Caesium-137, with a half-life of 30.17 years, is one of the two principal medium-lived fission products, along with 90Sr, which are responsible for most of the radioactivity of spent nuclear fuel after several years of cooling, up to several hundred years after use. It constitutes most of the radioactivity still left from the Chernobyl accident and is a major health concern for decontaminating land near the Fukushima nuclear power plant. 137Cs beta decays to barium-137m (a short-lived nuclear isomer) then to nonradioactive barium-137, and is also a strong emitter of gamma radiation.