|Decay mode||Decay energy|
|Beta decay||0.546 MeV|
Strontium-90 (90Sr) 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. Strontium-90 has applications in medicine and industry and is an isotope of concern in fallout from nuclear weapons and nuclear accidents.
Naturally occurring strontium is nonradioactive and nontoxic at levels normally found in the environment, but 90Sr is a radiation hazard. 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. 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 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%.
Strontium-90 is a "bone seeker" that exhibits biochemical behavior similar to calcium, the next lighter group 2 element. 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-90 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. Exposure to 90Sr can be tested by a bioassay, most commonly by urinalysis.
The biological half life of strontium-90 in humans has variously been reported as from 14 to 600 days, 1000 days, 18 years, 30 years and, at an upper limit, 49 years. 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.
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.
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. It is also used as a radioactive tracer in medicine and agriculture.
90Sr 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.
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 in 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.
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.
The Chernobyl disaster released roughly 10 PBq, or about 5% of the core inventory, of strontium-90 into the environment. The Fukushima Daiichi disaster released 0.1-1 PBq of strontium-90 in the form of contaminated cooling water into the Pacific Ocean.
Industrial and aerospace applications
90Sr finds use in industry as a radioactive source for thickness gauges.
Heat source for radioisotope thermoelectric generators
The radioactive decay of strontium-90 generates a significant amount of heat, 0.95 W/g in the form of pure strontium metal or approximately 0.23 W/g as strontium titanate 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. It was also used in the US "Sentinel" series of RTGs.
Nuclear Thermochemical Generator
Another use of the strontioum-90 beta decay has been found by a research team from the University of Missouri. It consists of direct transformation of beta particulae into electricity. This technology has been found to be far more efficient than the process of radio-to-thermal conversion followed by thermal-to-electric conversion of the energy used on RTGs. “Betavoltaics, a battery technology that generates power from radiation, has been studied as an energy source since the 1950s,” said Jae W. Kwon, an associate professor of electrical and computer engineering and nuclear engineering in the College of Engineering at MU. The team published a paper at Nature (journal) called Plasmon-assisted radiolytic energy conversion in aqueous solutions.
Proposed uses for this type of power source range from spacecraft to road cars. Nevertheless, this technology is on an early phase of development and its practical uses are not clear yet. For spacecraft use it would adapt quite well, although the life span of the Betavoltaics battery is going to be far shorter than that of the Plutonium powered RTGs, as well as the total energy released. This should impose serious constrains to interplanetary mission, although they may be overcomed by the much lower price and higher availability of Strontium-90 with respect of Plutonium-238.
Civilian/terrestrial uses of Betavoltaics batteries implies not only engineering, safety and practical issues, but a huge cultural impact. If personal devices (such as computers, cars, or house electrical systems) can be developed to last around 10 years with reasonable performance and negligible radiation, strong opposition may arise from different groups, like Greenpeace. As they state:
"End the nuclear age: Greenpeace has always fought - and will continue to fight - vigorously against nuclear power because it is an unacceptable risk to the environment and to humanity. The only solution is to halt the expansion of all nuclear power, and for the shutdown of existing plants."
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