Possibility of Localized Corrosion of PWR primary side materials in oxidative decontamination condition

Primary circuit of a PWR (radionuclides uptake in the inner oxide layer and oxide/metal interface occurred inevitably. Therefore, it is necessary to remove the inner oxide layer as well as the outer oxide layer to achieve excellent decontamination effects. It is known that the outer oxide layers are typically composed of Fe₃O₄ and NiFe₂O₄. On the other hand, the inner oxide layers are composed of Cr₂O₃, (Ni_1-xNi_x)(Cr_1-yFe_y)₂O₄, and FeCr₂O₄. Because of chromium in the trivalent oxidation state which is difficult to dissolve, the oxide layer has an excellent protectiveness and is hard to decontaminate. For the dissolution of chromium-rich oxide, there have been developed an alkaline permanganate (AP) or nitric permanganate (NP). A disadvantage of the AP process is the generation of a large volume of secondary waste. On the other hand, NP process is highly incompatible to the corrosion of the structure materials. In this study as a part of developing decontamination process, we investigated the corrosion behavior of the structure materials such as Inconel-600 and type 304 stainless steel in NP and AP oxidative decontamination conditions for the safe use of an oxidative phase in PWR system decontamination. The corrosion behavior was analyzed through the potential-pH equilibrium for the system of Cr-H₂O / Mn-H₂O at 90 .deg. C and potentiodynamic polarization in a typical AP and NP solution were evaluated. The AP or NP treated specimen surface was observed using an optical microscope and scanning electron microscopy (SEM) for an evaluation of the localized corrosion. The possibility of localized corrosion of PWR primary side materials under oxidative decontamination condition was evaluated using a potentiodynamic polarization technique, observation of localized corrosion morphology, and consideration of potential-pH diagrams at 90 .deg. C. From the results of these tests, we can conclude the following. The corrosion characteristics of a classical NP process rely highly on the acidity of the decontamination solution, and the typical process performed around a pH of 3 needs to be improved since it is vulnerable to corrosion. A small amount of copper (II) ion works as a corrosion inhibitor in the oxidative process and by the addition of 0.5mM copper (II) ion the general corrosion dropped to a level of 25% and local corrosion was completely inhibited. This modified nitric acid permanganate process was named MONAP