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Accumulation of uranium on austenitic stainless steel surfaces
Electrochimica Acta 52 (2007) 2542–2551
Accumulation of uranium on austenitic stainless steel surfaces P´eter Dombov´ari a , P´eter K´ad´ar a , Tibor Kov´acs a , J´anos Somlai a , Kriszti´an Rad´o a , Istv´an Varga a , Ren´ata Buj´ak a , K´alm´an Varga a,∗ , P´al Halmos b , J´anos Borsz´eki c , J´ozsef K´onya d , No´emi M. Nagy d , L´aszl´o K¨ov´er e , Dezs˝o Varga e , Istv´an Cserny e , J´ozsef T´oth e , Lajos Fodor f , Attila Horv´ath f , Tam´as Pint´er g , J´anos Schunk g a
Department of Radiochemistry, Pannon University, H-8201 Veszpr´em, P.O. Box 158, Hungary Analytical Chemistry Research Group, Hungarian Academy of Sciences, Veszpr´em, Hungary c Department of Analytical Chemistry, Pannon University, Veszpr´ em, Hungary d Department of Colloid- and, Environmental Chemistry, Isotope Laboratory, University of Debrecen, Debrecen, Hungary e Section of Electron Spectroscopy and Materials Science, Institute of Nuclear Research of the Hungarian Academy of Sciences (MTA ATOMKI), P.O. Box 51, H-4001 Debrecen, Hungary f Department of General and Inorganic Chemistry, Pannon University, Veszpr´ em, Hungary g Paks NPP Ltd., Paks, Hungary b
Received 25 August 2006; received in revised form 1 September 2006; accepted 1 September 2006 Available online 10 October 2006
Abstract The surface contamination by uranium in the primary circuit of PWR type nuclear reactors is a fairly complex problem as (i) different chemical forms (molecular, colloidal and/or disperse) of the uranium atoms can be present in the boric acid coolant, and (ii) only limited pieces of information about the extent, kinetics and mechanism of uranium accumulation on constructional materials are available in the literature. A comprehensive program has been initiated in order to gain fundamental information about the uranium accumulation onto the main constituents of the primary cooling circuit (i.e., onto austenitic stainless steel type 08X18H10T (GOSZT 5632-61) and Zr(1%Nb) alloy). In this paper, some experimental findings on the time and pH dependences of U accumulation obtained in a pilot plant model system are presented and discussed. The surface excess, oxidation state and chemical forms of uranium species sorbed on the inner surfaces of the stainless steel tubes of steam generators have been detected by radiotracer (alpha spectrometric), ICP-OES and XPS methods. In addition, the passivity, morphology and chemical composition of the oxide-layers formed on the studied surfaces of steel specimens have been analyzed by voltammetry and SEM-EDX. The experimental data imply that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. Some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 (Γ sample = 1.22 g cm−2 U∼ = 4 × 10−9 mol cm−2 UO2 (OH)2 ) exceeds a monolayer coverage. © 2006 Elsevier Ltd. All rights reserved. Keywords: Uranium; Accumulation; Stainless steel; Alpha spectrometry; XPS; ICP-OES; Voltammetry; SEM-EDX
1. Introduction 1.1. Objective As a consequence of the breakdown on April 10, 2003 there was a noteworthy uranium release (about 4 kg) and contamination in some technological units of the reactor block 2 of the
∗
Corresponding author. Tel.: +36 88 427 681; fax: +36 88 427 681. E-mail address: [email protected] (K. Varga).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.09.007
Paks Nuclear Power Plant (Paks NPP). Owing to the effective purification of the boric acid coolant the amount of uranium species exists at present time in the primary cooling circuit is very low. On analyzing the experimental data, it has become obvious that the limited pieces of information about the extent, chemical forms and kinetic behaviors of the uranium accumulated on the surface of the main structural materials (stainless steels, Zr–Nb alloys) of the primary circuit may cause difficulties in the better understanding of the transport processes and in the elaboration of effective decontamination procedures. The fact is that the uranium nuclides can be present in different chemical
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forms (molecular, colloidal and/or disperse) in the boric acid coolant enhances the complexity of the problem. In light of the above facts, a comprehensive program has been initiated in order to gain fundamental information about the uranium accumulation onto the main constituents of the primary cooling circuit; (i.e., onto austenitic stainless steel type 08X18H10T (GOSZT 5632-61) and Zr(1%Nb) alloy). In this paper, we give a brief summary of the uranium chemistry in boric acid solutions, as well as present and discuss some experimental findings concerning the time and pH dependences of U accumulation obtained in a pilot plant model system using stainless steel samples of the heat exchanger tube cut out from a steam generator (SG) of the Paks NPP. Within the frame of the present project, an alpha spectrometric detection procedure has also been worked out which is suitable for the measurement of the activity concentrations of uranium nuclides in liquid and surface phases. In addition, the main corrosion products (Fe, Cr and Ni) dissolved from steel samples into the boric acid solution in the course of the sorption experiments have been determined by ICP Optical Emission Spectroscopic (ICP-OES) method. Moreover, special attention has been paid to report some voltammetric and SEM-EDX results that reveal the corrosion properties, morphology and chemical composition of the oxide-layer formed on the studied surfaces of steel specimens. To obtain the more complex features of U accumulation, we have analyzed the oxidation state and chemical forms of uranium sorbed on the inner surfaces of stainless steel tubes by X-ray Photoelectron Spectroscopy (XPS). 1.2. Fundamental aspects of uranium chemistry in boric acid solution Before proceeding to a detailed analysis of the uranium accumulation on steel surfaces, it is useful to review some of its fundamental chemical properties in boric acid solution. The chemical behaviors of uranium species in boric acid solution can be influenced by several factors such as the inclination of uranium towards complex formation, the hydrolytic processes (that can frequently result in the formation of polymer ions) and the redox properties [1,2]. In aqueous solutions the uranyl cation (UO2 2+ ) is the most stable uranium species. The main characterizing data of uranyl cation can be described as follows: (i) Its chemical structure is linear and symmetric: (O U O)2+ (owing to the role of 5f atomic orbital in the formation of the chemical bond). The hexavalent U atom in the linear (O U O)2+ is only partially shielded by the two oxygen atoms; thus in the equatorial plane the effective charge of the uranium may be more than 3 (ca. 3.3 ± 0.1) [2–4]. (ii) UO2 2+ can coordinate several donor atoms in the equatorial plane and hereby can produce wide variety of complexes [2,5]. As uranyl cations, like other actinide ions, are strong acids, the electrostatic model can be used to describe their chemical bonds in the above complexes. Moreover, uranyl ions display strong preference to oxygen donors; accordingly, the chemical bonds formed exhibit predominantly ionic character [5].
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Independent literature data [6–8] reveal that the presence of uranyl cations and their interactions with anionic particles (formed from boric acid) should be considered as primary factors affecting the speciation in aqueous boric acid solutions. The complexation by boric acid is influenced by both pH and chemical concentration. At this point, it should be emphasized that there are no data available in the relevant literature about the interaction between uranyl cations and monomer (or polymer) borate ions. Therefore, it is unlikely that uranyl cations can form complexes with borate species in boric acid solutions. A careful inspection of the structure of the uranyl cation and B(OH)4 − borate ion may give a plausible explanation of the latter assumption. Namely the mono-borate anion has a symmetric tetrahedron structure like perchlorate anion, which is unable to form high stability complexes with metal ions, too. As we were not able to find any experimental evidences about uranium borate complexes, an attempt was made to collect information about the uranium perchlorate complexes that have a similar structure to the borate compounds. On studying the related references, it has been realized that there is no spectroscopic evidence of the formation of the uranyl perchlorate complex. It is to be noted that creation of some weak UO2 –ClO4 complex cannot be excluded in the presence of the great excess of perchlorate ions; however, its stability factor must be smaller than 10−1.1 (if any) [6]. All these data attest that the formation of uranyl borate complexes cannot affect notably the speciation of uranium nuclides in aqueous boric acid solutions. The hydrolysis of uranyl ion is an especially important factor in aqueous solutions. The references on this subject [6,7] provide evidences on the formation of the following complexes: - mononuclear complexes: UO2 (OH)+ , UO2 (OH)2 , UO2 (OH)3 − , UO2 (OH)4 2− . - polynuclear complexes: (UO2 )2 (OH)2 2+ , (UO2 )4 (OH)7 + , (UO2 )3 (OH)4 2+ , (UO2 )3 (OH)5 + , (UO2 )3 (OH)7 − . Besides the hydrolytic processes the strong carbonate complexation producing carbonate (UO2 CO3 , UO2 (CO3 )2 2− , UO2 (CO3 )3 4− ) and mixed hydroxo carbonate ((UO2 )2 CO3 (OH)3 − ) complexes is also considered, especially in geological environments [8]. It is of special interest to emphasize that CO2 in the air can also take part in the formation of the above carbonate and hydroxo carbonate complexes. Independent speciation calculations supporting some experimental data [6,7] reveal that the hydrolysis of uranyl cations in aqueous solutions begins at pH > 3.5. Moreover, at open to air condition the formation of hydroxo carbonate complexes commences at pH ≈ 5.5. Simultaneously, the solution may become oversaturated and uranyl hydroxide (schoepit) is precipitated in the pH range of 5.3–7.3. The relatively high solubility of uranium at higher pH is due to the negatively charged carbonate complexes dominating in the solution at pH > 8. When studying the fundamental aspects of the solution chemistry of uranium, numerous experiments have been carried out in order to gain a deeper insight into the sorption of uranyl species on solid surfaces (mainly in geological environment). Although a limited number of publications deal with uranium sorption on
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steel surfaces, the results of these sorption experiments could be useful during the estimation of the extent and character of U sorption on steel surfaces. The sorption behavior of uranyl species may be interpreted by the surface complexation model [9]. This model was firstly developed for oxide surfaces and later utilized for minerals and rocks, too. The model is based on the assumption that surface OH− sites are formed on the oxide/water interface and they are considered as proton donors as well as proton acceptors, respectively, depending on pH. Consequently, these surface sites are positively or negatively charged, and electrostatically interact with anions or cations in the aqueous solution. Both the chemical form of the species present in the solution and the charged active sites on surfaces play significant role in the model. If no specific adsorption phenomena occur, the electrostatic interactions (non-specific adsorption processes) between the oppositely charged surface and solution species determine the formation of surface complexes. The surface complexation model can explain the accumulation of uranium species on Fe oxide surfaces (see [10] and the references cited therein). Specifically, the extent of the sorption is primarily affected by at least two factors such as (i) the corrosion state (and dissolution behavior) of the Fe oxide surface influencing the active sites on the surface, and (ii) the fractions (and solubility) of uranium species in the solution. When pH is smaller than pzc (i.e., pH value of the zero surface charge), Fe oxide surface is positively charged, and the accumulation of anions can primarily be considered. On the other hand, at pH values above pzc, a significant cation adsorption takes place on the surface. In summary, when comparing the charge of uranium species with the surface charge of the adsorbent, the extent and character of the U accumulation can be estimated. However, it is important to note that the specific adsorption and deposition (colloidal and/or disperse particles) processes should also be taken into consideration. 2. Experimental 2.1. Experimental setup, chemicals and procedures A pilot plant model system elaborated earlier [11] was utilized for the dynamic studies of the accumulation processes on inner surfaces of the SG tube specimens provided by the Paks NPP. All parts of the circulation system were made of austenitic stainless steel (type: DIN 1.4541). A centrifugal pump (commercial available) with the smallest inner volume (18 cm3 ) was built into the circuit to provide the smallest available ration of the solution volume to the treated surface. In the present experiments, the above centrifugal pump equipped with a pump drive
unit was used to maintain the stable and long lasting (more than 30 h) circulation of the high temperature (up to 95 ◦ C) boric acid solution at a linear flow rate of 3.0 m s−1 . The uranium accumulation was studied by the so-called “solution” radioactive tracer method [11]. Specifically, the ex situ measurement of uranium was carried out by determining the radioactivity in aliquot volumes of a model solution of the primary coolant being in contact with the inner surfaces of the stainless steel tubes (type: 08X18H10T (GOST 5632-61), outer diameter: 16 mm, average wall thickness: 1.6 mm) originating from the SG of the reactor block 2 at Paks NPP. Main characteristics of the tube specimens as well as of the sorption experiments are summarized in Table 1. Following various time periods (1, 3, 5, 10, 20 and 30 h) 5-5 cm3 samples were taken from the solutions for analysis in each sorption experiment. After all samplings 5-5 cm3 volumes of boric acid solution containing 1 mg dm−3 238 U (at the given pH) were added into the circulation system to keep up the total solution volume of 90 cm3 . The activity concentrations (and chemical concentrations) of uranium in the solution samples were detected by alpha spectrometry. To improve the reliability of the “solution” method, activity of uranium accumulated onto the inner surface of the tube specimens during a period of 30 h was directly measured by alpha spectrometry. In addition, ICP-OES method was utilized to measure the chemical concentration of the main alloying components (Fe, Cr, Ni) dissolved from the surface into the solution during the U accumulation. Moreover, the oxidation state of the uranium sorbed onto the inner surface of the steel tube from model solution of pH 4.5 was identified with XPS. From the original tube samples, specimens of 20 mm were cut to be used for voltammetric and SEM-EDX studies. The tube pieces were cut into two halves along their axes and then – for the voltammetric studies only – flattened gently. In order to protect the oxide-layer on the specimens from possible chemical effect of organic solvents, the surfaces were not degreased. The corrosion properties of the stainless steel samples prepared by the above technique were studied mainly on their original inner surfaces. The metallographic cross-sections of tube specimens were prepared and analyzed by SEM-EDX method, too. 2.2. Analytical 2.2.1. Alpha spectrometric study of surface and electrolytes 2.2.1.1. Study of the uranium contamination on heat exchanger tubes. Following the 30 h period of uranium accumulation the 238 U activity on the flattened surface (2 cm long parts) of steel tube specimens was detected by alpha spectrometric method. For this measurement, in each sorption experiment 2-2 parallel
Table 1 Main characteristics of the sorption experiments Sample number
Composition of the model solution
Solution pH
SG tube specimens
Temperature (◦ C)
1 2 3 4
20 g dm−3 H3 BO3 + 1 mg dm−3 U 20 g dm−3 H3 BO3 + 1 mg dm−3 U 20 g dm−3 H3 BO3 + 1 mg dm−3 U 20 g dm−3 H3 BO3 + 1 mg dm−3 U
4.5 6.0 8.5 4.0
Length: 13.0 cm, inner surface area: 53.1 cm2 Length: 13.0 cm, inner surface area: 53.1 cm2 Length: 13.0 cm, inner surface area: 53.1 cm2 Length: 13.0 cm, inner surface area: 53.1 cm2
30 30 30 30
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samples having 4 cm2 of detectable surface area were used. Before activity measurements the steel samples were heated to 400 ◦ C for 30–60 min. The inner surfaces were detected in vacuum for 80,000 s using a semiconductor PIPS alpha spectrometric device (chamber: Oxford-Tennelec, detector: Eurisys PIPS detector, resolution: 19 keV, multichannel analyzer: Silena9302, data collecting software: EMCA-2000). This apparatus was calibrated by applying alpha calibration sources of 239 Pu, 241 Am and 244 Cm. The FWHM of the measured peaks was in the region of 60–80 keV, and the minimum detectable activity (MDA) was of about 3 mBq. The relative error of the measurement was estimated to be less than ±10%. Since the sample surfaces were originally contaminated by significant amount of alpha emitting nuclides (such as 241 Am and 238,239,240 Pu) which did exert a disturbing effect on the detection of the small amount of uranium, the oxide-layer had to be dissolved from the inner surface of the tube specimens. Owing to the passivity of the oxide film diluted sulfuric acid was not efficient for the above purpose; however, by using a mixture of hydrochloric acid and hydrogen peroxide the oxide-layer was dissolved completely. As the iron concentration in the solution was too high, Eichrom’s UTEVA type ion exchange resin was utilized for the purification the solution. As a consequence of the ion exchange procedure, the treated solution became rich in uranium and practically free of iron. (Later we shall return to a more detailed description of the ion exchange and sample preparation methods.) 2.2.1.2. Alpha spectrometric analysis of boric acid solution. In the course of the uranium accumulation, 5-5 cm3 samples were taken from boric acid solutions at the end of various adsorption periods. A part (3 cm3 ) was separated from each 5 cm3 solution sample to perform isotope specific sample treatment and to prepare alpha sources. In order to increase the accuracy of measurements 232 U internal tracer was added to the solution at the beginning of the sample treatment. Also, Fe(III) chloride and ammonia were introduced into the traced sample to form iron hydroxide precipitate which was capable of interacting with uranium nuclides effectively from the solution. The uranium reach precipitate was then filtered through a 0.45 m membrane and dissolved in 9 mol dm−3 HCl. The separation of uranium isotopes is based on selective bonding of the dominant uranium species on ion exchange resins. Owing to the high Fe content in the solutions, Eichrom’s UTEVA type resin having uranium specific organic material coating on the resin particles was used for uranium separation. (The UTEVA type resin (10–150 m) was conditioned with 8 mol dm−3 HNO3 and loaded into the columns of 2 cm3 volume.) After a preseparation by successive evaporation and dissolution the U containing sample was finally dissolved in 8 mol dm−3 HNO3 and introduced into the columns of ion exchange resins. Finally, 0.02 mol dm−3 HCl (40 cm3 ) was used to elute uranium species from the columns. After the ion exchange procedure the pH of the uranium containing solutions was set to 2.3, and alpha sources were prepared on stainless steel coupons by electrodeposition, as described in [12]. Following uranium electrodeposition the steel coupons were heated (to remove Po isotopes) and measured in vacuum
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for 80,000 s by the alpha spectrometric device outlined above. As a consequence of the above separation procedure the resolution of the alpha spectra became considerably better than earlier. During the 80,000 s measurements of the alpha sources the minimum detectable activity (MDA) was as small as 0.34 mBq. The relative error of the measurement was estimated to be less than ±10%. 2.2.2. Analysis of the chemical composition of boric acid solutions by ICP-OES During the uranium accumulation on the inner surface of tube specimens, the concentrations of the main alloying components (Fe, Cr, and Ni) dissolved from the surface oxide-layer into the solution were determined by an ICP-OES device (type GBC Integra XM). For this elemental analysis, solution samples having a volume of 2 cm3 were separated from the 5 cm3 boric acid samples taken after various time periods (1, 3, 5, 10, 20 and 30 h) in each adsorption series. The solution samples were stored in closed polyethylene vials. 2.2.3. XPS analysis of uranium accumulated on the surface of steel tubes The chemical composition of the outermost films formed on the inner surface of tube sample (number 1) during uranium accumulation was studied by X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out by the ESA-31 (ATOMKI, Hungary) type spectrometer [13] on “as received” samples as well as on surfaces obtained following sequential removal of the outermost surface layers by ion sputtering. For these studies, rectangular (10 mm × 10 mm) pieces of austenitic stainless steel were cut out from the tube specimens without mechanical deformation. Al K␣ radiation with energy of 1486.6 eV was used as the primary X-ray source. The sputtering was performed with 2 keV argon ions (the ion current density was 140 A/cm2 ) at a rate of about 1.4 nm/min. The total thickness of the oxide film removed from the surface of each sample by Ar ion sputtering was ca. 105 nm. The photoelectrons analyzed were emitted in a narrow solid angle around the normal vector of the sample surface. In the course of the XPS measurements the vacuum around the sample was about 10−7 Pa. The fix retarding ratios of R = 4 (for general spectra) and R = 8 (for detailed spectrums) were used to provide good intensity and energy resolution of about 1 eV. The energy calibration of the spectrometer was performed using photolines of Ag, Au, and Cu metal standards. 2.2.4. Investigation of the corrosion state of tube specimens by voltammetry The passivity of the inner surface of tube specimens was studied by potentiostatic polarization method. The experiments were carried out by the means of a VoltaLab 40 (RADIOMETER) type electrochemical measuring system controlled by PC. To perform these investigations a special electrochemical cell was developed. In the course of potentiostatic polarization experiments the potential (E) of the specimen (working electrode) was continuously shifted towards anodic direction at a constant rate of 10 mV min−1 and the current density (i) related to the
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inner surface area of the specimen was recorded. The measurements were carried out in boric acid solution (c = 20 g dm−3 ) in argon gas atmosphere (99.999, v/v% Ar). The schematic of the measuring system, the detailed experimental procedure and the determination of the corrosion parameters (such as corrosion potential (Ec ), corrosion current density (ic ), and corrosion rate (vc )) derived by so-called Stern method have been described in our earlier papers [14]. The electrode potential values quoted in this paper are given on the saturated calomel electrode (SCE) scale. 2.2.5. Study of the surfaces by SEM-EDX method The morphology and chemical composition of the oxidelayer developed on the inner surfaces of the stainless steel specimens were studied by scanning electron microscopy (SEM), equipped with an energy dispersive X-ray microanalyzer (EDX) (Type: JEOL JSM-50A, controlled with R¨ontec EDR 288 software). In case of each specimen at least two different surface areas were studied by making use of the combined SEM-EDX equipment. The comparative evaluation of surface morphology was performed by analyzing the SEM micrographs obtained at two different magnifications, M = 3000 and M = 1000, respectively. The chemical composition of the sample surfaces was determined on two different areas of 1 mm2 by EDX method [14]. In addition, the metallographic cross-sections of tube specimens were also studied. 3. Results 3.1. Alpha spectrometric study of surface and boric acid solutions By applying our version of the so-called “solution” technique, it has become obvious that the quantitative evaluation of surface excess (Γ ) values involves the optimization of the experimental and nuclear detection conditions (see Section 2) as well as the elaboration of a reliable calculation procedure. To elucidate the latter issue, we should recall some key features of the radiotracer method. The dynamic sorption measurements were carried out in a pilot plant model system at a solution volume of 90 cm3 . Following various time periods (1, 3, 5, 10, 20 and 30 h) 5-5 cm3 samples were taken from the solution for analysis in each sorption experiment. At all samplings 5-5 cm3 volumes of boric acid solution containing 1 mg dm−3 238 U were added into the circulation system to keep up the total solution volume. As a consequence of the above experimental procedure, it was of special importance to evaluate the corrected chemical concentration (ck ,i ) as well as the corrected activity concentration (Ak ,i ) of 238 U nuclides present in the boric acid solution after sampling number ‘i’. The cki and Aki values were computed by Eqs. (1) and (2), respectively: 85 5 + c0 90 90 85 5 + A0 = Ai 90 90
ck,i = ci Ak,i
where ci is the chemical concentration of 238 U measured in the boric acid solution at sampling number ‘i’ (mg dm−3 ), c0 the initial chemical concentration of 238 U in the boric acid solution before the sorption experiments (c0 = 1 mg dm−3 ), Ai the activity concentration measured in the boric acid solution at sampling number ‘i’ (mBq cm−3 ); A0 is the initial activity concentration of 238 U in the boric acid solution (A = 11.82 mBq cm−3 , as activity 0 of 1 mg 238 U corresponds to 11.82 Bq). In addition, it should be emphasized that: (i) At the first sampling the change of the chemical concentration as well as of the activity concentration corresponds to c0 − c1 , and A0 − A1 , respectively. Later on, at the consecutive samplings the above values are considered as ck(i − 1) − ci , and Ak(i − 1) − Ai , respectively. (ii) At each sampling the amount (or activity) of 238 U accumulated onto the inner surface of tube specimens in the recirculation system is provided by the product of the chemical concentration (or activity concentration) change and the volume of the model system (90 cm3 ). The cumulative amount (or cumulative activity) of the adsorbed uranium can be calculated as the sum of the values obtained at different samplings. Consequently, we provide the surface excess data as the cumulative amount of sorbed uranium related to the geometric surface area of tube specimens (see Table 1). In light of the above considerations, the pH and time dependence curves of uranium accumulation (Fig. 1) evaluated from the alpha spectrometric measurements of the solutions and steel surfaces (Fig. 2) are presented in Figs. 1–2. In addition, Ak (and ck ) versus time plots are compiled in Fig. 3. The experimental results shown in Figs. 1–3 reveal that: (i) A highly pH and time dependent accumulation of U species takes place on the inner surface of heat exchanger tubes in the model solution of the primary coolant (20 g dm−3 H3 BO3 containing 1 mg dm−3 238 U species). The quasiequilibrium surface excess of the deposited uranium is reached only after a period of 10 h.
(1) (2)
Fig. 1. Time dependence of 238 U accumulation on the inner surface of stainless steel tubes in solution containing 20 g dm−3 H3 BO3 + 1 mg dm−3 U: (1) pH 4.5, T = 30 ◦ C; (2) pH 6.0, T = 30 ◦ C; (3) pH 8.5, T = 30 ◦ C; (4) pH 4.0, T = 30 ◦ C.
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Fig. 2. Time dependence of measured 238 U activity concentration (A) and its chemical concentration (c) in 20 g dm−3 H3 BO3 in the course of uranium accumulation: (1) pH 4.5, T = 30 ◦ C; (2) pH 6.0, T = 30 ◦ C; (3) pH 8.5, T = 30 ◦ C; (4) pH 4.0, T = 30 ◦ C.
(ii) The extent of the uranium accumulation is primarily affected by the pH of the boric acid solution (i.e., by the quantity and distribution of the hydrolysis products) and also by the corrosion state (dissolution behavior) of the stainless steel surface. (In the next chapter, we shall return to a detailed interpretation of the latter statement.) (iii) The higher extent of the uranium accumulation on the inner surface of SG tubes can be detected at pH 6 of the model solution; the maximum surface excess corresponds to Γ sample = 1.22 g cm−2 . 3.2. Corrosion state and morphology of surface oxide-layer studied by voltammetry and SEM-EDX Prior to the detailed description of the dissolution characteristics of steel surfaces, it is worth mentioning that the general corrosion state, morphology and chemical composition of the heat exchanger tubes originating from different steam generators of the Paks NPP have been comprehensively studied recently [14]. In this section, only some voltammetric and SEM-EDX results characterizing the inner surface of steel samples are presented (see Figs. 4 and 5, respectively). The experimental data
Fig. 3. Time dependence of corrected activity concentration of 238 U (Ak ) and its corrected chemical concentration (ck ) in 20 g dm−3 H3 BO3 in the course of uranium accumulation: (1) pH 4.5, T = 30 ◦ C; (2) pH 6.0, T = 30 ◦ C; (3) pH 8.5, T = 30 ◦ C; (4) pH 4.04, T = 30 ◦ C.
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Fig. 4. Potentiostatic polarization curves measured at the inner surface of the tube samples studied in boric acid solution (c = 20 g dm−3 ). Scan rate: 10 mV min−1 . Curves 1–2 were obtained under identical experimental conditions on two parallel (different) steel samples.
shown in Figs. 4 and 5 in accordance with the statements in Ref. [14] reveal that the oxide-layer grown on the inner surfaces of tube specimens used in the present work is thick (the thickness is ranged between 2.5 and 5.7 m), and exhibits a “hybrid” structure of the amorphous and crystalline phases. In the outermost surface region of this film, besides amorphous Fe oxide (hydroxide), austenite and spinel phases of high Cr- and Nicontents can be found simultaneously. The inner surfaces of the tube samples have a passive character in a wide potential interval next to the corrosion potential (Ec ≈ 110 mV). The average corrosion rates for these surfaces in boric acid solution are very low (vc ≤ 0.8 m/year). In spite of the low corrosion current densities, the parallel polarization measurements performed on two different specimens reveal good reproducibility of both the Ec and vc values (see curves 1 and 2 in Fig. 4). 3.3. Investigation of the chemical composition of boric acid solutions by ICP-OES If we accept that the spontaneous passivation of stainless steel tubes is accompanied by the selective dissolution of certain alloying elements [14], a comparative study of the dissolution behavior of the main alloying components (Fe, Cr, Ni) at open circuit corrosion potential can be considered as a direct tool for the characterization of surface passivation in the pH region of sorption experiments. The maximum feature of the pH dependence of uranium accumulation shown in Fig. 1 may be interpreted on the basis of the time dependence of the dissolution of Fe, Cr, and Ni from steel surfaces during the sorption processes (Figs. 6 and 7). From the ICP-OES results summarized in Figs. 6 and 7 several implications naturally arise that: (a) The time dependence of the Fe and Cr concentrations measured in boric acid solutions at pH values of 4.5 and 6.0 (Fig. 4) differs significantly from those observed at pH 4.0 and pH 8.5 (Fig. 7). The Fe and Cr concentrations detected in the former case are definitely smaller and exhibit maximum character when plotted against time (see Fig. 6). It is to be noted that a dramatic decrease in the concentration of Fe
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Fig. 6. (a and b) Time dependence of the chemical concentrations of the corrosion products (Fe, Cr, Ni) dissolved into 20 g dm−3 H3 BO3 solution and of the pH in the course of uranium accumulation: (a) sample 1 (pH 4.5, T = 30 ◦ C); (b) sample 2 (pH 6.0, T = 30 ◦ C).
pH 8.5 exhibit saturation features (Fig. 7), and the surface excess of uranium accumulated onto steel surfaces at these pH values (Fig. 1) is significantly lower. All these results give a strong indication that the amount and the chemical forms of the Fe and Cr containing species dissolved from the steel surfaces play a determinative role in the extent and kinetics of uranium accumulation.
Fig. 5. An illustrative SEM micrograph (M = 3000×), an EDX spectrum and an image of the metallographic cross-section (at the bottom) of the tube sample, representing thickness, morphology and chemical composition of surface oxidelayer right before the adsorption experiments.
and Cr dissolved into boric acid solutions at pH values of 4.5 and 6.0 can be measured only after a period of 10 h. By taking into consideration that (i) higher extent of the uranium accumulation occurs at pH values of 4.5 and 6.0, and (ii) quasi-equilibrium surface excess of the deposited uranium is reached only after 10 h (see Fig. 1), it is immediately clear that coadsorption (coprecipitation) of the uranium species and the corrosion products containing Fe and Cr (presumably Fe and Cr hydroxides) takes place on the stainless steel surface. In contrast to this, concentration versus time curves of Fe and Cr dissolved into model solutions at pH 4.0 and
Fig. 7. (a and b) Time dependence of the chemical concentrations of the corrosion products (Fe, Cr, Ni) dissolved into 20 g dm−3 H3 BO3 solution and of the pH in the course of uranium accumulation: (a) sample 3 (pH 8.5, T = 30 ◦ C); (b) sample 4 (pH 4.04, T = 30 ◦ C).
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Fig. 8. The pH dependence of the surface excess (Γ sample ) and the corrected chemical concentration (ck ) of 238 U calculated after a sorption period of 30 h in the course of uranium accumulation. (The data presented in this figure have been derived from Figs. 1 and 3.)
(b) Beside dissolution of the Fe and Cr contents, a significant release of the Ni from the surface oxide-layer occurs in the course of the uranium accumulation. A comparison of Fig. 1 with Figs. 6 and 7 reveals that there is no direct correlation between the solution concentration of dissolved Ni and the accumulation of uranium. 4. Discussion The alpha spectrometric, voltammetric, SEM-EDX and ICPOES studies demonstrate without any doubt that the extent and the kinetics of uranium accumulation are primarily influenced by the pH of the boric acid solution (i.e., by quantity and distribution of the hydrolysis products) and also by the corrosion state and dissolution behavior of the stainless steel surface. The first statement is confirmed by the data compiled in Fig. 8. Fig. 8 provides a comparison between the pH dependency of the quasi-equilibrium surface excess (Γ sample ) and the quasiequilibrium solution concentration (ck ,) of uranium derived from Figs. 1 and 3, respectively. Under the assumption that hydrolysis and CO3 2− complexation are considered to be important factors in uranium sorption, the relative fractions of uranium species exist at various pH values of the model solution in the absence (Fig. 9) and presence of natural CO2 content (Fig. 10) were calculated from the equilibrium constants described in [7,8]. These calculations were performed by a rather complex modeling code [15] which uses speciation and solubility equilibrium relationships to calculate the concentration of different U species as well as their net solubility. Figs. 9 and 10 (in accordance with some experimental data [6,7]) indicate that the hydrolysis of uranyl cations in boric acid solution begins at pH > 3.5. Moreover, the model solution becomes oversaturated for uranyl hydroxide (schoepit, UO2 (OH)2 ·H2 O) to yield precipitate in the pH range of 4.2–10. As clearly seen from Figs. 9 and 10, roughly 90% of the uranyl hydroxide formed in the pH region of 5.5–8.0 exists as disperse (or colloidal) schoepit. The relatively higher solubility of uranium at higher pH is due to some negatively charged complexes that dominate in the solution above pH 8.0. The Γ sample versus pH relationship presented in Fig. 8 seems to be in accordance with the speciation calculations, and implies
Fig. 9. Relative fractions of uranium(VI) hydrolysis products in model solution in the absence of natural CO2 content (cU = 1 mg dm−3 , cCO3 2− = 0 mg dm−3 ).
that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. It is probable that some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 exceeds a monolayer coverage (assuming sorption of UO2 (OH)2 on geometric surface area). Additionally, the ICP-OES measurements provide direct evidence that amount and chemical forms of the Fe and Cr containing species dissolved from the steel surfaces do exert a major effect on the extent and kinetics of uranium accumulation. It is reasonable to assume that coadsorption (coprecipitation) of the uranyl hydroxide and the corrosion products containing Fe and Cr (presumably Fe and Cr hydroxides) takes place on the
Fig. 10. Relative fractions of uranium(VI) hydrolysis products in model solution with natural CO2 content (cU = 1 mg dm−3 , cCO3 2− = 60 mg dm−3 ).
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firm our assumptions that the uranium accumulation on stainless steel tubes mainly proceeds via some specific adsorption and deposition processes to yield passive oxide layer contaminated in 100 nm depth with U(VI) species. 5. Conclusion
Fig. 11. The average of XPS spectra of stainless steel surfaces (collected following each cycle of ion sputtering for sequential removal of surface layers) recorded after a sorption period of 30 h.
inner surface of the austenitic steel tubes. The surface excess of uranium at pH 8.5 is essentially smaller (see Fig. 8), which is presumably due to the decreasing amount of UO2 (OH)2 and also to the formation of some negatively charged complexes in the solution phase. As discussed in Section 1, no accumulation of negatively charged complexes can be considered at the pH values higher than the pzc (i.e., on the steel surface having negative excess charge). Finally, the question emerges of explaining the probable oxidation states of the uranium species accumulated onto steel surfaces. Considering the reduction potential of the U(VI)/U(IV) couple at studied pH interval [1,2] it cannot be ruled out that reduction reaction of the U(VI) species takes place in the course of the spontaneous passivation to yield steel surface covered by hardly soluble uranium dioxide (UO2 ). To elucidate this issue, chemical composition of the outermost films formed onto the inner surface of tube sample (number 1) during uranium accumulation was studied by XPS technique. Detailed XPS spectra were recorded after a sorption period of 30 h on “as received” sample as well as on surfaces obtained following ion sputtering. The average spectrum derived from the full scale XPS spectra on surfaces treated by ion sputtering is shown in Fig. 11. As seen in Fig. 11, the XPS peaks related to uranium can easily be identified next to the N 1s photoelectron line. The binding energy for U 4f7/2 peak found in spectra taken on “as received” sample appears at 381.7 eV. This value and the binding energy (382 eV) given for UO2 (NO3 )2 ·6H2 O in the XPS atlas of Perkin-Elmer Co. [16] are practically the same (i.e., the uncertainty of the evaluation of the binding energy is higher than the difference between the binding energy data mentioned above). It should be emphasized at this point that the binding energy values obtained for U 4f lines are independent of the dose applied by ion sputtering. Fig. 11 reveals that the center of U 4f7/2 peak detected on surfaces treated by ion sputtering is located at 380.7 eV (on the binding energy scale) which is smaller than 382 eV but higher than the values related to metallic uranium (377.2 eV). According to Ref. [16] the peak of uranium(IV) oxide is situated definitely below this value (380 eV), too. Therefore, the accumulated uranium is most likely present in the hexavalent state. All these results con-
In this paper, we have presented and discussed some experimental findings on the time and pH dependences of U accumulation obtained in a pilot plant model system using stainless steel samples of the heat exchanger tube cut out from a SG of the Paks NPP. Within the frame of the present project, an alpha spectrometric detection procedure has also been worked out which is suitable for the measurement of the activity concentrations of uranium nuclides in liquid and surface phases. The measured data imply that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. Some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 (Γ sample = 1.22 g cm−2 U ∼ = 4 × 10−9 mol cm−2 UO2 (OH)2 ) exceeds a monolayer coverage. It is reasonable to assume that coadsorption (coprecipitation) of the uranyl hydroxide and the corrosion products containing Fe and Cr (presumably Fe and Cr hydroxides) takes place on the inner surface of the austenitic steel tubes. In summary, to avoid the uranium contamination as much as possible: (i) the pH of the boric acid solution has to be below pH 4.0, or significantly higher than pH 8; (ii) the solution concentration of corrosion products containing Fe and Cr should be minimized (preferably by selective purification); and (iii) the dissolution rate of the steel surface has to be kept as minimal (i.e., ensuring that the formation of corrosion products and the reduction of U(VI) species are small). Acknowledgements This work was supported by the Paks NPP Co. Ltd. and the Hungarian Science Foundation (OTKA Grant No. 47219/2004). References [1] M. Pourbaix, Atlas of Electrochemical Equilibriums in Aqueous Solutions, Pergamon Press, Oxford, 1966. [2] G.R. Choppin, J. Rydberg, J.O. Liljenzin, Radiochemistry and Nuclear Chemistry, Butterworth-Heinemann Ltd., Oxford, 1995. [3] G.R. Choppin, Radiochim. Acta 32 (1983) 43. [4] W.R. Wadt, J. Am. Chem. Soc. 103 (1981) 6053. [5] G.F.R. Parkin (Ed.), Comprehensive Coordination Chemistry II, vol. 3, Elsevier, Amsterdam, 2004. [6] G.M. Nguyen-Trung, D. Begun, A. Palmer, Inorg. Chem. 31 (1992) 5280. [7] (a) M. Baraniak, M. Schmidt, G. Bernhard, H. Nitsche, Complex formation of hexavalent uranium with lignin degradation products, www.fz.rossendorf.de/pls/rois/cms?pNid-142, 1996; (b) C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, Wiley, New York, 1976. [8] C.L. Carnahan, Radiochim. Acta 44/45 (1987) 349.
P. Dombov´ari et al. / Electrochimica Acta 52 (2007) 2542–2551 [9] A.J. Davis, R.O. James, J.O. Leckie, J. Colloid Interface Sci. 63 (1978) 480. [10] T. Missana, M. Garcia-Gutierrez, V. Fernandez, Geochim. Cosmochim. Acta 67 (14) (2003) 2543. [11] (a) K. Varga, Z. N´emeth, J. Somlai, I. Varga, R. Sz´anth´o, J. Borsz´eki, P. Halmos, J. Schunk, P. Tilky, J. Radioanal, Nucl. Chem. 254 (3) (2002) 589; (b) K. Varga, The role of interfacial phenomena in the contamination and decontamination of nuclear reactors, in: G. Hor´anyi (Ed.), Radiotracer Studies of Interfaces, Interface Science and Technology, vol. 3, Elsevier B.V., Amsterdam, 2004, pp. 313–358. [12] (a) N.A. Talvitie, Anal. Chem. 44 (1972) 280; (b) L. Hallstadius, Nucl. Instrum. Methods 223 (1984) 266. [13] L. K¨ov´er, D. Varga, I. Cserny, J. T´oth, K. T˝ok´esi, Surf. Interface Anal. 19 (1992) 9.
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[14] (a) K. Varga, Z. N´emeth, Z. Homonnay, A. Szab´o, K. Rad´o, D. Oravetz, J. Schunk, P. Tilky, F. K˝or¨osi, J. Nucl. Mater. 348 (2006) 181; (b) Z. Homonnay, E. Kuzmann, S. Stichleutner, K. Varga, Z. N´emeth, A. ´ Mak´o, L. K¨ov´er, I. Cserny, D. Varga, J. T´oth, J. Szab´o, K. Rad´o, K.E. Schunk, P. Tilky, G. Patek, J. Nucl. Mater. 348 (2006) 191; (c) A. Szab´o, K. Varga, Z. N´emeth, K. Rad´o, D. Oravetz, K.E. Mak´o, Z. Homonnay, E. Kuzmann, P. Tilky, J. Schunk, G. Patek, Corrosion Sci. 48 (2006) 2727. [15] L. Z´ek´any, I. Nagyp´al, in: D.J. Legget (Ed.), Computational Methods for Determination of Formation Constants, Plenum Press, New York, 1985 (Chapter 8). [16] J. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Co., 1992.
Accumulation of uranium on austenitic stainless steel surfaces P´eter Dombov´ari a , P´eter K´ad´ar a , Tibor Kov´acs a , J´anos Somlai a , Kriszti´an Rad´o a , Istv´an Varga a , Ren´ata Buj´ak a , K´alm´an Varga a,∗ , P´al Halmos b , J´anos Borsz´eki c , J´ozsef K´onya d , No´emi M. Nagy d , L´aszl´o K¨ov´er e , Dezs˝o Varga e , Istv´an Cserny e , J´ozsef T´oth e , Lajos Fodor f , Attila Horv´ath f , Tam´as Pint´er g , J´anos Schunk g a
Department of Radiochemistry, Pannon University, H-8201 Veszpr´em, P.O. Box 158, Hungary Analytical Chemistry Research Group, Hungarian Academy of Sciences, Veszpr´em, Hungary c Department of Analytical Chemistry, Pannon University, Veszpr´ em, Hungary d Department of Colloid- and, Environmental Chemistry, Isotope Laboratory, University of Debrecen, Debrecen, Hungary e Section of Electron Spectroscopy and Materials Science, Institute of Nuclear Research of the Hungarian Academy of Sciences (MTA ATOMKI), P.O. Box 51, H-4001 Debrecen, Hungary f Department of General and Inorganic Chemistry, Pannon University, Veszpr´ em, Hungary g Paks NPP Ltd., Paks, Hungary b
Received 25 August 2006; received in revised form 1 September 2006; accepted 1 September 2006 Available online 10 October 2006
Abstract The surface contamination by uranium in the primary circuit of PWR type nuclear reactors is a fairly complex problem as (i) different chemical forms (molecular, colloidal and/or disperse) of the uranium atoms can be present in the boric acid coolant, and (ii) only limited pieces of information about the extent, kinetics and mechanism of uranium accumulation on constructional materials are available in the literature. A comprehensive program has been initiated in order to gain fundamental information about the uranium accumulation onto the main constituents of the primary cooling circuit (i.e., onto austenitic stainless steel type 08X18H10T (GOSZT 5632-61) and Zr(1%Nb) alloy). In this paper, some experimental findings on the time and pH dependences of U accumulation obtained in a pilot plant model system are presented and discussed. The surface excess, oxidation state and chemical forms of uranium species sorbed on the inner surfaces of the stainless steel tubes of steam generators have been detected by radiotracer (alpha spectrometric), ICP-OES and XPS methods. In addition, the passivity, morphology and chemical composition of the oxide-layers formed on the studied surfaces of steel specimens have been analyzed by voltammetry and SEM-EDX. The experimental data imply that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. Some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 (Γ sample = 1.22 g cm−2 U∼ = 4 × 10−9 mol cm−2 UO2 (OH)2 ) exceeds a monolayer coverage. © 2006 Elsevier Ltd. All rights reserved. Keywords: Uranium; Accumulation; Stainless steel; Alpha spectrometry; XPS; ICP-OES; Voltammetry; SEM-EDX
1. Introduction 1.1. Objective As a consequence of the breakdown on April 10, 2003 there was a noteworthy uranium release (about 4 kg) and contamination in some technological units of the reactor block 2 of the
∗
Corresponding author. Tel.: +36 88 427 681; fax: +36 88 427 681. E-mail address: [email protected] (K. Varga).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.09.007
Paks Nuclear Power Plant (Paks NPP). Owing to the effective purification of the boric acid coolant the amount of uranium species exists at present time in the primary cooling circuit is very low. On analyzing the experimental data, it has become obvious that the limited pieces of information about the extent, chemical forms and kinetic behaviors of the uranium accumulated on the surface of the main structural materials (stainless steels, Zr–Nb alloys) of the primary circuit may cause difficulties in the better understanding of the transport processes and in the elaboration of effective decontamination procedures. The fact is that the uranium nuclides can be present in different chemical
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forms (molecular, colloidal and/or disperse) in the boric acid coolant enhances the complexity of the problem. In light of the above facts, a comprehensive program has been initiated in order to gain fundamental information about the uranium accumulation onto the main constituents of the primary cooling circuit; (i.e., onto austenitic stainless steel type 08X18H10T (GOSZT 5632-61) and Zr(1%Nb) alloy). In this paper, we give a brief summary of the uranium chemistry in boric acid solutions, as well as present and discuss some experimental findings concerning the time and pH dependences of U accumulation obtained in a pilot plant model system using stainless steel samples of the heat exchanger tube cut out from a steam generator (SG) of the Paks NPP. Within the frame of the present project, an alpha spectrometric detection procedure has also been worked out which is suitable for the measurement of the activity concentrations of uranium nuclides in liquid and surface phases. In addition, the main corrosion products (Fe, Cr and Ni) dissolved from steel samples into the boric acid solution in the course of the sorption experiments have been determined by ICP Optical Emission Spectroscopic (ICP-OES) method. Moreover, special attention has been paid to report some voltammetric and SEM-EDX results that reveal the corrosion properties, morphology and chemical composition of the oxide-layer formed on the studied surfaces of steel specimens. To obtain the more complex features of U accumulation, we have analyzed the oxidation state and chemical forms of uranium sorbed on the inner surfaces of stainless steel tubes by X-ray Photoelectron Spectroscopy (XPS). 1.2. Fundamental aspects of uranium chemistry in boric acid solution Before proceeding to a detailed analysis of the uranium accumulation on steel surfaces, it is useful to review some of its fundamental chemical properties in boric acid solution. The chemical behaviors of uranium species in boric acid solution can be influenced by several factors such as the inclination of uranium towards complex formation, the hydrolytic processes (that can frequently result in the formation of polymer ions) and the redox properties [1,2]. In aqueous solutions the uranyl cation (UO2 2+ ) is the most stable uranium species. The main characterizing data of uranyl cation can be described as follows: (i) Its chemical structure is linear and symmetric: (O U O)2+ (owing to the role of 5f atomic orbital in the formation of the chemical bond). The hexavalent U atom in the linear (O U O)2+ is only partially shielded by the two oxygen atoms; thus in the equatorial plane the effective charge of the uranium may be more than 3 (ca. 3.3 ± 0.1) [2–4]. (ii) UO2 2+ can coordinate several donor atoms in the equatorial plane and hereby can produce wide variety of complexes [2,5]. As uranyl cations, like other actinide ions, are strong acids, the electrostatic model can be used to describe their chemical bonds in the above complexes. Moreover, uranyl ions display strong preference to oxygen donors; accordingly, the chemical bonds formed exhibit predominantly ionic character [5].
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Independent literature data [6–8] reveal that the presence of uranyl cations and their interactions with anionic particles (formed from boric acid) should be considered as primary factors affecting the speciation in aqueous boric acid solutions. The complexation by boric acid is influenced by both pH and chemical concentration. At this point, it should be emphasized that there are no data available in the relevant literature about the interaction between uranyl cations and monomer (or polymer) borate ions. Therefore, it is unlikely that uranyl cations can form complexes with borate species in boric acid solutions. A careful inspection of the structure of the uranyl cation and B(OH)4 − borate ion may give a plausible explanation of the latter assumption. Namely the mono-borate anion has a symmetric tetrahedron structure like perchlorate anion, which is unable to form high stability complexes with metal ions, too. As we were not able to find any experimental evidences about uranium borate complexes, an attempt was made to collect information about the uranium perchlorate complexes that have a similar structure to the borate compounds. On studying the related references, it has been realized that there is no spectroscopic evidence of the formation of the uranyl perchlorate complex. It is to be noted that creation of some weak UO2 –ClO4 complex cannot be excluded in the presence of the great excess of perchlorate ions; however, its stability factor must be smaller than 10−1.1 (if any) [6]. All these data attest that the formation of uranyl borate complexes cannot affect notably the speciation of uranium nuclides in aqueous boric acid solutions. The hydrolysis of uranyl ion is an especially important factor in aqueous solutions. The references on this subject [6,7] provide evidences on the formation of the following complexes: - mononuclear complexes: UO2 (OH)+ , UO2 (OH)2 , UO2 (OH)3 − , UO2 (OH)4 2− . - polynuclear complexes: (UO2 )2 (OH)2 2+ , (UO2 )4 (OH)7 + , (UO2 )3 (OH)4 2+ , (UO2 )3 (OH)5 + , (UO2 )3 (OH)7 − . Besides the hydrolytic processes the strong carbonate complexation producing carbonate (UO2 CO3 , UO2 (CO3 )2 2− , UO2 (CO3 )3 4− ) and mixed hydroxo carbonate ((UO2 )2 CO3 (OH)3 − ) complexes is also considered, especially in geological environments [8]. It is of special interest to emphasize that CO2 in the air can also take part in the formation of the above carbonate and hydroxo carbonate complexes. Independent speciation calculations supporting some experimental data [6,7] reveal that the hydrolysis of uranyl cations in aqueous solutions begins at pH > 3.5. Moreover, at open to air condition the formation of hydroxo carbonate complexes commences at pH ≈ 5.5. Simultaneously, the solution may become oversaturated and uranyl hydroxide (schoepit) is precipitated in the pH range of 5.3–7.3. The relatively high solubility of uranium at higher pH is due to the negatively charged carbonate complexes dominating in the solution at pH > 8. When studying the fundamental aspects of the solution chemistry of uranium, numerous experiments have been carried out in order to gain a deeper insight into the sorption of uranyl species on solid surfaces (mainly in geological environment). Although a limited number of publications deal with uranium sorption on
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steel surfaces, the results of these sorption experiments could be useful during the estimation of the extent and character of U sorption on steel surfaces. The sorption behavior of uranyl species may be interpreted by the surface complexation model [9]. This model was firstly developed for oxide surfaces and later utilized for minerals and rocks, too. The model is based on the assumption that surface OH− sites are formed on the oxide/water interface and they are considered as proton donors as well as proton acceptors, respectively, depending on pH. Consequently, these surface sites are positively or negatively charged, and electrostatically interact with anions or cations in the aqueous solution. Both the chemical form of the species present in the solution and the charged active sites on surfaces play significant role in the model. If no specific adsorption phenomena occur, the electrostatic interactions (non-specific adsorption processes) between the oppositely charged surface and solution species determine the formation of surface complexes. The surface complexation model can explain the accumulation of uranium species on Fe oxide surfaces (see [10] and the references cited therein). Specifically, the extent of the sorption is primarily affected by at least two factors such as (i) the corrosion state (and dissolution behavior) of the Fe oxide surface influencing the active sites on the surface, and (ii) the fractions (and solubility) of uranium species in the solution. When pH is smaller than pzc (i.e., pH value of the zero surface charge), Fe oxide surface is positively charged, and the accumulation of anions can primarily be considered. On the other hand, at pH values above pzc, a significant cation adsorption takes place on the surface. In summary, when comparing the charge of uranium species with the surface charge of the adsorbent, the extent and character of the U accumulation can be estimated. However, it is important to note that the specific adsorption and deposition (colloidal and/or disperse particles) processes should also be taken into consideration. 2. Experimental 2.1. Experimental setup, chemicals and procedures A pilot plant model system elaborated earlier [11] was utilized for the dynamic studies of the accumulation processes on inner surfaces of the SG tube specimens provided by the Paks NPP. All parts of the circulation system were made of austenitic stainless steel (type: DIN 1.4541). A centrifugal pump (commercial available) with the smallest inner volume (18 cm3 ) was built into the circuit to provide the smallest available ration of the solution volume to the treated surface. In the present experiments, the above centrifugal pump equipped with a pump drive
unit was used to maintain the stable and long lasting (more than 30 h) circulation of the high temperature (up to 95 ◦ C) boric acid solution at a linear flow rate of 3.0 m s−1 . The uranium accumulation was studied by the so-called “solution” radioactive tracer method [11]. Specifically, the ex situ measurement of uranium was carried out by determining the radioactivity in aliquot volumes of a model solution of the primary coolant being in contact with the inner surfaces of the stainless steel tubes (type: 08X18H10T (GOST 5632-61), outer diameter: 16 mm, average wall thickness: 1.6 mm) originating from the SG of the reactor block 2 at Paks NPP. Main characteristics of the tube specimens as well as of the sorption experiments are summarized in Table 1. Following various time periods (1, 3, 5, 10, 20 and 30 h) 5-5 cm3 samples were taken from the solutions for analysis in each sorption experiment. After all samplings 5-5 cm3 volumes of boric acid solution containing 1 mg dm−3 238 U (at the given pH) were added into the circulation system to keep up the total solution volume of 90 cm3 . The activity concentrations (and chemical concentrations) of uranium in the solution samples were detected by alpha spectrometry. To improve the reliability of the “solution” method, activity of uranium accumulated onto the inner surface of the tube specimens during a period of 30 h was directly measured by alpha spectrometry. In addition, ICP-OES method was utilized to measure the chemical concentration of the main alloying components (Fe, Cr, Ni) dissolved from the surface into the solution during the U accumulation. Moreover, the oxidation state of the uranium sorbed onto the inner surface of the steel tube from model solution of pH 4.5 was identified with XPS. From the original tube samples, specimens of 20 mm were cut to be used for voltammetric and SEM-EDX studies. The tube pieces were cut into two halves along their axes and then – for the voltammetric studies only – flattened gently. In order to protect the oxide-layer on the specimens from possible chemical effect of organic solvents, the surfaces were not degreased. The corrosion properties of the stainless steel samples prepared by the above technique were studied mainly on their original inner surfaces. The metallographic cross-sections of tube specimens were prepared and analyzed by SEM-EDX method, too. 2.2. Analytical 2.2.1. Alpha spectrometric study of surface and electrolytes 2.2.1.1. Study of the uranium contamination on heat exchanger tubes. Following the 30 h period of uranium accumulation the 238 U activity on the flattened surface (2 cm long parts) of steel tube specimens was detected by alpha spectrometric method. For this measurement, in each sorption experiment 2-2 parallel
Table 1 Main characteristics of the sorption experiments Sample number
Composition of the model solution
Solution pH
SG tube specimens
Temperature (◦ C)
1 2 3 4
20 g dm−3 H3 BO3 + 1 mg dm−3 U 20 g dm−3 H3 BO3 + 1 mg dm−3 U 20 g dm−3 H3 BO3 + 1 mg dm−3 U 20 g dm−3 H3 BO3 + 1 mg dm−3 U
4.5 6.0 8.5 4.0
Length: 13.0 cm, inner surface area: 53.1 cm2 Length: 13.0 cm, inner surface area: 53.1 cm2 Length: 13.0 cm, inner surface area: 53.1 cm2 Length: 13.0 cm, inner surface area: 53.1 cm2
30 30 30 30
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samples having 4 cm2 of detectable surface area were used. Before activity measurements the steel samples were heated to 400 ◦ C for 30–60 min. The inner surfaces were detected in vacuum for 80,000 s using a semiconductor PIPS alpha spectrometric device (chamber: Oxford-Tennelec, detector: Eurisys PIPS detector, resolution: 19 keV, multichannel analyzer: Silena9302, data collecting software: EMCA-2000). This apparatus was calibrated by applying alpha calibration sources of 239 Pu, 241 Am and 244 Cm. The FWHM of the measured peaks was in the region of 60–80 keV, and the minimum detectable activity (MDA) was of about 3 mBq. The relative error of the measurement was estimated to be less than ±10%. Since the sample surfaces were originally contaminated by significant amount of alpha emitting nuclides (such as 241 Am and 238,239,240 Pu) which did exert a disturbing effect on the detection of the small amount of uranium, the oxide-layer had to be dissolved from the inner surface of the tube specimens. Owing to the passivity of the oxide film diluted sulfuric acid was not efficient for the above purpose; however, by using a mixture of hydrochloric acid and hydrogen peroxide the oxide-layer was dissolved completely. As the iron concentration in the solution was too high, Eichrom’s UTEVA type ion exchange resin was utilized for the purification the solution. As a consequence of the ion exchange procedure, the treated solution became rich in uranium and practically free of iron. (Later we shall return to a more detailed description of the ion exchange and sample preparation methods.) 2.2.1.2. Alpha spectrometric analysis of boric acid solution. In the course of the uranium accumulation, 5-5 cm3 samples were taken from boric acid solutions at the end of various adsorption periods. A part (3 cm3 ) was separated from each 5 cm3 solution sample to perform isotope specific sample treatment and to prepare alpha sources. In order to increase the accuracy of measurements 232 U internal tracer was added to the solution at the beginning of the sample treatment. Also, Fe(III) chloride and ammonia were introduced into the traced sample to form iron hydroxide precipitate which was capable of interacting with uranium nuclides effectively from the solution. The uranium reach precipitate was then filtered through a 0.45 m membrane and dissolved in 9 mol dm−3 HCl. The separation of uranium isotopes is based on selective bonding of the dominant uranium species on ion exchange resins. Owing to the high Fe content in the solutions, Eichrom’s UTEVA type resin having uranium specific organic material coating on the resin particles was used for uranium separation. (The UTEVA type resin (10–150 m) was conditioned with 8 mol dm−3 HNO3 and loaded into the columns of 2 cm3 volume.) After a preseparation by successive evaporation and dissolution the U containing sample was finally dissolved in 8 mol dm−3 HNO3 and introduced into the columns of ion exchange resins. Finally, 0.02 mol dm−3 HCl (40 cm3 ) was used to elute uranium species from the columns. After the ion exchange procedure the pH of the uranium containing solutions was set to 2.3, and alpha sources were prepared on stainless steel coupons by electrodeposition, as described in [12]. Following uranium electrodeposition the steel coupons were heated (to remove Po isotopes) and measured in vacuum
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for 80,000 s by the alpha spectrometric device outlined above. As a consequence of the above separation procedure the resolution of the alpha spectra became considerably better than earlier. During the 80,000 s measurements of the alpha sources the minimum detectable activity (MDA) was as small as 0.34 mBq. The relative error of the measurement was estimated to be less than ±10%. 2.2.2. Analysis of the chemical composition of boric acid solutions by ICP-OES During the uranium accumulation on the inner surface of tube specimens, the concentrations of the main alloying components (Fe, Cr, and Ni) dissolved from the surface oxide-layer into the solution were determined by an ICP-OES device (type GBC Integra XM). For this elemental analysis, solution samples having a volume of 2 cm3 were separated from the 5 cm3 boric acid samples taken after various time periods (1, 3, 5, 10, 20 and 30 h) in each adsorption series. The solution samples were stored in closed polyethylene vials. 2.2.3. XPS analysis of uranium accumulated on the surface of steel tubes The chemical composition of the outermost films formed on the inner surface of tube sample (number 1) during uranium accumulation was studied by X-ray photoelectron spectroscopy (XPS). The XPS measurements were carried out by the ESA-31 (ATOMKI, Hungary) type spectrometer [13] on “as received” samples as well as on surfaces obtained following sequential removal of the outermost surface layers by ion sputtering. For these studies, rectangular (10 mm × 10 mm) pieces of austenitic stainless steel were cut out from the tube specimens without mechanical deformation. Al K␣ radiation with energy of 1486.6 eV was used as the primary X-ray source. The sputtering was performed with 2 keV argon ions (the ion current density was 140 A/cm2 ) at a rate of about 1.4 nm/min. The total thickness of the oxide film removed from the surface of each sample by Ar ion sputtering was ca. 105 nm. The photoelectrons analyzed were emitted in a narrow solid angle around the normal vector of the sample surface. In the course of the XPS measurements the vacuum around the sample was about 10−7 Pa. The fix retarding ratios of R = 4 (for general spectra) and R = 8 (for detailed spectrums) were used to provide good intensity and energy resolution of about 1 eV. The energy calibration of the spectrometer was performed using photolines of Ag, Au, and Cu metal standards. 2.2.4. Investigation of the corrosion state of tube specimens by voltammetry The passivity of the inner surface of tube specimens was studied by potentiostatic polarization method. The experiments were carried out by the means of a VoltaLab 40 (RADIOMETER) type electrochemical measuring system controlled by PC. To perform these investigations a special electrochemical cell was developed. In the course of potentiostatic polarization experiments the potential (E) of the specimen (working electrode) was continuously shifted towards anodic direction at a constant rate of 10 mV min−1 and the current density (i) related to the
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inner surface area of the specimen was recorded. The measurements were carried out in boric acid solution (c = 20 g dm−3 ) in argon gas atmosphere (99.999, v/v% Ar). The schematic of the measuring system, the detailed experimental procedure and the determination of the corrosion parameters (such as corrosion potential (Ec ), corrosion current density (ic ), and corrosion rate (vc )) derived by so-called Stern method have been described in our earlier papers [14]. The electrode potential values quoted in this paper are given on the saturated calomel electrode (SCE) scale. 2.2.5. Study of the surfaces by SEM-EDX method The morphology and chemical composition of the oxidelayer developed on the inner surfaces of the stainless steel specimens were studied by scanning electron microscopy (SEM), equipped with an energy dispersive X-ray microanalyzer (EDX) (Type: JEOL JSM-50A, controlled with R¨ontec EDR 288 software). In case of each specimen at least two different surface areas were studied by making use of the combined SEM-EDX equipment. The comparative evaluation of surface morphology was performed by analyzing the SEM micrographs obtained at two different magnifications, M = 3000 and M = 1000, respectively. The chemical composition of the sample surfaces was determined on two different areas of 1 mm2 by EDX method [14]. In addition, the metallographic cross-sections of tube specimens were also studied. 3. Results 3.1. Alpha spectrometric study of surface and boric acid solutions By applying our version of the so-called “solution” technique, it has become obvious that the quantitative evaluation of surface excess (Γ ) values involves the optimization of the experimental and nuclear detection conditions (see Section 2) as well as the elaboration of a reliable calculation procedure. To elucidate the latter issue, we should recall some key features of the radiotracer method. The dynamic sorption measurements were carried out in a pilot plant model system at a solution volume of 90 cm3 . Following various time periods (1, 3, 5, 10, 20 and 30 h) 5-5 cm3 samples were taken from the solution for analysis in each sorption experiment. At all samplings 5-5 cm3 volumes of boric acid solution containing 1 mg dm−3 238 U were added into the circulation system to keep up the total solution volume. As a consequence of the above experimental procedure, it was of special importance to evaluate the corrected chemical concentration (ck ,i ) as well as the corrected activity concentration (Ak ,i ) of 238 U nuclides present in the boric acid solution after sampling number ‘i’. The cki and Aki values were computed by Eqs. (1) and (2), respectively: 85 5 + c0 90 90 85 5 + A0 = Ai 90 90
ck,i = ci Ak,i
where ci is the chemical concentration of 238 U measured in the boric acid solution at sampling number ‘i’ (mg dm−3 ), c0 the initial chemical concentration of 238 U in the boric acid solution before the sorption experiments (c0 = 1 mg dm−3 ), Ai the activity concentration measured in the boric acid solution at sampling number ‘i’ (mBq cm−3 ); A0 is the initial activity concentration of 238 U in the boric acid solution (A = 11.82 mBq cm−3 , as activity 0 of 1 mg 238 U corresponds to 11.82 Bq). In addition, it should be emphasized that: (i) At the first sampling the change of the chemical concentration as well as of the activity concentration corresponds to c0 − c1 , and A0 − A1 , respectively. Later on, at the consecutive samplings the above values are considered as ck(i − 1) − ci , and Ak(i − 1) − Ai , respectively. (ii) At each sampling the amount (or activity) of 238 U accumulated onto the inner surface of tube specimens in the recirculation system is provided by the product of the chemical concentration (or activity concentration) change and the volume of the model system (90 cm3 ). The cumulative amount (or cumulative activity) of the adsorbed uranium can be calculated as the sum of the values obtained at different samplings. Consequently, we provide the surface excess data as the cumulative amount of sorbed uranium related to the geometric surface area of tube specimens (see Table 1). In light of the above considerations, the pH and time dependence curves of uranium accumulation (Fig. 1) evaluated from the alpha spectrometric measurements of the solutions and steel surfaces (Fig. 2) are presented in Figs. 1–2. In addition, Ak (and ck ) versus time plots are compiled in Fig. 3. The experimental results shown in Figs. 1–3 reveal that: (i) A highly pH and time dependent accumulation of U species takes place on the inner surface of heat exchanger tubes in the model solution of the primary coolant (20 g dm−3 H3 BO3 containing 1 mg dm−3 238 U species). The quasiequilibrium surface excess of the deposited uranium is reached only after a period of 10 h.
(1) (2)
Fig. 1. Time dependence of 238 U accumulation on the inner surface of stainless steel tubes in solution containing 20 g dm−3 H3 BO3 + 1 mg dm−3 U: (1) pH 4.5, T = 30 ◦ C; (2) pH 6.0, T = 30 ◦ C; (3) pH 8.5, T = 30 ◦ C; (4) pH 4.0, T = 30 ◦ C.
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Fig. 2. Time dependence of measured 238 U activity concentration (A) and its chemical concentration (c) in 20 g dm−3 H3 BO3 in the course of uranium accumulation: (1) pH 4.5, T = 30 ◦ C; (2) pH 6.0, T = 30 ◦ C; (3) pH 8.5, T = 30 ◦ C; (4) pH 4.0, T = 30 ◦ C.
(ii) The extent of the uranium accumulation is primarily affected by the pH of the boric acid solution (i.e., by the quantity and distribution of the hydrolysis products) and also by the corrosion state (dissolution behavior) of the stainless steel surface. (In the next chapter, we shall return to a detailed interpretation of the latter statement.) (iii) The higher extent of the uranium accumulation on the inner surface of SG tubes can be detected at pH 6 of the model solution; the maximum surface excess corresponds to Γ sample = 1.22 g cm−2 . 3.2. Corrosion state and morphology of surface oxide-layer studied by voltammetry and SEM-EDX Prior to the detailed description of the dissolution characteristics of steel surfaces, it is worth mentioning that the general corrosion state, morphology and chemical composition of the heat exchanger tubes originating from different steam generators of the Paks NPP have been comprehensively studied recently [14]. In this section, only some voltammetric and SEM-EDX results characterizing the inner surface of steel samples are presented (see Figs. 4 and 5, respectively). The experimental data
Fig. 3. Time dependence of corrected activity concentration of 238 U (Ak ) and its corrected chemical concentration (ck ) in 20 g dm−3 H3 BO3 in the course of uranium accumulation: (1) pH 4.5, T = 30 ◦ C; (2) pH 6.0, T = 30 ◦ C; (3) pH 8.5, T = 30 ◦ C; (4) pH 4.04, T = 30 ◦ C.
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Fig. 4. Potentiostatic polarization curves measured at the inner surface of the tube samples studied in boric acid solution (c = 20 g dm−3 ). Scan rate: 10 mV min−1 . Curves 1–2 were obtained under identical experimental conditions on two parallel (different) steel samples.
shown in Figs. 4 and 5 in accordance with the statements in Ref. [14] reveal that the oxide-layer grown on the inner surfaces of tube specimens used in the present work is thick (the thickness is ranged between 2.5 and 5.7 m), and exhibits a “hybrid” structure of the amorphous and crystalline phases. In the outermost surface region of this film, besides amorphous Fe oxide (hydroxide), austenite and spinel phases of high Cr- and Nicontents can be found simultaneously. The inner surfaces of the tube samples have a passive character in a wide potential interval next to the corrosion potential (Ec ≈ 110 mV). The average corrosion rates for these surfaces in boric acid solution are very low (vc ≤ 0.8 m/year). In spite of the low corrosion current densities, the parallel polarization measurements performed on two different specimens reveal good reproducibility of both the Ec and vc values (see curves 1 and 2 in Fig. 4). 3.3. Investigation of the chemical composition of boric acid solutions by ICP-OES If we accept that the spontaneous passivation of stainless steel tubes is accompanied by the selective dissolution of certain alloying elements [14], a comparative study of the dissolution behavior of the main alloying components (Fe, Cr, Ni) at open circuit corrosion potential can be considered as a direct tool for the characterization of surface passivation in the pH region of sorption experiments. The maximum feature of the pH dependence of uranium accumulation shown in Fig. 1 may be interpreted on the basis of the time dependence of the dissolution of Fe, Cr, and Ni from steel surfaces during the sorption processes (Figs. 6 and 7). From the ICP-OES results summarized in Figs. 6 and 7 several implications naturally arise that: (a) The time dependence of the Fe and Cr concentrations measured in boric acid solutions at pH values of 4.5 and 6.0 (Fig. 4) differs significantly from those observed at pH 4.0 and pH 8.5 (Fig. 7). The Fe and Cr concentrations detected in the former case are definitely smaller and exhibit maximum character when plotted against time (see Fig. 6). It is to be noted that a dramatic decrease in the concentration of Fe
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Fig. 6. (a and b) Time dependence of the chemical concentrations of the corrosion products (Fe, Cr, Ni) dissolved into 20 g dm−3 H3 BO3 solution and of the pH in the course of uranium accumulation: (a) sample 1 (pH 4.5, T = 30 ◦ C); (b) sample 2 (pH 6.0, T = 30 ◦ C).
pH 8.5 exhibit saturation features (Fig. 7), and the surface excess of uranium accumulated onto steel surfaces at these pH values (Fig. 1) is significantly lower. All these results give a strong indication that the amount and the chemical forms of the Fe and Cr containing species dissolved from the steel surfaces play a determinative role in the extent and kinetics of uranium accumulation.
Fig. 5. An illustrative SEM micrograph (M = 3000×), an EDX spectrum and an image of the metallographic cross-section (at the bottom) of the tube sample, representing thickness, morphology and chemical composition of surface oxidelayer right before the adsorption experiments.
and Cr dissolved into boric acid solutions at pH values of 4.5 and 6.0 can be measured only after a period of 10 h. By taking into consideration that (i) higher extent of the uranium accumulation occurs at pH values of 4.5 and 6.0, and (ii) quasi-equilibrium surface excess of the deposited uranium is reached only after 10 h (see Fig. 1), it is immediately clear that coadsorption (coprecipitation) of the uranium species and the corrosion products containing Fe and Cr (presumably Fe and Cr hydroxides) takes place on the stainless steel surface. In contrast to this, concentration versus time curves of Fe and Cr dissolved into model solutions at pH 4.0 and
Fig. 7. (a and b) Time dependence of the chemical concentrations of the corrosion products (Fe, Cr, Ni) dissolved into 20 g dm−3 H3 BO3 solution and of the pH in the course of uranium accumulation: (a) sample 3 (pH 8.5, T = 30 ◦ C); (b) sample 4 (pH 4.04, T = 30 ◦ C).
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Fig. 8. The pH dependence of the surface excess (Γ sample ) and the corrected chemical concentration (ck ) of 238 U calculated after a sorption period of 30 h in the course of uranium accumulation. (The data presented in this figure have been derived from Figs. 1 and 3.)
(b) Beside dissolution of the Fe and Cr contents, a significant release of the Ni from the surface oxide-layer occurs in the course of the uranium accumulation. A comparison of Fig. 1 with Figs. 6 and 7 reveals that there is no direct correlation between the solution concentration of dissolved Ni and the accumulation of uranium. 4. Discussion The alpha spectrometric, voltammetric, SEM-EDX and ICPOES studies demonstrate without any doubt that the extent and the kinetics of uranium accumulation are primarily influenced by the pH of the boric acid solution (i.e., by quantity and distribution of the hydrolysis products) and also by the corrosion state and dissolution behavior of the stainless steel surface. The first statement is confirmed by the data compiled in Fig. 8. Fig. 8 provides a comparison between the pH dependency of the quasi-equilibrium surface excess (Γ sample ) and the quasiequilibrium solution concentration (ck ,) of uranium derived from Figs. 1 and 3, respectively. Under the assumption that hydrolysis and CO3 2− complexation are considered to be important factors in uranium sorption, the relative fractions of uranium species exist at various pH values of the model solution in the absence (Fig. 9) and presence of natural CO2 content (Fig. 10) were calculated from the equilibrium constants described in [7,8]. These calculations were performed by a rather complex modeling code [15] which uses speciation and solubility equilibrium relationships to calculate the concentration of different U species as well as their net solubility. Figs. 9 and 10 (in accordance with some experimental data [6,7]) indicate that the hydrolysis of uranyl cations in boric acid solution begins at pH > 3.5. Moreover, the model solution becomes oversaturated for uranyl hydroxide (schoepit, UO2 (OH)2 ·H2 O) to yield precipitate in the pH range of 4.2–10. As clearly seen from Figs. 9 and 10, roughly 90% of the uranyl hydroxide formed in the pH region of 5.5–8.0 exists as disperse (or colloidal) schoepit. The relatively higher solubility of uranium at higher pH is due to some negatively charged complexes that dominate in the solution above pH 8.0. The Γ sample versus pH relationship presented in Fig. 8 seems to be in accordance with the speciation calculations, and implies
Fig. 9. Relative fractions of uranium(VI) hydrolysis products in model solution in the absence of natural CO2 content (cU = 1 mg dm−3 , cCO3 2− = 0 mg dm−3 ).
that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. It is probable that some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 exceeds a monolayer coverage (assuming sorption of UO2 (OH)2 on geometric surface area). Additionally, the ICP-OES measurements provide direct evidence that amount and chemical forms of the Fe and Cr containing species dissolved from the steel surfaces do exert a major effect on the extent and kinetics of uranium accumulation. It is reasonable to assume that coadsorption (coprecipitation) of the uranyl hydroxide and the corrosion products containing Fe and Cr (presumably Fe and Cr hydroxides) takes place on the
Fig. 10. Relative fractions of uranium(VI) hydrolysis products in model solution with natural CO2 content (cU = 1 mg dm−3 , cCO3 2− = 60 mg dm−3 ).
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firm our assumptions that the uranium accumulation on stainless steel tubes mainly proceeds via some specific adsorption and deposition processes to yield passive oxide layer contaminated in 100 nm depth with U(VI) species. 5. Conclusion
Fig. 11. The average of XPS spectra of stainless steel surfaces (collected following each cycle of ion sputtering for sequential removal of surface layers) recorded after a sorption period of 30 h.
inner surface of the austenitic steel tubes. The surface excess of uranium at pH 8.5 is essentially smaller (see Fig. 8), which is presumably due to the decreasing amount of UO2 (OH)2 and also to the formation of some negatively charged complexes in the solution phase. As discussed in Section 1, no accumulation of negatively charged complexes can be considered at the pH values higher than the pzc (i.e., on the steel surface having negative excess charge). Finally, the question emerges of explaining the probable oxidation states of the uranium species accumulated onto steel surfaces. Considering the reduction potential of the U(VI)/U(IV) couple at studied pH interval [1,2] it cannot be ruled out that reduction reaction of the U(VI) species takes place in the course of the spontaneous passivation to yield steel surface covered by hardly soluble uranium dioxide (UO2 ). To elucidate this issue, chemical composition of the outermost films formed onto the inner surface of tube sample (number 1) during uranium accumulation was studied by XPS technique. Detailed XPS spectra were recorded after a sorption period of 30 h on “as received” sample as well as on surfaces obtained following ion sputtering. The average spectrum derived from the full scale XPS spectra on surfaces treated by ion sputtering is shown in Fig. 11. As seen in Fig. 11, the XPS peaks related to uranium can easily be identified next to the N 1s photoelectron line. The binding energy for U 4f7/2 peak found in spectra taken on “as received” sample appears at 381.7 eV. This value and the binding energy (382 eV) given for UO2 (NO3 )2 ·6H2 O in the XPS atlas of Perkin-Elmer Co. [16] are practically the same (i.e., the uncertainty of the evaluation of the binding energy is higher than the difference between the binding energy data mentioned above). It should be emphasized at this point that the binding energy values obtained for U 4f lines are independent of the dose applied by ion sputtering. Fig. 11 reveals that the center of U 4f7/2 peak detected on surfaces treated by ion sputtering is located at 380.7 eV (on the binding energy scale) which is smaller than 382 eV but higher than the values related to metallic uranium (377.2 eV). According to Ref. [16] the peak of uranium(IV) oxide is situated definitely below this value (380 eV), too. Therefore, the accumulated uranium is most likely present in the hexavalent state. All these results con-
In this paper, we have presented and discussed some experimental findings on the time and pH dependences of U accumulation obtained in a pilot plant model system using stainless steel samples of the heat exchanger tube cut out from a SG of the Paks NPP. Within the frame of the present project, an alpha spectrometric detection procedure has also been worked out which is suitable for the measurement of the activity concentrations of uranium nuclides in liquid and surface phases. The measured data imply that the uranium sorption is significant in the pH range of 4–8 where the intense hydrolysis of uranyl cations in boric acid solution can be observed. Some specific adsorption and deposition of (mainly colloidal and disperse) uranyl hydroxide to be formed in the solution prevail over the accumulation of other U(VI) hydroxo complexes. The maximum surface excess of uranium species measured at pH 6 (Γ sample = 1.22 g cm−2 U ∼ = 4 × 10−9 mol cm−2 UO2 (OH)2 ) exceeds a monolayer coverage. It is reasonable to assume that coadsorption (coprecipitation) of the uranyl hydroxide and the corrosion products containing Fe and Cr (presumably Fe and Cr hydroxides) takes place on the inner surface of the austenitic steel tubes. In summary, to avoid the uranium contamination as much as possible: (i) the pH of the boric acid solution has to be below pH 4.0, or significantly higher than pH 8; (ii) the solution concentration of corrosion products containing Fe and Cr should be minimized (preferably by selective purification); and (iii) the dissolution rate of the steel surface has to be kept as minimal (i.e., ensuring that the formation of corrosion products and the reduction of U(VI) species are small). Acknowledgements This work was supported by the Paks NPP Co. Ltd. and the Hungarian Science Foundation (OTKA Grant No. 47219/2004). References [1] M. Pourbaix, Atlas of Electrochemical Equilibriums in Aqueous Solutions, Pergamon Press, Oxford, 1966. [2] G.R. Choppin, J. Rydberg, J.O. Liljenzin, Radiochemistry and Nuclear Chemistry, Butterworth-Heinemann Ltd., Oxford, 1995. [3] G.R. Choppin, Radiochim. Acta 32 (1983) 43. [4] W.R. Wadt, J. Am. Chem. Soc. 103 (1981) 6053. [5] G.F.R. Parkin (Ed.), Comprehensive Coordination Chemistry II, vol. 3, Elsevier, Amsterdam, 2004. [6] G.M. Nguyen-Trung, D. Begun, A. Palmer, Inorg. Chem. 31 (1992) 5280. [7] (a) M. Baraniak, M. Schmidt, G. Bernhard, H. Nitsche, Complex formation of hexavalent uranium with lignin degradation products, www.fz.rossendorf.de/pls/rois/cms?pNid-142, 1996; (b) C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, Wiley, New York, 1976. [8] C.L. Carnahan, Radiochim. Acta 44/45 (1987) 349.
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[14] (a) K. Varga, Z. N´emeth, Z. Homonnay, A. Szab´o, K. Rad´o, D. Oravetz, J. Schunk, P. Tilky, F. K˝or¨osi, J. Nucl. Mater. 348 (2006) 181; (b) Z. Homonnay, E. Kuzmann, S. Stichleutner, K. Varga, Z. N´emeth, A. ´ Mak´o, L. K¨ov´er, I. Cserny, D. Varga, J. T´oth, J. Szab´o, K. Rad´o, K.E. Schunk, P. Tilky, G. Patek, J. Nucl. Mater. 348 (2006) 191; (c) A. Szab´o, K. Varga, Z. N´emeth, K. Rad´o, D. Oravetz, K.E. Mak´o, Z. Homonnay, E. Kuzmann, P. Tilky, J. Schunk, G. Patek, Corrosion Sci. 48 (2006) 2727. [15] L. Z´ek´any, I. Nagyp´al, in: D.J. Legget (Ed.), Computational Methods for Determination of Formation Constants, Plenum Press, New York, 1985 (Chapter 8). [16] J. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Co., 1992.