The Chernobyl accident is considered the worst nuclear disaster in history. It occurred on 26th April 1986 during a low-power test with the No.4 RBMK-type reactor. The core overheated, causing a chain reaction of explosions that released huge amounts of radiation into the atmosphere. It is estimated that thousands of people were affected by the disaster, some developing blood disorders or cancers. The health consequences of the disaster are poorly understood. In Kiev, Ukraine, 35,000 people are being studied to determine the true effects.