COMPARATIVE STUDY OF THE CORROSION AND SURFACE CHEMICAL EFFECTS OF THE DECONTAMINATION TECHNOLOGIES

Transcription

1 COMPARATIVE STUDY OF THE CORROSION AND SURFACE CHEMICAL EFFECTS OF THE DECONTAMINATION TECHNOLOGIES Thesis of the PhD dissertation Bernadett Baja PhD School of Chemical and Environmental Sciences Supervisor: Dr. Varga Kálmán University of Pannonia Institute of Radiochemistry and Radioecology Veszprém, 2011.

2 1. INTRODUCTION, AIMS Decontamination technologies are mainly developed to reduce the collective dose of the maintenance personnel at NPPs. The highest efficiency (i.e., the highest DF values) 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 The AP-CITROX procedure has mainly been used for the decontamination of the primary coolant circuit s components (e.g. main circulating pump (MCP) and steam generators (SGs)) at the Paks NPP. While decontamination of the dismountable devices (e.g. MCP) has been carried out in big tanks with heating potential, separable equipments (e.g. SGs) have been treated with special decontamination facilities. The AP- CITROX procedure was used 24 times for the SGs decontamination at units 1-3 of the Paks NPP during the period of The analysis of the typical operation data and the comprehensive study of the corrosion effects of the technology reveal that the non-regenerative version of the AP-CITOX technology is not an adequate method for the chemical decontamination of any reactor equipments having large steel surfaces. In 2003 R&D projects focused on the investigation of the effects of AP- CITROX technology on the corrosion and surface characteristics of the heat exchanger tubes in the long run was started. In addition beginning in 2005 a new R&D program on the elaboration of base chemical decontamination technology was granted by the Paks NPP. During the period of , the base-technology was improved for specific applications such as decontamination of some dismountable devices and separable equipment and the total decontamination prior to future decommissioning. In the time period of I joined to the above mentioned project as a PhD student. The primary aim of my work was the investigation of the long-term trends in the corrosion state and surface properties of the stainless steel tubes of steam generators decontaminated chemically. In addition I compared the corrosion and surface chemical effects of the improved versions of base-technology. The main aims of my work are as follows: Analysing the corrosion and metallographic features of 13 austenitic stainless steel specimens (type: 08X18H10T (GOSZT )) originating from different SGs of the Paks NPP by electrochemical (voltammetry) and surface analytical technique (SEM-EDX, CEMS). An improvement the data base of the steam generators. Studying some obvious tendencies in the structure and corrosion state of the oxide layer covering the inner surfaces of heat exchanger tubes cut out from the same SG at different time periods after decontamination by AP-CITROX procedure. Participation in the comprehensive studies of the decontamination technologies under laboratory conditions. Comparing the efficiency, corrosion and surface chemical effects of the improved versions of base-technology. 2. EXPERIMENTAL METHODS AND INSTRUMENTS In the course of the comparative study the corrosion and surface chemical effects of the decontamination technologies the experimental findings were obtained by voltammetry, CEMS and SEM-EDX methods. The efficiency of the decontamination technologies were investigated on austenitic stainless steel samples originating from of the heat exchanger tubes of the steam generators of the Paks NPP. Dosimetry (γ-spectrometric) and solution chemical (ICP-OES) methods were used to demonstrate the effectiveness of the chemical technologies on the steam generator tube samples. 1 2

3 3. THESIS 1. Tendencies in the corrosion state of the steam generators at Paks NPP On the basis of the results of electrochemical (voltammetric) thematic research between I have concluded that there is no adverse tendency in the corrosion characteristics (E k, i k, i p és v k ) of the heat exchanger tubes of steam generators with the operation time. I have demonstrated that the stability of the oxide structure on the surface of the decontaminated heat exchanger tubes improved during the time of operation, and today all the steam generators of the reactor blocks 1-4 shows favourable erosion-corrosion conditions (removable thickness of the oxide layer d ~ 0,1 µm). However, the fact that the major part of the corrosion product removed from heat exchanger surfaces is still given by the disperse phase (solid residue) must be emphasized. Evaluating the findings obtained by surface analytical methods (CEMS, SEM-EDX) I proved that there is a layer of spinel-structure magnetite rich in Cr and Ni in the near-surface region of the decontaminated tube samples, which enhance the passivity of oxide layer grown on the corrosion resistant steel surfaces. On the basis of the improving data of mobility as well as the favourable morphological and surface structure changes in relation to the operation time after the 2001 mass application of the AP-CITROX technology we can assume that the protective features (solubility, resistance to chemicals) of the heat exchanger tubes of decontaminated steam generators and its mechanical stability goes through a favourable change. 2. The efficiency of chemical decontamination technologies and their surface chemical and corrosion effects 2.1. The efficiency of the decontamination technologies in the knowledge of the surface characteristics of the treated steel tubes Examining the procedures applied for the decontamination of different apparatus in the primary circle developed on the basis of the decontamination base technology elaborated in the IRR PE I realized that the efficiency of these technologies depends on the thickness, the chemical and phase composition, the morphology (particle size and specific surface area) of oxide layer and on the decontamination history of the steel tubes surface to a great extent. I proved that - there is a basic difference in the decontamination ability between the austenitic steel samples originating from the SGs in Block 4 and the SGs in Block the efficiency of the chemical treatments in both cases is primarily affected by the removability of the 60 Co radionuclide Consequently, it is vital in the case of the industrial implementation of decontamination procedures for the appropriate efficiency to be aware of the complex knowledge of the surface characteristics of the equipment to be contaminated The interpretation of the decontamination ability of the austenitic steel surfaces By the comparative analysis of the experiment results and the literature data I concluded that the decontamination ability of the steel surfaces in Block 4 (type: 08X18H10T (GOST ) which corresponds to AISI 321 and DIN ) greatly depends on the chemical composition and morphology of the crystalline deposits covering the surfaces. I proved that the Cr content, particle size and specific surface are determining as far as the decontamination ability of the surfaces is concerned. Knowing these parameters allows us to estimate the radioactivity of the steel surfaces and the potential order of their chemical resistance. To measure the amount of the corrosion product radionuclides carried by the crystals I used the so-called activation and transport mechanism. At the same time, it can be stated (independently from the dominant mechanism of the decontamination of surfaces) that the crystal with higher Cr content carries more activity, however, it is more resistant chemically, so it is more difficult to remove during the 3 4

4 decontamination treatment. This is supported by the following experimental results: a) During the chemical decontamination of surfaces covered by crystals of small Cr content depending on the particle size of the crystals the removability of 60 Co from the surface is 98-99% feasible. b) During the decontamination of surfaces covered by the crystalline oxide deposits having high Cr content the efficiency of the removability of 60 Co is %. However, the efficiency can be improved by increasing the amount of the specific active agent of the decontamination solution The interpretation of the decontamination ability of austenitic steel samples decontaminated in the previous years at Paks NPP. It has been revealed that steel tubes from the SGs of Block 1-3 (type: 08X18H10T (GOST ) which corresponds to AISI 321 and DIN ) can be more efficiently decontaminated (the removability of radionuclide 60 Co is higher than 99%). This may be explained by the fact that crystalline deposit on the surface underwent a partial dissolution through the industrial decontamination in the previous years and a thick (5-11 µm) so-called hybrid oxide structure was formed. The distribution of the radionuclides is inhomogeneous in the hybrid structure oxide layer, and the efficiency data reached show that the oxide layer has lower resistance during the decontamination than the original surface exempt from treatment The corrosion and surface chemical effect analysis of the procedures passive condition of the surfaces treated did not deteriorate compared to the original surface. No adverse tendency in the corrosion characteristics of the sample surfaces has been exhibited. After the decontamination treatment the average corrosion rate of the surfaces is less than 0,45 µm year -1. b) As a result of the decontamination treatment the morphology of the sample surfaces favourably changed. In the case of the samples where the whole oxide layer was removed the hoarseness of the surfaces decreased, and the chemical composition of the surfaces treated matches that of the bulk steel phase. After the decontamination procedure the Cr content of the oxide residue on the sample surfaces is significant, so these are chemically excellently resistant. The ICP-OES results prove that the amount of metal removed in the last steps of the technology are in all cases at least two times less in magnitude than the concentrations of the alloy components which got in the solution phase in the pre-oxidation period. On the basis of these it is to be presumed that the mobility of the oxide residue on the surfaces treated is low. No significant amount of chemical residues (manganese-dioxide and organic carbon) of the decontamination solution are detected on the surfaces treated. 4. INDUSTRIAL APPLICATION This work was supported by Paks NPP Ltd. Significant elements of the present results have been used at the comprehensive investigation (chemical, corrosion) program prior to the life-extension of reactor units. a) On the basis of the electrochemical (voltammetry) and surface analytical (SEM-EDX) results I have concluded that after the application of the improved versions of base technology the surfaces display no selective corrosion changes (pitting, crevice corrosion and intergranular stress corrosion cracking ). The 5 6

5 5. RELEVANT PUBLICATIONS Publications in foreign languages 1. A. Szabó Nagy, K. Varga, B. Baja, Z. Németh, D. Oravetz, Z. Homonnay, E. Kuzmann, J. Schunk: Long-term effects of a chemical decontamination procedure on the corrosion state of the heat exchanger tubes of steam generators. Nova Biotechnologica VII-1, (2007) 2. B. Baja, K. Varga, Z. Németh, P. Kádár, N. A. Szabó, D. Oravetz, Z. Homonnay, E. Kuzmann, J. Schunk, G. Patek: 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. Corrosion Sci. 51, (2009) (IP:2,293) 3. Z. Németh, B. Baja, K. Radó, E. H. Deák, K. Varga, A. N. Szabó, J. Schunk, G. Patek: Comparative study of the corrosion and surface analytical effects of the decontamination technologies. J. Radioanal. Nucl. Chem., 286(3), (2010) (IP: 0,631) 4. P. Kádár, K. Varga, B. Baja, Z. Németh, N. Vajda, Zs. Stefánka, L. Kövér, D. Varga, I. Cserny, J. Tóth, T. Pintér, J. Schunk: Accumulation of uranium, transuranium and fission products on stainless steel surfaces. II. Sorption studies in a laboratory model system J. Radioanal. Nucl. Chem., 288, (2011) Other publication in foreign languages 5. Sz. E. Bárdos, B. Baja, E. Horváth, A. Horváth: Photocatalytic decomposition of L-serine and L-aspartic acid over bare and silver deposited TiO 2 J. Photochem. and Photobiol. 213, (2010) (IP: 2,553) Publications in Hungarian 1. Baja B., Németh Z., Kádár P., Varga K., Nagyné Szabó A., Oravetz D., Homonnay Z., Kuzmann E., Kövér L., Varga D., Cserny I., Tóth J., Schunk J., Patek G.: Korróziós-eróziós tendenciák a paksi atomerőmű gőzfejlesztőiben, Korróziós Figyelő 47(3), (2007) 2. Kádár P., Varga K., Baja B., Németh Z., Vajda N., Stefánka Zs., Kövér L., Varga D., Cserny I., Tóth J., Pintér T. Schunk J.: Urán és transzurán nuklidok, valamint hasadványtermékek akkumulációja acélfelületeken. II. Szorpciós vizsgálatok dinamikus laboratóriumi modellrendszerben. Korróziós Figyelő 48(3), (2008) 3. Baja B., Horváthné Deák E., Berkesi K., Varga K., Radó K., Németh Z., Szeiler G., Nagyné Szabó A., Oravetz D., Schunk J., Patek G., Baradlai P.: Atomerőművi gőzfejlesztők kémiai mentesítése III. Dekontaminációs technológiák korróziós és felületkémiai hatásainak Korróziós Figyelő 49(6), (2009) 4. Varga K., Horváthné Deák E., Nagyné Szabó A., Felföldi V., Baja B., Németh Z., Schunk J., Patek G.: Atomerőművi kémiai dekontaminációs technológia fejlesztése Nukleon 3(3)70, 1-6 (2010) 5. Varga K., Baja B., Horváthné Deák E., Kristóf T., Vajda N., Horváth L. G., Pintér T., Patek G., Schunk J.: Radioaktív korróziótermékek keletkezése és transzportja nyomottvizes atomreaktorokban Nukleon (Megjelenés alatt) Presentations in foreign languages 1. B. Baja, K. Varga, Z. Németh, K. Radó, D. Oravetz, K. É. Makó, Z. Homonnay, E. Kuzmann, J. Schunk: Long-term effects of the AP- CITROX decontamination procedure on the protective oxide-layer formed on stainless steel. EUROCORR 2007, Freiburg im Breisgau, Germany, 9-13 September,

6 2. K. Varga, B. Baja, Z. Németh, K. Radó, D. Oravetz, K.E. Makó, Z. Homonnay, E. Kuzmann, J. Schunk: Long-term effects of the AP- CITROX decontamination procedure on the oxide-layer formed on stainless steel tubes of steam generators. RER 9/076 IAEA Expert Meeting: Crud deposition on fuel cladding. Paks-Bikács Hungary, July 2-4, B. Baja, K. Varga, Z. Németh K. Radó, D. Oravetz, K.E. Makó, Z. Homonnay, E. Kuzmann, J. Schunk, G. Patek: Corrosion-erosion tendencies on the primary side of the steam generators at Paks NPP. Seventh International Conference on Nuclear and Radiochemistry (NRC7), Budapest, Hungary, August, B. Baja, K. Radó, Z. Németh, K. Varga, N. A. Szabó, J. Schunk, G. Patek: Comparative study of the corrosion and surface analytical effects of the decontamination technologies EUROCORR 2009, Nice, France, 6-10 September, A. Szabó Nagy, K. Varga, B. Baja, Z. Németh, D. Oravetz, Z. Homonnay, E. Kuzmann, J. Schunk, G. Patek: Comprehensive investigation of the corrosion state and surface properties of the stainless steel tubes of steam generators. 16th Radiochemical Conference, April 2010 Marianské Lasné, Czech Republik. Booklet of Abstracts. Chem. Listy 104, 107. (2010) ISSN E. H. Deák, A. Szabó Nagy, K. Varga, B. Baja, Z. Németh, D. Oravetz, J. Schunk, G. Patek: Comprehensive studies of corrosion processes of austenitic stainless steel and carbon steel in permanganate solutions The 61st Annual Meeting of the International Society of Electrochemistry (ISE 2010), Nice, France, September 26- October 1, E. H. Deák, A. Szabó Nagy, B. Baja, K. Varga, Z. Németh, D. Oravetz, Z. Homonnay, E. Kuzmann, J. Schunk, G. Patek: Long-term trends in the corrosion state of the stainless steel tubes of steam generators decontaminated chemically The 61st Annual Meeting of the International Society of Electrochemistry (ISE 2010), Nice, France, September 26- October 1, Presentations in Hungarian 1. Baja B., Németh Z., Varga K., Kádár P., Nagyné Szabó A., Oravetz D., Homonnay Z., Kuzmann E., Schunk J., Patek.G.: Korróziós-eróziós tendenciák a paksi atomerőmű gőzfejlesztőiben Őszi Radiokémiai Napok 2007 Sopron, október Baja B., Radó K., Németh Z., Varga K., Schunk J., Patek G.: Kémiai dekontaminációs technológiák korróziós és felületkémiai hatásainak Őszi Radiokémiai Napok 2008 Hajdúszoboszló, október Radó K., Baja B., Kádár P., Németh Z., Varga K., Schunk J., Patek G.: Kémiai dekontaminációs technológiák fejlesztésének újabb irányai és eredményei Őszi Radiokémiai Napok 2008 Hajdúszoboszló, október Németh Z., Baja B., Kádár P., Varga K., Nagyné Szabó A., Schunk J., Patek G.: Gőzfejlesztő hőátadó csövek primer oldali felületének állapotelemzése. VII. Nukleáris Technikai Szimpózium (Magyar Nukleáris Társaság), Budapest, december Baja B., Radó K., Kádár P., Németh Z., Varga K., Nagyné Szabó A., Schunk J. Patek G.: Kémiai dekontaminációs technológiák hatékonyságának valamint korróziós és felületkémiai hatásainak Őszi Radiokémiai Napok 2009 Pécs, október Baja B., Varga K., Németh Z., Horváthné Deák E., Nagyné Szabó A., Patek G., Baradlai P., Schunk J.: Kémiai dekontaminációs technológiák hatékonyságának és felületkémiai hatásainak Őszi Radiokémiai Napok 2010 Keszthely október