Scoping Study Investigating PWR Instrumentation during a Severe Accident Scenario

Abstract

The accidents at the Three Mile Island Unit 2 (TMI-2) and Fukushima Daiichi Units 1, 2, and 3 nuclear power plants demonstrate the critical importance of accurate, relevant, and timely information on the status of reactor systems during a severe accident. These events also highlight the critical importance of understanding and focusing on the key elements of system status information in an environment where operators may be overwhelmed with superfluous and sometimes conflicting data. While progress in these areas has been made since TMI-2, the events at Fukushima suggest there may still be a potential need to ensure critical plant information is available to plant operators. Recognizing the significant technical and economic challenges associated with plant modifications, it is important to focus on instrumentation that can efficiently address these critical information needs. As part of a program initiated by the Department of Energy, Office of Nuclear Energy, a scoping effort was initiated to assess critical information needs identified for severe accident management and mitigation in commercial light water reactors, to quantify the environment instruments monitoring this data would have to survive, and to identify gaps where predicted environments exceed conditions for instrumentation Environmental Qualification (EQ). Results from the Pressurized Water Reactor (PWR) scoping evaluation are documented in this report. The PWR evaluations were limited in this scoping evaluation to quantifying the environmental conditions for an unmitigated Short-Term Station BlackOut (STSBO) sequence in one unit at the Surry nuclear power station. Quantification was based on results obtained using the MELCOR models developed for the State of the Art Consequence Assessment project sponsored by the US Nuclear Regulatory Commission. Critical instrumentation considered in this scoping evaluation included sensors proposed by the PWR Owners Group (PWROG) in new Severe Accident Management Guidelines (SAMGs) supplemented with alternate generic PWR instrumentation that should be available at the Surry plant. Equipment locations and EQ values were estimated based on input from prior Surry plant evaluations and generic PWR plant information. There are limitations associated with the information available for this scoping evaluation. The current study was limited to only one sequence, and plant specific instrumentation information was not available. However, results indicate that some instrumentation identified to provide critical information would be exposed to conditions that significantly exceed EQ values for extended time periods in the low frequency STSBO sequence evaluated. It is recognized that the estimated core damage frequency of this STSBO sequence would be considerably lower at some plants if evaluations considered new accident mitigation measures being implemented by industry, including measures to assure survivability of key instrumentation for an extended loss of alternating current power event. Furthermore, it is not clear that degradation of instrumentation systems exposed to conditions that exceed their EQ values would preclude the success of new SAMGs being proposed by industry. The use of alternate methods, such as alternate sensor information and ‘trending’ of degraded instrumentation, may be able to address critical information needs for implementing actions to mitigate such accidents. Nevertheless, because of uncertainties in instrumentation response when exposed to conditions beyond EQ ranges and challenges associated with different sequences that may present unique challenges to sensor performance, it is recommended that additional evaluations be completed to provide confidence that operators have access to accurate, relevant, and timely information on the status of reactor systems for a broad range of challenges associated with risk important severe accident sequences. iii INL/EXT-15-35940