Kyshtym disaster

From the very beginning, the reactor at the Mayak complex faced a number of problems.

An excerpt from the article 42 facts about Kyshtym disaster

Inadequate cooling of the reactor caused the fuel to fuse with graphite. Pressure from Stalin meant that the reactor's operation was not halted, and with the reactor on, inexperienced workers at the combine tried to fix the malfunction. The level of radiation around the reactor far exceeded standards.

Despite the constant malfunctions and high radiation, after six months of operation, they managed to produce enough weapons-grade plutonium to construct the first bomb.