In nuclear safety, the source term is introduced to provide adequate isolation of the nuclear hazards from the public, by establishing a concept of ‘effective distance.’ This combines a geographical distance to the site boundary and an effective ‘distance’ with the use of engineered safety features (i.e., a containment system and its cooling system), combined with evacuation procedures to prevent radiation injury. Severe accidents occur when these safety systems failed to function. This basic safety approach was once again jeopardized by the Fukushima accident, which followed the Chernobyl accident. The factors that mitigated the effluent releases however depend greatly on the intrinsic safety features combined with the accident management. The multi-layered retention/decontamination factors that a nuclear power plant possesses should be incorporated in specifying the environmental source term. The Fukushima accident provides a reasonable upper bound with respect to environmental releases due to a LUHS (loss of ultimate heat sink), which triggered a prolonged SBO (station blackout).

Due to the anticipated radiological consequences, the Japanese Government issued a series of evacuation orders, resulting in the evacuation of approximately 160,000 people from the Fukushima area. The prolonged evacuation is believed to be the cause of over one thousand “disaster-related (pre-mature) deaths (DRDs)” which have been reported among the evacuees due to psychosomatic effects (48%) and the disruption of medical and social welfare facilities (18%). In the future these types of deaths should be avoided.

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