The authorities have warned thousands of residents near the site of the accident to stay indoors and to wash off any rain that falls on them. There are concerns about radiation in the surrounding areas.
Criticality is the point at which a nuclear chain reaction becomes self-sustaining.
Three workers from the plant have been taken to hospital and hundreds have been forced to leave their homes. One of the three workers in hospital is reported to be in a serious condition, suffering from continuous vomiting. Prime Minister Keizo Obuchi has set up an emergency task force to tackle the accident. A government request for help from US military forces in Japan for help was turned down. The US said its forces were not equipped to handle such accidents.