On April 26,1986, there was an accident at Unit 4 of the Chernobyl Atomic Power Station in the Ukraine, Soviet Union. Substantial quantities of radioactive materials were released for 10 days. Thirty-one workers and emergency personnel exposed at the reactor site died within two months of the accident from trauma, burns, and radiation effects. Excessive exposure to radiation during fire fighting and rescue operations resulted in the hospitalization of more than 200 personnel who were onsite. No acute radiation sickness was found in the offsite population, but over 135,000 persons were evacuated from a 30-kilometer zone around the reactor because of high ambient radiation levels. Since it was by far the worst nuclear power plant accident in history, Chernobyl was an important subject of inquiry by the Commission.