IMPROVEMENT OF PLANT SAFETY AND AVAILABILITY BY A CHANGE OF THE CONTROL CIRCUIT DESIGN

摘要

On 16 October 1993, an unscheduled reactor shutdown due to closure of the main feed-water isolation valve (MFIV) occurred in one of the Korean nuclear power plants (NPPs). The cause of the unscheduled trip was found to be the failure of a fuse in the control card for the solid state interposing logic system (SSILS) which controls the MFIVs. Among all Korean plants, there had been 17 reactor trips due to failure of this control logic circuit. In order to reduce the frequency of unscheduled reactor shutdown, the control logic design was revised by installing double SSILS circuits with an AND logic. In the revised design, the main steam isolation valves (MSIVs) or the MFIVs can be closed only when both SSILS cards operate correctly, and so it was expected that there would be some reduction in plant safety due to an increase in the possibility of a failure of the MSIVs and MFIVs to close. Therefore, a probabilistic safety assessment (PSA) was performed to compare the risk of the revised design and that of the old one. Two types of failure which can affect plant safety were considered for this PSA. One type of failure was spurious operation of the control circuit which can cause an unscheduled trip. The other type was a failure of the MSIVs and MFIVs to close when this was required for isolating a faulted steam generator during accidents such as steam generator tube rupture. The PSA showed that the core damage frequency of the revised design was approximately half of that of the old design. Thus, the revised design was adopted for four Korean NPPs in 1994. Since then, no unscheduled reactor shutdowns due to spurious closing of the MSIVs and MFIVs have occurred in those plants.