THE REDUNDANCY MATRIX: A QUALITATIVE METHOD FOR ASSESSING THE RISK OF MULTIPLE SYSTEM DEGRADATIONS

摘要

Multiple system degradations may occur as a result of planned (preventive) and/or emergent (corrective) maintenance activities. In most cases, the resulting changes in system failure rates are not known, hence the associated risk is difficult to quantify with PSA models. Nevertheless, a systematic method for controlling the risk of system degradations is desirable (see NUMARC 93-01, the Industry Guideline for implementation of the NRC Maintenance Rule). This paper presents a qualitative method for assessing the risk of multiple system degradations. The method is based on a "Redundancy Matrix" which displays system redundancies as analyzed in the PSA. Both "function" and "event" redundancies are considered in the matrix. The "event" redundancies are of particular interest since they highlight system relationships which might not be apparent from a design-basis perspective. The Redundancy Matrix is currently being used at the Vermont Yankee Nuclear Power Station, where it has been found to be a practical and robust tool for assessing the risk of removing equipment from service for on-line maintenance.