- Email: [email protected]
The influence of dissolved hydrogen on the surface composition of doped uranium dioxide under aqueous corrosion conditions
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 602 (2007) 8–16 www.elsevier.com/locate/jelechem
The influence of dissolved hydrogen on the surface composition of doped uranium dioxide under aqueous corrosion conditions M.E. Broczkowski *, J.J. Noe¨l, D.W. Shoesmith
*
Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, Ont., Canada N6A 5B7 Received 28 August 2006; received in revised form 8 November 2006; accepted 21 November 2006 Available online 4 January 2007
Abstract The influence of oxic (O2-purged), anoxic (Ar-purged) and potentially reducing (5% H2/95% Ar-purged) conditions on the corrosion of UO2 (nuclear fuel) has been studied on SIMFUEL specimens in 0.1 mol L1 KCl (pH 9.5) solutions at 60 °C using corrosion potential measurements and X-ray photoelectron spectroscopy. A number of SIMFUEL specimens, doped to simulate various degrees of inreactor burn-up were used. The doping yielded specimens containing REIII (rare-earth) ions at UIV lattice sites within the UO2 matrix and noble metal (epsilon) particles interspersed throughout the solid. Under oxic and anoxic conditions, the corrosion potential and surface composition did not vary significantly with the degree of simulated burn-up. For potentially reducing conditions, both the corrosion potential and the extent of surface oxidation decreased as the degree of simulated burn-up increased. This was attributed to the increased number of noble metal particles on which H2 oxidation is possible. Since these particles are galvanically coupled to the rare-earth doped UO2 matrix, this suppresses the corrosion potential of the matrix, thereby preventing its oxidation. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Electrochemistry; SIMFUEL; Hydrogen; Reduction; X-ray photoelectron spectroscopy
1. Introduction Canada’s Nuclear Waste Management Organization has recommended to the federal government an Adaptive Phased Management Approach for the long-term management of Canada’s used nuclear fuel [1], which includes centralized containment and isolation of the used fuel in a deep geologic repository. In such a repository, the used fuel would be sealed in metallic containers and the containers would be surrounded by compacted bentonite clay, with excess space within the repository backfilled with a mixture of clay and crushed rock. Performance assessment calculations indicate that containers, fabricated with an outer shell of copper (for corrosion protection) and an inner liner of carbon steel (for mechanical strength) should not fail by * Corresponding authors. Tel.: +1 519 661 2111; fax: +1 519 661 3022 (D.W. Shoesmith), Tel.: +1 519 661 2111 (M.E. Broczkowski). E-mail addresses: [email protected] (M.E. Broczkowski), [email protected] (D.W. Shoesmith).
0022-0728/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.11.021
corrosion [2,3]. However, the possibility of failure due to fabrication defects remains, and considerable effort is being expended on understanding the consequences of such failures. Failure could lead to the exposure of both the fuel wasteform and the carbon steel to groundwater. This ground water would be expected to be anoxic, since environmental oxidants (e.g. dissolved O2) would have been rapidly consumed by the corrosion and mineral/biological oxidation processes in the materials used to backfill the waste repository [4]. Consequently, the main source of oxidants to drive fuel corrosion within the failed waste container will be produced by the radiolytic decomposition of water. Recently, we have developed a model to predict fuel corrosion rates in a failed waste container [5]. This model is based on mixed potential principles and assumes that only the long-lived alpha (a) radiation fields produce oxidants; i.e. the container lifetime will be sufficient to protect the fuel wasteform until b/c radiation fields have decayed to insignificant levels (300–1000 years).
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
In this model fuel corrosion is driven by the alpha radiolysis of water, which produces H2O2 close to the fuel surface, UO2 þ H2 O2 ! UO2þ 2 þ 2OH
ð1Þ
and steel corrosion by reaction with water to produce, Fe2+, Fe3O4, and H2, Fe þ 2H2 O ! Fe2þ þ 2OH þ H2
ð2Þ
3Fe þ 4H2 O ! Fe3 O4 þ H2
ð3Þ
These interfacial corrosion processes are summarized in Fig. 1, which shows the essential reactions anticipated within the failed container. A more detailed description of the reactions in the model is given elsewhere [5]. Since the repository will be sealed, the on-going corrosion of carbon steel could lead to substantial hydrogen pressures and dissolved hydrogen concentrations. This introduces the possibility that H2 could scavenge radiolytic H2O2, Fig. 1, thereby suppressing the corrosion of the fuel and delaying radionuclide release. Despite the expectation that H2 will not be reactive at anticipated repository temperatures (<100 °C), a range of studies suggests that it is. Spent fuel leaching studies in the presence of H2 have shown it to have a very strong influence on radionuclide leaching and fuel corrosion rates [6]. Recent studies demonstrate a decrease in corrosion rate of over four orders of magnitude (compared to the rates under oxidizing conditions) in experiments in the presence of 1 atmosphere (0.8 mmol L1 dissolved) H2 [7]. Two possible explanations were proposed; (i) a reductive influence of hydrogen radicals (H), produced by a catalytic effect of the UO2 surface on H2 decomposition, on fuel corrosion; and (ii) the scavenging of radiolytic oxidants by reaction with H 2. This second process is consistent with the predictions of standard radiolytic models [8], which calculate that moder-
9
ate hydrogen concentrations (<105 mol L1) are sufficient to suppress H2O2 production. Electrochemical and surface analytical evidence exists to demonstrate the suppression of fuel oxidation by H2 in the presence of alpha [9] and gamma [10] radiation. Using XPS, Sunder et al. [9] showed that, at a temperature of 100 °C, the presence of dissolved H2 suppressed the oxidation process, UO2 ! UO2þx ! UO3 yH2 O
ð4Þ
driven by a-radiolysis. Using external gamma sources and an H2 pressure of 5 MPa, King et al. [10] found the corrosion potential of a UO2 electrode suppressed to 700 mV (vs. SCE), a value well below that for the onset of UO2 oxidation. This very negative value suggests that the influence of H2 is not confined to the scavenging of radiolytic oxidants but exerts a distinct reductive effect on the UO2 surface. In a recent paper [11] we showed that the presence of noble metal (e) particles in the UO2 fuel matrix had a very significant effect on the corrosion potential (ECORR) in 0.1 mol L1 KCl at 60 °C when the system was purged with a 5% H2/95% Ar gas mixture. ECORR was suppressed to increasingly more negative values as the partial pressure of H2 was increased. Since a similar effect was not observed on UO2 electrodes not containing e-particles, it was proposed that H2 oxidation on e-particles prevented oxidation on the galvanically coupled UO2 matrix. Subsequently, we demonstrated that when holding the potential of a SIMFUEL electrode at 1.5 V, H2O reduction to H2 (as opposed to H2 oxidation to H2O under natural corrosion conditions) occurred at localized sites (presumed to be eparticles) on the fuel surface [12]. In this paper, we present a more extensive study of the influence of noble metal e-particles on fuel corrosion using electrochemical and surface analytical techniques. The primary goal is to determine whether or not the presence of hydrogen dissolved in solution does prevent oxidation of the UO2 surface and to what degree this effect is influenced by the presence of noble metal (e) particles in the fuel. 2. Experimental 2.1. Electrode materials and preparation, and solutions
Fig. 1. Schematic showing the corrosion scenario inside a failed nuclear waste container.
Experiments were performed on SIMFUEL electrodes cut from pellets fabricated by Atomic Energy of Canada Limited (Chalk River, Ontario, Canada). SIMFUEL is an unirradiated analogue of used nuclear fuel, produced by doping natural UO2 with a series of stable elements (Ba, Ce, La, Mo, Sr, Y, Rh, Pd, Ru, Nd, Zr) in proportions appropriate to replicate the chemical effects of irradiation of UO2 fuel in a CANDU reactor to various burn-ups [13,14]. As a consequence of this doping procedure, holes are injected into the 5f band, due to the substitution of trivalent rare-earth species (e.g. NdIII, YIII) for UIV in the UO2 fluorite lattice, which leads to an increase in electronic
10
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
conductivity. The noble metal elements (Mo, Ru, Rh, Pd), insoluble in the oxide lattice, congregate in metallic e-particles. This phase consists of small, spherical precipitates (0.5–1.5 lm diameter) randomly distributed in the UO2 matrix [13]. The SIMFUEL used in these studies mimics UO2 fuel irradiated to 1.5, 3, or 6 at.% burn-up. As well, a SIMFUEL sample doped to 3.0 at.% but containing no e-particles was used. The essential properties of these electrodes are listed in Table 1. The designation in Table 1 is used to refer to these electrodes in the subsequent text. Slices approximately 3 mm thick and 12 mm in diameter were cut from the SIMFUEL pellets and the electrodes pre-
pared as previously described [11]. Prior to the start of each experiment, the electrode was polished with 220 grit, and then 1200 grit SiC paper and rinsed with deaerated water. All solutions were prepared with distilled deionized water (resistivity, q = 18.2 MX cm) purified by using a Millipore milli-Q-plus unit to remove organic and inorganic impurities, and subsequently passed through milli-Q-plus ion exchange columns. All experiments were performed at 60 °C in 0.1 mol L1 KCl solution adjusted to pH 9.5, saturated with Ar, O2, or 5% H2/95% Ar gas. This solution was chosen to simulate the slightly alkaline, moderately saline conditions anticipated in a waste repository [2,3].
Table 1 Properties of electrode materials
2.2. Electrochemical cell and equipment
Designation
Material
Resistivity (q) (X cm)
SF 1.5 SF 3.0 SF 6.0 SS 3.0
1.5 at.% SIMFUEL 3.0 at.% SIMFUEL 6.0 at.% SIMFUEL 3 at.% SIMFUEL (oxides only)
182 81 1120 174
Experiments were conducted using a three-electrode, three-compartment electrochemical cell fitted with a jacket to allow control of cell temperature using a Haacke circulating water bath. The reference electrode was a commercial saturated calomel electrode (SCE), with a reference
Fig. 2. ECORR measurements on 1.5 at.% SIMFUEL (A), 3.0 at.% (B), 6.0 at.% (C) and 3.0 at.% SIMFUEL with no e-particles (D) in Ar (1), O2 (2), or 5% H2/95% Ar (3) solutions at 60 °C and pH 9.5. The horizontal dashed line at 0.4 V indicates the threshold potential for the onset of oxidation of the UO2 surface.
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
11
potential of 216 mV (i.e. compared to the standard hydrogen electrode) at 60 °C, while the counter electrode was a platinum foil with a surface area of 13 cm2, spot-welded to a platinum wire. The cell was housed in a grounded Faraday cage to minimize external noise. A Solartron model 1287 potentiostat was used to control applied potentials and to record current responses. Corrwareä software (supplied by Scribner Associates) was used to control the instruments and to analyze the data. 2.3. Experimental procedure
Fig. 3. ECORR as a function of SIMFUEL sample in either O2 (m), Ar (d) or 5% H2/95% Ar () purged solutions. The horizontal dashed line at 0.4 V indicates the threshold potential for the onset of oxidation of the UO2 surface.
In all experiments the electrode was cathodically cleaned at a potential of 1.2 V for 5 min. Subsequently, corrosion potential (ECORR) measurements were recorded for various lengths of time, but generally until a steady-state value was achieved. X-ray photoelectron spectroscopy (XPS) was performed after ECORR measurements to determine the surface film composition as a function of ECORR and the purge gas
Fig. 4. The U 4f7/2 XPS peak resolved into contributions from UIV, UV, and UVI for surfaces exposed to 5% H2/95% Ar (A), Ar (B), and O2 (C) purged solutions. The solid line encompassing the individual peaks is the measured spectrum, and the dotted line is the sum of the three individual peaks (fitted spectrum).
12
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
(Ar, O2 or Ar/H2) used. An SSX100 spectrometer was used to record all XPS spectra. Spectra were excited using AlKaradiation to bombard the surface with high energy monochromatic X-rays (hv = 1486.6 eV). The position of the C (1s) line at 285.0 eV was recorded and used to correct for surface charging. The O 2p peak, U (4f7/2), satellites associated with this region, and the valence band regions of the UO2 spectra were recorded. The uranium (4f7/2) peak was resolved into contributions from UIV, UV, and UVI and the satellite structure in the vicinity of this peak used to check the validity of the fit. The O 2p peak was resolved into contributions from O2, OH, and H2O. A detailed description of our analytical and spectral fitting procedures has been published elsewhere [15]. 3. Results Fig. 2A–D shows ECORR recorded on each SIMFUEL electrode in the three different purge gases. The horizontal line at 0.4 V indicates the threshold potential for the
Fig. 6. Relative fractions of uranium oxidation states as a function of ECORR in 5% H2/95% Ar-purged solutions for SF 6.0 (r), SF 3.0 (d), SF 1.5 (n), SF 1.5 after purge-gas switch from Ar to 5% H2/95% Ar ( ), SF 1.5 after purge-gas switch from O2 to 5% H2/95% Ar (w), and SS 3.0 ().
Fig. 5. XPS data collected after ECORR experiments on SIMFUEL sample SF 1.5 (A), SF 3.0 (B), SF 6.0 (C), and SS 3.0 (D) compared to calibration plots determined after electrochemical control of the SF 1.5 electrode at various individual potentials. The vertical dashed line at 0.4 V indicates the threshold potential for the onset of oxidation of the UO2 surface.
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
onset of oxidation of the UO2 surface (see below). In all cases, ECORR increased rapidly from the cathodic cleaning potential and eventually achieved a steady-state value. With the exception of SF 6.0 in an Ar-purged solution, steady-state is approached in 10–20 h. For all electrodes the steady-state ECORR value is very similar in Ar and O2-purged solutions, Fig. 3. For Arpurged solutions the value of ECORR for SS 3.0 is slightly greater than the values recorded on the three SIMFUEL electrodes containing e-particles. The values recorded in O2-purged solutions are substantially more positive than those recorded in Ar-purged solutions. For 5% H2/95% Ar purged solutions, however, the influence of electrode composition is significant. For the SS 3.0 electrode, which contains no e-particles, the steady-state ECORR value is the same as that measured in the Ar-purged solutions, as previously observed [11]. By contrast, the steady-state ECORR values recorded on the three SIMFUEL electrodes are lower in 5% H2/95% Ar purged solu-
13
tions, and the absolute value of ECORR decreases with an increase in extent of simulated burn-up, and achieves the oxidation threshold (0.4 V) for SF 6.0, Fig. 3. Fig. 4A–C shows the measured 4f7/2 peak for the SF 1.5 electrode after corrosion in the three different purge solutions. Also shown are the contributions of UIV, UV, and UVI determined by fitting the spectra using the procedure described in Ref. [15]. The dashed line in these plots indicates the fitted spectra. The change in dominant surface oxidation states as the purge gas is changed is clear, with oxidized states (UV and UVI) dominating in O2-purged solutions and the reduced UIV state being dominant in the 5% H2/95% Ar-purged solution. Similar XPS analyses were performed after all the experiments. Fig. 5A–D show the compositions of the electrode surfaces expressed as fractions of the individual oxidation states compared to a calibration plot of the composition of the UO2 surface as a function of potential. This calibration plot was determined by XPS analysis of the surface of
Fig. 7. The O 2p XPS peak resolved into contributions from O2, OH, and H2O for SIMFUEL sample SF 1.5 exposed to 5% H2/95% Ar (A), Ar (B), or O2 (C)-purged solutions. The solid line encompassing the individual peaks is the measured spectrum, and the dotted line is the sum of the three individual peaks (fitted spectrum).
14
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
an SF 1.5 electrode after a 1-h potentiostatic oxidation at individual potentials in the range 500 mV to +500 mV. The details of the construction of this calibration plot will be published elsewhere. The agreement between the surface compositions of corroded surfaces and the calibration plot is generally good, confirming that the ECORR values indicate distinct differences in surface composition for electrodes exposed to the three different purge gases. As expected from the ECORR measurements, the composition varies little between specimens exposed to solutions with either Ar or O2-purging. Surfaces exposed to Ar-purged solutions are only slightly oxidized, while those exposed to O2 are predominantly composed of the oxidized UV/UVI states. For the SS 3.0 electrode (no e-particles), the composition of the surface after experiments in Ar and 5% H2/ 95% Ar-purged solutions is effectively the same, consistent with the similar ECORR values observed in these two experiments. For all three SF electrodes, the extent of surface oxidation is significantly suppressed after exposure to a 5% H2/95% Ar-purged solution compared to an Ar-purged solution, and the fraction of total oxidized states ((UV + UVI)/UT) decreased with an increase in simulated burn-up. Fig. 6 shows the fraction of each oxidation state plotted as a function of ECORR clearly demonstrating the relationship between composition and potential. The O 2p XPS spectra were also recorded after each experiment and deconvoluted into contributions from O2, OH, and H2O. Fig. 7 shows the spectra for the SF 1.5 electrode recorded after corrosion in the three different purge solutions. Comparison of these spectra to the 4f7/2 spectra from the same experiments (Fig. 4) shows that an increase in the fraction of oxidized U states in the electrode surface is accompanied by an increase in surface OH/H2O content. Similar behaviour was observed for the other electrodes. This is as expected based on previously published observations on electrochemically oxidized surfaces [15]. The poor fit to the spectra in the high binding energy region suggests the state of adsorbed water on these specimens is not completely represented by the reference binding energies used to fit the spectra [15], and that we may be underestimating the amount of water associated with the surface. In an additional experiment with the SF 1.5 specimen, the electrode was first exposed to either an O2-purged or an Ar-purged solution and ECORR recorded until a steady-state value was achieved, Fig. 8. The ECORR values attained were consistent with those recorded in the previous experiments. Subsequently, the purge gas was switched to 5% H2/95% Ar and ECORR again followed until a steady-state value was obtained. In both cases, ECORR decreased to a new steady-state value, which was approximately independent of whether the initial purge gas was Ar or O2. XPS analysis was performed on completion of both experiments and the fraction of the individual oxidation states of U are plotted and compared to the calibration plot in Fig. 9. These fractions are also included in Fig. 6.
Fig. 8. ECORR recorded on SF 1.5 in either O2 or Ar-purged solution for 70 h followed by a switch to purging with 5% H2/95% Ar gas.
Fig. 9. XPS data collected after ECORR experiments on SF 1.5 in either O2 or Ar purged solutions, followed by a switch to purging with 5% H2/95% Ar gas.
4. Discussion Based on a wide range of electrochemical and surface analytical experiments, we have previously defined the behaviour of UO2 as a function of potential as illustrated in Fig. 10. The onset of observable oxidation occurs around 400 mV (vs. SCE), the lowest potential for which an increase in UV content of the fuel surface is observed by XPS, as illustrated in Fig. 5, and previously demonstrated for room temperature [15]. Support for this conclusion comes from our observation that the electrode surface composition at 500 mV is identical to that obtained after extensive electrochemical reduction of the electrode surface at 1200 mV. The residual UV/UVI content of the electrode surface after extensive reduction could be due to a slight degree of surface oxidation on transferring the electrode from the electrochemical cell to the vacuum chamber of the spectrometer. Based on Fig. 5 we can claim that a
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
Fig. 10. Composition and corrosion behaviour of UO2 as a function of UO2 corrosion potential.
potential of 400 mV represents a threshold for the onset of oxidation of the UO2, a claim consistent with our previous photoeletrochemical [5,15] and photothermal deflection spectroscopy observations [16]. Over the potential range 400 mV to 0 mV, the ratio of UV/UIV in the UO2 surface increases due to the formaV tion of a thin surface layer of UIV 12x U2x O2 + x. According to the calibration plot in Fig. 5, the formation of UVI species on the UO2 surface at 60 °C begins as early as 300 mV compared to 0 mV at room temperature [15]. A possibility is that this could be due to a more rapid air oxidation of the surface after extraction from the cell at 60 °C compared to extraction from the cell at room temperature. However, our ability to observe a ‘‘fully reduced’’ surface after potential control at 500 mV suggests the increase in UVI content of the surface for potentials P300 mV is due to oxidation within the cell. This is supported by our photothermal deflection spectroscopy results, which showed that dissolution as UO2þ 2 could be detected at potentials as low as 300 mV. Then, given the insolubility of UVI at a pH of 9.5 [16], the formation of a UVI surface deposit (UO3 Æ yH2O) is to be expected. The decrease in UVI surface concentration for potentials >+100 mV can be attributed to the enhanced solubility of UVI at acidified locations on the electrode surface, as previously characterized at room temperature [15]. In Ar-purged solutions only a slight oxidation of the V UO2 surface to UIV 12x U2x O2 + x with a low coverage of VI U is observed. Since the formation of such a layer involves the incorporation of O2 into interstitial sites within the surface of the UO2 fluorite lattice we would expect the O2 state to dominate the O 2p XPS spectra as observed, Fig. 7. The SS 3.0 electrode appears to be slightly more oxidized than the three SF electrodes, Fig. 5, suggesting a possible role of e-particles in suppressing UO2 oxidation of the latter under anoxic conditions. For O2-purged solutions, the ECORR values are consider-
15
ably more positive, consistent with the domination of oxidized states (UV/UVI) present as hydrated species in the electrode surface. No clearly apparent influence of e-particles on either ECORR or surface composition is observed for Ar or O2 purged solutions. For exposure to 5% H2/95% Ar-purged solutions, a very distinct influence of electrode composition on both ECORR and surface composition is observed. As the size and number of e-particles in the UO2 increases, ECORR is significantly reduced, eventually attaining the threshold value of 400 mV for SF 6.0, Fig. 3. Consistent with this observation, the fraction of oxidized surface states decreases accompanied by an increased dominance of O2 over OH/H2O in the electrode surface. This combination of surface compositional features indicates a stabilization of the UO2 fluorite lattice at the expense of hydrated higher oxidation states. This is a clear demonstration that the redox chemistry of H2, catalyzed on the noble metal particles, suppresses UO2 oxidation via galvanic coupling between the e-particles and the conductive, rare-earth doped, UO2 lattice, as previously claimed [11]. In the experiments in which the purge gas was changed from either Ar or O2-purged to 5% H2/95% Ar-purged, we would expect the surface composition to reflect the ECORR achieved during the initial purge if oxidation of the UO2 surface was irreversible. However, the switch to 5% H2/ 95% Ar-purge leads to a decrease in ECORR in both cases, indicating that the H2 present in solution is polarizing the electrode surface. Thus, the question arises as to whether such a polarization leads to a reduction of oxidized states produced during the initial purge. When the analyzed surface compositions are compared to those obtained in the other 5% H2/95% Ar-purged experiments, Fig. 6, the surface compositions after the Ar-5% H2/95% Ar-purged experiment fit the expected trend with potential while those obtained after the O2-5% H2/95% Ar-purged experiment do not. For the Ar-5% H2/95% Ar-purge this could be taken to indicate that the switch in purge gas did lead to reduction of the SF 1.5 surface. For the O2-5% H2/95% Ar-purged experiment the surface is substantially more oxidized than would be expected based on the ECORR (250 mV) achieved after switching to 5% H2/95% Ar-purge, but less oxidized that would be expected at the ECORR (60 mV) achieved after the initial O2 purge. It is possible that the H2 introduced on switching the purge gas partially reduced oxidized surface states. However, it is also possible that UVI states partially chemically dissolved and no reduction occurred. Until a more extensive database is available, there is no conclusive evidence that the presence of H2 can lead to reduction of a preoxidized surface. 5. Summary and conclusions The influence of oxic (O2-purged), anoxic (Ar-purged), and potentially reducing (5% H2/95% Ar) conditions on the corrosion of UO2 has been studied on SIMFUEL spec-
16
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
imens in slightly alkaline KCl solutions at 60 °C. The specimens contained different levels of rare-earth (RE+III) doping and various numbers of noble metal (epsilon) particles. (i) Under oxic conditions the steady-state ECORR values achieved were positive, compared to the threshold ECORR for the onset of UO2 oxidation, and the electrode extensively oxidized to UV/UVI. The number density of noble metal particles in the UO2 did not appear to exert a measurable effect on the extent of surface oxidation. (ii) Under anoxic conditions, lower steady-state ECORR values were achieved. The presence of noble metal particles appeared to suppress ECORR and reduce the extent of oxidation of the surface. However, this suppression was not measurably dependent on the number density of particles. (iii) In 5% H2/95% Ar-purged solutions, both the steadystate ECORR and the extent of oxidation of the UO2 surface were dependent on the number density of the noble metal particles in the SIMFUEL. As the number density increased, ECORR was suppressed to more negative values, and the UV/UVI content of the surface decreased. This effect is attributed to the reversible decomposition of H2 to adsorbed H atoms on the noble metal particles. Since these particles are galvanically coupled to the UO2 matrix, this causes ECORR to be suppressed and fuel oxidation to be avoided. (iv) There is no conclusive evidence to date, that the presence of dissolved H2 can lead to the reduction of preformed oxidized states on the UO2 surface. Acknowledgements This research is funded under the Industrial Chair agreement between the Canadian Natural Sciences and Engi-
neering Research Council (NSERC) and Ontario Power Generation (OPG), Toronto, Canada. Surface Science Western is gratefully acknowledged for the use of their XPS instrument. References [1] Nuclear Waste Management Organization (NWMO), Choosing a Way Forward: The Future Management of Canada’s Used Nuclear Fuel, November 2005, This report available at www.nwmo.ca. [2] F. King, M. Kolar, Ontario Power Generation Report No. 00819REP-01200-10041-ROO, 1999. [3] L.H. Johnson, D.M. LeNeveu, F. King, D.W. Shoesmith, M. Kolar, D.W. Oscarson, S. Sunder, C. Onofrei, J.L. Crosthwaite, Atomic Energy of Canada Limited Report, AECL-11494-2, COG-95-552-2, 2000. [4] F. King, M. Kolar, Mat. Res. Soc. Symp. Proc. 412 (1996) 547. [5] D.W. Shoesmith, J. Nucl. Mater. 282 (2000) 1. [6] K. Spahiu, L. Werme, U.-B. Eklund, Radio Chim. Acta 88 (2000) 507. [7] S. Ro¨llin, K. Spahiu, U.-B. Eklund, J. Nucl. Mater. 297 (2001) 231. [8] J.C. Tait, L.H. Johnson, in: Proceedings of the 2nd Canadian Nuclear Society, Toronto, 1986, p. 611. [9] S. Sunder, G.D. Boyer, N.H. Miller, J. Nucl. Mater. 175 (1990) 147. [10] F. King, M.J. Quinn, N.H. Miller, Swedish Nuclear Fuel Company (SKB) Report, SKB TR-99-27, 1999. [11] M.E. Broczkowski, J.J. Noe¨l, D.W. Shoesmith, J. Nucl. Mater. 346 (2005) 16. [12] M.E. Broczkowski, R. Zhu, Z. Ding, J.J. Noe¨l, D.W. Shoesmith, Mater. Res. Soc. Symp. Proc. 932 (2005) 449. [13] P.G. Lucuta, R.A. Verrall, H. Matzke, B.J. Palmer, J. Nucl. Mater. 178 (1991) 48. [14] A.F. Gerwing, F.E. Doern, W.H. Hocking, Surf. Interface Anal. 14 (1989) 559. [15] B.G. Santos, H.W. Nesbitt, J.J. Noel, D.W. Shoesmith, Electrochim. Acta 49 (2004) 1863. [16] J.D. Rudnicki, R. Russo, D.W. Shoesmith, J. Electroanal. Chem. 372 (1994) 3.
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 602 (2007) 8–16 www.elsevier.com/locate/jelechem
The influence of dissolved hydrogen on the surface composition of doped uranium dioxide under aqueous corrosion conditions M.E. Broczkowski *, J.J. Noe¨l, D.W. Shoesmith
*
Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, Ont., Canada N6A 5B7 Received 28 August 2006; received in revised form 8 November 2006; accepted 21 November 2006 Available online 4 January 2007
Abstract The influence of oxic (O2-purged), anoxic (Ar-purged) and potentially reducing (5% H2/95% Ar-purged) conditions on the corrosion of UO2 (nuclear fuel) has been studied on SIMFUEL specimens in 0.1 mol L1 KCl (pH 9.5) solutions at 60 °C using corrosion potential measurements and X-ray photoelectron spectroscopy. A number of SIMFUEL specimens, doped to simulate various degrees of inreactor burn-up were used. The doping yielded specimens containing REIII (rare-earth) ions at UIV lattice sites within the UO2 matrix and noble metal (epsilon) particles interspersed throughout the solid. Under oxic and anoxic conditions, the corrosion potential and surface composition did not vary significantly with the degree of simulated burn-up. For potentially reducing conditions, both the corrosion potential and the extent of surface oxidation decreased as the degree of simulated burn-up increased. This was attributed to the increased number of noble metal particles on which H2 oxidation is possible. Since these particles are galvanically coupled to the rare-earth doped UO2 matrix, this suppresses the corrosion potential of the matrix, thereby preventing its oxidation. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Electrochemistry; SIMFUEL; Hydrogen; Reduction; X-ray photoelectron spectroscopy
1. Introduction Canada’s Nuclear Waste Management Organization has recommended to the federal government an Adaptive Phased Management Approach for the long-term management of Canada’s used nuclear fuel [1], which includes centralized containment and isolation of the used fuel in a deep geologic repository. In such a repository, the used fuel would be sealed in metallic containers and the containers would be surrounded by compacted bentonite clay, with excess space within the repository backfilled with a mixture of clay and crushed rock. Performance assessment calculations indicate that containers, fabricated with an outer shell of copper (for corrosion protection) and an inner liner of carbon steel (for mechanical strength) should not fail by * Corresponding authors. Tel.: +1 519 661 2111; fax: +1 519 661 3022 (D.W. Shoesmith), Tel.: +1 519 661 2111 (M.E. Broczkowski). E-mail addresses: [email protected] (M.E. Broczkowski), [email protected] (D.W. Shoesmith).
0022-0728/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.11.021
corrosion [2,3]. However, the possibility of failure due to fabrication defects remains, and considerable effort is being expended on understanding the consequences of such failures. Failure could lead to the exposure of both the fuel wasteform and the carbon steel to groundwater. This ground water would be expected to be anoxic, since environmental oxidants (e.g. dissolved O2) would have been rapidly consumed by the corrosion and mineral/biological oxidation processes in the materials used to backfill the waste repository [4]. Consequently, the main source of oxidants to drive fuel corrosion within the failed waste container will be produced by the radiolytic decomposition of water. Recently, we have developed a model to predict fuel corrosion rates in a failed waste container [5]. This model is based on mixed potential principles and assumes that only the long-lived alpha (a) radiation fields produce oxidants; i.e. the container lifetime will be sufficient to protect the fuel wasteform until b/c radiation fields have decayed to insignificant levels (300–1000 years).
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
In this model fuel corrosion is driven by the alpha radiolysis of water, which produces H2O2 close to the fuel surface, UO2 þ H2 O2 ! UO2þ 2 þ 2OH
ð1Þ
and steel corrosion by reaction with water to produce, Fe2+, Fe3O4, and H2, Fe þ 2H2 O ! Fe2þ þ 2OH þ H2
ð2Þ
3Fe þ 4H2 O ! Fe3 O4 þ H2
ð3Þ
These interfacial corrosion processes are summarized in Fig. 1, which shows the essential reactions anticipated within the failed container. A more detailed description of the reactions in the model is given elsewhere [5]. Since the repository will be sealed, the on-going corrosion of carbon steel could lead to substantial hydrogen pressures and dissolved hydrogen concentrations. This introduces the possibility that H2 could scavenge radiolytic H2O2, Fig. 1, thereby suppressing the corrosion of the fuel and delaying radionuclide release. Despite the expectation that H2 will not be reactive at anticipated repository temperatures (<100 °C), a range of studies suggests that it is. Spent fuel leaching studies in the presence of H2 have shown it to have a very strong influence on radionuclide leaching and fuel corrosion rates [6]. Recent studies demonstrate a decrease in corrosion rate of over four orders of magnitude (compared to the rates under oxidizing conditions) in experiments in the presence of 1 atmosphere (0.8 mmol L1 dissolved) H2 [7]. Two possible explanations were proposed; (i) a reductive influence of hydrogen radicals (H), produced by a catalytic effect of the UO2 surface on H2 decomposition, on fuel corrosion; and (ii) the scavenging of radiolytic oxidants by reaction with H 2. This second process is consistent with the predictions of standard radiolytic models [8], which calculate that moder-
9
ate hydrogen concentrations (<105 mol L1) are sufficient to suppress H2O2 production. Electrochemical and surface analytical evidence exists to demonstrate the suppression of fuel oxidation by H2 in the presence of alpha [9] and gamma [10] radiation. Using XPS, Sunder et al. [9] showed that, at a temperature of 100 °C, the presence of dissolved H2 suppressed the oxidation process, UO2 ! UO2þx ! UO3 yH2 O
ð4Þ
driven by a-radiolysis. Using external gamma sources and an H2 pressure of 5 MPa, King et al. [10] found the corrosion potential of a UO2 electrode suppressed to 700 mV (vs. SCE), a value well below that for the onset of UO2 oxidation. This very negative value suggests that the influence of H2 is not confined to the scavenging of radiolytic oxidants but exerts a distinct reductive effect on the UO2 surface. In a recent paper [11] we showed that the presence of noble metal (e) particles in the UO2 fuel matrix had a very significant effect on the corrosion potential (ECORR) in 0.1 mol L1 KCl at 60 °C when the system was purged with a 5% H2/95% Ar gas mixture. ECORR was suppressed to increasingly more negative values as the partial pressure of H2 was increased. Since a similar effect was not observed on UO2 electrodes not containing e-particles, it was proposed that H2 oxidation on e-particles prevented oxidation on the galvanically coupled UO2 matrix. Subsequently, we demonstrated that when holding the potential of a SIMFUEL electrode at 1.5 V, H2O reduction to H2 (as opposed to H2 oxidation to H2O under natural corrosion conditions) occurred at localized sites (presumed to be eparticles) on the fuel surface [12]. In this paper, we present a more extensive study of the influence of noble metal e-particles on fuel corrosion using electrochemical and surface analytical techniques. The primary goal is to determine whether or not the presence of hydrogen dissolved in solution does prevent oxidation of the UO2 surface and to what degree this effect is influenced by the presence of noble metal (e) particles in the fuel. 2. Experimental 2.1. Electrode materials and preparation, and solutions
Fig. 1. Schematic showing the corrosion scenario inside a failed nuclear waste container.
Experiments were performed on SIMFUEL electrodes cut from pellets fabricated by Atomic Energy of Canada Limited (Chalk River, Ontario, Canada). SIMFUEL is an unirradiated analogue of used nuclear fuel, produced by doping natural UO2 with a series of stable elements (Ba, Ce, La, Mo, Sr, Y, Rh, Pd, Ru, Nd, Zr) in proportions appropriate to replicate the chemical effects of irradiation of UO2 fuel in a CANDU reactor to various burn-ups [13,14]. As a consequence of this doping procedure, holes are injected into the 5f band, due to the substitution of trivalent rare-earth species (e.g. NdIII, YIII) for UIV in the UO2 fluorite lattice, which leads to an increase in electronic
10
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
conductivity. The noble metal elements (Mo, Ru, Rh, Pd), insoluble in the oxide lattice, congregate in metallic e-particles. This phase consists of small, spherical precipitates (0.5–1.5 lm diameter) randomly distributed in the UO2 matrix [13]. The SIMFUEL used in these studies mimics UO2 fuel irradiated to 1.5, 3, or 6 at.% burn-up. As well, a SIMFUEL sample doped to 3.0 at.% but containing no e-particles was used. The essential properties of these electrodes are listed in Table 1. The designation in Table 1 is used to refer to these electrodes in the subsequent text. Slices approximately 3 mm thick and 12 mm in diameter were cut from the SIMFUEL pellets and the electrodes pre-
pared as previously described [11]. Prior to the start of each experiment, the electrode was polished with 220 grit, and then 1200 grit SiC paper and rinsed with deaerated water. All solutions were prepared with distilled deionized water (resistivity, q = 18.2 MX cm) purified by using a Millipore milli-Q-plus unit to remove organic and inorganic impurities, and subsequently passed through milli-Q-plus ion exchange columns. All experiments were performed at 60 °C in 0.1 mol L1 KCl solution adjusted to pH 9.5, saturated with Ar, O2, or 5% H2/95% Ar gas. This solution was chosen to simulate the slightly alkaline, moderately saline conditions anticipated in a waste repository [2,3].
Table 1 Properties of electrode materials
2.2. Electrochemical cell and equipment
Designation
Material
Resistivity (q) (X cm)
SF 1.5 SF 3.0 SF 6.0 SS 3.0
1.5 at.% SIMFUEL 3.0 at.% SIMFUEL 6.0 at.% SIMFUEL 3 at.% SIMFUEL (oxides only)
182 81 1120 174
Experiments were conducted using a three-electrode, three-compartment electrochemical cell fitted with a jacket to allow control of cell temperature using a Haacke circulating water bath. The reference electrode was a commercial saturated calomel electrode (SCE), with a reference
Fig. 2. ECORR measurements on 1.5 at.% SIMFUEL (A), 3.0 at.% (B), 6.0 at.% (C) and 3.0 at.% SIMFUEL with no e-particles (D) in Ar (1), O2 (2), or 5% H2/95% Ar (3) solutions at 60 °C and pH 9.5. The horizontal dashed line at 0.4 V indicates the threshold potential for the onset of oxidation of the UO2 surface.
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
11
potential of 216 mV (i.e. compared to the standard hydrogen electrode) at 60 °C, while the counter electrode was a platinum foil with a surface area of 13 cm2, spot-welded to a platinum wire. The cell was housed in a grounded Faraday cage to minimize external noise. A Solartron model 1287 potentiostat was used to control applied potentials and to record current responses. Corrwareä software (supplied by Scribner Associates) was used to control the instruments and to analyze the data. 2.3. Experimental procedure
Fig. 3. ECORR as a function of SIMFUEL sample in either O2 (m), Ar (d) or 5% H2/95% Ar () purged solutions. The horizontal dashed line at 0.4 V indicates the threshold potential for the onset of oxidation of the UO2 surface.
In all experiments the electrode was cathodically cleaned at a potential of 1.2 V for 5 min. Subsequently, corrosion potential (ECORR) measurements were recorded for various lengths of time, but generally until a steady-state value was achieved. X-ray photoelectron spectroscopy (XPS) was performed after ECORR measurements to determine the surface film composition as a function of ECORR and the purge gas
Fig. 4. The U 4f7/2 XPS peak resolved into contributions from UIV, UV, and UVI for surfaces exposed to 5% H2/95% Ar (A), Ar (B), and O2 (C) purged solutions. The solid line encompassing the individual peaks is the measured spectrum, and the dotted line is the sum of the three individual peaks (fitted spectrum).
12
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
(Ar, O2 or Ar/H2) used. An SSX100 spectrometer was used to record all XPS spectra. Spectra were excited using AlKaradiation to bombard the surface with high energy monochromatic X-rays (hv = 1486.6 eV). The position of the C (1s) line at 285.0 eV was recorded and used to correct for surface charging. The O 2p peak, U (4f7/2), satellites associated with this region, and the valence band regions of the UO2 spectra were recorded. The uranium (4f7/2) peak was resolved into contributions from UIV, UV, and UVI and the satellite structure in the vicinity of this peak used to check the validity of the fit. The O 2p peak was resolved into contributions from O2, OH, and H2O. A detailed description of our analytical and spectral fitting procedures has been published elsewhere [15]. 3. Results Fig. 2A–D shows ECORR recorded on each SIMFUEL electrode in the three different purge gases. The horizontal line at 0.4 V indicates the threshold potential for the
Fig. 6. Relative fractions of uranium oxidation states as a function of ECORR in 5% H2/95% Ar-purged solutions for SF 6.0 (r), SF 3.0 (d), SF 1.5 (n), SF 1.5 after purge-gas switch from Ar to 5% H2/95% Ar ( ), SF 1.5 after purge-gas switch from O2 to 5% H2/95% Ar (w), and SS 3.0 ().
Fig. 5. XPS data collected after ECORR experiments on SIMFUEL sample SF 1.5 (A), SF 3.0 (B), SF 6.0 (C), and SS 3.0 (D) compared to calibration plots determined after electrochemical control of the SF 1.5 electrode at various individual potentials. The vertical dashed line at 0.4 V indicates the threshold potential for the onset of oxidation of the UO2 surface.
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
onset of oxidation of the UO2 surface (see below). In all cases, ECORR increased rapidly from the cathodic cleaning potential and eventually achieved a steady-state value. With the exception of SF 6.0 in an Ar-purged solution, steady-state is approached in 10–20 h. For all electrodes the steady-state ECORR value is very similar in Ar and O2-purged solutions, Fig. 3. For Arpurged solutions the value of ECORR for SS 3.0 is slightly greater than the values recorded on the three SIMFUEL electrodes containing e-particles. The values recorded in O2-purged solutions are substantially more positive than those recorded in Ar-purged solutions. For 5% H2/95% Ar purged solutions, however, the influence of electrode composition is significant. For the SS 3.0 electrode, which contains no e-particles, the steady-state ECORR value is the same as that measured in the Ar-purged solutions, as previously observed [11]. By contrast, the steady-state ECORR values recorded on the three SIMFUEL electrodes are lower in 5% H2/95% Ar purged solu-
13
tions, and the absolute value of ECORR decreases with an increase in extent of simulated burn-up, and achieves the oxidation threshold (0.4 V) for SF 6.0, Fig. 3. Fig. 4A–C shows the measured 4f7/2 peak for the SF 1.5 electrode after corrosion in the three different purge solutions. Also shown are the contributions of UIV, UV, and UVI determined by fitting the spectra using the procedure described in Ref. [15]. The dashed line in these plots indicates the fitted spectra. The change in dominant surface oxidation states as the purge gas is changed is clear, with oxidized states (UV and UVI) dominating in O2-purged solutions and the reduced UIV state being dominant in the 5% H2/95% Ar-purged solution. Similar XPS analyses were performed after all the experiments. Fig. 5A–D show the compositions of the electrode surfaces expressed as fractions of the individual oxidation states compared to a calibration plot of the composition of the UO2 surface as a function of potential. This calibration plot was determined by XPS analysis of the surface of
Fig. 7. The O 2p XPS peak resolved into contributions from O2, OH, and H2O for SIMFUEL sample SF 1.5 exposed to 5% H2/95% Ar (A), Ar (B), or O2 (C)-purged solutions. The solid line encompassing the individual peaks is the measured spectrum, and the dotted line is the sum of the three individual peaks (fitted spectrum).
14
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
an SF 1.5 electrode after a 1-h potentiostatic oxidation at individual potentials in the range 500 mV to +500 mV. The details of the construction of this calibration plot will be published elsewhere. The agreement between the surface compositions of corroded surfaces and the calibration plot is generally good, confirming that the ECORR values indicate distinct differences in surface composition for electrodes exposed to the three different purge gases. As expected from the ECORR measurements, the composition varies little between specimens exposed to solutions with either Ar or O2-purging. Surfaces exposed to Ar-purged solutions are only slightly oxidized, while those exposed to O2 are predominantly composed of the oxidized UV/UVI states. For the SS 3.0 electrode (no e-particles), the composition of the surface after experiments in Ar and 5% H2/ 95% Ar-purged solutions is effectively the same, consistent with the similar ECORR values observed in these two experiments. For all three SF electrodes, the extent of surface oxidation is significantly suppressed after exposure to a 5% H2/95% Ar-purged solution compared to an Ar-purged solution, and the fraction of total oxidized states ((UV + UVI)/UT) decreased with an increase in simulated burn-up. Fig. 6 shows the fraction of each oxidation state plotted as a function of ECORR clearly demonstrating the relationship between composition and potential. The O 2p XPS spectra were also recorded after each experiment and deconvoluted into contributions from O2, OH, and H2O. Fig. 7 shows the spectra for the SF 1.5 electrode recorded after corrosion in the three different purge solutions. Comparison of these spectra to the 4f7/2 spectra from the same experiments (Fig. 4) shows that an increase in the fraction of oxidized U states in the electrode surface is accompanied by an increase in surface OH/H2O content. Similar behaviour was observed for the other electrodes. This is as expected based on previously published observations on electrochemically oxidized surfaces [15]. The poor fit to the spectra in the high binding energy region suggests the state of adsorbed water on these specimens is not completely represented by the reference binding energies used to fit the spectra [15], and that we may be underestimating the amount of water associated with the surface. In an additional experiment with the SF 1.5 specimen, the electrode was first exposed to either an O2-purged or an Ar-purged solution and ECORR recorded until a steady-state value was achieved, Fig. 8. The ECORR values attained were consistent with those recorded in the previous experiments. Subsequently, the purge gas was switched to 5% H2/95% Ar and ECORR again followed until a steady-state value was obtained. In both cases, ECORR decreased to a new steady-state value, which was approximately independent of whether the initial purge gas was Ar or O2. XPS analysis was performed on completion of both experiments and the fraction of the individual oxidation states of U are plotted and compared to the calibration plot in Fig. 9. These fractions are also included in Fig. 6.
Fig. 8. ECORR recorded on SF 1.5 in either O2 or Ar-purged solution for 70 h followed by a switch to purging with 5% H2/95% Ar gas.
Fig. 9. XPS data collected after ECORR experiments on SF 1.5 in either O2 or Ar purged solutions, followed by a switch to purging with 5% H2/95% Ar gas.
4. Discussion Based on a wide range of electrochemical and surface analytical experiments, we have previously defined the behaviour of UO2 as a function of potential as illustrated in Fig. 10. The onset of observable oxidation occurs around 400 mV (vs. SCE), the lowest potential for which an increase in UV content of the fuel surface is observed by XPS, as illustrated in Fig. 5, and previously demonstrated for room temperature [15]. Support for this conclusion comes from our observation that the electrode surface composition at 500 mV is identical to that obtained after extensive electrochemical reduction of the electrode surface at 1200 mV. The residual UV/UVI content of the electrode surface after extensive reduction could be due to a slight degree of surface oxidation on transferring the electrode from the electrochemical cell to the vacuum chamber of the spectrometer. Based on Fig. 5 we can claim that a
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
Fig. 10. Composition and corrosion behaviour of UO2 as a function of UO2 corrosion potential.
potential of 400 mV represents a threshold for the onset of oxidation of the UO2, a claim consistent with our previous photoeletrochemical [5,15] and photothermal deflection spectroscopy observations [16]. Over the potential range 400 mV to 0 mV, the ratio of UV/UIV in the UO2 surface increases due to the formaV tion of a thin surface layer of UIV 12x U2x O2 + x. According to the calibration plot in Fig. 5, the formation of UVI species on the UO2 surface at 60 °C begins as early as 300 mV compared to 0 mV at room temperature [15]. A possibility is that this could be due to a more rapid air oxidation of the surface after extraction from the cell at 60 °C compared to extraction from the cell at room temperature. However, our ability to observe a ‘‘fully reduced’’ surface after potential control at 500 mV suggests the increase in UVI content of the surface for potentials P300 mV is due to oxidation within the cell. This is supported by our photothermal deflection spectroscopy results, which showed that dissolution as UO2þ 2 could be detected at potentials as low as 300 mV. Then, given the insolubility of UVI at a pH of 9.5 [16], the formation of a UVI surface deposit (UO3 Æ yH2O) is to be expected. The decrease in UVI surface concentration for potentials >+100 mV can be attributed to the enhanced solubility of UVI at acidified locations on the electrode surface, as previously characterized at room temperature [15]. In Ar-purged solutions only a slight oxidation of the V UO2 surface to UIV 12x U2x O2 + x with a low coverage of VI U is observed. Since the formation of such a layer involves the incorporation of O2 into interstitial sites within the surface of the UO2 fluorite lattice we would expect the O2 state to dominate the O 2p XPS spectra as observed, Fig. 7. The SS 3.0 electrode appears to be slightly more oxidized than the three SF electrodes, Fig. 5, suggesting a possible role of e-particles in suppressing UO2 oxidation of the latter under anoxic conditions. For O2-purged solutions, the ECORR values are consider-
15
ably more positive, consistent with the domination of oxidized states (UV/UVI) present as hydrated species in the electrode surface. No clearly apparent influence of e-particles on either ECORR or surface composition is observed for Ar or O2 purged solutions. For exposure to 5% H2/95% Ar-purged solutions, a very distinct influence of electrode composition on both ECORR and surface composition is observed. As the size and number of e-particles in the UO2 increases, ECORR is significantly reduced, eventually attaining the threshold value of 400 mV for SF 6.0, Fig. 3. Consistent with this observation, the fraction of oxidized surface states decreases accompanied by an increased dominance of O2 over OH/H2O in the electrode surface. This combination of surface compositional features indicates a stabilization of the UO2 fluorite lattice at the expense of hydrated higher oxidation states. This is a clear demonstration that the redox chemistry of H2, catalyzed on the noble metal particles, suppresses UO2 oxidation via galvanic coupling between the e-particles and the conductive, rare-earth doped, UO2 lattice, as previously claimed [11]. In the experiments in which the purge gas was changed from either Ar or O2-purged to 5% H2/95% Ar-purged, we would expect the surface composition to reflect the ECORR achieved during the initial purge if oxidation of the UO2 surface was irreversible. However, the switch to 5% H2/ 95% Ar-purge leads to a decrease in ECORR in both cases, indicating that the H2 present in solution is polarizing the electrode surface. Thus, the question arises as to whether such a polarization leads to a reduction of oxidized states produced during the initial purge. When the analyzed surface compositions are compared to those obtained in the other 5% H2/95% Ar-purged experiments, Fig. 6, the surface compositions after the Ar-5% H2/95% Ar-purged experiment fit the expected trend with potential while those obtained after the O2-5% H2/95% Ar-purged experiment do not. For the Ar-5% H2/95% Ar-purge this could be taken to indicate that the switch in purge gas did lead to reduction of the SF 1.5 surface. For the O2-5% H2/95% Ar-purged experiment the surface is substantially more oxidized than would be expected based on the ECORR (250 mV) achieved after switching to 5% H2/95% Ar-purge, but less oxidized that would be expected at the ECORR (60 mV) achieved after the initial O2 purge. It is possible that the H2 introduced on switching the purge gas partially reduced oxidized surface states. However, it is also possible that UVI states partially chemically dissolved and no reduction occurred. Until a more extensive database is available, there is no conclusive evidence that the presence of H2 can lead to reduction of a preoxidized surface. 5. Summary and conclusions The influence of oxic (O2-purged), anoxic (Ar-purged), and potentially reducing (5% H2/95% Ar) conditions on the corrosion of UO2 has been studied on SIMFUEL spec-
16
M.E. Broczkowski et al. / Journal of Electroanalytical Chemistry 602 (2007) 8–16
imens in slightly alkaline KCl solutions at 60 °C. The specimens contained different levels of rare-earth (RE+III) doping and various numbers of noble metal (epsilon) particles. (i) Under oxic conditions the steady-state ECORR values achieved were positive, compared to the threshold ECORR for the onset of UO2 oxidation, and the electrode extensively oxidized to UV/UVI. The number density of noble metal particles in the UO2 did not appear to exert a measurable effect on the extent of surface oxidation. (ii) Under anoxic conditions, lower steady-state ECORR values were achieved. The presence of noble metal particles appeared to suppress ECORR and reduce the extent of oxidation of the surface. However, this suppression was not measurably dependent on the number density of particles. (iii) In 5% H2/95% Ar-purged solutions, both the steadystate ECORR and the extent of oxidation of the UO2 surface were dependent on the number density of the noble metal particles in the SIMFUEL. As the number density increased, ECORR was suppressed to more negative values, and the UV/UVI content of the surface decreased. This effect is attributed to the reversible decomposition of H2 to adsorbed H atoms on the noble metal particles. Since these particles are galvanically coupled to the UO2 matrix, this causes ECORR to be suppressed and fuel oxidation to be avoided. (iv) There is no conclusive evidence to date, that the presence of dissolved H2 can lead to the reduction of preformed oxidized states on the UO2 surface. Acknowledgements This research is funded under the Industrial Chair agreement between the Canadian Natural Sciences and Engi-
neering Research Council (NSERC) and Ontario Power Generation (OPG), Toronto, Canada. Surface Science Western is gratefully acknowledged for the use of their XPS instrument. References [1] Nuclear Waste Management Organization (NWMO), Choosing a Way Forward: The Future Management of Canada’s Used Nuclear Fuel, November 2005, This report available at www.nwmo.ca. [2] F. King, M. Kolar, Ontario Power Generation Report No. 00819REP-01200-10041-ROO, 1999. [3] L.H. Johnson, D.M. LeNeveu, F. King, D.W. Shoesmith, M. Kolar, D.W. Oscarson, S. Sunder, C. Onofrei, J.L. Crosthwaite, Atomic Energy of Canada Limited Report, AECL-11494-2, COG-95-552-2, 2000. [4] F. King, M. Kolar, Mat. Res. Soc. Symp. Proc. 412 (1996) 547. [5] D.W. Shoesmith, J. Nucl. Mater. 282 (2000) 1. [6] K. Spahiu, L. Werme, U.-B. Eklund, Radio Chim. Acta 88 (2000) 507. [7] S. Ro¨llin, K. Spahiu, U.-B. Eklund, J. Nucl. Mater. 297 (2001) 231. [8] J.C. Tait, L.H. Johnson, in: Proceedings of the 2nd Canadian Nuclear Society, Toronto, 1986, p. 611. [9] S. Sunder, G.D. Boyer, N.H. Miller, J. Nucl. Mater. 175 (1990) 147. [10] F. King, M.J. Quinn, N.H. Miller, Swedish Nuclear Fuel Company (SKB) Report, SKB TR-99-27, 1999. [11] M.E. Broczkowski, J.J. Noe¨l, D.W. Shoesmith, J. Nucl. Mater. 346 (2005) 16. [12] M.E. Broczkowski, R. Zhu, Z. Ding, J.J. Noe¨l, D.W. Shoesmith, Mater. Res. Soc. Symp. Proc. 932 (2005) 449. [13] P.G. Lucuta, R.A. Verrall, H. Matzke, B.J. Palmer, J. Nucl. Mater. 178 (1991) 48. [14] A.F. Gerwing, F.E. Doern, W.H. Hocking, Surf. Interface Anal. 14 (1989) 559. [15] B.G. Santos, H.W. Nesbitt, J.J. Noel, D.W. Shoesmith, Electrochim. Acta 49 (2004) 1863. [16] J.D. Rudnicki, R. Russo, D.W. Shoesmith, J. Electroanal. Chem. 372 (1994) 3.