What was the first nuclear power plant accident?

Chalk River, December 12, 1952. Control rod failure, combined with operator errors, caused a power excursion and a buildup of hydrogen, that was not handled properly by the gas control system, which subsequently exploded, releasing about 30 kg of uranium and 4500 tons of radioactive water. The operators purged the heavy water, D2O, which is used as a moderator, shutting down the reaction in about 30 seconds, but the hydrogen explosion damaged the core. The core was repaired, and placed back into service about two years later. There were no fatalities or injuries, and a followup study in 1982 revealed no long term impact to exposed workers.