Strontium-90

Strontium-90 (90
Sr
) is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.8 years. It undergoes β− decay into yttrium-90, with a decay energy of 0.546 MeV.[1] Strontium-90 has applications in medicine and industry and is an isotope of concern in fallout from nuclear weapons and nuclear accidents.[2]

Strontium-90, 90Sr
General
Symbol90Sr
Namesstrontium-90, Sr-90
Protons38
Neutrons52
Nuclide data
Natural abundancesyn
Half-life28.79 years
Decay products90Y
Decay modes
Decay modeDecay energy (MeV)
Beta decay0.546
Isotopes of strontium
Complete table of nuclides

Radioactivity

Naturally occurring strontium is nonradioactive and nontoxic at levels normally found in the environment, but 90Sr is a radiation hazard.[3] 90Sr undergoes β− decay with a half-life of 28.79 years and a decay energy of 0.546 MeV distributed to an electron, an anti-neutrino, and the yttrium isotope 90Y, which in turn undergoes β− decay with half-life of 64 hours and decay energy 2.28 MeV distributed to an electron, an anti-neutrino, and 90Zr (zirconium), which is stable.[4] Note that 90Sr/Y is almost a pure beta particle source; the gamma photon emission from the decay of 90Y is so infrequent that it can normally be ignored.

90Sr has a specific activity of 5.21 TBq/g.[5]

Medium-lived
fission products
Prop:
Unit:		 t½
(a)		 Yield
(%)		 Q *
(keV)		 βγ *
155Eu 4.76 0.0803 252 βγ
85Kr 10.76 0.2180 687 βγ
113mCd 14.1 0.0008 316 β
90Sr 28.9 4.505 2826 β
137Cs 30.23 6.337 1176 βγ
121mSn 43.9 0.00005 390 βγ
151Sm 88.8 0.5314 77 β

Fission product

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 5.7%, from U-233 6.6%, but from Pu-239 only 2.0%.[6]

Biological effects

Biological activity

Strontium-90 is a "bone seeker" that exhibits biochemical behavior similar to calcium, the next lighter group 2 element.[3][7] After entering the organism, most often by ingestion with contaminated food or water, about 70–80% of the dose gets excreted.[2] Virtually all remaining strontium-90 is deposited in bones and bone marrow, with the remaining 1% remaining in blood and soft tissues.[2] Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia. Exposure to 90Sr can be tested by a bioassay, most commonly by urinalysis.[3]

The biological half-life of strontium-90 in humans has variously been reported as from 14 to 600 days,[8][9] 1000 days,[10] 18 years,[11] 30 years[12] and, at an upper limit, 49 years.[13] The wide-ranging published biological half life figures are explained by strontium's complex metabolism within the body. However, by averaging all excretion paths, the overall biological half life is estimated to be about 18 years.[14]

The elimination rate of strontium-90 is strongly affected by age and sex, due to differences in bone metabolism.[15]

Together with the caesium isotopes 134Cs, 137Cs, and iodine isotope 131I it was among the most important isotopes regarding health impacts after the Chernobyl disaster. As strontium has an affinity to the calcium-sensing receptor of parathyroid cells that is similar to that of calcium, the increased risk of liquidators of the Chernobyl power plant to suffer from primary hyperparathyroidism could be explained by binding of strontium-90.[16]

Uses

Radioisotope Thermoelectric Generators (RTGs)

The radioactive decay of strontium-90 generates a significant amount of heat, 0.536 W/g in the form of pure strontium metal or approximately 0.256 W/g as strontium titanate[17] 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 titanate.[18] It was also used in the US "Sentinel" series of RTGs.[19]

Industrial applications

90Sr finds use in industry as a radioactive source for thickness gauges.[2]

Medical applications

90Sr finds extensive use in medicine as a radioactive source for superficial radiotherapy of some cancers. Controlled amounts of 90Sr and 89Sr can be used in treatment of bone cancer, and to treat coronary restenosis via vascular brachytherapy. It is also used as a radioactive tracer in medicine and agriculture.[2]

Aerospace applications

90Sr is used as a blade inspection method in some helicopters with hollow blade spars to indicate if a crack has formed.[20]

90Sr contamination in the environment

Strontium-90 is not quite as likely as caesium-137 to be released as a part of a nuclear reactor accident because it is much less volatile, but is probably the most dangerous component of the radioactive fallout from a nuclear weapon.[21]

A study of hundreds of thousands of deciduous teeth, collected by Dr. Louise Reiss and her colleagues as part of the Baby Tooth Survey, found a large increase in 90Sr levels through the 1950s and early 1960s. The study's final results showed that children born in St. Louis, Missouri, in 1963 had levels of 90Sr in their deciduous teeth that was 50 times higher than that found in children born in 1950, before the advent of large-scale atomic testing. Commentators on the study said that the fallout was likely to cause increased cases of diseases in those who absorb strontium-90 into their bones.[22]

An article with the study's initial findings was circulated to U.S. President John F. Kennedy in 1961, and helped convince him to sign the Partial Nuclear Test Ban Treaty with the United Kingdom and Soviet Union, ending the above-ground nuclear weapons testing that placed the greatest amounts of nuclear fallout into the atmosphere.[23]

The Chernobyl disaster released roughly 10 PBq, or about 5% of the core inventory, of strontium-90 into the environment.[24] The Fukushima Daiichi disaster released 0.1-1 PBq of strontium-90 in the form of contaminated cooling water into the Pacific Ocean.[25]

References
  1. "Table of Isotopes decay data". Lund University. Retrieved 2014-10-13.
  2. "Strontium | Radiation Protection | US EPA". EPA. 24 April 2012. Retrieved 18 June 2012.
  3. TOXICOLOGICAL PROFILE FOR STRONTIUM (PDF), Agency for Toxic Substances and Disease Registry, April 2004, retrieved 2014-10-13
  4. Decay data from National Nuclear Data Center at the Brookhaven National Laboratory in the US.
  5. Delacroix, D.; Guerre, J. P.; Leblanc, P.; Hickman, C. (2002). Radionuclide and Radiation Protection Data Handbook 2002 (2nd ed.). Nuclear Technology Publishing. ISBN 978-1-870965-87-3.
  6. "Livechart - Table of Nuclides - Nuclear structure and decay data". IAEA. Retrieved 2014-10-13.
  7. "NRC: Glossary -- Bone seeker". US Nuclear Regulatory Commission. 7 May 2014. Retrieved 2014-10-13.
  8. Tiller, B. L. (2001), "4.5 Fish and Wildlife Surveillance" (PDF), Hanford Site 2001 Environmental Report, DOE, retrieved 2014-01-14
  9. Driver, C.J. (1994), Ecotoxicity Literature Review of Selected Hanford Site Contaminants (PDF), DOE, doi:10.2172/10136486, retrieved 2014-01-14
  10. "Freshwater Ecology and Human Influence". Area IV Envirothon. Retrieved 2014-01-14.
  11. "Radioisotopes That May Impact Food Resources" (PDF). Epidemiology, Health and Social Services, State of Alaska. Retrieved 2014-01-14.
  12. "Human Health Fact Sheet: Strontium" (PDF). Argonne National Laboratory. October 2001. Retrieved 2014-01-14.
  13. "Biological Half-life". HyperPhysics. Retrieved 2014-01-14.
  14. Glasstone, Samuel; Dolan, Philip J. (1977). "XII: Biological Effects" (PDF). The effects of Nuclear Weapons. p. 605. Retrieved 2014-01-14.
  15. Shagina, N B; Bougrov, N G; Degteva, M O; Kozheurov, V P; Tolstykh, E I (2006). "An application of in vivo whole body counting technique for studying strontium metabolism and internal dose reconstruction for the Techa River population". Journal of Physics: Conference Series. 41: 433–440. doi:10.1088/1742-6596/41/1/048. ISSN 1742-6588.
  16. Boehm BO, Rosinger S, Belyi D, Dietrich JW (August 2011). "The Parathyroid as a Target for Radiation Damage". New England Journal of Medicine. 365 (7): 676–678. doi:10.1056/NEJMc1104982. PMID 21848480.
  17. Harris, Dale; Epstein, Joseph (1968), Properties of Selected Radioisotopes (PDF), NASA
  18. Standring, WJF; Selnæs, ØG; Sneve, M; Finne, IE; Hosseini, A; Amundsen, I; Strand, P (2005), Assessment of environmental, health and safety consequences of decommissioning radioisotope thermal generators (RTGs) in Northwest Russia (PDF), Østerås: Norwegian Radiation Protection Authority
  19. "Power Sources for Remote Arctic Applications" (PDF). Washington, DC: U.S. Congress, Office of Technology Assessment. June 1994. OTA-BP-ETI-129.
  20. "Wireless blade monitoring system and process".
  21. "Nuclear Fission Fragments". HyperPhysics. Retrieved 18 June 2012.
  22. Schneir, Walter (April 25, 1959). "Strontium-90 in U.S. Children". The Nation. 188 (17): 355–357.
  23. Hevesi, Dennis. "Dr. Louise Reiss, Who Helped Ban Atomic Testing, Dies at 90", The New York Times, January 10, 2011. Accessed January 10, 2011.
  24. "II: The release, dispersion and deposition of radionuclides", Chernobyl: Assessment of Radiological and Health Impacts (PDF), NEA, 2002
  25. Povinec, P. P.; Aoyama, M.; Biddulph, D.; et al. (2013). "Cesium, iodine and tritium in NW Pacific waters – a comparison of the Fukushima impact with global fallout". Biogeosciences. 10 (8): 5481–5496. Bibcode:2013BGeo...10.5481P. doi:10.5194/bg-10-5481-2013. ISSN 1726-4189.
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