INVESTIGATION OF RADIOACTIVE CONTAMINATION PROCESSES ON STRUCTURAL MATERIAL SURFACES. László Péter Kádár 1. INTRODUCTION, GOALS

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1 1. INTRODUCTION, GOALS INVESTIGATION OF RADIOACTIVE CONTAMINATION PROCESSES ON STRUCTURAL MATERIAL SURFACES Theses of the PhD dissertation László Péter Kádár Chemical Doctoral School Supervisor: Dr. Kálmán Varga Full professor For the safe operation of pressurized water reactors it is fundamental to know the contamination and corrosion processes of the structural materials (stainless steels, Zr-Nb alloys) in boric acid coolant contaminated by uranium and transuranium (Pu, Am, Cm) nuclides. As a consequence of the breakdown on April 10, 2003 of the reactor block 2 of Paks Nuclear Power Plant (PNPP) it was of special importance to estimate the amount of the above radionuclides in the fuel-transfer pond as well as the extent and kinetics of the contamination phenomena on structural material surfaces. Within the frame of a joint project started in the year 2005 and supervised by the Institute of Radiochemistry and Radioecology, University of Pannonia four domestic institutes have cooperated in order to study the accumulation of uranium and transuranium (Pu, Am, Cm) species on constructional materials of the primary cooling circuit (such as heat exchanger tubes (SGs) and the stainless steel can material). 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 kinetics and mechanism of uranium accumulation on stainless steel and Zr- Nb alloy surfaces. In addition I studied the extent, kinetics and mechanism of the accumulation of some transuranium (Pu, Am, Cm) nuclides on the above steel surfaces. To accomplish the above aims a three years-long comprehensive research program has been elaborated. The main issues of the program are as follows: University of Pannonia Institute of Radiochemistry and Radioecology Veszprém, 2009 I. A brief overview of the relevant literature data. II. Investigation of the extent and the basic sorption characteristics of uranium accumulation (sorption-desorption) in boric acid solution on austenitic stainless steel surfaces (type: 08X18H10T (GOSZT )) in dynamic model system. III. Investigation of the extent and kinetics of the uranium and transuranium (Pu, Am, Cm) accumulation (sorption-desorption) in in- 2

2 dustrial samples of boric acid coolant on stainless steel surfaces in dynamic model system. IV. Investigation of the extent and kinetics of the uranium accumulation (sorption-desorption) in industrial samples of boric acid coolant on Zr-Nb alloy surfaces in static model system. 2. EXPERIMENTAL METHODS AND INSTRUMENTS The surface excess as well as solution chemical and activity concentrations of the uranium and transuranium species (particle, colloid and dissolved) were analyzed by ICP-MS and alpha-spectrometric method. In order to investigate the mechanism of the sorption phenomena the corrosion state, morphology and chemical composition of the inner surfaces of the sample surfaces were studied by voltammetry and SEM-EDX method. The oxidation state and chemical form of the uranium accumulated on surfaces were identified by XPS method. ICP-OES method was used to analyze the chemical concentration of the dissolved corrosion products (Fe, Cr, Ni, Zr, Nb). 3. THESES I. Comprehensive investigation of the uranium and transuranium accumulation on austenitic stainless steel surfaces in dynamic model system I.1. Investigation of the uranium accumulation in boric acid model solutions contain 20 g dm -3 H 3 BO mg dm -3 U(VI) species at different ph values [1]: a.) The accumulation of uranium is mainly determined by the ph of the boric-acid solution and the corrosion state of the surfaces. The saturation surface excess of the uranium-species adsorbed on the steel surface was attained after a sorption period of 10 hours (see table 1. column 6. row 1-4.). Significant U-sorption was measured in a ph range of 4 to 8, where intensive hydrolysis of uranyl-ions was observed in the solution. The accumulation of uranium is presumably determined by specific adsorption of U(VI)-hydroxo-complexes (mainly the dissolved and colloid-size UO 2 (OH) 2 ) formed in the solution. The maximum sorption of U was measured at ph 6, (Γ sample = 1.22 µg cm -2 (see table 1. column 4. row 3.) U mol cm -2 UO 2 (OH) 2 ). This maximum surface excess approaches to a monolayer coverage (assuming the sorption of (UO 2 (OH) 2 )). b.) To avoid the uranium contamination as much as possible: - the ph of the boric-acid solution has to be below ph=4.0, or significantly higher than ph=8, (see table 1. column 4. row 1-4.) and, - the solution concentration of corrosion products containing Fe and Cr and the general corrosion rate of the steel surfaces should be minimized. I.2. Investigation of the uranium and transuranium (Pu, Am, Cm) accumulation in industrial sample of boric acid coolant [2-3]: a.) On the positively charged surfaces of steel can material the dominant interaction is the so-called non-specific (electrostatic) adsorption and the deposition of the colloidal and/or disperse form uranium and transuranium (Cm) isotopes. In contrast to this, significant accumulation of cations takes place on the surface of SG tube sample, giving a strong indication of some specific adsorption processes,, It should however be noted that, the simultaneous deposition of the colloidal and/or disperse forms of radionuclides cannot be excluded.. b.) The surface excess of the studied radionuclides except the uranium isotopes is extremely low, the cumulative surface excess concentration of transuranium isotopes does not exceed the 1 % of the monolayer coverage even in the case of the SG tube specimen. The calculated surface excess of uranium species measured after 30 hours on SG tube surfaces is significantly higher, exceeds a monolayer coverage assuming sorption of UO 2 (see table 1. column 4. row 5.). 3 4

3 c.) The deviations in the accumulation processes can be interpreted by considering the differences of the morphology (roughness), chemical composition and thickness of individual oxide-layers. II. Comprehensive investigation of the uranium accumulation on Zr-Nb alloy surfaces in industrial sample of boric acid coolant in static model system a.) The time dependence of uranium sorption shows saturation character on both Zr-Nb alloy surfaces studied. The accumulation of 238 U isotope is higher on the fuel cassette material (Zr+2.5%Nb), than the fuel cladding (Zr+1%Nb) material (see table 1. column 4. row 7-8.). b.) The uranium sorption can basically be considered as the accumulation of colloid particles owing to the fact that the accumulation of the cationic uranium species on the positively charged surface of Zr-Nb alloy can be ruled out c.) It is probable that the corrosion of Zr-Nb alloy surfaces and the dissolution of the surface contaminants (e.g. Fe) have a significant effect on the extent and kinetics of the uranium contamination. In our case the above mentioned phenomena can not be dominant since the surface analysis (XPS, SEM-EDX) reveal the passive state of both investigated surface. 5

4 Table 1.: The comparison of the adsorption behaviors of the investigated steel surfaces Modell solution 20 g dm -3 H 3 BO mg dm -3 U Coolant solution 20 g dm -3 H 3 BO 3 ~ 0.8 mg dm -3 U Coolant solution 20 g dm -3 H 3 BO 3 ~ 1 mg dm -3 U Steel Steel Zr-Nb alloy Surface 2/2 SG* ph Γ max / A max / Time / μg cm -2 mbq cm -2 h /5 SG* CAN MATERIAL Cladding material (Zr + 1% Nb) Cassette material (Zr + 2,5% Nb) Chemical form of sorbed species UO 2 2- (according to the XPS measurements) UO 2 (according to the XPS measurements) *: the first number indicates the reactor block, the second number denotes the steam generator. Interaction - specific adsorption, -colloid deposition, - specific adsorption - deposition of colloid and disperse particles, - non specific adsorption - deposition of colloid and disperse particles - non specific adsorption - colloid deposition 6

5 4. INDUSTRIAL APPLICATION This work was supported by Paks NPP Ltd. To perform the safe removal of the damaged fuel started in October 2006 (and finished successfully in January 2007) it was of special importance to estimate the amount of the emerged radionuclides in the fuel-transfer pond and the extent and kinetics of the contamination on structural material surfaces. 5. PUBLICATION LIST Publications in foreign languages 1. P. Dombóvári, P. Kádár, T. Kovács, J. Somlai, K. Radó, I. Varga, R. Buják, K. Varga, P. Halmos, J. Borszéki, J. Kónya, N. M. Nagy, L. Kövér, D. Varga, I. Csernyi, J. Tóth, L. Fodor, A. Horváth, T. Pintér, J. Schunk: Accumulation of uranium on austenitic stainless steel surfaces. Electrochim. Acta 52, (2007). (IP: 2.95). 2. P. Kádár, K. Varga, Z. Németh, N. Vajda, T. Pintér, J. Schunk: Accumulation of uranium, transuranium and fission products on stainless steel surfaces. Part I. A comprehensive view of the experimental parameters influencing the extent and character of the contamination. J. Radioanal. Nucl. Chem. (submitted for publication) 3. 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.part II. Sorption studies in dynamic model system. J. Radioanal. Nucl. Chem. (submitted for publication) erators decontaminated chemically in VVER type nuclear reactors. Corr Sci. (in press, on-line accessibility: (IP: 2.29). Publications in Hungarian 1. Dombóvári P., Kádár P., Kovács T., Radó K., Varga I., Varga K., Halmos P., Borszéki J., Kónya J., M. Nagy N., Kövér L., Varga D., Cserny I., Tóth J., Fodor L., Horváth A., Pintér T., Schunk J.: Urán-vegyületek akkumulációja ausztenites korrózióálló acélfelületeken. Korróziós Figyelő 45(6), , (2005). 2. Kádár P., Varga K., Németh Z., Vajda N., Pintér T. Schunk J.: Urán és transzurán nuklidok, valamint hasadványtermékek akkumulációja acélfelületeken. I. A kontamináció jellegét és mértékét befolyásoló tényezők. Korróziós Figyelő 47(5), (2007). 3. Kádár P., Varga K., Baja Bernadett, 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). 4. 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). 4. B. Baja, K. Varga, Z. Németh, P. Kádár, N. A. Szabó, D. Oravetz, Z. Homonnay, E. Kuzmann, J. Schunk, G. Patek: Long terms in the corrosion state and surface properties of the stainless steel tubes of steam gen- 7 8

6 Other publications 1. K. Varga, P. Dombóvári, P. Kádár, T. Kovács, K. Radó, I. Varga, P. Halmos, J. Borszéki, J. Kónya, N. M. Nagy, L. Kövér, D. Varga, I. Cserny, J. Tóth, L. Fodor, A. Horváth, T. Pintér, J. Schunk: Accumulation of uranium compounds on austenitic stainless steel surfaces EUROCORR 2006, Maastricht, Neederland, September 23-29, Proceedings (on CD-ROM) 2. P. Kádár, K. Varga, I. Varga, Z. Németh, K. Radó, N. Vajda, Zs. Stefanka, T. Pintér, J. Schunk: Comprehensive study of uranium and transuranium (Pu, Am, Cm) accumulation on stainless steeel sufraces. EUROCORR 2007, Freiburg im Breisgau, Germany, September 9-13, Proceedings (on CD-ROM). 3. P. Kádár, K. Varga, I. Varga, Z, Németh, K. Radó, N. Vajda, Zs. Stefánka, T. Pintér, J. Schunk: Comprehensive study of uranium and transuranium (Pu, Am, Cm) accumulation on stainless steel surfaces. Seventh International Conference on Nuclear and Radiochemistry (NRC7), Budapest, Hungary, August 24-29, Proceedings (on CD- ROM). 4. 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 4-5. Presentations in foreign languages 1. P. Kádár, K. Varga, I. Varga, Z. Németh, K. Radó, N. Vajda, Zs. Stefanka, T. Pintér, J. Schunk: Comprehensive study of uranium and transuranium (Pu, Am, Cm) accumulation on stainless steeel sufraces. EUROCORR 2007, Freiburg im Breisgau, Germany, September 9-13, P. Kádár, K. Varga, I. Varga, Z, Németh, K. Radó, N. Vajda, Zs. Stefánka, T. Pintér, J. Schunk: Comprehensive study of uranium and transuranium (Pu, Am, Cm) accumulation on stainless steel surfaces. Seventh International Conference on Nuclear and Radiochemistry (NRC7), Budapest, Hungary, August 24-29, Presentations in Hungarian 1. Kádár P., Dombóvári P., Kovács T., Radó K., Varga I., Varga K., Halmos P., Borszéki J., Kónya J., M. Nagy N., Kövér L., Varga D., Cserny I., Tóth J., Fodor L., Horváth A., Pintér T., Schunk J.: Urán-vegyületek akkumulációja ausztenites korrózióálló acélfelületeken. Őszi Radiokémiai Napok 2005 az MTA Radiokémiai Bizottság és MKE Radioanalitikai Szakcsoport tudományos ülése, Mátraháza, október Kádár P., Varga K., Varga I., Németh Z., Somlai J., Radó K., Oravetz D., Vajda N., Stefanka Zs., Pintér T., Schunk J.: Urán- és transzurán (Pu, Cm) nuklidok, valamint hasadványtermékek akkumulációja acélfelületeken. A MTA Elektrokémiai Munkabizottság, az MTA Izotópalkalmazási Munkabizottság és a HUNKOR Nukleáris Korróziós Munkabizottság tudományos ülése Veszprém, május Kádár P., Varga K., Varga I., Németh Z., Somlai J., Radó K., Oravetz D., Vajda N. Stefanka Zs., Pintér T., Schunk J.: Urán és transzurán (Pu, Cm) nuklidok akkumulációjának vizsgálata primerköri szerkezeti anyag felületeken. Őszi Radiokémiai Napok 2006 az MTA Radiokémiai Bizottság és MKE Radioanalitikai Szakcsoport tudományos ülése. Siófok, október

7 4. Kádár P., Varga K., Varga I., Németh Z., Vajda N. Stefanka Zs., Pintér T., Schunk J.: Az urán és transzurán (Pu, Cm) izotópok akkumulációjának vizsgálatára korrózióálló acél tokanyagon illetve inaktív fűtőelem burkolat (Zr+Nb) felülteken. Őszi Radiokémiai Napok 2007 az MTA Radiokémiai Bizottság és MKE Radioanalitikai Szakcsoport tudományos ülése. Sopron, október Kádár P., Varga K., Varga I., Németh Z., Kristóf T.,Vajda N. Stefanka Zs., Pintér T., Schunk J.: Urán és transzurán (Pu, Cm) izotópok akkumulációjának vizsgálata korrózióálló acél és Zr+Nb fűtőelem burkolat felületeken. Őszi Radiokémiai Napok 2008 az MTA Radiokémiai Bizottság és MKE Radioanalitikai Szakcsoport tudományos ülése. Hajdúszoboszló október