János Schunk
7003590134
Publications - 7
Impact assessment of chemical decontamination base-technology's preoxidation step on different steel surfaces
Publication Name: Korrozios Figyelo
Publication Date: 2012-12-14
Volume: 52
Issue: 4
Page Range: 73-80
Description:
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.
Open Access: Yes
DOI: DOI not available
Comparative study of the corrosion and surface chemical effects of the decontamination technologies
Publication Name: Journal of Radioanalytical and Nuclear Chemistry
Publication Date: 2010-12-01
Volume: 286
Issue: 3
Page Range: 815-821
Description:
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.
Open Access: Yes
Chemical decontamination of steam generators used in nuclear power plants III. Comparative study of the corrosion and surface analytical effects of the decontamination technologies
Publication Name: Korrozios Figyelo
Publication Date: 2009-12-01
Volume: 49
Issue: 6
Page Range: 119-129
Description:
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.
Open Access: Yes
DOI: DOI not available
Chemical decontamination of steam generators used in nuclear power plants II. Comprehensive investigation of the efficiency of used in nuclear power plants II. Comprehensive investigation of the efficiency of chemical decontamination technologies
Publication Name: Korrozios Figyelo
Publication Date: 2009-12-01
Volume: 49
Issue: 5
Page Range: 86-94
Description:
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.
Open Access: Yes
DOI: DOI not available
Comprehensive investigation of the corrosion state of the heat exchanger tubes of steam generators
Publication Name: European Corrosion Congress 2009 Eurocorr 2009
Publication Date: 2009-12-01
Volume: 2
Issue: Unknown
Page Range: 1212-1220
Description:
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.
Open Access: Yes
DOI: DOI not available
Long-term trends in the corrosion state and surface properties of the stainless steel tubes of steam generators decontaminated chemically in VVER-type nuclear reactors
Publication Name: Corrosion Science
Publication Date: 2009-12-01
Volume: 51
Issue: 12
Page Range: 2831-2839
Description:
At some VVER-type pressurized water nuclear reactors (Russian-type PWR) different versions of the so-called AP-CITROX method have been widely used for the chemical decontamination of the heat exchanger tubes of steam generators. In the period of 2000-2007, within the frame of a joint-project dealing with the comprehensive investigation of the corrosion state of the steam generators of the Paks Nuclear Power Plant, Hungary, effects of the AP-CITROX chemical decontamination procedure on the corrosion and surface characteristics of the heat exchanger tubes have been studied. These studies provide evidences that some adverse features (formation of a "hybrid" layer with accelerated corrosion rate and great mobility) can be detected after 1-3 years of applying the AP-CITROX procedure. The present work is a continuation of the above program and focused on the long-term trends in the corrosion state and structure of protective oxide-layer grown on the decontaminated surfaces. The results of electrochemical (voltammetric), surface analytical (SEM-EDX, CEMS) and mobility (ICP-OES) studies have revealed that (1) some beneficial changes in the corrosion characteristics, mobility and chemical composition of the inner surfaces of decontaminated heat exchanger tubes can be observed in the long run, and (2) the passivity of the oxide-layers formed on decontaminated surfaces of steel tubes exhibits favorable tendency after 4-7 years under normal operation conditions. © 2009 Elsevier Ltd. All rights reserved.
Open Access: Yes
Long-term effects of the ap-citrox decontamination procedure on the protective oxide-layer formed on stainless steel
Publication Name: Korrozios Figyelo
Publication Date: 2007-07-09
Volume: 47
Issue: 3
Page Range: 62-72
Description:
Our previous studies have revealed that a "hybrid" structure of the amorphous and crystalline phases is formed in the outermost surface region of the austenitic stainless steel tubes of steam generators (SGs) as an undesired consequence of the industrial application of the AP-CITROX decontamination technology. The formation of this mobile oxide-layer increased the amount of the corrosion products in the primary circuit significantly (∼80 kg/block), resulting in magnetite deposition on fuel assemblies. As deposits blocked the cooling channels, the flow rate of water coolant through the reactor core decreased. Consequently, the power capacity of three nuclear reactor units had to be reduced, and full core fuel replacement became necessary. In the light of the above events, the present work gives a brief overview on the general corrosion state of the heat exchanger tubes of SGs, concerning the long-term effects of the AP-CITROX procedure on the chemical composition and structure of the protective oxide-layer. 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 project based on sampling as well as on ex-situ electrochemical and surface analytical measurements was elaborated. Within the frame of this project, comprehensive investigation of the general corrosion state and metallographic features of 36 stainless steel specimens, cut out from various locations of the four steam generators of the Paks NPP has been performed. Electrochemical (voltammetric) and surface analytical (SEM-EDX, CEMS, and XRD) results reveal that some beneficial changes in the corrosion properties, morphology and chemical composition of the inner surfaces of heat exchanger tubes can be observed in the time period of 2000-2006.
Open Access: Yes
DOI: DOI not available
