It did not explode. There was no meltdown, no dramatic plume of smoke. The incident at the Tsuruga Nuclear Power Plant on April 25, 1981, was a silent, invisible misstep during what should have been a controlled operation. Workers were manually cleaning a sludge tank connected to the plant's spent fuel pool. The procedure involved flushing the tank with water. A critical error in the sequence of valve operations—a bypass of a crucial overflow mechanism—allowed the tank to fill beyond its safe limit.
As the water level rose, it began to act as a reflector for neutrons emitted by the radioactive sludge at the bottom. In the space of a few minutes, the tank achieved criticality. A self-sustaining nuclear fission chain reaction began inside the concrete building. It lasted for approximately 40 minutes. There was no blast, only a surge of gamma and neutron radiation that permeated the work area. The radiation alarms sounded. The workers, 103 of them, were evacuated. They had received varying doses, some significantly above annual safety limits, though none were immediately fatal.
The aftermath was a study in containment of a different kind. The plant's operator, Japan Atomic Power Company, was criticized for inadequate procedures and worker training. The accident was rated a Level 2 on the International Nuclear Event Scale—an 'incident,' not a 'major accident.' This technical classification, combined with its occurrence in the pre-Chernobyl era of greater nuclear optimism and before the digital age of instant global news, allowed it to fade from international memory. It became a footnote, a near-miss that demonstrated how catastrophe could begin not with a structural failure, but with a simple, erroneous manual action in a room filling quietly with water.