Nuclear ship Mutsu and the 1974 radiation leak Incident: Technical background and management lessons

During its experimental voyage in September 1974, Japan’s first nuclear-powered ship, Mutsu, experienced abnormal radiation during a low-power reactor test conducted approximately 800 km east of Cape Shiriya in Aomori Prefecture. The immediate technical mechanism involved fast-neutron streaming and secondary gamma-ray generation caused by an inadequate shielding configuration. Although the event did not involve major fuel damage or a large release of radioactive material, it developed into a serious operational and political problem, leading to resistance to the ship’s return and to a government investigation. This paper reconstructs the incident and reviews the technical and organizational conditions that shaped its occurrence and subsequent handling. Drawing on official reports, technical records, and prior studies, it examines how inadequate project integration, limited design review, contractual separation, and communication failures complicated what began as a shielding-design problem. Using action system theory of management as a heuristic framework, we organize key task-level findings and relate them to recurrent patterns in major public technology projects. The case indicates that, in prototype nuclear systems, technical adequacy alone is insufficient: integrated review, clear responsibility, and effective communication are also essential to safe development and testing.