noun
criticality accident
Technical term used in the nuclear industry for an uncontrolled nuclear chain reaction, typically involving fissile material.
1999年に茨城県で臨界事故が発生した。
A criticality accident occurred in Ibaraki Prefecture in 1999.
臨界 means 'criticality' or 'critical state' in nuclear physics, while 臨界事故 specifically refers to an accident involving criticality.
Compound of 臨界 (criticality) and 事故 (accident). The exact historical derivation is uncertain; the term is a modern technical coinage.