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 7 long-lived fission products. Its 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.
Se-79 decays by emitting a beta particle with no attendant gamma radiation. The low specific activity and relatively low energy of its beta particle have been said to limit the radioactive hazards of this isotope.
Performance assessment calculations for the Belgian deep geological repository estimated 79Se 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 79Se." (p. 179)
Neutron absorption cross sections for 79Se have been estimated at 50 barns for thermal neutrons and 60.9 barns for resonance integral.
Se-80 and Se-82 have higher fission yields, about 20 times the yield of Se-79 in the case of U-235, 6 times in the case of Pu-239 or U-233, and 14 times in the case of Pu-241.
Mobility of selenium in the environment
Due to redox-disequilibrium, selenium could be very reluctant to chemical reduction and would be released from the waste (spent fuel or vitrified waste) as soluble selenate, a species not sorbed onto clay minerals. Without solubility limit and retardation for aqueous selenium, the dose of 79Se is comparable to that of 129I. Moreover, selenium is an essential micronutrient for many organisms (protection of cell membrane against oxidative damages) and can be easily bioconcentrated in the food chain. In the presence of nitrate, even reduced forms of selenium could be easily oxidised and mobilised.