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{{Short description|Critical radionuclides for the long-term safety of nuclear waste repositories}}
'''Long-lived fission products''' (LLFPs) are radioactive materials with a long [[half-life]] (more than 200,000 years) produced by [[nuclear fission]] of [[uranium]] and [[plutonium]]. Because of their persistent [[Ionizing radiation|radiotoxicity]], it is necessary to isolate them from humans and the biosphere and to confine them in nuclear waste repositories for geological periods of time. The focus of this article is radioisotopes ([[radionuclide]]s) generated by [[fission reactor]]s.

==Evolution of radioactivity in nuclear waste==
[[Nuclear fission]] produces [[fission products]], as well as [[actinide]]s from [[nuclear fuel]] nuclei that capture neutrons but fail to fission, and [[activation products]] from [[neutron activation]] of reactor or environmental materials.

===Short-term===
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 due to short-lived isotopes such as [[Iodine-131|<sup>131</sup>I]] and <sup>140</sup>Ba, after about four months <sup>141</sup>Ce, <sup>95</sup>Zr/<sup>95</sup>Nb and <sup>89</sup>Sr constitute the largest contributors, while after about two or three years the largest share is taken by <sup>144</sup>Ce/<sup>144</sup>Pr, <sup>106</sup>Ru/<sup>106</sup>Rh and <sup>147</sup>Pm.
Note that in the case of a release of radioactivity from a power reactor or used fuel, 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===
{{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 yield]]s, lower [[decay energy]], and several (<sup>151</sup>Sm, <sup>155</sup>Eu, <sup>113m</sup>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 <sup>137</sup>Cs and <sup>90</sup>Sr. These are sometimes known as medium-lived fission products.<ref>{{cite book|title=Nuclear Wastes: Technologies for Separations and Transmutation|url=https://books.google.com/books?id=iRI7Cx2D4e4C&pg=PA23 | isbn=978-0-309-05226-9 | year=1996 | publisher=National Academies Press}}</ref><ref>{{cite web|title=The Nuclear Alchemy Gamble: An Assessment of Transmutation as a Nuclear Waste Management Strategy|url=https://ieer.org/resource/reports/nuclear-alchemy-gamble/ |work=[[Institute for Energy and Environmental Research]] |first=Hisham |last=Zerriffi |first2=Annie |last2=Makhijani |date=May 2000}}</ref>

[[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 <sup>85</sup>Kr will have decayed.
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===Actinides===
{{Actinidesvsfissionproducts}}
After <sup>137</sup>Cs and <sup>90</sup>Sr have decayed to low levels, the bulk of radioactivity from spent fuel come not from fission products but [[actinides]], notably [[plutonium-239]] (half-life 24&nbsp;[[kiloannus|ka]]), [[plutonium-240]] (6.56&nbsp;ka), [[americium-241]] (432&nbsp;years), [[americium-243]] (7.37&nbsp;ka), [[curium]]-245 (8.50&nbsp;ka), and curium-246 (4.73&nbsp;ka). These can be recovered by [[nuclear reprocessing]] (either before or after most <sup>137</sup>Cs and <sup>90</sup>Sr decay) and fissioned, offering the possibility of greatly reducing waste radioactivity in the time scale of about 10<sup>3</sup> to 10<sup>5</sup> years. <sup>239</sup>Pu is usable as fuel in existing [[thermal reactor]]s, but some [[minor actinides]] like <sup>241</sup>Am, as well as the non-[[fissile]] and less-[[fertile material|fertile]] isotope [[plutonium-242]], are better destroyed in [[fast reactor]]s, accelerator-driven [[subcritical reactor]]s, or [[fusion reactor]]s. [[Americium-241]] has some industrial applications and is used in [[smoke detector]]s and is thus often separated from waste as it fetches a price that makes such separation economic.

===Long-lived fission products===
On scales greater than 10<sup>5</sup> years, fission products, chiefly [[Technetium-99|<sup>99</sup>Tc]], again represent a significant proportion of the remaining, though lower radioactivity, along with longer-lived actinides like [[neptunium-237]] and [[plutonium-242]], if those have not been destroyed.

The most abundant long-lived fission products have total [[decay energy]] around 100–300&nbsp;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 decay]]s, each with decay energy around 4–5&nbsp;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==
{{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.
#[[Technetium-99]] produces the largest amount of LLFP radioactivity. It emits [[beta particles]] of low to medium energy but no [[gamma ray]]s, so has little hazard on external exposure, but only if ingested. However, technetium's chemistry allows it to form [[anion]]s ([[pertechnetate]], TcO<sub>4</sub><sup>−</sup>) that are relatively mobile in the environment.
#[[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 <sup>126</sup>Sn is only about 5% as much as from <sup>99</sup>Tc 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.
#[[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.
#[[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. <sup>93</sup>Zr 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 almost-stable xenon-136 before it can decay to caesium-135. If 90% of <sup>135</sup>Xe is destroyed, then the remaining <sup>135</sup>Cs's decay energy per unit time is initially only about 1% as great as that of the <sup>99</sup>Tc. In a fast reactor, less of the Xe-135 may be destroyed.<br />[[Cs-135|<sup>135</sup>Cs]] 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, <sup>135</sup>Cs 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 <sup>135</sup>Cs.<br />Fission caesium contains not only <sup>135</sup>Cs but also stable but neutron-absorbing <sup>133</sup>Cs (which wastes neutrons and forms [[Cs-134|<sup>134</sup>Cs]] which is radioactive with a half-life of 2 years) as well as the common fission product [[Cs-137|<sup>137</sup>Cs]] which does not absorb neutrons but is highly radioactive, making handling more hazardous and complicated; for all these reasons, transmutation disposal of <sup>135</sup>Cs would be more difficult.
#[[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 <sup>235</sup>U fission, or 2000 for 65% <sup>235</sup>U+35% <sup>239</sup>Pu. Palladium is a [[noble metal]] and extremely inert.
#[[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 <sup>99</sup>Tc. However, radioactive [[iodine]] is a disproportionate biohazard because the [[thyroid gland]] concentrates iodine. <sup>129</sup>I has a half-life nearly a [[1,000,000,000 (number)|billion]] times as long as its more hazardous sister isotope <sup>131</sup>I; therefore, with a shorter half-life and a higher decay energy, <sup>131</sup>I is approximately a billion times more radioactive than the longer-lived <sup>129</sup>I.

==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]] [[Neutron cross-section|cross section]]s, although transmutation is still 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. Given that stable Caesium-133 is also produced in nuclear fission and both it and its neutron activation product {{chem|134|Cs}} are [[neutron poison]]s, transmutation of {{chem|135|Cs}} might necessitate [[isotope separation]]. {{Chem|99|Tc}} is particularly attractive for transmutation not only due to the undesirable properties of the product to be destroyed and the relatively high neutron absorption cross section but also because {{chem|100|Tc}} rapidly [[beta decay]]s to stable {{chem|100|Ru}}. [[Ruthenium]] has no radioactive isotopes with half lives much longer than a year and the price of ruthenium is relatively high, making the destruction of {{chem|99|Tc}} into a potentially lucrative source of producing a precious metal from an undesirable feedstock.

==References==
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{{Nuclear Technology}}

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