A novel efficient "soft" chemical decontamination technology has been developed - supported by the Paks Nuclear Power Plant - at the Institute of Radiochemistry and Radioecology of the University of Pannonia. The present work gives a brief overview on the corrosion and dissolution processes of austenitic stainless steel and carbon steel specimens that occur in the oxidative pretreatment step (mixture of 1.0 g·dm^-3 HMnO4 + 0.2 mol·dm^-3 HNO3, pH = 0.8-1.0) of the novel base-technology. The oxidative pretreatment of steel specimens was performed under laboratory conditions in an electrochemical cell, and the open circuit (corrosion) potential of treated surfaces was studied by a VoltaLab 40 (RADIOMETER) type electrochemical measuring system controlled by PC. In addition, the redoxi potential in the decontamination solution was measured on-line by Consort C861 type electrochemistry meter. In the course of the chemical procedure the concentration of the main alloying components (stainless steel: Fe, Cr, Ni, carbon steel: Fe) dissolved from the surface oxide layer into the decontamination solutions was determined by ICP-OES method. The morphology and chemical composition of the oxide layer formed on the surfaces of steel specimens were studied by scanning electron microscopy (SEM), equipped with an energy dispersive X-ray microanalyzer (EDX). The results of our laboratory experiments have revealed that the acidic dissolution of austenitic stainless steel surfaces by nitric acid is negligible (at room temperature), while that the carbon steel surface is significant and intensive. After addition of permanganic acid, a moderate dissolution of the protective oxide layer of the austenitic stainless steel surface was observed; however, the reaction is blocked by MnO2 formed on the steel surface. On the other hand, the dissolution rate of carbon steel surface is not increased following the addition of permanganic acid into nitric acid solution, and the formation of surface MnO2 is marginal.

Our earlier publications have revealed that version of the AP-CITROX technology applied for chemical decontamination of the steam generators (SGs) of the Paks NPP was not adequately developed considering its chemical, analytic and corrosion aspects. To replace the AP-CITROX procedure, novel "soft" chemical decontamination technologies have been elaborated at the Institute of Radiochemistry and Radioecology of the University of Pannonia. Present paper deals with the comparative study of the corrosion and surface chemical effects of the novel chemical decontamination technologies. The chemical composition and morphology of the oxide layer formed on the inner side of the austenitic stainless steel specimens before and after the full decontamination process were studied by scanning electron microscopy (SEM), equipped with an energy dispersive X-ray microanalyzer (EDX). The passivity of the inner surfaces of the stainless steel samples was studied by voltammetry. The full decontamination cycle has been performed under laboratory conditions in a pilot plant circulation system elaborated earlier. In various steps of the chemical procedure the concentration of the main alloying components (Fe, Cr, Ni) dissolved from the surface oxide layer into the decontamination solutions was determined by ICP optical emission spectrometric (ICP-OES) method. Based upon the ICP-OES results the average thickness of the oxide layer removed from the surface into the solutions was calculated. The activity concentrations of the radionuclides (⁶⁰Co, ⁵⁸Co, ¹¹⁰Ag, ⁵⁴Mn) measured in the decontamination solutions allow us to draw conclusions concerning the efficiency of the main steps of the technology and depth distribution of the radionuclides on the treated steel surfaces.

Our earlier publications have been focused on detailed explanation of the effects of the applied AP-CITROX chemical decontamination technology on the corrosion state and surface morphology of heat exchanger tubes of steam generators (SG). These studies have revealed that the industrial application of the decontamination technology results in the inhomogeneous dissolution of stainless steel surface, leading to the formation of oxide layer with undesired chemical composition and structure which exhibits potentially greater mobility in the primary coolant. Laboratory examinations have provided an independent verification of the formation of the so called "hybrid" structure. The first part of our three-part series has presented some selected findings on the corrosion and surface chemical effects of the AP-CITROX technology as well as the fundamental issues of the technology development. Present paper deals with the efficiency (mass transport and decontamination characteristics) of the novel chemical decontamination technologies elaborated for the treatment of the inner surface of SGs and regenerative heat exchangers of the water purification system No. 1. The third part of our series is scheduled to highlight some results obtained by laboratorial analysis of the effects of the two novel technologies on the corrosion state as well as on the structure and composition of surface oxide layer. Determining the radioactivity of specified nuclides in the solutions used for decontamination, important conclusions on the efficiency of the main technological steps as well as on the distribution of radionuclides in the depth of treated surface can be drawn.