United States Nuclear Regulatory Commission - Protecting People and the Environment

GSI-191 PWR Sump Screen Blockage Chemical Effects Tests: Thermodynamic Simulations (NUREG/CR-6912)

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Publication Information

Manuscript Completed: November 2006
Date Published:
December 2006

Prepared by:
J. McMurry, V. Jain, X. He, D. Pickett
R. Pabalan, Y.-M. Pan

Center for Nuclear Waste Regulatory Analyses
Southwest Research Institute
6220 Culebra Road
San Antonio, TX 78238-5166

B.P. Jain, 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 Codes N6121 and N6278

Availability Notice

Abstract

This report summarizes chemical modeling studies and experiments performed to support the resolution of GSI–191. Along with entrained debris components, the formation of secondary precipitates and gels have the potential to impede the performance of Emergency Core Cooling System pumps, Containment Spray System pumps, or other components downstream of the sump strainer after a loss-of-coolant accident (LOCA). The purpose of this study was to examine the use of chemical modeling software as a tool in predicting whether secondary precipitates would be likely to form in specific post-LOCA chemical environments. Within the limits of the available thermodynamic data for the model, the software also identified which solids would be expected to form and their quantities, and it indicated how the containment water chemistry was affected by these reactions. Several existing, widely available chemical modeling programs—EQ3/6 (Lawrence Livermore National Laboratory, 1995), OLI Systems Stream Analyzer (OLI Systems, Inc., 2005), The Geochemist's Workbench® REACT (RockWare, Inc., 2004), and PHREEQC (U.S. Geological Survey, 2003)—and their accompanying thermodynamic database files were evaluated to simulate the potential formation of precipitates under post-LOCA conditions. Detailed simulations were performed for five representative post- LOCA environments, in which alkaline or neutral borated containment waters interacted with metals, concrete, and insulation materials at 60 °C [140 °F] for times up to 720 hours. The modeled conditions corresponded to the Integrated Chemical Effects Test (ICET) experiments conducted at the University of New Mexico, and results of the experiments were used to benchmark and calibrate the simulations. The input water compositions for the simulations were estimated from specified initial containment water compositions, previously derived corrosion rates for the metals of interest, and dissolution rates from new experiments involving insulation materials and concrete. The modeling programs EQ3/6 and PHREEQC were used to perform blind predictions of the experiment results. Analytical data and qualitative observations of precipitation (or lack of it) from the ICET experiments were used to refine the conceptual model. Revised dissolution rates were obtained from additional experiments at the Center for Nuclear Waste Regulatory Analyses, after which informed simulations were performed using Stream Analyzer and PHREEQC. A more detailed simulation considered the gradual changes in chemistry of the solution water over time, based on kinetic reaction rates with the reactive materials and ongoing equilibration (precipitation) with oversaturated secondary phases.

The study determined that the most important requirements for developing more accurate chemical effects simulations were (i) a realistic estimate of starting water compositions and dissolution rates, and (ii) the availability of an adequate set of thermodynamic data, particularly for amorphous or metastable solids that would be expected to form under the simulated conditions. The study concluded that the codes as tested were broadly useful in assessing whether precipitation of secondary solid phases was likely under the specified conditions and the quantity of material that was predicted to form. In applying chemical modeling software to other plant-specific sets of conditions, the effectiveness of the simulations and confidence in their predictions would be considerably improved by a more complete characterization of source-term materials and release rates for the conditions of interest, and by development of an appropriate thermodynamic database for modeling purposes that includes more realistic amorphous or metastable solids for the conditions of interest.

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