Long-lived fission product

Long-lived fission products (LLFPs) are radioactive materials with a long

Evolution of radioactivity in nuclear waste

Medium-lived fission products

After several years of cooling, most radioactivity is from the fission products caesium-137 and strontium-90, which are each produced in about 6% of fissions, and have half-lives of about 30 years. Other fission products with similar half-lives have much lower fission product yields, lower decay energy, and several (¹⁵¹Sm, ¹⁵⁵Eu, ^113mCd) are also quickly destroyed by neutron capture while still in the reactor, so are not responsible for more than a tiny fraction of the radiation production at any time. Therefore, in the period from several years to several hundred years after use, radioactivity of spent fuel can be modeled simply as exponential decay of the ¹³⁷Cs and ⁹⁰Sr. These are sometimes known as medium-lived fission products.^[1][2]

Krypton-85, the 3rd most active MLFP, is a noble gas which is allowed to escape during current nuclear reprocessing; however, its inertness means that it does not concentrate in the environment, but diffuses to a uniform low concentration in the atmosphere. Spent fuel in the U.S. and some other countries is not likely to be reprocessed until decades after use, and by that time most of the ⁸⁵Kr will have decayed.

Actinides

After ¹³⁷Cs and ⁹⁰Sr have decayed to low levels, the bulk of radioactivity from spent fuel come not from fission products but

The most abundant long-lived fission products have total decay energy around 100–300 keV, only part of which appears in the beta particle; the rest is lost to a neutrino that has no effect. In contrast, actinides undergo multiple alpha decays, each with decay energy around 4–5 MeV.

Only seven fission products have long half-lives, and these are much longer than 30 years, in the range of 200,000 to 16 million years. These are known as long-lived fission products (LLFP). Three have relatively high yields of about 6%, while the rest appear at much lower yields. (This list of seven excludes isotopes with very slow decay and half-lives longer than the age of the universe, which are effectively stable and already found in nature, as well as a few nuclides like technetium-98 and samarium-146 that are "shadowed" from beta decay and can only occur as direct fission products, not as beta decay products of more neutron-rich initial fission products. The shadowed fission products have yields on the order of one millionth as much as iodine-129.)

The 7 long-lived fission products

The first three have similar half-lives, between 200 thousand and 300 thousand years; the last four have longer half-lives, in the low millions of years.

1. anions (pertechnetate
   , TcO₄⁻) that are relatively mobile in the environment.
2. thermal neutrons, so the energy per unit time from ¹²⁶Sn is only about 5% as much as from ⁹⁹Tc for U-235 fission, or 20% as much for 65% U-235+35% Pu-239. Fast fission may produce higher yields. Tin
   is an inert metal with little mobility in the environment, helping to limit health risks from its radiation.
3. Selenium-79 is produced at low yields and emits only weak radiation. Its decay energy per unit time should be only about 0.2% that of Tc-99.
4. Zirconium-93 is produced at a relatively high yield of about 6%, but its decay is 7.5 times slower than Tc-99, and its decay energy is only 30% as great; therefore its energy production is initially only 4% as great as Tc-99, though this fraction will increase as the Tc-99 decays. ⁹³Zr does produce gamma radiation, but of a very low energy, and zirconium
   is relatively inert in the environment.
5. ¹³⁷Cs
   which does not absorb neutrons but is highly radioactive, making handling more hazardous and complicated; for all these reasons, transmutation disposal of ¹³⁵Cs would be more difficult.
6. Palladium-107 has a very long half-life, a low yield (though the yield for plutonium fission is higher than the yield from uranium-235 fission), and very weak radiation. Its initial contribution to LLFP radiation should be only about one part in 10000 for ²³⁵U fission, or 2000 for 65% ²³⁵U+35% ²³⁹Pu. Palladium is a noble metal
   and extremely inert.
7. billion
   times as long as its more hazardous sister isotope ¹³¹I; therefore, with a shorter half-life and a higher decay energy, ¹³¹I is approximately a billion times more radioactive than the longer-lived ¹²⁹I. (What relevance ¹³¹I has in this coverage of LLFPs is debatable.)