Risk assessment approach for Rokkasho reprocessing plant

Full text: It is desirable that the operation and maintenance of Rokkasho Reprocessing Plant (RRP) be established and conducted with maximum effectiveness and efficiency, making the best use of risk information to help the plant achieve further enhanced safety. Risk assessment is applied for RRP, and upgraded risk information is established. In the basic design phase, the potential incidents and accidents that might occur in the plant were identified systematically and exhaustively adopting the HAZOP method. After screening the potential for occurrence, the design basis accidents (DBAs) were identified and it was confirmed that the plant would not put the general public at risk of significant radiation exposure in the case of such accidents, even when assuming the single failure of dynamic apparatus in the prevention and mitigation systems. To support the deterministic safety assessment mentioned above, the risk assessment was conducted during the basic design phase. Of the DBAs and out-of-design basis accidents excluded from DBAs because of extremely rare occurrence possibilities, the risk assessment was conducted for such accidents which might cause relatively high consequence for the general public. The risk assessment was conducted using the PSA method generally used for nuclear power plants. After that, a review of the occurrence frequency assessment for some of the accidents was made, taking into account information relating to detailed design and operation procedures. Typical examples are a loss of the hydrogen scavenging function in the plutonium solution tank and a loss of cooling capability in the high-active liquid waste storage tank. The occurrence frequency for a loss of the hydrogen scavenging function was less than 10-5 /year. The occurrence frequency for a loss of cooling capability was less than 10-7/year. In addition, an importance assessment (FV index, Risk Achievement Worth) was conducted, such as a contribution to the occurrence frequency for various types of equipment and piping and for operations performed by operators, and was obtained through sensitivity analysis, such as the influence of change in the maintenance periods of the occurrence frequency. This information might be of use for operation management. On the other hand, in order to have grasp of the risk profile at a reprocessing plant and in order for us to make good use of the risk information, it is also necessary to conduct risk assessments for incidents other than major accidents. Since it is appropriate to use a Graded Approach to assess the risks of such a large number of incidents effectively and efficiently, a simplified method, called ''improved risk index method'' was developed. It based on the Risk Index Method recommended by U.S. NRC. The Risk Index Method is simple and is appropriate for fuel cycle facilities where a wide variety of accidents may occur, but it is characterized by the fact that it requires engineering judgments by analysts when assessing the occurrence frequency, and the common components between the safety measures (the subordinacy) are difficult to clarify. In order to solve the problems inherent in the Risk Index Method, quantification, flexibility and traceability have been improved while maintaining its advantages. Currently, risk assessments for a wide variety of incidents and accidents are being conducted using the above mentioned methods and their results will be used for the improvement of operation management. Furthermore, risk assessments for external incidents (earthquakes, etc.) are being planned