STRONTIUM

ARJUN MAKHIJANI, INSTITUTE FOR ENERGY & ENVIRONMENTAL RESEARCH

Strontium was named after a Scottish town, Strontian, in the Argyllshire district, where strontium carbonate mineral was first identified in the late-18th century. It is a metal with four stable isotopes that give flares and fireworks their crimson color.

Despite this useful and pleasing property, strontium is perhaps best known today in its dangerous radioactive incarnation, Sr-90. There are also a number of other radioactive isotopes of strontium, but only Sr-90 has a long enough half-life, 29.1 years, to present a persistent hazard.

Sr-90 is created during nuclear explosions or in nuclear reactors in the process of fission of heavy nuclei, such as uranium-235 and plutonium-239. Strontium belongs in the same group of metals as calcium--and that is the principal source of its danger. When it is ingested or inhaled, radioactive strontium is processed by the body in the same way as calcium and winds up in bones, creating the risk of cancer, especially bone cancer and leukemia.

Sr-90 ionizes molecules in the body by the emission of a high-energy -particle. The specific activity of Sr-90 is about 138 curies per g. It decays into Y-90, which is also a -emitter, with which it is normally in equilibrium, thus doubling the specific activity of the material.

Biological harm can also be caused by Sr-90 from outside the body. When -particles it emits are stopped in air, liquids, or solids, the frictional or braking energy that slows down the electrons is emitted in the form of X-rays called brehmstrahlung (German for braking) radiation. This radiation, like the dentist's X-rays, can penetrate the body and cause the ionizing of molecules and increased cancer risk.

	LIGHT SHOW Strontium imparts a crimson color to fireworks. © FIRST NIGHT BOSTON/PHOTO BY SUSAN COLE KELLY

In the U.S., the Greater St. Louis Citizens Committee for Nuclear Information issued a call for people to send in baby teeth, where Sr-90 accumulation was expected. Positive results seemed to clinch the matter. In 1963, the U.S., the Soviet Union, and Great Britain signed a treaty banning atmospheric testing.

Sr-90 also pollutes soil and water at some reprocessing plants, such as those at the Savannah River Site in South Carolina, the Hanford Site in Washington, and the Mayak plant in Russia's southern Urals. At Mayak, an explosion of a tank containing high-level wastes in 1957 released 20 million curies of radioactive fission products into the environment. About 2 million curies of this was deposited over an area of 15,000 to 23,000 km², necessitating the evacuation of more than 30 towns and villages. About 5% of this was Sr-90 and yttrium-90.

Sr-90 has been used to make radioisotope thermoelectric generators (RTGs). The energy of the -particles is captured as heat, some of which is converted to electricity using thermocouples. RTGs have been used as power sources in space missions both by the U.S. and the Soviet Union. They were deployed as power sources for powering remote seismic stations in Alaska and, far more commonly, in remote areas of the former Soviet Union. RTGs using plutonium-238, with a half-life of 87 years, are now preferred, since these devices can be made more compact because they need less shielding.

After the disintegration of the Soviet Union, the system for keeping track of Sr-90 power sources fell into some disarray. They are now a source of danger to the local population. (Hunters in the Republic of Georgia have been accidentally irradiated, for instance.) There is also the risk that they could be discovered by or sold to terrorists for use in radiological weapons. Each RTG typically contains tens of thousands of curies--several hundred grams--of Sr-90.

A little over 10 µg of Sr-90, if inhaled in insoluble form, would give a sufficient dose to cause cancer with high probability. The risk is mitigated by the fact that it would be difficult to make radioactive dispersal weapons using Sr-90, given the potentially severe radiation risk to those who might want to fabricate such devices without expert help or considerable experience. A considerable effort to locate and secure Sr-90 RTGs is now under way. Ironically, the use of radiological weapons was first conceptualized by the U.S. during World War II as part of the Manhattan Project. Fortunately, the idea was never implemented.

Despite the crimson brilliance of stable strontium, its radioactive variety has given element 38 an air of risk and notoriety.

Arjun Makhijani is president of the Institute for Energy & Environmental Research. He has a Ph.D. from the University of California, Berkeley, where he specialized in nuclear fusion. He is principal editor of "Nuclear Wastelands," MIT Press, 1995 and 2000, which was nominated for the Pulitzer Prize.

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