Recently, the USNRC has proposed a voluntary alternative to the pressurized thermal shock (PTS) rule (10CFR50.61(a)) that may be used instead of the existing PTS rule (10CFR50.61) by power reactor licensees to demonstrate the safety of their RPVs. The reference temperature (RT) screening limits in 10CFR50.61(a) are less restrictive than those of 10CFR50.61; this relaxation being justified by the much more realistic models that underlie the technical basis of the revised rule. One of the many models that constitute this technical basis is used to estimate the effect of irradiation damage on the fracture toughness transition temperature. This relationship, which is called the embrittlement trend curve, or ETC, can change as time goes on due to the continued development of new knowledge (e.g., from on-going reactor vessel surveillance programs, from test reactor studies aimed at better understanding the physical mechanisms of irradiation damage, etc.). In this paper we present information that quantifies the effect of different ETCs on the through wall cracking frequencies that are calculated using the probabilistic fracture mechanics computer code that was used to establish the RT limits that have been proposed in 10CFR50.61(a).

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