A radioactive isotope of the element strontium (see radioactivity). The symbol yeeseng Sr, abbreviated Sr. Strontium 90 is a pure beta decay nuclide with a maximum energy of 0.546 MeV for beta rays. Strontium 90 is one of the fission products of uranium, has a half-life of 28 years, and generally comes from nuclear explosions or nuclear fuel products, and is not highly diffusible.