Scientific root-causes of component degradation of aged LWRs. Radiation-induced 'long-cell action' corrosion

In a series of previously published papers the author has identified that 'long cell action' corrosion plays a pivotal role in practically all unresolved corrosion issues for all types of nuclear power plants. Some of these unresolved issues are IGSCC, PWSCC, AOA and FAC. In conventional corrosion science it is well established that 'long cell action' can seriously accelerate or suppress the local cell corrosion activities. Although long cell action is another fundamental mechanism of corrosion, especially in a 'soil corrosion' arena, potential involvement of this corrosion process has never been studied in nuclear and fossil power plants as far as the author has been able to establish. The author believes that the omission of this basic corrosion mechanism is the root cause of practically all un-resolved corrosion issues. Among the basic corrosion configurations discussed in the above reference, the radiation induced corrosion cell is an important mechanism in nuclear power plants, as it plays a major role in the corrosion issues found in the primary water. The author recently developed a unified theory, which enables the estimation of redox potential differences induced by radiation in the reactor water. The author's 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' (electric) 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 (Faraday's law). The author continued his studies in this paper 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 well clarified. In trying to respond to the basic arguments on whether the radiation-chemistry processes, which occur primarily in the bulk of water, can be reconciled with the electro-chemical framework which begin with reactions on the surface of the electrodes. The theoretical results included in this paper assumes that the hydrated electrons can be described also as a free energy change through charge transfer reactions as in the case of electrode reactions. The verification results generally support this assumption, although the weight of the perturbation terms was not consistent with the anticipated charge transfer numbers. In spite of these uncertainties, he believes that there is no doubt that the 'long-cell' action takes place in LWRs, although scientific details are still debatable. (author)