_in_tin.jpg) Strontium-90 test source intin | |
| General | |
|---|---|
| Symbol | 90Sr |
| Names | strontium-90 |
| Protons(Z) | 38 |
| Neutrons(N) | 52 |
| Nuclide data | |
| Natural abundance | syn |
| Half-life(t1/2) | 28.91 years[1] |
| Isotope mass | 89.907728[2]Da |
| Decay products | 90Y |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Beta decay | 0.546[3] |
| Isotopes of strontium Complete table of nuclides | |
Strontium-90 (90
Sr) is aradioactiveisotope of strontium produced bynuclear fission, with ahalf-life of 28.91 years. It undergoesβ− decay intoyttrium-90, with adecay energy of 0.546 MeV. Strontium-90 has applications in medicine and industry and is anisotope of concern infallout fromnuclear weapons,nuclear weapons testing, andnuclear accidents.[4]
Naturally occurringstrontium is nonradioactive and nontoxic at levels normally found in the environment, but90Sr is a radiation hazard.[5]90Sr undergoesβ− decay with ahalf-life of 28.91 years and adecay energy of 0.546MeV distributed to anelectron, anantineutrino, and theyttrium isotope90Y, which in turn undergoesβ− decay with a half-life of 64.05 hours and a decay energy of 2.28 MeV distributed to an electron, an antineutrino, and occasionally agamma ray, leaving stable90Zr.[6] The gamma-emitting branches are so weak that for most purposes90Sr/90Y can be considered a purebeta particle source.
| Nuclide | t1⁄2 | Yield | Q[a 1] | βγ |
|---|---|---|---|---|
| (a) | (%)[a 2] | (keV) | ||
| 155Eu | 4.74 | 0.0803[a 3] | 252 | βγ |
| 85Kr | 10.73 | 0.2180[a 4] | 687 | βγ |
| 113mCd | 13.9 | 0.0008[a 3] | 316 | β |
| 90Sr | 28.91 | 4.505 | 2826[a 5] | β |
| 137Cs | 30.04 | 6.337 | 1176 | βγ |
| 121mSn | 43.9 | 0.00005 | 390 | βγ |
| 151Sm | 94.6 | 0.5314[a 3] | 77 | β |
| ||||
90Sr is a product ofnuclear fission. It is present in significant amount inspent nuclear fuel, inradioactive waste fromnuclear reactors and innuclear fallout fromnuclear tests.Forthermal neutron fission as in today's nuclear power plants, thefission product yield fromuranium-235 is 5.7%, fromuranium-233 6.6%, but fromplutonium-239, only 2.0%.[7] (Commercial reactors derive energy both from uranium-235 and plutonium-239 in comparable amounts.)
Strontium-90 is classified as high-level waste. Its 29-year half-life means that it can take hundreds of years to decay to negligible levels. Exposure from contaminated water and food may increase the risk ofleukemia andbone cancer.[8] Reportedly, thousands of capsules of radioactive strontium containing millions of curies are stored atHanford Site's Waste Encapsulation and Storage Facility.[9]
Algae has shown selectivity for strontium in studies, where most plants used inbioremediation have not shown selectivity between calcium and strontium, often becoming saturated with calcium, which is greater in quantity and also present in nuclear waste.[8]
Researchers have looked at the bioaccumulation ofstrontium byScenedesmus spinosus (algae) in simulated wastewater. The study claims a highly selectivebiosorption capacity for strontium ofS. spinosus, suggesting that it may be appropriate for use of nuclear wastewater.[10]
A study of the pond algaClosterium moniliferum using stable strontium found that varying the ratio ofbarium tostrontium in water improved strontium selectivity.[8]
Strontium-90 is a "bone seeker" that exhibits biochemical behavior similar tocalcium, the next lightergroup 2 element.[5][11] After entering the organism, most often by ingestion with contaminated food or water, about 70–80% of the dose gets excreted.[4] Virtually all remaining strontium-90 is deposited inbones andbone marrow, with the remaining 1% remaining in blood and soft tissues.[4] Its presence in bones can causebone cancer, cancer of nearby tissues, andleukemia.[12] Exposure to90Sr can be tested by abioassay, most commonly byurinalysis.[5]
Thebiological half-life of strontium-90 in humans has variously been reported as from 14 to 600 days,[13][14] 1000 days,[15] 18 years,[16] 30 years[17] and, at an upper limit, 49 years.[18] 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.[19]
The elimination rate of strontium-90 is strongly affected by age and sex, due to differences inbone metabolism.[20]
Together with the caesium isotopes134Cs and137Cs, and the iodine isotope131I, it was among the most important isotopes regarding health impacts after theChernobyl disaster.As strontium has an affinity to thecalcium-sensing receptor ofparathyroidcells that is similar to that of calcium, the increased risk ofliquidators of theChernobyl power plant to suffer fromprimary hyperparathyroidism could be explained by binding of strontium-90.[21]
The radioactive decay of strontium-90 generates a significant amount of heat, 0.920 W/g in the form of pure strontium metal or 0.445 W/g asstrontium titanate[22] and is cheaper than the alternative238Pu. It is used as a heat source in many Russian/Sovietradioisotope thermoelectric generators, usually in the form of strontium titanate.[23] It was also used in the US "Sentinel" series of RTGs.[24] Startup company Zeno Power is developing RTGs that use strontium-90 from theDOD, and is aiming to ship product by 2026.[25]
90Sr finds use in industry as a radioactive source for thickness gauges.[4]
90Sr finds extensive use in medicine as a radioactive source for superficialradiotherapy of some cancers. Controlled amounts of90Sr or of89Sr can be used in treatment ofbone cancer, and to treat coronaryrestenosis viavascular brachytherapy. It is also used as aradioactive tracer in medicine and agriculture.[4]
90Sr is used as ablade inspection method in some helicopters with hollow blade spars to indicate if a crack has formed.[26]
In April 1943,Enrico Fermi suggested toRobert Oppenheimer the possibility of using the radioactive byproducts from enrichment to contaminate the German food supply. The background was fear that the German atomic bomb project was already at an advanced stage, and Fermi was also skeptical at the time that an atomic bomb could be developed quickly enough. Oppenheimer discussed the proposal withEdward Teller, who suggested the use of strontium-90.James Bryant Conant andLeslie R. Groves were also briefed, but Oppenheimer wanted to proceed with the plan only if enough food could be contaminated with the weapon to kill half a million people.[27]
Strontium-90 is not quite as likely ascaesium-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.[28]
A study of hundreds of thousands ofdeciduous teeth, collected by Dr.Louise Reiss and her colleagues as part of theBaby Tooth Survey, found a large increase in90Sr levels through the 1950s and early 1960s. The study's final results showed that children born inSt. Louis, Missouri, in 1963 had levels of90Sr 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. Reviewers of the study predicted that the fallout would cause increased incidence of disease in those who absorbed strontium-90 into their bones.[29] However, no follow up studies of the subjects have been performed, so the claim is untested.
An article with the study's initial findings was circulated to U.S. PresidentJohn F. Kennedy in 1961, and helped convince him to sign thePartial Nuclear Test Ban Treaty with theUnited Kingdom andSoviet Union, ending the above-groundnuclear weapons testing that placed the greatest amounts ofnuclear fallout into the atmosphere.[30]
TheChernobyl disaster released roughly 10 PBq, or about 5% of the core inventory, of strontium-90 into the environment.[31] TheKyshtym disaster released strontium-90 and other radioactive material into the environment. It is estimated to have released 20 MCi (800 PBq) of radioactivity. TheFukushima Daiichi disaster had from the accident until 2013 released 0.1 to 1 PBq of strontium-90 in the form of contaminated cooling water into thePacific Ocean.[32]