Strontium-90 What Is Strontium-90? Strontium is a naturally occurring metallic element found in rocks, soil, coal, and dust. However, Strontium-90  is a radioactive isotope produced during nuclear fission in reactors and weapons testing. With a half-life of approximately 29 years, it persists in the environment for decades, allowing it to circulate through ecosystems, food chains, and human populations long after initial release. Because it behaves chemically similar to calci