Transition temperature considerations when extrapolating safety-related equipment post-accident operating times

Qualified life is the amount of aging time prior to an accident that an item of safety-related equipment is assured by test and analysis to still be capable of performing its safety function even when exposed to the harsh environment of that accident (assumed to be a postulated high energy pipe break). Qualified post-accident operating time is the amount of time after an accident begins that an item of safety-related equipment can be assured by test and analysis to continue to operate and perform its safety function while exposed to the accident's harsh environment. It is desirable that both periods of time be established by test alone; however in reality because of economic considerations, one and often both require extrapolation analysis using Arrhenius (accelerated aging) techniques. There are two conditions critical to the success and acceptability of the analytical result when extrapolating tested post-accident operating time to a longer period of time. The first condition requires the equipment to be in thermal equilibrium with the elevated temperature conditions of the test. The second condition requires that the aging regime at the tested temperature is the same or sufficiently similar to the aging regime at the extrapolated temperature to provide a valid result. For example, at high temperatures, aging may be dominated by melting or annealing but at lower temperatures aging may take the form of embrittlement. Thus, the second condition requires a careful consideration of the component material transition temperatures. This paper will report on an effective approach we have developed for performing post-accident operating time extrapolations and the results when applied to several items of safety-related equipment commonly used at nuclear power plants.