fissile , less-fertile isotope plutonium-242, better destroyed in fast reactors, accelerator-driven subcritical reactors, or fusion reactors.

long-lived fission products

on scales greater 10 years, fission products, chiefly tc, again represent significant proportion of remaining, though lower radioactivity, along longer-lived actinides neptunium-237 , plutonium-242, if have not been destroyed.

the abundant long-lived fission products have total decay energy around 100-300 kev, part of appears in beta particle; rest lost neutrino has no effect. in contrast, actinides undergo multiple alpha decays, each decay energy around 4-5 mev.

only 7 fission products have long half-lives, , these longer 30 years, in range of 200,000 16 million years. these known long-lived fission products (llfp). 2 or 3 have relatively high yields of 6%, while rest appear @ lower yields. (this list of 7 excludes isotopes slow decay , half-lives longer age of universe, stable , found in nature; few nuclides technetium-98 , samarium-146 shadowed beta decay , can occur direct fission products, not beta decay products of more neutron-rich initial fission products. shadowed fission products have yields on order of 1 millionth as iodine-129.)