Effects of Insulation Debris on Throttle-Valve Flow Performance: A Subtask of GSI-191 (NUREG/CR-6902,LA-UR-05-7631)

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Publication Information

Manuscript Completed: March 2006
Date Published: March 2006

Principal Investigators:
Crystal B. Dale, Pratap Sadasivan, Bruce C. Letellier
Los Alamos National Laboratory
Los Alamos, New Mexico 87545

Prepared by:
Pratap Sadasivan¹, Crystal B. Dale¹, Arup K. Maji², Kerry Howe²,
Felix Carles², Clifford E. Anderson² , Bruce C. Letellier¹

¹Los Alamos National Laboratory
Los Alamos, New Mexico 87545

²Department of Civil Engineering (Subcontractor)
University of New Mexico
Albuquerque, New Mexico 87110

E. Geiger, NRC Project Manager

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

NRC Job Code Y6871

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Abstract

Generic Safety Issues 191, "Assessment of Debris Accumulation on PWR Sump Performance," addressed the transport of debris to pressurized-water-reactor (PWR) sump screens following a loss-of-coolant accident, potentially impacting the emergency core cooling systems and containment spray system. NUREG/CR 6885, "Screen Penetration Test Report," describes tests conducted to assess the propensity of insulation debris to penetrate the sump screens. This document describes a series of tests conducted to assess the potential for loss-of-coolant-accident (LOCA)-generated debris to be trapped in the HPSI throttle valve downstream of the sump screen.

Trapping of debris in the valve has important consequences for emergency-core-cooling-system (ECCS) operation because it may result in unacceptably high pressure losses in the system and consequent degradation of ECCS performance. Tests have been performed using a range of loadings and compositions of insulation introduced either as a single batch or as a set of successive batches. The tests used a surrogate throttle valve designed to simulate a range of representative valve configurations in use within United States pressurized-water reactors. This test program was the second in a series of Nuclear-Regulatory-Commission-sponsored tests that were conducted to address the effects downstream of the ECCS sump screens.

The first test program in this series addressed the potential for LOCA-generated debris materials to penetrate the sump screen. The current tests addressed the downstream effects of the debris that was able to penetrate the sump screen in these earlier tests. The test data provided information on the potential blockage of the high-pressure safety-injection throttle valves caused by single slugs of unmixed debris, as well as the potential for enhanced blockage caused by single or multiple batches of combinations of debris types. The insulation debris that was tested included calcium silicate (CalSil) insulation, NUKON™ fiberglass insulation, and reflective metallic insulation (RMI); however, many other types of insulation exist in plants. The range of debris sizes was based on the results of the screen penetration tests.

Debris blockage in the valve was gauged using the valve-loss-coefficient K, which was calculated using measured data for the pressure drop across the valve, the flow rate through the valve, and the temperature of the water. As the effective flow area of the valve decreased because of blockage, the loss coefficient increased. The overall approach was first to establish baseline loss coefficients for each valve configuration of interest and then to compare loss coefficients for various debris flow conditions with the data to get an indication of the extent of blockage caused by the debris. In addition, baseline loss coefficients were determined for selected known blockages (blockage-area fractions simulated using shims) to determine the relationship between K and the blocked-area fraction, as well as the blockage detection threshold of the system (~5%–8%). Loss coefficients for debris flow conditions then were compared with those for shim blockage data to obtain estimates of the blockage-area fractions.

Data from tests with single batches of unmixed debris showed that, in general, higher debris loadings and larger debris sizes (relative to the throttle-valve opening) resulted in higher observed increases in K. The K increases were higher for RMI than for NUKON for equivalent mass loadings. However, NUKON is judged to be more likely than RMI or CalSil to cause throttle valve blockage because of the propensity for NUKON to transport and penetrate the sump screen.

Tests using CalSil-RMI mixtures were the only two-component combinations that exhibited clear increases in K when compared with results from analogous single-debris CalSil and RMI tests. The results of tests performed using NUKON-RMI or CalSil-NUKON mixtures did not differ significantly from results for analogous separate tests, with one possible exception. One mixture test performed using unsieved CalSil with NUKON showed an appreciable increase in valve blockage compared with single-debris NUKON tests. However, it is unclear if this result is attributed to clumping within the unsieved CalSil or to retention by NUKON fibers within the valve.

The three-component mixture tests were divided into two types of tests: (1) homogeneous mixtures of RMI, CalSil, and NUKON; and (2) sequential additions of each debris type using different ordering. Tests using homogeneous mixtures of RMI, CalSil, and NUKON showed an increase in valve blockage when compared with analogous single-debris RMI tests. However, no particular debris introduction sequence resulted in increases in valve blockage compared with results for homogeneous mixtures. Further, in the tests where NUKON was introduced first in the debris sequence, the blockage was much less than for homogeneous mixtures.

Three accumulation tests were performed to investigate the potential for a cumulative increase in valve clogging as a result of a stream of debris batches reaching the valve. In these tests, multiple batches of debris were introduced at ~15-min intervals over a period of 3 h. Three debris types and loadings were tested. The tests with 25 g each of successive additions of NUKON-CalSil showed a sustained increase in K over time as more and more debris reached the valve. However, consistent with the variability observed in other tests, the increase in K was not observed following all additions of debris. Some debris additions did not result in any increase in K, suggesting that no net increase in valve blockage occurred at that step. Accumulation tests with periodic additions of CalSil alone (after early introduction of NUKON) also showed that some CalSil additions triggered increases in K, whereas others did not. Relative to single-debris CalSil tests, larger K increases were observed after some CalSil additions, which suggests that the potential exists for CalSil to be trapped by NUKON or RMI that may be present in the valve.

The results for replicated single-debris, multiple-debris, and accumulation tests exhibited significant test-to-test variability. This variability is consistent with the inherent randomness involved in the process; the propensity for trapping of debris in the valve gap is a function of the random orientation of the individual pieces as they enter the valve gap. Further, the bending or thrashing of the debris pieces inside the valve also is a random process. This variability makes it difficult to quantify trends in these results because only a limited number of replicate tests were performed for any single condition.