This paper presents a risk-informed assessment of fatigue pipe rupture (break) criterion with an application to a 12-inch Reactor Recirculation (RRC) System Class 1 pipe. In this case, a deterministic fatigue usage break criterion is established that will ensure: 1) low break probability, 2) adequate damage tolerance, and 3) insignificant risk. A bounding high stress-low cycle fatigue loading over a 60-year design life is assumed and the environmental fatigue life correlations in NUREG/CR-6909 are applied. Deterministic damage tolerance calculations examine the time for multiple cracks to initiate and grow through the pipe thickness. Probabilistic fracture mechanics models, coupled with industry service experience, relate small and large break loss of coolant accident (LOCA) probabilities and frequencies with cumulative fatigue usage factor (CUF). Risk impact calculations quantify small and large break core damage frequency contributions associated with alternative risk-informed CUF break criterion.
Sample calculations in this paper show that when component fatigue damage is limited to a 60-year environmental CUF <1.0 through-wall crack damage tolerance greater than 60 year design life, extremely low LOCA probabilities and negligible risk impacts can be expected. Additional reductions in the break probabilities and risk impacts can be obtained if 10-year inservice inspections are accounted for. These results suggest that a new criterion based on risk-informed principles can be used to support increasing the fatigue break criterion from 0.1 to the ASME Code design fatigue limit (CUF <1.0). Such criterion, if adopted could save the industry millions of dollars in subsequent dynamic analyses and the elimination of unnecessary pipe whip restraints.