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

Qualified life is the period of operational aging time prior to an accident event (assumed to be a postulated high energy pipe break) that an item of safety-related equipment is assured by test and analysis to remain capable of performing its post-accident safety function. 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, economic considerations with respect to test performance often forces the use of 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. 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, melting or annealing may dominate aging, 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 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.