Analysis of the Effects of Multiple Radionuclide Plumes under Severe Accident Conditions on Environment around a Site Boundary

In TEPCO’s Fukushima Dai-ichi Nuclear Power Station accident on March 11, 2011, a massive number of radionuclides were released from Unit–1, Unit–2, and Unit–3 to the environment. After the accident, the effects of multiple sources became one of the major topics of discussion in safety assessment. However, these discussions mainly focused on core damage frequencies. In the case of simultaneous accidents at multiple units, it is important to consider not only core damage frequencies but also risks because radionuclides are released from multiple units to the environment. In this study, we analyzed dominant parameters that affect health in the case of multiple sources with a simplified configuration of units. The analytical results show that the probabilities of early health effects near the site boundary depend on the locations of units and the wind direction at the site. They also show that the probabilities of early health effects due to releases from multiple units cannot be calculated by the arithmetic summation of probabilities of early health effects due to releases from single units because, for example, of the nonlinearity due to the threshold of the occurrence probability of early health effects.