Technical Evaluation Report on the Seven Main Transformer Failures at the North Anna Power Station, Units 1 and 2 (Docket Nos. 50-338, 50-339)

摘要

This report documents technical evaluations on various aspects pertaining to the seven main transformer failures at the North Anna Power Station, Units 1 and 2. These reports cover the subjects of Probability Risk Assessment (PRA), Failure Modes and Effects Analysis (FMEA), Root Causes, Protection Systems, Modifications, Failure Statistics, and Generic Aspects. The PRA determined that the contribution from a main transformer failure affecting plant safety systems so as to increase the risk to the public health and safety is negligible. The FMEA determined that a main transformer failure can have primary and secondary effects on plant safety system operation. The evaluation of the Root Causes found that no single common cause contributed to the seven failures. Each failure was found to have specific circumstances for initiating the failure. Both the generator and transformer primary protection systems were found to perform correctly and were designed within industry standards and practices. The proposed modifications resulting from the analyses of the failures will improve system reliability and integrity, and will reduce potentially damaging effects. The failure statistic survey found very limited data bases from which a meaningful correlation could be ascertained. The statistical comparison found no appreciable anomalies with the NAPS failures. The evaluation of all the available information and the results of the separate reports on the main transformer failures found that several generic concerns exist. (ERA citation 09:026059)