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Reactor Coolant Pumps (RCPs) of various types are used to circulate the primary coolant through the reactor in most reactor designs. RCPs generally contain mechanical seals to limit the leakage of pressurized reactor coolant along the pump drive shaft into the containment. The relatively large number of RCP seal and seal auxiliary system failures experienced at US operating plants during the 1970's and early 1980's raised concerns from the US Nuclear Regulatory Commission (NRC) that gross failures may lead to reactor core uncovery and subsequent core damage. Some seal failure events resulted in a loss of primary coolant to the containment at flow rates greater than the normal makeup capacity of Pressurized Water Reactor (PWR) plants. This is an example of RCP seal failures resulting in a small Loss of Coolant Accident (LOCA). This paper discusses observed and potential causes of RCP seal failure and the recommendations for limiting the likelihood of a seal induced small LOCA. Issues arising out of the research supporting these recommendations and subsequent public comments by the utility industry on them, serve as lessons learned, which are applicable to the design of new reactor plants.
Over the last 30 years, reactor safety technology has evolved not so much from a need to recover from accidents or incidents, but primarily from many groups in the nuclear community asking hypo thetical, searching (what if) ~uestions. This ~uestioning has indeed paid off in establishing preventive measures for many types of events and potential accidents. Conditions, such as reactivity excursions, large break, loss of coolant, core melt, and contain ment integrity loss, to name a few, were all at one time topics of protracted discussions on hypothesized events. Historically, many of these have become multiyear, large-scale research programs aimed at resolving the "what ifs. " For the topic of anticipated and abnormal plant transients, how ever, the searching ~uestions and the research were not so prolific until the mid-1970s. At that time, probabilistic risk methodolo gies began to tell us we should change our emphasis in reactor safety research and development and focus more on small pipe breaks and plant transients. Three Mile Island punctuated that message in 1979. The plant transient topic area is a multidisciplinary subject involving not only the nuclear, fluid flow, and heat transfer technologies, but also the synergistics of these with the reactor control systems, the safety s;,"stems, operator actions, maintenance and even management and the economic considerations of a given plant.
This document presents an investigation of the safety impact resulting from mechanical- and maintenance-induced reactor coolant pump (RCP) seal failures in nuclear power plants. A data survey of the pump seal failures for existing nuclear power plants in the US from several available sources was performed. The annual frequency of pump seal failures in a nuclear power plant was estimated based on the concept of hazard rate and dependency evaluation. The conditional probability of various sizes of leak rates given seal failures was then evaluated. The safety impact of RCP seal failures, in terms of contribution to plant core-melt frequency, was also evaluated for three nuclear power plants. For leak rates below the normal makeup capacity and the impact of plant safety were discussed qualitatively, whereas for leak rates beyond the normal make up capacity, formal PRA methodologies were applied. 22 refs., 17 figs., 19 tabs.