1989 In the Kramatorsk radiological accident

A small capsule containing highly radioactive 137Cs was found inside the concrete wall in an apartment building in Kramatorsk, Ukrainian SSR. It is believed that the capsule, originally contained in a measurement device, was lost sometime during the late 1970s and ended up mixed in with gravel used to construct that building in 1980. By the time the capsule was discovered, six residents of the building had died from leukemia and 17 more received varying doses of radiation.


 * Sources: https://en.wikipedia.org/wiki/List_of_civilian_nuclear_accidents and https://www.wikiwand.com/en/List_of_civilian_radiation_accidents