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 this 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) equipments. 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 2 or 3 consecutive cycles. The electrochemical studies have provided evidences that no unfavorable tendencies in the general corrosion state of the tube samples can be detected in the course of the chemical treatments. © 2010 Akadémiai Kiadó, Budapest, Hungary.

Evaluating the water chemistry in the primary circuit and the effect of chemical decontamination of the heat exchanger tubes performed by the AP-CITROX procedure at Paks NPP (Hungary), a project dealing with the comprehensive investigation of the general corrosion state of the steam generators (SGs) has been initiated. Owing to the fact that there is no investigation method available for the in-situ monitoring of the inner surfaces of heat exchanger tubes, a research program based on sampling as well as on ex-situ electrochemical and surface analytical measurements were developed and elaborated. In the time period of 2000-2008 - within the frame of the above project - 45 stainless steel specimens, cut out from various locations of the steam generators of the Paks NPP were investigated. Besides to the corrosion characteristics (corrosion rate, thickness and chemical composition of the protective oxide-layer) surface properties (morphology, chemical and phase compositions) of the passive layer formed on the inner surface of above heat exchanger tubes were studied, too. The passivity of the inside surface of the stainless steel specimens was measured by voltammetry, the morphology, chemical and phase compositions of the oxide layer formed on the surface were analyzed by SEM-EDX, XRD and CEMS methods. The great number of experimental results allowed us to develop an electronic database which involves the results of the above corrosion experiments, and also some special characteristics of the tubes (e.g. location in the SGs, surface pretreatment by decontamination, if any, etc). Evaluating the main relations among these parameters may contribute to the identification of important processes affecting the corrosion state of steam generators, and highly decisive concerning a life time prolongation project of VVER-type nuclear reactors. In the present work we provide a brief overview on these experiments, some characteristic results, the database developed, as well as some novel findings.