PNL technical review of pressurized thermal-shock issues. [PWR]

Abstract

Pacific Northwest laboratory (PNL) was asked to develop and recommend a regulatory position that the Nuclear Regulatory Commission (NRC) should adopt regarding the ability of reactor pressure vessels to withstand the effects of pressurized thermal shock (PTS). Licensees of eight pressurized water reactors provided NRC with estimates of remaining effective full power years before corrective actions would be required to prevent an unsafe operating condition. PNL reviewed these responses and the results of supporting research and concluded that none of the eight reactors would undergo vessel failure from a PTS event before several more years of operation. Operator actions, however, were often required to terminate a PTS event before it deteriorated to the point where failure could occur. Therefore, the near-term (less than one year) recommendation is to upgrade, on a site-specific basis, operational procedures, training, and control room instrumentation. Also, uniform criteria should be developed by NRC for use during future licensee analyses. Finally, it was recommended that NRC upgrade nondestructive inspection techniques used during vessel examinations and become more involved in the evaluation of annealing requirements.Pacific Northwest Laboratory (PNL) was asked by the Nuclear Regulatory Commission (NRC) to develop and recommend a near-term (<1 year) regulatory position that NRC should adopt to avoid or mitigate pressurized thermal shock (PTS) at nuclear power plants. The PNL technical staff and several independent consultants, who provided an overview of the program, evaluated what corrective actions, if any, must be taken before longer-term PTS generic resolution and acceptance criteria are established. Responses to NRC's request for information are still being received from licensees and owners groups. In this regard, the PNL review is limited to information available through May 1982.The responses considered several classes of overcooling scenarios which could lead to a PTS event. For all scenarios, it was concluded that none of the eight reactors under review would undergo vessel failure should a PTS event occur before several more years of operation and, in most cases, before the end of reactor life. However, in many scenarios, operator actions were required to terminate the event before it deteriorated to a state where the conditions necessary for vessel failure were present. The NRC evaluation of PTS procedures and operator training at two of the eight plants indicated deficiencies in these areas. Therefore, it is recommended that procedures, training, and control room instrumentation be changed on a site-specific basis in the nearto long-term period.In addition, the responses differed in terms of event conditions, assumptions, and acceptance criteria beyond what would be expected because of plantspecific situations. It is therefore recommended that uniform criteria be used to evaluate the effective full power years (EFPY) remaining before further corrective actions are required. Adopting these...

PNL Technical Review of Pressurized Thermal Shock Issues Supplement 1: Technical Critique of the NRC Near-Term Screening Criteria

“…Certain ORNL tests have shown that a rapid growth in crack length could transform a crack of relatively small aspect ratio into a long flaw. Accordingly, the acceptance criteria proposed by PNL in NUREG-2837 (6) recommended that the 6:1 postulated flaw be treated as a long axial or circumferential flaw once crack growth initiates.…”