# Selenium-79

Selenium-79 is a radioisotope of selenium present in spent nuclear fuel and the wastes resulting from reprocessing this fuel. It is one of only seven long-lived fission products. Its fission yield is low (about 0.04%), as it is near the lower end of the mass range for fission products. Its half-life has been variously reported as 650,000 years, 65,000 years, 1.13 million years, 480,000 years, 295,000 years, 377,000 years and most recently with best current precision, 327,000 years.
$^{79}$Se decays to $^{79}$Br by emitting a beta particle with no attendant gamma radiation (i.e., 100% β decay). This complicates its detection and liquid scintillation counting (LSC) is required for measuring it in environmental samples. The low specific activity (5.1 × 10$^8$ Bq/g) and relatively low energy (151 keV) of its beta particles have been said to limit the radioactive hazards of this isotope.
Performance assessment calculations for the Belgian deep geological repository estimated $^{79}$Se may be the major contributor to activity release in terms of becquerels (decays per second), "attributable partly to the uncertainties about its migration behaviour in the Boom Clay and partly to its conversion factor in the biosphere." (p. 169). However, "calculations for the Belgian safety assessments use a half-life of 65 000 years" (p. 177), much less than the currently estimated half-life, and "the migration parameters ... have been estimated very cautiously for $^{79}$Se." (p. 179)
Neutron absorption cross sections for $^{79}$Se have been estimated at 50 barns for thermal neutrons and 60.9 barns for resonance integral.
Selenium-80 and selenium-82 have higher fission yields, about 20 times the yield of $^{79}$Se in the case of uranium-235, 6 times in the case of plutonium-239 or uranium-233, and 14 times in the case of plutonium-241.

## Mobility of selenium in the environment

Due to redox-disequilibrium, selenium could be very reluctant to abiotic chemical reduction and would be released from the waste (spent fuel or vitrified waste) as selenate (SeO$^{2–}$ $_4$), a soluble Se(VI) species, not sorbed onto clay minerals. Without solubility limit and retardation for aqueous selenium, the dose of $^{79}$Se is comparable to that of $^{129}$I. Moreover, selenium is an essential micronutrient as it is present in the catalytic centers in the glutathione peroxidase, an enzyme needed by many organisms for the protection of their cell membrane against oxidative stress damages; therefore, radioactive $^{79}$Se can be easily bioconcentrated in the food web. In the presence of nitrate (NO$^–$ $_3$) released in deep geological clay formations by bituminized waste issued from the spent fuel dissolution step during their reprocessing, even reduced forms of selenium could be easily oxidised and mobilised.
Long-lived fission productsvte :
| Nuclide | t$_{1⁄2}$ | Yield | Q | βγ |
| | (Ma) | (%) | (keV) | |
| $^{99}$Tc | 0.211 | 6.1385 | 294 | β |
| $^{126}$Sn | 0.230 | 0.1084 | 4050 | βγ |
| $^{79}$Se | 0.327 | 0.0447 | 151 | β |
| $^{135}$Cs | 1.33 | 6.9110 | 269 | β |
| $^{93}$Zr | 1.53 | 5.4575 | 91 | βγ |
| $^{107}$Pd | 6.5 | 1.2499 | 33 | β |
| $^{129}$I | 16.14 | 0.8410 | 194 | βγ |
| ↑ Decay energy is split among β, neutrino, and γ if any. ↑ Per 65 thermal neutron fissions of $^{235}$U and 35 of $^{239}$Pu. ↑ Has decay energy 380 keV, but its decay product $^{126}$Sb has decay energy 3.67 MeV. ↑ Lower in thermal reactors because $^{135}$Xe, its predecessor, readily absorbs neutrons. |||||