CHARACTERIZATION OF IN-CORE WATER CHEMISTRY FOR CORROSION CONTROL OF LWRS

摘要

This paper updates scientific bases of water chemistry in applying the author's recent theory, which integrates the elemental radiation- and electro-chemistry reactions in the "Butlar-Volmer equation," presented in ICONE21-16525. For the past several years the author has been trying to establish that the "long-cell" (a kin to macro-cell) corrosion mechanism is inducing practically all sorts of accelerated corrosion phenomena widely observed in water-cooled reactors, especially in aged plants. The theoretical electrochemical potential differences have been benchmarked with the published in-pile test results for both PWR- and BWR water chemistry environments. However the author's previous verification efforts were limited to the extent that the curves were fitted with experimental results at a single point. The author re-formulated the basic theory and found that the redox potential difference consists of an electrochemical part (e.g., Nernst equation of dissolved hydrogen or oxygen) and radiation-induced perturbation term, the latter diminishes to zero without radiation. The author continued his studies to clarify whether our current scientific knowledge is sufficient to explain the in-core "chemistry" to reproduce the experimental results without the fitting parameter. Through his study he realized that the basic mechanism of the potential difference is still not sufficiently known. No fitting parameter was used for the PWR water chemistry in the DH region for practical engineering applications, although it is indispensable to confirm the results with an in-pile test loop. In the BWR-NWC the theoretical redox potential out of core was still necessary to be fitted with the experimental results, due to an effect of residual hydrogen peroxide detected by the reference electrode. In addition the calculated potential shift is several times larger than the experimental observation. With the reformulation the scientific validity of the author's theory is further confirmed. He believes that there is no doubt that the "long-cell" takes place in LWRs, although details are still debatable.