United States Nuclear Regulatory Commission - Protecting People and the Environment

GSI-191: The Impact of Debris Induced Loss of ECCS Recirculation on PWR Core Damage Frequency (NUREG/CR-6771)

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Publication Information

Manuscript Completed: August 2001
Date Published: August 2002

Prepared by:
J. L. Darby, W. Thomas,* D. V. Rao
B. C. Letellier, S. G. Ashbaugh, M. T. Leonard**

Los Alamos National Laboratory
Los Alamos, NM 87545

*ARES Corporation
851 University Blvd. S.E.
Albuquerque, NM 87106

**dycoda LLC
70 Andres Sanchez Road
Belen, NM 87002

M. L. Marshall, NRC Project Manager

Prepared for:
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code Y6041

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This report documents a risk significance study that supported a parametric evaluation of operating U. S. pressurized water reactors (PWR) plants to access whether or not Emergency Core Cooling Systems (ECCS) recirculation sump failure is a plausible concern. This evaluation was part of the Nuclear Regulatory Commission (NRC) Generic-SafetyIssue (GSI) 191 study tasked to determine if the transport and accumulation of debris in a containment following a loss-of-coolant accident (LOCA) will impede the operation of the ECCS in operating PWRs. The parametric evaluation identified a range of conditions in which PWR ECCS could fail in the recirculation mode of operation. These conditions stem from the destruction and transport of piping insulation materials, containment surface coatings (paint), and particulate matter (e.g., dirt) by the steam/water jet emerging from a postulated break in reactor coolant piping. The likelihood that sufficient quantities could transport and accumulate on the recirculation sump screen to severely impede recirculation flow is plant specific and a review of PWR plant design features indicated adverse conditions exist in several plants.

The specific goal of the risk significance study was to estimate the amount by which the core damage frequency (CDF) would increase if failure of PWR ECCS recirculation cooling due to debris accumulation on the sump screen were accounted for in a manner that reflects the results of recent experimental and analytical work. Further, the estimate was made in a manner that reflected the total population of U. S. PWR plants. Results suggest the conditional probability of recirculation sump failure (given a demand for recirculation cooling) is sufficiently high at many U. S. plants to cause an increase in the total CDF of an order of magnitude or more.

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