Accident at Chernobyl Nuclear Power Station

On April 26, 1986, a large accident occurred in No.4 plant of Chernobyl Nuclear Power Station in Ukraine, USSR. This accident was an unprecedented one in the history of atomic energy development in view of any of the state of reactor damage, the number of casualties, the amount of radioactivity release and so on, and gave a large shock to the whole world. Also in Japan far away, the radioactive substance that seemed to be originated from the accident was detected though a minute amount. In the countries of Europe as well as the surroundings of the Power Station, the contamination with radioactive substances was serious, and the measures such as the limitation of intake and the limitation of distribution of foods were taken. The international conference was held for five days from August 25 at IAEA, and USSR made the report on the accident. Respective countries and international organization began the evaluation of the accident and the work to extract the lessons based on this report. However, as to the details of the accident, still many unknown points remain. The No.4 plant is RBMK-1000 type, graphite moderated, boiling light water cooled, pressure tube type of 1000 MWe output. The features of the damaged reactor, the course of the accident, and the evaluation of the accident and the lessons are described. (Kako, I.)