Evolution of radioactivity in nuclear waste
produces fission products
, as well as actinides
from nuclear fuel
nuclei that capture neutrons but fail to fission, and activation products
from neutron activation
of reactor or environmental materials.
The high short-term radioactivity
of spent nuclear fuel
is primarily from fission products with short half-life
The radioactivity in the fission product mixture is mostly short lived isotopes such as I-131 and 140Ba, after about four months 141Ce, 95Zr/95Nb and 89Sr take the largest share, while after about two or three years the largest share is taken by 144Ce/144Pr, 106Ru/106Rh and 147Pm.
Note that in the case a release of radioactivity from a power reactor or used fuel that only some elements are released, as a result the isotopic signature of the radioactivity is very different from an open air nuclear detonation where all the fission products are dispersed.
Medium-lived fission products
After several years of cooling, most radioactivity is from the fission products caesium-137
, 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 (151
Cd) 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 137
Cs and 90
Sr. These are sometimes known as medium-lived fission products
Krypton-85, the 3rd most active MLFP, is a noble gas which escapes 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 US and some other countries is not likely to be reprocessed until decades after use, and by that time most of the Kr-85 will have decayed.
After Cs-137 and Sr-90 have decayed to low levels, the bulk of radioactivity from spent fuel is from not fission products but actinides
, notably plutonium-239
-245, and curium-246. These can be recovered by nuclear reprocessing
(either before or after most Cs-137 and Sr-90 decay) and fissioned, offering the possibility of greatly reducing waste radioactivity in the time scale of about 103
years. Pu-239 is usable as fuel in existing thermal reactors
, but some minor actinides
like Am-241, as well as the non-fissile
, are better destroyed in fast reactors
, accelerator-driven subcritical reactors
, or fusion reactors
Long-lived fission products
On scales greater than 105
years, fission products, chiefly 99
Tc, again represent a significant proportion of the remaining, though lower, radioactivity, along with longer-lived actinides like neptunium-237
, if those have not been destroyed.
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). Two or 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 halflives 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 comparable halflives, between 200 thousand and 300 thousand years; the last four have longer halflives, in the low millions of years.
- 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.
- Tin-126 has a large decay energy (due to a following short-halflife decay) 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.
- 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.
- 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.
- 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 nonradioactive 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 halflife 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.
- Palladium-107 has a very long halflife, 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.
- Iodine-129 has the longest halflife, 15.7 million years. Initially it has only about 1% as intense radioactivity as Tc-99. However, radioactive iodine is a disproportionate biohazard because the thyroid gland concentrates iodine. I-129 has a halflife nearly a billion times as long as its sister isotope iodine-131 which is a hazard from nuclear explosions, and a smaller decay energy, so is only about a billionth as radioactive per unit mass.
LLFP radioactivity compared
In total, the other six LLFPs, in thermal reactor spent fuel, initially release only a bit more than 10% as much energy per unit time as Tc-99 for U-235 fission, or 25% as much for 65% U-235+35% Pu-239. About 1000 years after fuel use, radioactivity from the medium-lived fission products Cs-137 and Sr-90 drops below the level of radioactivity from Tc-99 or LLFPs in general. (Actinides, if not removed, will be emitting more radioactivity than either at this point.) By about 1 million years, Tc-99 radioactivity will have declined below that of Zr-93, though immobility of the latter means it is probably still a lesser hazard. By about 3 million years, Zr-93 decay energy will have declined below that of I-129.
Nuclear transmutation is under consideration as a disposal method, primarily for Tc-99 and I-129 as these both represent the greatest biohazards and have the greatest neutron capture cross sections, although transmutation is still relatively slow compared to fission of actinides in a reactor. Transmutation has also been considered for Cs-135, but is almost certainly not worthwhile for the other LLFPs.