Use of reliability and risk standards as bases for acceptance in licensing and regulation of nuclear power plants

The prevention and mitigation of accidents in PWR and BWR type reactor plants is essentially based on the sufficient operability of certain primary safety systems. To ensure sufficient reliability, the authority sets forth numerical reliability goal for the most important safety functions of new nuclear power plants. The unreliability of the function in question shall remain below this goal. To calculate the reliability of systems, one uses a data base which has been collected from the operating experiences of similar plants. If a data base of this kind is not available, a generic data base is utilized. However no fixed numerical acceptance standards are set forth in advance for the probabilities of a core melt, but the estimated probabilities are compared with the known probabilities of a core melt at other nuclear power plants. As concerns the operating plants it is defined in the Technical Specifications under what conditions the operation of the plant can be continued when faults have been detected in components important to safety. However, the Technical Specifications do not cover all situations in plant operation. Especially if there is a hidden potential fault discovered after the commissioning of the plant, for example an initiating event that may damage systems important to safety, the decision concerning the shutdown of the plant and the urgency of the repair must be made utilizing a probabilistic criteria. In cases like this, the authority requires that the power company estimate the annual risk of a core melt caused by the safety deficiency in question. The authority will perform an independent estimate and considers the shutdown of the plant and the urgency of the repair by comparing the risk of a core melt caused by the safety deficiency with the risk of a core melt estimated in connection with the PSA. 2 refs, 2 figs, 1 tab