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February 21, 2003 Dr. William D. Travers
Dear Dr. Travers: During the 499th meeting of the Advisory Committee on Reactor Safeguards, February 6-8, 2003, we reviewed a draft report that the NRC's Office of Nuclear Regulatory Research (RES) staff has prepared to document its work to develop technical bases for revising the pressurized thermal shock screening criteria in the PTS rule (10 CFR 50.61). Our Subcommittee on Materials and Metallurgy also reviewed this matter on February 5, 2003. During our review, we had the benefit of discussions with representatives of the NRC staff and the documents referenced. CONCLUSIONS AND RECOMMENDATIONS
DISCUSSION The PTS Rule 10 CFR 50.61 was established to ensure the integrity of irradiation-embrittled reactor pressure vessels. Reactor pressure vessel steels undergo a transition from highly ductile behavior at high temperatures to brittle behavior at low temperatures. This change in behavior occurs abruptly over a narrow range of temperatures, and a temperature RTNDT can be defined to characterize the transition in fracture behavior. Under irradiation, the transition temperature RTNDT increases, making the vessel susceptible to brittle fracture at higher temperatures. Estimation of the frequency of vessel
failure requires (1) identification of sequences that could lead to rapid cooling
of the vessel and estimation of their frequencies of occurrence; The studies conducted by the PTS Reevaluation Project to assess the frequency of vessel failure are much more comprehensive than those done in the early 1980s. These recent studies include systematic consideration of uncertainties in (1) the frequency of initiating events for PTS scenarios, (2) the thermal-hydraulic conditions that provide the driving forces for crack propagation and initiation, and (3) the assessment of the fracture toughness of the vessel materials. Substantial work has also been done to develop more realistic distributions for flaw density and geometry and improve the accuracy and rigor of the probabilistic fracture mechanics code, FAVOR, which is used in these analyses. The results from detailed plant-specific studies of Oconee Nuclear Station, Unit 1; Palisades Plant; and Beaver Valley Power Station, Unit 1, show that the current PTS screening criteria are very conservative for these plants. Two of these plants are among the most susceptible to irradiation embrittlement in the reactor fleet. Moreover, the staff has presented good arguments as to why these results can be considered representative of the entire fleet of pressurized water reactors. The staff also currently has additional studies under way to further confirm the generic applicability of these results. The distributions of the predicted vessel failure frequency are very broad. There are about three orders of magnitude between the 5th and 95th percentiles of the failure frequency. The distributions are also highly skewed, so that the mean and 95th percentiles are virtually identical. At embrittlement levels corresponding to the current screening criterion, the mean frequency of vessel failure is about 1 x 10-8/year. This is a factor of about 500 lower than the current acceptance level. For plant lifetimes of 60-80 years, the predicted mean vessel failure frequencies will range from 5 x 10-10/year to 5 x 10-8/year. Based on current estimates, 10 plants will be within 20 F of the current screening criteria at the end of their original 40-year licenses. Because the transition temperature increases about 1 F per year of operation, revision of the current PTS screening criteria could significantly impact the licensees decisions regarding whether to pursue license renewal for these plants. The staff has concurred with our recommendation in our report of July 18, 2002, that a risk-informed acceptance criterion for vessel failure frequency should be based on considerations of large early-release frequency and not on core damage frequency. The scoping studies presented by the staff suggest that it is likely that the performance of containment systems after vessel failure will be adequate to ensure that a vessel failure frequency criterion of 1 x 10-6/year will be adequate to ensure that the risk due to PTS is acceptably low. These studies also provide an approach for developing a risk-informed failure frequency criterion. Nevertheless, further consideration of the possibility of late containment failure may be needed and should be pursued if rulemaking is undertaken. The documentation of the technical bases is currently inadequate
and incomplete. Topical reports on some important technical tasks have not yet
been completed. For example, no referenceable reports are available on the experiments
and analyses that were performed to assess the potential for strong temperature
gradients in the downcomer region near the beltline region that would invalidate
the one-dimensional treatment of the thermal boundary conditions used in the probabilistic
fracture mechanics analyses. Similarly, no referenceable reports are available
on the studies undertaken at the University of Maryland that were used to develop
a method to address thermal-hydraulic uncertainties, or to document the methods
and approaches used for the probabilistic risk assessments used to determine the
frequency of PTS events. A meaningful peer review cannot be performed without
more complete documentation. The staff also needs to revise the discussion of the treatment of uncertainties. Although the studies have attempted to distinguish between aleatory and epistemic uncertainties and the FAVOR code implements methods that account for the different ways they impact the failure frequencies, the current document does not always make clear that the epistemic and aleatory uncertainties were correctly handled. We
commend the staff for an outstanding multidisciplinary study and look forward
to reviewing the staff's final reports.
References:
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