Selenium-79 is a radioisotope of selenium present in spent nuclear fuel and the

wastes Waste (or wastes) are unwanted or unusable materials. Waste is any substance discarded after primary use, or is worthless, defective and of no use. A by-product, by contrast is a joint product of relatively minor Value (economics), economic v ...

resulting from reprocessing this fuel. It is one of only 7

long-lived fission product 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 radiotoxicity it is necessary to isolate them from man ...

s. Its fission yield is low (about 0.04%), as it is near the lower end of the mass range for

fission products Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...

. 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. ⁷⁹Se decays to ⁷⁹Br by emitting a

beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β� ...

with no attendant

gamma radiation A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...

(''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. 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 Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors which change the measured quantity value without changing its effects. Overview The process ...

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. Selenium-80 and

selenium-82 Selenium (34Se) has six natural isotopes that occur in significant quantities, along with the trace isotope 79Se, which occurs in minute quantities in uranium ores. Five of these isotopes are stable: 74Se, 76Se, 77Se, 78Se, and 80Se. The last thr ...

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.

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 (), a soluble Se(VI) species, not sorbed onto

clay minerals Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay mineral ...

. Without solubility limit and

retardation Retardation is the act or result of delaying; the extent to which anything is retarded or delayed; that which retards or delays. Retardation or retarded or similar may refer to: Medicine and biology * Mental retardation, also known as inte ...

for aqueous selenium, the dose of ⁷⁹Se is comparable to that of ¹²⁹I. Moreover, selenium is an essential

micronutrient Micronutrients are nutrient, essential dietary elements required by organisms in varying quantities throughout life to orchestrate a range of physiological functions to maintain health. Micronutrient requirements differ between organisms; for exam ...

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 ⁷⁹Se can be easily bioconcentrated in the food web. In the presence of

nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...

() 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.

References

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Mobility of selenium in the environment

References

See also