Physics:Selenium-79

Short description: Long-lived radioisotope of selenium

Selenium-79, ⁷⁹Se

General
Symbol	79Se
Names	selenium-79, Se-79
Protons	34
Neutrons	45
Nuclide data
Natural abundance	trace
Half-life	327000±28000 years
Decay products	79Br
Spin	7/2+
Excess energy	−75917.46±0.22 keV
Binding energy	8695.592±0.003 keV
Decay modes
Decay mode	Decay energy (MeV)
Beta decay	0.1506
Isotopes of Chemistry:selenium Complete table of nuclides

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 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.

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.

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.

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.

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 (SeO2–4), a soluble Se(VI) 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 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 (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 products

Nuclide	t​1⁄2	Yield	Decay energy	Decay mode
	(Ma)	(%)	(keV)
99Tc	0.211	6.1385	294	β
126Sn	0.230	0.1084	4050	βγ
79Se	0.327	0.0447	151	β
93Zr	1.53	5.4575	91	βγ
135Cs	2.3	6.9110	269	β
107Pd	6.5	1.2499	33	β
129I	15.7	0.8410	194	βγ

See also

• Isotopes of selenium
• ANL factsheet
• Journal of Analytical Atomic Spectrometry