Strontium-90

Strontium-90 (⁹⁰Sr) is a radioactive isotope of strontium, with a half life of 28.8 years. ⁹⁰Sr undergoes beta decay with decay energy of 0.546 MeV to the yttrium isotope ⁹⁰Y, which in turn undergoes beta decay with half life of 64 hours and decay energy 2.28 MeV for beta particles to ⁹⁰Zr (zirconium), which is stable^[1]. Note that ⁹⁰Sr/Y is almost a perfectly pure beta source, the gamma photon emission from the decay of ⁹⁰Y is so weak that it can normally be ignored.

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⁹⁰Sr finds extensive use in medicine and industry, as a radioactive source for thickness gauges and for superficial radiotherapy of some cancers. Controlled amounts of ⁹⁰Sr and ⁸⁹Sr can be used in treatment of bone cancer. As the radioactive decay of strontium-90 generates significant amount of heat, and is cheaper than the alternative ²³⁸Pu, it is used as a heat source in many Russian/Soviet radioisotope thermoelectric generators, usually in the form of strontium fluoride. It is also used as a radioactive tracer in medicine and agriculture. It is obtained during nuclear reprocessing of spent nuclear fuel.

⁹⁰Sr is a product of nuclear fission. It is present in significant amount in spent nuclear fuel and in radioactive waste from nuclear reactors and in nuclear fallout from nuclear tests. For thermal neutron fission as in today's nuclear power plants, the fission product yield from U-235 is 5.8%, from U-233 6.8%, but from Pu-239 only 2.1%.

Together with cesium isotopes ¹³⁴Cs, ¹³⁷Cs, and iodine isotope ¹³¹I it was between the most important isotopes regarding health impacts after the Chernobyl disaster. Slightly elevated levels of ⁹⁰Sr may be present in the vicinity of nuclear power plants.

Strontium has biochemical behavior similar to calcium. After entering the organism, most often by ingestion with contaminated food or water, about 70-80% of the dose gets excreted. Virtually all remaining strontium is deposited in bones and bone marrow, with the remaining 1% remaining in blood and soft tissues. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia. Exposition to ⁹⁰Sr can be tested by a bioassay, most commonly by urinalysis.

In the vicinity of nuclear waste and nuclear test sites, strontium also enters the metabolism of plants in lieu of calcium. For example, specimens of chamisa growing in Bayo Canyon, near Los Alamos, New Mexico, exhibit a concentration of radioactive strontium 300,000 times higher than normal plants. Their roots reach into a nuclear waste treatment area that has been closed since 1963; the radioactive shrubs are "indistinguishable from other shrubs without a Geiger counter" ^[2]. The same happens with the tumbleweed plants at the Hanford Site; "crews armed with pitchforks" are employed to prevent the contaminated plants from spreading ^[2].

Accidental mixing of radioactive sources containing strontium with metal scrap can result in production of radioactive steel. Discarded radioisotope thermoelectric generators are a major source of ⁹⁰Sr contamination in the area of the former Soviet Union.

References

External links

• NLM Hazardous Substances Databank – Strontium, Radioactive