Discussion on probability of cesium-137 release exceeding 100 TBq as a part of the consideration of nuclear power plant probabilistic risk criteria for environmental protection

Though the fundamental safety objective of the IAEA is to protect both people and the environment from harmful radiation effects, probabilistic risk criteria for environmental protection has only been discussed recently after the occurrence of the Fukushima Accident. Preceding studies confirmed the appropriateness of using 100 TBq cesium-137 release as a part of the probabilistic risk criterion since it has limited consequences to the environment. This study aims to propose the way to derive the probability of this release when used as a probabilistic risk criterion. Literature survey revealed issues of existing criteria. As most criteria are not systematically derived, it is difficult to justify the contribution to the fundamental safety objective and the appropriateness of the probability used in the criteria. Accident consequence mitigative features or actions were not also taken into account in the derivation process. The way to address these issues is to simply follow the same process as the derivation of the probabilistic risk criteria for human protection. This will ensure the required level of safety without posing unnecessary burden onto the operator, and will enable the operator to concentrate its efforts on safety actions that contribute to nuclear safety.