Double-ended breaks in reactor primary piping

Abstract

The US Nuclear Regulatory Commission (NRC) is reevaluating current design criteria for light water reactor plants that require postulation of a double-ended guillotine break (DEGB) in reactor coolant piping. In support of this reevaluation, the Lawrence Livermore National Laboratory has estimated the probability of occurrence of a DEGB, and has assessed the effect that earthquakes have on DEGB probability. This report describes a probabilistic evaluation of reactor coolant loop piping in pressurized water reactor (PWR) plants having nuclear steam supply systems designed by Westinghouse and by Combustion Engineering. Two causes of pipe break are considered: pipe fracture due to the growth of cracks at welded joints (“direct” DEGB), and pipe rupture indirectly caused by the seismically-induced failure of heavy component supports (“indirect” DEGB). The probability of direct DEGB was estimated using a probabilistic fracture mechanics model. The probability of indirect DEGB was estimated by convolving seismic hazard and support fragility. The results of this study indicate that the probability of a DEGB from either cause is very low for these plants, and that the NRC should therefore consider eliminating reactor coolant loop DEGB as a plant design basis in favor of more realistic criteria.