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.^[1][2]

⁷⁹Se decays to ⁷⁹Br 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 × 10⁸ Bq/g) and relatively low energy (151 keV) of its beta particles have been said to limit the radioactive hazards of this isotope.^[3]

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).^[4] 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.^[5]

Selenium-80 and selenium-82 have higher fission yields, about 20 times the yield of ⁷⁹Se 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.^[6]