Argonne Model Boiler Test Results (NUREG/CR-6994)

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Publication Information

Manuscript Completed: November 2008
Date Published: December 2009

Prepared by:
C.B. Bahn, K.E. Kasza, J. Park, C. Vulyak, and W.J. Shack
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439

C. Harris, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code Y6588

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Abstract

Various corrosion phenomena have been observed in the steam generator (SG) tubes of pressurized water reactors. One such type of corrosion involves impurity concentration in the narrow gap between SG tubes and supporting structures or sludge piles (“crevices”). The purpose of this study is to characterize accumulation of impurities in the crevices for varying Na-to-Cl molar ratios in water, temperature, and packing type (diamond or magnetite). This characterization is based on tests carried out at Argonne National Laboratory in a model boiler system which can simulate prototypic SG conditions. Diamond powder, which has a higher thermal conductivity than magnetite powder, can enhance the boiling rate and lead to a rapid rate of impurity accumulation. Magnetite-packed crevices, which have lower permeability, are more appropriate for the simulation of actual SG crevices than a diamond-packed crevice. A radial chemistry gradient was observed in a crevice packed tightly with magnetite powder, a finding supported by earlier experimental work. Near the tube wall, the crevice chemistry tends to vary actively because of the increased volatility effect of Cl at the heated tube wall where boiling occurs. Initially, the crevice pH near the tube wall appears to be alkaline. As the concentration progresses, however, the crevice pH becomes neutralized and even acidic because of preferential Cl concentration, enabled by a reduced boiling rate near the tube wall due to the presence of a Na-rich liquid film. Based on the test results, the chemistry variation in actual SG crevice deposits near the tube wall was estimated. Unless some impurities remain and accumulate in the crevice after each fuel cycle, during most of a typical fuel cycle, the crevice chemistry would be in a transient rather than a steady-state condition because of low impurity concentrations in the secondary system. The kinetic data obtained for the crevice chemistry evolution with low bulk impurity concentration is valuable for the estimation of actual SG crevice chemistries. Based on the crevice and bulk solution sample analyses, the volatility effect of Cl in the diamond-packed crevices becomes significant as the molar ratio decreases. Data are limited but it is likely that the volatility effect of Cl in the magnetite-packed crevices is not strongly influenced by the molar ratio variation in the bulk solution. Tests showed that once a tube crack is formed, the crack itself can act as a crevice, and in the presence of NaOH bulk chemistry, it can grow even if the sludge or debris is cleaned out of the SG.