Aging assessment of bistables and switches in nuclear power plants

Bistables and process switches play vital roles in the instrumentation and control logic of a nuclear power station. To understand the aging characteristics of these components, more than 5,000 NPRDS events and more than 1200 LERs were reviewed and analyzed. Telephone surveys were conducted and nuclear plant site visits were made to collect information on operating experience and the current status of these devices. Interaction with the equipment manufacturers provided further details on the designs of bistables and switches. The aging characteristics studied included the effects of aging on failure frequency, failure mode, and failure cause. This study found that two groups of bistables are in operation. The first group which consists of older bistables, needs attention in the near future concerning the decision on replacement or refurbishment; however, the second group, which is newer, still has time to manage aging concerns. Most of the original switches employed in reactor protection systems have been replaced with transmitters and bistables, and the trend shows that the remaining ones will be replaced in the near future. Based on the results of the analyses, recommendations for better aging management are made, and the areas for future studies are identified.