The nuclear criticality accident in Tokaimura demonstrated the terrifying speed and physics of liquid prompt criticality. Technicians preparing enriched uranium fuel bypassed safety protocols by manually pouring dissolved uranyl nitrate solution into an unshielded precipitation vessel using stainless steel buckets. Crucially, the tall, narrow geometry designed to prevent accidental critical mass was abandoned in favor of a wide mixing basin.
When the concentration of uranyl nitrate in the vessel reached approximately 16 kilograms of enriched uranium, the fluid surpassed its critical mass. An uncontrolled nuclear chain reaction erupted instantly, accompanied by an intense blue flash caused by Cherenkov radiation as high-energy particles passed through the aqueous liquid faster than light travels in water. The liquid solution acted as both fuel and neutron moderator, sustaining intermittent criticality pulses for 20 hours.
Thermal expansion and boiling in the liquid created bubbles that momentarily disrupted criticality, only for the reaction to restart as the solution cooled and settled. The event highlighted the critical importance of geometric containment control in wet nuclear processing facilities.