Comparative Study of the Corrosion and Surface Analytical Effects of the Decontamination Technologies

Decontamination technologies are generally developed to reduce the collective dose of the maintenance and operation personnel at nuclear power plants (NPP). The highest efficiency (i.e. the highest decontamination factors) available without detrimental modification of the treated surface of structural material is the most important goal in the course of the application of a decontamination technology. At the Paks NPP the AP-CITROX procedure has been utilized for the decontamination of the primary coolant circuit's components (e.g. main circulating pump (MCP) and steam generators (SGs). Our previous studies have revealed that a 'hybrid' structure of the amorphous and crystalline phases was formed in the outermost surface region of the austenitic stainless steel tubes of SGs as an undesired consequence of the industrial application of the AP-CITROX decontamination technology during the period of 1993-2001. In the paper, we report some comparative findings on the corrosion and surface chemical effects of the AP-CITROX procedure and the novel decontamination technology elaborated at our institution. On optimizing the operational parameters the latter technology may become suitable for the effective decontamination of both dismountable (e.g. MCP swivel) and separable (e.g. SGs) equipment. For this purpose experiments were performed. In this laboratory scale experiments, the passivity, morphology and chemical compositions of the treated surfaces of tube specimens were investigated by voltammetry, and SEM-EDX methods, respectively. The SEM-EDX results have revealed that the oxide removal is surprisingly uniform even after two or three consecutive cycles. The electrochemical studies have provided evidence that no unfavourable tendencies in the general corrosion state of the tube samples can be detected in the course of the chemical treatments. (author)