Risk-based test frequency optimization of a turbine generator protection system

The turbine generator of any nuclear power plant includes a protection system, which is required to shut down the steam supply to the turbine during adverse conditions such as turbine overspeed, loss of load, loss of bearing oil, thrust-bearing failure, or loss of (condenser) vacuum. During refueling and maintenance outages and during normal operation, the protection system of the turbine generator is tested to detect potential dormant (wear-out) failures of the protective devices to ensure proper function in the case of a sudden demand. The problem is to identify and implement the most effective testing policy to maximize plant safety, minimize the financial risk of severe damage to the turbine generator, and to maximize the generating production capacity, under operating and maintenance (O ampersand M) budget constraints. This paper presents a methodology to develop plant-specific testing strategies and acceptance criteria of the turbine generator protection system, using a multivariate safety-cost-benefit approach based on risk assessment. Based on plant-specific data of the design and component failure rates, augmented by industry data, the methodology is currently being applied to Salem units 1 and 2 with an optimal on-line test frequency of three per year