| General | |
|---|---|
| Symbol | 79Se |
| Names | selenium-79 |
| Protons(Z) | 34 |
| Neutrons(N) | 45 |
| Nuclide data | |
| Natural abundance | trace |
| Half-life(t1/2) | 327000±28000 years[1] |
| Spin | 7/2+ |
| Excess energy | −75917.46±0.22keV |
| Binding energy | 8695.592±0.003 keV |
| Decay products | 79Br |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Beta decay | 0.151[2] |
| Isotopes of selenium Complete table of nuclides | |
Selenium-79 is aradioisotope ofselenium present inspent nuclear fuel and thewastes resulting fromreprocessing this fuel. It is one of only sevenlong-lived fission products. Itsfission yield is low (about 0.04%), as it is near the lower end of the mass range forfission products. Itshalf-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 and the best current value, 327,000 years.[3][4]
79Se decays to79Br by emitting abeta particle with no attendantgamma radiation (i.e., 100% β decay). This complicates its detection andliquid scintillation counting (LSC) is required for measuring it in environmental samples. The lowspecific activity (5.1 × 108 Bq/g) and relatively low energy (maximum 151 keV) of its beta particles limit the radioactive hazards of this isotope.[5]
Performance assessment calculations for the Belgiandeep geological repository estimated79Se may be the major contributor to activity release in terms ofbecquerels (decays per second), "attributable partly to the uncertainties about its migration behaviour in the Boom Clay and partly to itsconversion factor in thebiosphere." (p. 169).[6] 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 for79Se." (p. 179)
Neutron absorptioncross sections for79Se have been estimated at 50barns forthermal neutrons and 60.9 barns forresonance integral.[7]
Selenium-80 andselenium-82 have higherfission yields, about 20 times the yield of79Se in the case ofuranium-235, 6 times in the case ofplutonium-239 oruranium-233, and 14 times in the case ofplutonium-241.[8]
Due toredox-disequilibrium, selenium could be very resistant toabiotic chemical reduction and be released from the waste (spent fuel or vitrified waste) asselenate (SeO2–
4), a soluble Se(VI) species, notsorbed ontoclay minerals. Without considering anysolubility limit andretardation for aqueous selenium, the dose of79Se is comparable to that of129I. Moreover, selenium is an essentialmicronutrient as it is present in the catalytic centers in theglutathione peroxidase, anenzyme needed by many organisms for the protection of theircell membrane againstoxidative stress damages; therefore, radioactive79Se can be easilybioconcentrated in thefood web. In the presence ofnitrate, a common groundwater contaminant, even reduced forms of selenium could be easilyoxidised and mobilised.[9]
| Nuclide | t1⁄2 | Yield | Q[a 1] | βγ |
|---|---|---|---|---|
| (Ma) | (%)[a 2] | (keV) | ||
| 99Tc | 0.211 | 6.1385 | 294 | β |
| 126Sn | 0.23 | 0.1084 | 4050[a 3] | βγ |
| 79Se | 0.33 | 0.0447 | 151 | β |
| 135Cs | 1.33 | 6.9110[a 4] | 269 | β |
| 93Zr | 1.61 | 5.4575 | 91 | βγ |
| 107Pd | 6.5 | 1.2499 | 33 | β |
| 129I | 16.1 | 0.8410 | 194 | βγ |
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