Long-lived Fission Product - The 7 Long-lived Fission Products

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. Technetium-99 produces the largest amount of LLFP radioactivity. It emits beta particles of low to medium energy but no gamma rays, so has little hazard on external exposure, but only if ingested. However, technetium's chemistry allows it to form anions (pertechnate, TcO₄-) that are relatively mobile in the environment. Tons of technetium-99 have been released into the ocean.
2. Tin-126 has a large decay energy (due to its following short half-life decay product) and is the only LLFP that emits energetic gamma radiation, which is an external exposure hazard. However, this isotope is produced in very small quantities in fission by 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 limit health risks from its radiation.
3. Selenium-79 is produced at low yields and has 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. Caesium-135's predecessor xenon-135 is produced at a high rate of over 6% of fissions, but is an extremely potent absorber of thermal neutrons (neutron poison), so that most of it is transmuted to almost-stable xenon-136 before it can decay to caesium-135. If 90% of 135Xe is destroyed, then the remaining 135Cs's decay energy per unit time is initially only about 1% as great as that of the 99Tc. In a fast reactor, less of the Xe-135 may be destroyed.
   135Cs is the only alkaline or electropositive LLFP; in contrast, the main medium-lived fission products and the minor actinides other than neptunium are all alkaline and tend to stay together during reprocessing; with many reprocessing techniques such as salt solution or salt volatilization, 135Cs will also stay with this group, although some techniques such as high-temperature volatilization can separate it. Often the alkaline wastes are vitrified to form high level waste, which will include the 135Cs.
   Fission caesium contains not only 135Cs but also stable but neutron-absorbing 133Cs (which wastes neutrons and forms 134Cs which is radioactive with a half-life of 2 years) as well as the common fission product 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 U-235 fission, or 2000 for 65% U-235+35% Pu-239. Palladium is a noble metal and extremely inert.
7. Iodine-129 has the longest half-life, 15.7 million years, and due to its higher half life, lower fission fraction and decay energy it produces only about 1% the intensity of radioactivity as Tc-99. However, radioactive iodine is a disproportionate biohazard because the thyroid gland concentrates iodine. I-129 has a half-life nearly a billion times as long as its more hazardous sister isotope iodine-131, therefore with a shorter high life, I-131 is approximately a billion times more radioactive than the longer lived I-129. Together with the longer more stable nature(longer half life) of I-129, and its lower decay energy, than its sister isotope I-131, I-129 is only about a billionth as radioactive as I-131.