Since the Fukushima Daiichi accident occurred, the probabilistic safety assessment on multi-unit sites has been studied so far. Several studies addressed to reflect the factor that can affect an adjacent unit, called inter-unit dependency, in the assessment and various research has been made to investigate or evaluate the de-pendencies. In this study, researches on inter-unit dependency are implemented to internal and seismic level 1 site risk assessment in the purpose of the evaluation on their effect on the probabilistic safety assessment. The assessments are conducted on the hypothetical site in Korea with 4 units of APR-1400. To reflect the redundancy changes in safety system by operation state, the approach is suggested based on the study of dependent accidents. The result showed that the earthquake and loss of offsite power would induce most of the multi-unit core damage. The inter-unit dependency models related to the electric system were sensitive to the result and the synergies between the models were found. The seismic correlation made site core damage frequency decrease. The expectation is proposed as alternative metric for the risk of site and the necessity of site risk assessment is pointed out in this study.
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