Before Chernobyl, the Soviets had another massive nuclear disaster that occurred on 29 September 1957 and contaminated over 20,000 square km. The area was turned into a preserve to cover up the accident. The CIA knew of the accident, but also covered it up in order to protect the fledgling US nuclear industry from hysteria.
In 1989, a small capsule, originally a part of a measurement device, containing highly radioactive caesium-137 was found inside the concrete wall of a Kramatorsk apartment building. By the time the capsule was discovered, 6 residents of the building died from leukemia and 17 more received varying doses of radiation.