Yokohama, 165 miles south of Fukushima. Here’s what Wikipedia says about strontium-90: Strontium-90 ( 90 Sr) is a radioactive isotope of strontium , with a half-life of 28.8 years. Natural strontium is nonradioactive and nontoxic, but 90 Sr is a radioactivity hazard. Strontium-90 is a “ bone seeker ” that exhibits