Chemico-physical causes of radiation-induced “long-cell” action corrosion in water-cooled reactors

In the previous papers, the author has established various ‘long-cell’ (akin to ‘macrocell’) corrosion configurations that exist in nuclear power plants starting with the pioneering works by the USSRʹs G.V. Akinov in 1945 and USAʹs R. Pope in 1956, based on 45 years of research by the U.S. National Bureau of Standards (NBS). With these corrosion mechanisms in place, the plant can be characterized as an assembly of gigantic short-circuited electrical batteries, enhancing electrochemical corrosion at depassivated anodic sites. To prevent the resultant component degradation, it is indispensable to disconnect the closed circuit by inserting electrical insulation to isolate cathodic components with large surface areas, such as fuel assemblies, low-pressure heat exchangers and steam condensers. The remarkable effectiveness of electrical insulation installed in flanges for prevention of corrosion problems in power plants was demonstrated by an US electrical engineer H.W. Wahlquist in 1952. thor recently developed a unified theory, which enables the estimation of redox potential differences induced by radiation in the reactor water for all types of LWRs (e.g., PWR, BWR, VVER, and RBMK). The authorʹs previous calculations showed reasonable agreement with the published in-pile experimental results with respect to potential differences. However, the calculated DO and DH resulted in significant deviations from the input data. Following these observations, the author tried to fortify the verification through another approach, by incorporating electrochemical cathodic reactions occurring on the surface of electrodes. The author found that the deviation is induced by the “long-cell” (electron) current, which is flowing out from the cathodic site. This induces a significant amount of additional solute species (e.g., DH and DO) through the electrolysis (Faradayʹs law). This process is inducing surprisingly strong “long cell” current (i.e., cathode current), which determines many of the basic corrosion parameters, including ECP, redox potentials, pH, DO and DH, as well as over-potentials, which are adjusted through water chemistry specifications. The author believes that existence of both the potential difference and the electrical current flowing through structures should prove that this mode of corrosion is playing the major role in degradation of various corrosion phenomena occurring in LWRs.