The influence of silicate on the development of acidity in corrosion product deposits on SIMFUEL (UO2)

Corrosion Science 48 (2006) 3852–3868 www.elsevier.com/locate/corsci

The influence of silicate on the development of acidity in corrosion product deposits on SIMFUEL (UO2) B.G. Santos *, J.J. Noe¨l, D.W. Shoesmith Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, Ont., Canada N6A 5B7 Received 2 December 2005; accepted 26 February 2006 Available online 22 May 2006

Abstract The influence of silicate on the formation of corrosion products and secondary phase deposits on corroding/dissolving nuclear fuel surfaces under waste disposal conditions were investigated. SIMFUEL (doped uranium dioxide) has been characterized over the potential range 100–500 mV (vs. SCE). Through the use of X-ray photoelectron spectroscopy (XPS) the surface composition over this potential range has been determined. The presence of silicate was found to have little influence on the V oxidation of UO2 to produce the oxidized surface layer UIV 12x U2x O2þx . However, depending on its V concentration it could suppress the conversion reaction UIV U 12x 2x O2þx ! UO3  yH2 O for potentials 6250 mV (vs. SCE). This suppression appears due to the adsorption of silicate on the fuel surface. The accumulation of a hydrated UVI silicate on the fuel surface occurs when UVI is formed at higher anodic potentials, leading to a suppression of anodic dissolution. The UO2þ 2 formed in, and subsequently transported out of, the acidified sites accumulates on the outer fuel surface as a hydrated UVI silicate.  2006 Elsevier Ltd. All rights reserved. Keywords: Corrosion product deposits; X-ray photoelectron spectroscopy; Silicate; Nuclear waste disposal; Uranium dioxide

*

Corresponding author. E-mail address: [email protected] (B.G. Santos).

0010-938X/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2006.02.012

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1. Introduction The release of the majority of radionuclides from spent nuclear fuel under permanent disposal conditions will be controlled by the rate of dissolution of the UO2 fuel matrix. The nature of the fuel dissolution process, and hence its rate, will evolve as environmental conditions within the waste vault change with time. Since the solubility of uranium is many orders of magnitude greater under oxidizing, as opposed to non-oxidizing, conditions [1], the redox condition is an important parameter in determining fuel dissolution and, hence, radionuclide release. An extensive effort has been expended to measure the dissolution rate of UO2 in simple solutions under well-defined conditions [2–14] in order to facilitate the development of performance assessment models to predict fuel corrosion rates inside failed waste containers. Under granitic conditions, groundwater oxidants, such as dissolved O2, will be rapidly scavenged by reaction with the Cu container material, oxidizable minerals, or organic matter present in the repository environment [15]. Since the container is expected to easily outlive the oxic period, anoxic ground water conditions should prevail at the time of container penetration. However, radiolytic oxidants produced directly at, or in the environment immediately adjacent to, the wetted fuel surface could potentially cause significant fuel corrosion [15,16]. Since redox conditions at the surface of the fuel will change with time over the repository lifetime as a-radiation fields decay, the composition of the fuel surface should evolve. The influence of groundwater constituents on the evolution of the corroding/dissolving nuclear fuel surfaces needs clarification, since it will influence the formation of corrosion/dissolution product deposits on the surface of the fuel. The accumulation of deposits could have a number of distinct effects; (i) it could block the fuel dissolution process; (ii) the deposits formed could incorporate radionuclides released by fuel dissolution, thereby preventing, or at least delaying, their release to groundwaters; (iii) the trapping of a-emitting radionuclides could influence local redox conditions at the fuel surface, and potentially promote the corrosion/alteration of the fuel. The combination of redox conditions, temperature and groundwater composition will determine the physical and chemical properties of deposits formed on fuel surfaces. A review of available literature [17–23] shows that a complicated range of phases is possible as redox conditions evolve from initially oxidizing to eventually anoxic. The incorporation of Na+ and Ca2+, and the formation of silicates are a common feature of these phases, indicating that these groundwater constituents could exert a major influence on fuel corrosion over extended periods of time. Also, since the alteration phases (deposits) formed on fuel surfaces in long term laboratory experiments are similar to those observed in the geological alteration of natural uraninite deposits, there are good grounds to expect these phases are likely to control fuel alteration in the long-term [19,24,25]. In addition, the corrosion rate of UO2 in single pass flow-through experiments, when the formation of deposits on the fuel surface would not be expected, have been shown to be directly influenced by the species Ca2+ and SiO4 4 . Uranium concentrations measured with unirradiated UO2 pellet fragments in these experiments showed that the effect of adding 15 mg L1 of Ca2+ to a 2 · 103 mol L1 HCO 3 solution decreased the concentration of U in the effluent solution by a factor of 3 [26]. A subsequent addition of 30 mg L1 of

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SiO4 4 increased this suppression by a further factor of 100, indicating a much stronger influence of silicate than Ca2+. A similar suppression of the UO2 corrosion rate by a factor of 200 was observed by Tait and Luht [27] in similar experiments. Furthermore, they showed that the corrosion rate of spent fuel and the release of radionuclides were suppressed by a similar factor in a groundwater containing Ca2+ and SiO4 4 . Our previous work [23] showed that Ca2+ consistently suppressed the anodic currents observed in potentiostatic experiments when compared to similar currents measured in Na+ solutions. It was concluded that the primary influence of Ca2+ was its ability to interfere electrostatically with the fuel dissolution process, and that this could be accounted for by one, or both, of two possible mechanisms: (a) adsorption of Ca2+ inhibits the stabilization of the cation precursor to dissolution, (UO2(OH)2)ads; (b) by denying protons access to surface anion sites, Ca2+ adsorption blocks the O2 anion transfer reaction from the fuel surface. Since its influence is expected to be greater than that of Ca2+, we have been studying the influence of silicate on fuel dissolution. In this paper we present the results of our electrochemical/X-ray photoelectron spectroscopic studies. 2. Experimental 2.1. Fuel specimens The electrodes used in this study were 1.5 at.% SIMFUEL made from pellets fabricated at Chalk River Laboratories, Ontario. These are natural UO2 pellets doped with nonradioactive elements (Ba, Ce, La, Mo, Sr, Y, Zr, Rh, Pd, Ru, and Nd) to replicate the chemical effects of a CANDU reactor irradiation to 1.5 at.% burn up [28]. Simulated fuels of this type provide a practical means to study the effect on fuel corrosion of the impurities created by in-reactor fission [29–31]. 2.2. Electrode preparation The SIMFUEL electrodes were prepared using a methodology previously described [32]. Prior to use, the electrode was polished (wet) with a 600 grit SiC paper, rinsed with distilled deionized water and repolished (wet) with 1200 grit SiC paper. 2.3. Solutions The solutions used were 0.1 mol L1 NaCl containing various amounts of Na2SiO3 Æ 5H2O with the pH adjusted to 9.5. All solutions were prepared with distilled deionized water purified using a Millipore milli-Q-plus unit to remove organic and inorganic impurities. All chemicals were reagent grade and purchased from Fisher Scientific. The solution pH was monitored with an Orion model 720A pH meter, and adjusted, when desired, with reagent grade HCl or NaOH.

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2.4. Electrochemical cell and equipment All experiments were conducted in a standard three-electrode, three compartment cell. The central chamber of the cell was separated from the reference and counter electrode chambers by glass frits. The counter electrode was made of Pt foil spot welded to a Pt wire. All potentials were measured, and are quoted, against a saturated calomel reference electrode (SCE). All electrochemical experiments were performed with a Solartron 1287 potentiostat controlled by Corrware software. 2.5. Surface analysis X-ray photoelectron spectroscopy (XPS) was used to characterize the surface films formed electrochemically. An SSX-100 spectrometer was used to record the XPS spectra. The U (4f7/2), satellites associated with U (4f5/2), and the valence band regions of the UO2 spectrum were recorded. The procedure used to analyze these regions involved a fitting routine which was 70% Gaussian and 30% Lorentzian with a Shirley background correction [33]. The XPS band at 380 eV, resulting from the ejected 4f7/2 electrons, was used to monitor the chemical state of uranium atoms. The proportions of uranium oxidation states UIV, UV and UVI in the electrode surface were determined by fitting the 4f7/2 peak using accepted binding energies (UIV 379.9 eV, UV 380.4 eV and UVI 381.3 eV) and spectral satellite analyses as previously described [32,34]. The position of the C (1s) line at 284.6 eV was recorded and used, when necessary, to correct for surface charging. The peak position used for Si (2s) was 153.1 eV [35]. This fitting routine was also applied to the O (1s) peak, which was resolved into contributions from O2, OH, Si–O and H2O [36,37] using the peak characteristics shown in Table 1. 3. Results 3.1. Constant potential current-time behaviour (1 h) Fig. 1 shows potentiostatic current-time curves recorded over a 1 h period at potentials in the range 100–500 mV in 0.1 mol L1 NaCl plus 0.1 mol L1 Na2SiO3 Æ 5H2O. The general features of the current responses are similar to those observed previously in NaCl [23] and CaCl2 [32]. At lower applied potentials, the current decreases logarithmically with time, consistent with the growth of a thin UO2+x surface layer. As the potential is increased, the currents at short times initially appear to be approaching a steady-state value before decaying to a similar value for all potentials. As in NaCl solutions the time at which this change in slope occurs, decreases with increasing potential. However, in

Table 1 O (1s) binding energy values and peak parameters Chemical state 2

Oxygen (O ) Hydroxyl (OH) Silicon (Si–O) Attached H2O

Binding energy (eV)

FWHM (eV)

529.76 530.32 530.60 531.58

1.20 1.50 1.70 2.00

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(v)

10-3

i / A cm-2

(iv) 10-4

(iii)

(ii) 10-5

(i) 100

101

102

103

t/s

Fig. 1. Potentiostatic current–time curves recorded for 1 h in 0.1 mol L1 silicate + 0.1 mol L1 NaCl at potentials of (i) 100 mV, (ii) 200 mV, (iii) 300 mV, (iv) 400 mV, (v) 500 mV.

contrast to previous observations in NaCl [32], no tendency for the current to increase significantly again at longer times is observed for potentials P300 mV. Fig. 2(A)–(C), show direct comparisons of current–time curves recorded in the presence and absence of silicate at 100 mV (A), 200 mV (B) and 500 mV (C). Fig. 3(A)–(C) compares the charges, obtained by integration of the current–time plots, for the same potential range. The linear log I–log t plots for potentials around 100 mV (Fig. 2(C)) have previously been V attributed to oxidation of the UO2 surface to UIV 12x U2x O2þx involving the incorporation of 2 O at interstitial sites in the UO2 lattice accompanied by oxidation of UIV atoms in adjacent sites to UV [28,38,39]. The similarity between the log I–log t plots then suggests little influence of silicate on this oxidation process and this is confirmed by the similarity in the rates of charge accumulation in the two solutions, Fig. 3(A). At 200 mV, our previous V studies indicate that the oxidation of the substrate surface to UIV 12x U2x O2þx is accompanied VI by the formation of U species and their accumulation in an outer layer on the surface. The current in the presence of silicate is suppressed up to 600 s, beyond which the currents are effectively identical. This suppression is more clearly evident in the charges in Fig. 3(B). At 500 mV the influence of silicate is very marked, Figs. 2(C) and 3(C). The current–time behaviours at short times are effectively identical, but once the shoulder in the log I–log t begins to appear (around 10 s) the current in silicate solution is subsequently suppressed to values similar to those observed at the lower potential of 200 mV. By contrast, the current in NaCl alone is sustained. This sustained current at high potentials at long times has been shown to be due to the development of acidity within local sites on the rough fuel surface due to rapid uranyl ion dissolution and hydrolysis [32,40]. This was shown to occur only when UVI species began to accumulate on the fuel surface. The very marked decay in current in silicate containing solution suggests that an accumulation of UVI on the fuel surface occurs but does not lead (on this time scale) to the development of acidity. 3.2. X-ray photoelectron spectroscopy after oxidation (1 h) Fig. 4 shows an XPS survey spectrum taken after anodically oxidizing the UO2 surface at 100 mV for 1 h. A Si (2s) peak is found at 153.1 eV [35]. Fig. 5(A) shows the relative

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A

i / A cm-2

1E-3

1E-4

(ii)

1E-5

(i) 1E-6

1

2

10

3

10

10

t/s

B

1E-3

i / A cm-2

(ii) 1E-4

(i)

1E-5

100

101

102

103

t/s

C

i / A cm-2

1E-3

(ii) 1E-4

1E-5

(i) 100

101

102

103

t/s Fig. 2. Potentiostatic current–time curves recorded for one hour in 0.1 mol L1 silicate + 0.1 mol L1 NaCl (i) and in 0.1 mol L1 NaCl alone (ii) at potentials of 100 mV (A), 200 mV (B) and 500 mV (C).

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0.030

(ii)

A

QA / C cm-2

0.025

(i)

0.020

0.015

0.010

0.005

0.000

0

500

1000

1500

2000

2500

3000

3500

t/s 0.05

(ii)

B

QA / C cm-2

0.04

0.03

(i) 0.02

0.01

0.00

0

500

1000

1500

2000

2500

3000

3500

t/s

1.75

(ii)

C

QA / C cm-2

1.50 1.25 1.00 0.75 0.50 0.25

(i) 0.00

0

500

1000

1500

2000

2500

3000

3500

t/s Fig. 3. Comparison of anodic charges obtained by the integration of the current-time curves from Fig. 2 for 0.1 mol L1 silicate + 0.1 mol L1 NaCl (i) and 0.1 mol L1 NaCl (ii).

proportions of the three oxidation states as a function of applied potential after 1 h of anodic oxidation in silicate plus 0.1 mol L1 NaCl for the entire potential range investigated. The behaviour observed in silicate is generally different to that obtained in NaCl [32] over the same potential range, and a number of points are emphasized:

B.G. Santos et al. / Corrosion Science 48 (2006) 3852–3868

Intensity / cps x 103

6

U 4f5/2 Oa

3859

U 4f7/2

O 1s

5

4

U 4d 3

2

C 1s Si 2s U 5d

1

900

800

700

600

500

400

300

200

100

0

Binding Energy / eV Fig. 4. XPS survey spectrum recorded on a UO2 surface after 1 h in 0.1 mol L1 silicate + 0.1 mol L1 NaCl at 100 mV.

(i) The high UV content of the surface at 100 mV demonstrates the presence of the V UIV 12x U2x O2þx layer, consistent with the linearity of the log I–log t plot (Fig. 1) and the claim that silicate does not impede its formation; (ii) The increasing dominance of UVI at higher potentials is consistent with the development of the shoulder in the log I–log t plot attributed to the accumulation of UVI on VI V the UIV content of the surface is suppressed over 12x U2x O2þx surface. In silicate the U the potential range 100–200 mV as compared to NaCl but enhanced at higher potentials, Fig. 5(B). The lack of a marked shoulder in the log I–log t plots at 200 mV V (Fig. 2(B)) suggests the conversion UIV 12x U2x O2þx ! UO3  yH2 O is inhibited initially in silicate solution, possibly due to the adsorption of silicate. The enhancement in UVI content at higher potentials, along with the steep decay in the log I–log t plot (Fig. 2(C)) suggests that, as soon as UVI is formed, it is stabilized on the electrode surface by silicate leading to the suppression of further oxidative dissolution. This suppression prevents local acidification, which would require the on-going anodic dissolution-uranyl ion hydrolysis to be sustained; (iii) The drop in UVI and slight increase in UIV and UV at the highest applied potential, 500 mV, indicates the re-dissolution of UVI and exposure of the underlying UIVUVO2+x surface due to local acidification [32,40,41]. Fig. 6(A) shows that the amount of silicate found on the surface of the electrode steadily increased from 13% (100 mV) to 23% (400 mV), and Fig. 6(B) shows a clear correlation between the accumulation of Si and that of UVI on the electrode surface, consistent with the formation of a current-suppressing UVI-silicate solid. Fig. 7(A) and (B) shows the O (1s) spectra deconvoluted into the contributions from O2, OH, Si–O and H2O, recorded after one hour oxidations at 100 mV and 400 mV, and Fig. 8 shows the proportions of these O-containing species as a function of applied potential over the full potential range investigated. A comparison of Figs. 5 and 8 reveals that, when the surface is predominantly UVI (E P 200 mV) the Si–O content of the surface increases markedly. A similar increase in H2O content of the surface with increasing potential is also found. These observations, taken together, show that the decrease in

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UIV UV UVI

70

Relative Fraction / %

60 50 40 30 20

A

10 100

200

300

400

500

E / mV 70

NaCl NaCl + silicate Relative Fraction of UVI (%)

60

50

40

30

20

B

10 100

200

300

400

500

E / mV Fig. 5. Relative fractions of all three U oxidation states as a function of applied potential after anodic oxidation for 1 h in 0.1 mol L1 silicate + 0.1 mol L1 NaCl (A) and a comparison of the relative fractions of UVI as a function of applied potential (1 hour) in 0.1 mol L1 silicate + 0.1 mol L1 NaCl and 0.1 mol L1 NaCl (pH 9.5) (B).

current with time in this potential region (Fig. 1), can be attributed to the accumulation of a hydrated UVI silicate on the fuel surface. Consistent with this claim is the general conversion from a surface comprising predominantly O2 and OH at E < 200 mV, Fig. 8 (the potential region expected to be dominated by a UIVUVO2+x layer, Fig. 5) to one dominated by Si–O and H2O at higher potentials. Further inspection of Figs. 5, 6(A) and 8 show that the UVI, Si–O and H2O contents of the surface decrease slightly at the highest applied potential of 500 mV, an indication that, perhaps, some re-dissolution of the dissolution inhibiting phase due to local acidification is occurring.

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24

Atomic % of Si

22

20

18

16

14

A 12

100

200

300

400

500

E / mV 70

Relative Fraction of UVI / %

60

50

40

30

20

B 10 12

14

16

18

20

22

24

Amount of Si / % Fig. 6. Atomic % of Si in the surface of 1.5 at.% SIMFUEL after 1 h of anodic oxidation in 0.1 mol L1 silicate + 0.1 mol L1 NaCl solutions as a function of potential (A). Correlation plot of the relationship between the amounts of UVI and Si, in the electrode surface (B).

3.3. Constant potential current–time behaviour (up to 24 h) To investigate whether local acidity could eventually develop despite the presence of silicate, a series of anodic dissolution experiments was performed in 0.1 mol L1 silicate solutions for various times. A potential of 250 mV was chosen since the development of local acidity was observed in NaCl solutions at this potential, but it is still in the potential range where the XPS results, Fig. 5(B), show silicate suppresses the formation of the surface UVI species shown to be a prerequisite for acidification [32]. Fig. 9 shows that an extension of the period of anodic oxidation eventually leads to the generation of noise and a tendency for the current to increase, clear indications of the development of acidity. Therefore, while silicate may initially suppress the development

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36 A

32 Intensity / cps x 103

OHO2-

28 H2O 24

Si-O

20 16 12 532

36

531 530 Binding Energy / eV

529

B

32 Intensity / cps x 103

Si-O 28

H2O

24 OH-

20

O2--

16 12 532

531 530 Binding Energy / eV

529

Fig. 7. The O (1s) XPS peak resolved into contributions from O2, OH, Si–O and H2O for surfaces anodically oxidized at 100 mV (A) and 400 mV (B) for 1 h in a 0.1 mol L1 silicate + 0.1 mol L1 NaCl solution.

of acidity it is unable to prevent it completely. The XPS results in Fig. 10 confirm that this local acidification may enhance dissolution but does not prevent the accumulation of UVI on the surface once neutral conditions beyond the acidified site are encountered. 3.4. Anodic oxidations at 250 mV with varying [Silicate] (1 h) To further investigate the indication (Sections 3.1 and 3.2) that silicate can suppress dissolution under moderately oxidizing conditions a series of anodic oxidations were performed at 250 mV as a function of silicate concentration. These experiments, Fig. 11, were limited to 1 h to avoid the longer-term development of acidity observed in the experiments described in Section 3.3. The shape of the curves is very reproducible and not very

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3863

60

O2OHSi-O H2O

Relative Fraction / %

50

40

30

20

10

0

100

200

300

400

500

E / mV

i / A cm-2

Fig. 8. Relative fractions of O2, OH, Si–O and H2O as a function of applied potential after 1 h of anodic oxidation in 0.1 mol L1 silicate + 0.1 mol L1 NaCl solution.

10

-3

10

-4

10

-5

(v)

(ii) (iv) 10

(i)

-6

10

1

10

2

10

3

(iii) 10

4

10

5

t/s Fig. 9. Potentiostatic current–time curves recorded on 1.5 at.% SIMFUEL in 0.1 mol L1 silicate + 0.1 mol L1 NaCl at an applied potential of 250 mV for (i) 1 h, (ii) 4 h, (iii)12 h, (iv) 18 h and (v) 24 h.

dependent on the added silicate concentration. Over the first 100 s the slope of the plot is close to 1 consistent with the occurrence of the film growth process V UO2 ! UIV 12x U2x O2þx . The closeness of the current values and the similarity in slopes of the log I–log t relationships as a function of silicate concentration confirm the claim

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U IV

Fraction of Surface Oxidation / %

60

50

U

V

U

VI

40

30

20

10 0

4

8

12

16

20

24

t/h Fig. 10. Relative fractions of all three U oxidation states in the surface of 1.5 at.% SIMFUEL oxidized at 250 mV as a function of time in 0.1 mol L1 silicate + 0.1 mol L1 NaCl.

i / A cm-2

10 -3

10 -4

(i) 10

(ii)

-5

(iii) (iv)

10 -6 10

0

10

1

10

2

103

t/s Fig. 11. Potentiostatic current–time curves recorded on 1.5 at.% SIMFUEL in 0.1 mol L1 NaCl with varying concentrations (mol L1) of silicate at an applied potential of 250 mV for 1 h (i) 0.025, (ii) 0.050, (iii) 0.075, (iv) 0.1.

(above) that the presence and concentration of silicate does not significantly influence this film growth process. However, a subtle influence of silicate does exist. The location of the shoulder, taken to be the onset of the formation of a dissolution-inhibiting UVI layer on the electrode surface, shifts to shorter times indicating a more rapid passivation of the surface as the silicate concentration increases. This effect is very clear in Fig. 12, which shows that the total anodic charge is markedly decreased with increasing silicate concentration.

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0.050

QA / C cm-2

0.045

0.040

0.035

0.030 0.025

0.050

0.075

[silicate] / mol L

0.100

-1

Fig. 12. Anodic charges for dissolution obtained by the integration of I  t curves from Fig. 11.

0

-50

I / μA cm-2

2 (iv)

-100

(ii) (iii) -150

(i) 1

-200

-250 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

E/V Fig. 13. Cathodic stripping voltammograms performed after 1 h of oxidation at 250 mV in 0.1 mol L1 NaCl with varying concentrations (mol L1) of silicate (i) 0.025, (ii) 0.050, (iii) 0.075, and (iv) 0.1.

Although terminating the experiments after one hour was designed to avoid local acidification within the fuel, the cathodic stripping voltammograms (measured in a separate, but identical set of experiments) in Fig. 13 show that such a process did occur. The peak V (1) at 0.8 V is due to the reduction of the UIV 12x U2x O2þx /UO3 Æ yH2O layer [40] while the shoulder at 0.3 V ((2) in Fig. 13) has been shown to be the combined reduction of protons and surface adsorbed dissolution intermediates present under acidic conditions

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Fraction of Surface Oxidation / %

60

50 IV

U V U VI U

40

30

20

10

0.025

0.050

0.075

[silicate] / mol L

0.100

-1

Fig. 14. Relative fractions of all three U oxidation states in the surface of 1.5 at.% SIMFUEL after oxidation at 250 mV for 1 h in 0.1 mol L1 NaCl solutions containing various concentrations of silicate.

[41]. While peak 1 is present at all silicate concentrations the shoulder beginning around – 0.3 V decreases as the concentration of silicate is increased, especially in 0.1 mol L1 silicate solution. Fig. 14 shows the relative proportions of UIV, UV and UVI in the electrode surface following the 1 h oxidations presented in Fig. 11. There is an obvious increase in the amount of UV and accompanying decrease in the amount of UVI on the surface as the silicate con-

Atomic Percent of Si / %

20

18

16

14

12 0.025

0.050

0.075

0.100

[silicate] / mol L-1 Fig. 15. Atomic % of Si in the surface of 1.5 at.% SIMFUEL after oxidation at 250 mV for 1 h in 0.1 mol L1 NaCl containing various concentrations of silicate.

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V centration increases, clearly indicating that the conversion of the mixed UIV 12x U2x O2þx layer to the corrosion product deposit UO3 Æ yH2O is suppressed as the concentration of silicate is increased. This effect is accompanied by an increase in Si content of the surface, Fig. 15, clearly indicating that the suppression of the conversion reaction can be attributed to V silicate adsorption on the UIV 12x U2x O2þx surface.

4. Conclusions The presence of silicate has little influence on the oxidation of UO2 to produce an oxiV dized surface layer UIV 12x U2x O2þx . However, depending on its concentration it suppresses V the conversion reaction UIV 12x U2x O2þx !UO3 Æ yH2O for potentials 6250 mV. This suppression appears due to the adsorption of silicate on the fuel surface. Once UVI is formed anodically at higher potentials the accumulation of a hydrated UVI silicate on the fuel surface occurs, leading to a suppression of anodic dissolution. While the presence of silicate can suppress the development of acidity in localized sites in the fuel surface it cannot totally prevent it. Once the UO2þ 2 dissolved in these acidified sites is transported out of the sites it again accumulates on the outer fuel surface as a hydrated UVI silicate.

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