Current State of Knowledge of Water Radiolysis Effects

AECL 12 082

KADIOACIIVfc WASTE CURRENT STATE OF KNOWLEDGE OF MANAGtMf NT WATER RADIOLYSIS EFFECTS ON AND DISPOSAL KEYWORDS: nuclear-fuel corro- SPENT NUCLEAR FUEL CORROSION sion, radiolysis. uranium dioxide

H. CHRISTENSEN Studsvik Material AB S-61182 Nykdping, Sweden

S. SUNDER* Atomic Energy of Canada Limited CA0200110 Fuel Safety Branch, Chalk River, Ontario, Canada KOJ I JO

Received October 5, 1999 Accepted for Publication January 18, 2000

dionuclides to migrate from the used fuel in the disposal vault to the biosphere. Literature data on the effect of water radiolysis prod- The release rate of radionuclides in the fuel will de- ucts on spent-fuel oxidation and dissolution are re- pend on the dissolution rate of the used fuel. Used nu- viewed. Effects of gamma radiolysis, alpha radiolysis, and clear fuel is mainly UO2 (^95%), which has a very low 16 17 dissolved Oi or H2O2 in unirradiated solutions are dis- solubility in water under reducing conditions. - Al- cussed separately. Also, the effect of carbonate in gamma- though groundwaters are generally reducing at the ex- irradiated solutions and radiolysis effects on leaching of pected depth of a disposal vault,18 the redox conditions spent fuel are reviewed. In addition, a kinetic model for may be altered because of radiolysis of water by ionizing calculating the corrosion rates of UO2 in solutions un- radiation associated with the fuel, as the radiolysis of wa- dergoing radiolysis is discussed. The model gives good ter produces both oxidants and reductants.19'20 The im- agreement between calculated and measured corrosion portance of radiolysis for oxidation and dissolution of 1 3 410 12 rates in the case of gamma radiolysis and in unirradi- UO2 has been discussed previously. ' ' ' ated solutions containing dissolved oxygen or hydrogen In this paper, a review is given of radiolysis effects, peroxide. However, the model fails to predict the results including discussion of the effects of gamma and alpha of alpha radiolysis. In a recent study, it was shown that radiation, of O2 and H2O2 (without irradiation), and of the model gave good agreement with measured corro- carbonate on UO2 fuel corrosion. Effects of radiolysis sion rates of spent fuel exposed in deionized water. The have been noticed in some studies of spent-fuel leaching. applications of radiolysis studies for geologic disposal Development of a kinetic model, based on one pro- of used nuclear fuel are discussed. posed by Christensen and Bjergbakke,1 to predict the ox- 11 12 idation rates of UO2 fuel due to water radiolysis ' is discussed as well as its applications to the corrosion behavior of used fuel in a disposal vault.

I. INTRODUCTION II. EFFECT OF GAMMA RADIOLYSIS

The concept of geological disposal of used nuclear It was shown as early as 1981 by Gromov21 that the fuel in corrosion-resistant containers is being investi- dissolution rates of UO2 and U3O8 were increased by gated in several countries, including Canada, Finland, Ger- gamma radiation when samples were immersed in aque- many, Spain, Sweden, and the United States.1 ~1S The ous solutions of carbonate (pH ~ 10) or H+ (0.1 N purpose of the disposal concept is to minimize and delay H2SO4). At a dose rate of 1 Gy-s~\ the increase in the the entry of radionuclides into the biosphere so that their dissolution rate of UO2 in carbonate (pH 10) is 2 X effects on man and the environment are negligible. Trans- HT'ing-m-^min"1 (3 X 10~Vg.cm -day"1). (Gro- port by groundwater is the most likely pathway for ra- mov did not specifically report his dissolution rate in unirradiated carbonate solutions, but his data suggests that *E-raail: [email protected] at a 100-times-lower dose rate, the increase is 10 times

102 NUCLEAR TECHNOLOGY VOL.131 JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER lower.) The dissolution rate increased with increasing dose by the reactions of O2 with e^. Therefore, to a first rates. A systematic study of the effect of the various wa- approximation, the number of O2 radicals formed in ter radiolysis products on UO2 oxidation and dissolution formate-containing solutions is twice the number of OJ has been carried out by Sunder et al.3'410 Different scav- radicals formed in the solutions containing J-butanol engers were used to optimize the production of specific under the same conditions.10'12 However, the use of radicals in order to study their effects on UO2 corrosion. formate also results in the formation of CO2, and hence A solution saturated with N2O favors the formation CO3", an anion known to accelerate the oxidative disso- 17 of OH radicals, as N2O scavenges hydrated electrons to lution of uranium oxides. Note, O2 is the basic form of form OH radicals: H02 with a pKa of 4.8. Experiments were also carried out in Ar-purged solutions, with no scavenger, resulting H2O e N2O -> N2 OH (1) in the formation of a mixture of OH and e^ radicals, and in O2-saturated solutions, with no scavenger, to produce A solution with mainly O2 radicals was obtained by 3410 radiolysis of oxygenated solutions containing either a mixture of OH and O2 radicals. f-butanol, (CH3)3COH, or formate ions, HCOO", as ex- The dose rate of the irradiated solutions was varied plained in reactions shown in Eqs. (2) through (5): between 1 and 300 Gy/h, using a 192Ir gamma source {t\ = 74 days). The oxidation of UO2 was monitored by OH + (CH3)3COH -» H2O + (CH3)2(CH2)COH , (2) recording the open-circuit corrosion potential (ECORR) of the UO2 electrode as a function of time. The thickness of OH + HCOO -> CO2 + H2O (3) the film formed on the UO2 electrode, as a result of oxidation during radiolysis, was determined by measuring CO2 O2 -> CO2 + 07 (4) the cathodic charge Qc needed to reduce the film in and cathodic-stripping voltammetry (CSV). An additional freshly polished UO2 pellet was placed in the electro- el, + O2 •o; (5) chemical cell to determine, using X-ray photoelectron The formate and f-butanol are used to scavenge the spectroscopy (XPS), the surface composition of the ox- OH radicals. Thus, in formate-containing solutions, OJ idized layer formed on UO2 during radiolysis. radicals are formed by two reactions, i.e., the reac- Figures 1 and 2 show the corrosion potential of UO2 tions of O2 with e~q and CO2 radicals, respectively. In measured as a function of time in Ar-purged or oxygen- 1 f-butanol solutions, the OJ radicals are formed mainly ated 0.1 mol-^" NaC104 solution (pH = 9.5) at various

200- 200 i

I ° + + o , § • * • o Q D • o * i. a ° ° D |-200- */ a °° I-200 S n a x x x x £ Q X X X X X xxx*x 0 °xx x 0 xx isio n E -400 a * ~ x _ g f (-400 1 0 -600- 0 -600 4 8 12 16 20 24 -za -ts -to -016 OJO as to ts Time (hours) Log (Time/hoirs) (a) (b) Fig. 1. Corrosion potential of a UO2 electrode in 0.1 mol-l l NaClO4 solution, pH = 9.5, Ar purge, in gamma fields with absorbed dose rates of 280 Gy/h [+], 29 Gy/h [o], 11 Gy/h [*], 6.4 Gy/h [•], and no field (x), respectively; as a function of (a) time and (b) log time. The arrow indicates the time at which the gamma field was removed. (From Ref. 10, reprinted with permission of the copyright holder.)

NUCLEAR TECHNOLOGY VOL.131 JULY 2000 103 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

200 200

I -400 o

-600-1 -600-1 -2.0 -15 -1.0 -0.5 0.0 0.5 1.0 1.5 Log (Time / hours) (b) 1 Fig. 2. Corrosion potential of a UO2 electrode as a function of time in O2-saturated 0.1 mol-€~' NaClO4 +0.01 mol-^" HCOONa solution, pH = 9.5, in gamma fields with absorbed dose rates of 280 Gy/h [+], 23.5 Gy/h [O], 9.5 Gy/h [*], 0.87 Gy/h [•]; and O2-saturated 0.1 mol-€"' NaClC>4 solution no field (X), respectively; as a function of (a) time and (b) log time. (From Ref. 10, reprinted with permission of the copyright holder.)

absorbed dose rates.10 The aerated solution also contained ysis products is exemplified by the difference between 1 0.01 mol-^" sodium formate to maximize the concen- steady-state ECORR values recorded at low dose rates and tration of O2 radicals, reactions shown in Eqs. (3) and (4). those recorded in either unirradiated Ar-purged solution, In both solutions, ECORR rose rapidly with time, this rise Fig. la, or unirradiated aerated solution, Fig. 2a. Once a being faster in aerated than in Ar-purged solutions, see steady-state ECORR is achieved, switching off the radia- Figs, lb and 2b. Also, the effect of absorbed dose rate is tion field produces only a minor decrease in ECORR > apparent for lower potentials in aerated than in Ar-purged Fig. la, confirming that stage 2 of oxidation is irreversible.

solutions. At the lowest dose rates used for each solution, Steady-state values of ECORR are plotted as a func- the rise of ECORR was arrested over the potential range tion of the logarithm of absorbed dose rates in Fig. 3 - 500 mV < ECORR ^ -100 mV versus the Saturated Cal- (Ref. 10). This plot includes values measured in N2O- omel Electrode (SCE) in both Ar-purged and oxygenated purged solutions, as well as in aerated solutions contain- solutions. ing f-butanol. The hatched and shaded areas in this figure The shape of the curves in Figs. 1 and 2 suggests show the range of steady-state ECORR values recorded on that oxidation of the UO2 electrode surface occurs in two other UO2 electrodes in many previous experiments in separate stages: (a) stage 1, up to around —100 mV ver- aerated and Ar-purged solutions, respectively.10'22"25

sus SCE, in which ECORR rises approximately exponen- The steady-state ECORR values achieved in the pres- tially with time, and (b) stage 2, above around —100 mV ence of gamma radiation (shown in Figs. 1, 2, and 3) are 22 versus SCE, in which ECORR changes are approximately higher than those achieved in either aerated solutions linear with time until the final approach to steady state. or in solutions containing hydrogen peroxide,24'25 strongly These stages are more distinct in oxygenated solutions, suggesting that oxidation and dissolution of the UO2 by Fig. 2, as is the dose-rate dependence of stage 2. For radiolytically produced radicals is significant. This con- ECORR values below -100 mV (stage 1), data in the two clusion is also demonstrated in Figs. 4 and 5 (Ref. 3). figures (Figs. 1 and 2) can be modeled by a single direct Calculations have shown that the concentration of H2O2 logarithmic relationship. There is a definite change in the is decreased when a gamma field is introduced to solu- exponent on the log-log plots for more positive ECORR tions containing H2O2. At the same time (see Fig. 5), the values. corrosion rate is increased (as indicated by the increase The attainment of steady state can take from ~2 to in the ECORR value), clearly demonstrating that the rad- ~28 h, depending on the solution redox potential (i.e., icals produced by radiolysis are more effective than H2O2 whether oxygen is present) and the absorbed dose rate. in oxidation of UO2. (The reasons for the plateau seen at The importance of even small concentrations of radiol- short times in Fig. 5, showing the corrosion potential of

104 NUCLEAR TECHNOLOGY VOL. I3l JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

200-

-400- -1 Log (Dose Rate / Gy/h) Fig. 3. Corrosion potential of a UO2 electrode as a function of log absorbed dose rate after corrosion for —18 h in (^-saturated 1 1 1 0.1 mol-^" NaClO4 +0.01 mol-^" HCOONa[+], O2-saturated +0.01 mol^" f-butanol(A),N2O-saturated0.1 mol-€"' 1 NaClO4 [*] and Ar-purged 0.1 mol-^" NaClO4 solution [O]; all at pH 9.5. The shaded areas show the range of steady- state ECORR values recorded on other UO2 electrodes in previous experiments in aerated (hatched area) or Ar-purged (darker shaded area) solutions. (From Ref. 10, reprinted with permission of the copyright holder.)

a UO2 electrode in H2O2 solutions, are not completely and 27). The value Qc is a measure of the thickness of understood at present. Note, similar behavior in corro- the oxidized films formed on the electrode surface. The sion potential-time plots was seen, at very short times, thickness of these films is directly related to ECORR, up for UO2 electrodes in solutions undergoing radiolysis, to ~0 mV, irrespective of the composition of the solu- Figs, lb and 2b.) tion (i.e., the nature of the predominant radiolytic oxi- Higher corrosion potentials are achieved, at a given dant) or whether the oxidation was achieved rapidly or absorbed dose rate, in solutions containing dissolved ox- slowly. At higher potentials, even though a steady-state ygen (and formate ions or r-butanol) than in oxygen-free ECORR is achieved, the thickness of the oxidized surface solutions (obtained by purging with argon or N2O), Fig. 3. film continues to increase. VI IV The solutions containing dissolved oxygen produce O2 Figure 7 shows the U :U ratio obtained using XPS radicals [Eqs. (3), (4), and (5) and Ref. 26]. This sug- analyses of UO2 specimens exposed to the same solu- 10 gests that the O2 is an effective agent for UO2 fuel oxi- tions for equivalent times. Consequently, the surface dation3-4-10 composition of these specimens is expected to reflect that VI IV Figure 6 shows the cathodic charge Qc measured by of the electrodes. The U :U ratio increases slowly over CSV as a function of the value of ECORR achieved by the the ECORR range —400 mV to —100 mV versus SCE, dur- time the experiment was terminated.27 The plot includes ing which the film thickness increases linearly with time, ECORR values measured in all fout of the solutions over a see Fig. 6. Once steady-state ECORR values are achieved wide range of absorbed dose rates and for various dura- around —100 mV versus SCE, the film thickening that tions of the experiment. In many cases, the ECORR values occurs at these potentials is accompanied by a large VI IV VI are not steady-state values but close to it, as most of the increase in U :U —demonstrating that U oxides form experiments were terminated after ~18 h (Refs. 4, 10, at the surface at these potentials.

NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 105 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

200 1 1 1 1 1 o AIR PURGE + ARGON PURGE o

8 + ** o o o o 0 .oo o oo° ° r • > of + E 8 o o o o -200 - o 0 i o o a 1§ *t - o 4 -

-400 -

•f 1 1 1 10 20 -600 -400 -200 200 HOURS Corrosion Potential (mV vs. SCE) Fig. 4. Corrosion potential of a UO2 electrode in 0.1 mol • £ ' NaClO4 solution, pH = 9.5, (a) air purged, [o]; and Fig. 6. Cathodic charge Qc measured by cathodic-stripping (b) argon purged, [+]; gamma fields were introduced voltammetry at 20 mV/s as a function of the corro- after about 18 h of corrosion without any radiation. The sion potential ECORR achieved under natural corro- 1 increase in the corrosion potential, on introducing the sion conditions in 0.1 mol^" NaClO4 (pH = 9.5) radiation field represents the effects of radiolysis prod- solution containing various additives and gamma- ucts. (From Ref. 3, reprinted with permission of the irradiated at various absorbed dose rates for various copyright holder.) times. (From Ref. 10, reprinted with permission of the copyright holder.)

FIELD 35.1 Gy/h, ()H = 9.6, [H202]= io-'M 1 I '

(ft -

+ + -300 + FIELD ON AT I6.83h x OFF AT 41.1 t o

J is O is" • O- -600 -

1 - 1 i i 16 32 48 HOURS Fig. 5. Corrosion potential of a UO2 electrode in 0.1 mol • i ' -600 -400 -200 0 200 NaClO4 solution containing H2O2, argon purge, gamma Corrosion Potential (mV vs. SCE) field was introduced after —17 h of corrosion without any radiation field. The increase in the corrosion po- Fig. 7. UVI:UIV ratio in the surface of a UO2 specimen after tential, on introducing the radiation field, represents the exposure to gamma-irradiated, 0.1 mol • €"' NaClO4 so- effects of radiolysis products. (From Ref. 3, reprinted lution with pH = 9.5. (From Ref. 10, reprinted with with permission of the copyright holder.) permission of the copyright holder.)

106 NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

The oxidation of UO2 in the presence of oxidants pro- Experimental dissolution rates were deduced from duced by the gamma radiolysis of water occurs in two the measured steady-state corrosion potentials using an distinct stages: electrochemistry-based empirical dissolution model (see the Appendix). The following equation was used to ob- 1. The formation of a thin layer of UO2+i with a tain the corrosion rates: stoichiometry close to UO2.33. This process occurs over the following potential range: —500 mV < ECORR ^ (6) -100 mV versus SCE. where 2. The subsequent oxidative dissolution of this sur- CR = corrosion rate in /tg-cm"2-day~1 face layer to produce soluble UVI species and secondary E = exponent of 10 phases, probably hydrated schoepite (UO3 • J:H2O), on the electrode surface. This process starts around ECORR at ECORR = steady-state corrosion potential in volts approximately —100 mV versus SCE and eventually (versus SCE). achieves steady state at a value of ECORR determined by the absorbed dose rate. Experimental corrosion rates, obtained from mea-

sured values of ECORR using Eq. (6), were compared In unirradiated aerated solutions, the formation of with the calculated corrosion rates obtained using a ki- secondary phases appears to be concentrated initially at netic model (described in Sec. IV) Table I. The experi- grain boundaries.22 Their confinement to these sites made mental results varied between ~10"4 /jbg-cm~2-da.y~l them slow to redissolve when the solutions were sub- in Ar-purged solutions irradiated at 5 Gy-h"1 up to 2 1 sequently purged with nitrogen. A similar concentration 0.75 yu,g-cm- -day- in O2-purged solutions irradiated 1 of hydrated UO3 phases at grain boundaries, and their at 280 Gy-h- . subsequent slow redissolution, could explain the slow In experiments by Christensen, the oxidation and dis- relaxation in ECORR observed in formate solutions once solution of UO2 pellets immersed in gamma-irradiated the radiation field is switched off. water, containing various scavengers, were measured.33

TABLE I Comparison of Calculated and Measured Corrosion Rates*

Corrosion Rate (fig • cm 2 • day ') Dose Rateb Solutiona (Gyh"1) Calculations Experiments

Ar-purged 5 0.003 ~8E-5C Ar-purged 120 4.7E-3 Ar-purged 280 0.12 0.10

N2O-purged 5 0.04 -8E-5 N2O-purged 110 4.4E-2 N2O-purged 280 0.84 0.36 N2O-purged 600 3

O2-purged 0 2.7E-3 3.8E-3 O2-purged 5 0.06 0.13 O2-purged 100 0.17 O2-purged 280 0.84 0.75 O2-purged 600 3

6 d ooo o H2O2 lO" M 2.2E-4 2.1E-4 4 d H2O2 2 X 10" M 0.13 0.13 4 d H2O2 3 X 10- M 0.18 2 d H2O2 5 X 10- M 13 5

•Results from Refs. 6, 10, and 12. 1 "Base solution is 0.1 mol^" NaC104, pH 9.5. bGamma-radiation. cRead as 8 X 10"5. dAr-purged, M is 1

NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 107 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

TABLE II Corrosion Rates in Alpha-Irradiated, Ar-Saturated Solutions*

Alpha Source Corrosion Rate (fig • cm 2 • day ') Corrosion Potential (MCi) (Gy/h) (mV SCE) Measured Calculated 4.7 37 -90 1.1 E-4a 3.2 E-2 50 400 10 4.5 E-3 0.33 100 790 40 1.4 E-2 250 1980 90 8.9 E-2 1.5 686 5440 115 0.21 2.9

*Results from Refs. 6, 25, 37, and 38. "Read as 1.1 X 10"4.

The composition and thickness of the oxidized surface TABLE III layer were measured using secondary ion mass spectrom- G Values for Gamma and Alpha Irradiation etry (SIMS) and XPS. Uranium in solution and on walls of Neutral Water Used in the Model was measured by chemical analysis. The corrosion rate 2 caused by OH and O2 radicals was ~3 /i,g-cm~ -day~' G Value at a dose rate of 600 Gy -h"1; see Table I. Species Gamma Alpha

OH 2.67 0.24 III. EFFECT OF ALPHA RADIOLYSIS p~~ 2.66 0.06 H+ 2.76 0.30 The effects of products from alpha radiolysis of wa- H 0.55 0.21 ter on UO2 oxidation and dissolution have been studied H2 0.45 1.30 by Shoesmith and Sunder,6 Shoesmith et al.,24 Sunder 34 36 37 H2O2 0.72 0.985 et al. " and Bailey et al. Since alpha activity within OH" 0.1 0.02 used fuel decays slowly in comparison with gamma and H2O -6.87 -2.71 beta activity, there is a possibility that oxidative dissolu- 0 0.22 tion will be sustained by alpha radiolysis for a long time. or Corrosion potentials were measured in a thin-layer electrochemical cell that allows an alpha source of known strength to be brought within ~25 fim of a UO2 elec- 24 E ORR trode. Using steady-state C values measured for Fig. 5. (Further details of effects of H2O2 on UO2 are alpha sources of various strengths, values for the disso- discussed in Sec. V.B). lution rate as a function of alpha source strength have In the experiments of Bailey etal.,37 an alpha source been obtained in Ar-purged solution. was placed —30 /u,m from the UO2 electrode. When the The results for various alpha sources are given in separation distance was increased to 750 /tin, the corro- Table II (Refs. 24, 25, and 35 through 37). The corrosion rate decreased by almost two orders of magnitude. sion rates (as obtained from ECORR values; see the Ap- The interpretation given was that H2O2 caused corrosion pendix) are considerably lower than those observed for in the narrow gap, but it was swept out by diffusion when comparable gamma dose rates; see Table I. The differ- the gap was opened. At the larger distance, radiolysis prod- ence may partly be explained by the difference in initial ucts did not reach the electrode. Thus, it seems likely that radiolysis yields, as seen from Table III. For low-linear- the behavior of UO2 in the presence of alpha radiolysis energy-transfer (LET) radiation, such as gamma and beta, will be similar to its behavior in H2O2. the yields of radicals are higher and the yields of mol- Sunder et al.34 studied the effect of alpha radiolysis ecules (H2 and H2O2) are lower. The opposite is true of water on the oxidation and dissolution of UO2 at 100°C for high-LET radiation, such as alpha radiation. There- as a function of alpha-field strength and water chemistry fore, the effect of alpha radiolysis may be caused mainly using XPS. In N2-purged solutions, the oxidation of UO2 by H2O2, the oxidizing effect of which has been found increased with the strength of the alpha flux: An alpha to be considerably lower than the effect of the radicals flux greater than or equal to that from a 250-/ACJ 241Am OH and O2, as can be seen from results shown in source led to oxidation of UO2 beyond the UO2.33 (U3O7)

108 NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

0.2

Dissolution with Secondary Phase Formation

+ (UOf )SURF<

*UO3-2H,O

Onset of Oxidative Dissolution

Oxidation of UO2 Surface

-0.3 ua

Time »• Fig. 8. Schematic diagram showing the behavior of the corrosion potential measured on UO2 electrodes in 0.1 mol-^"1 NaClCU solution (pH around 9.5). The stages of oxidation and dissolution are also illustrated. (From Ref. 12, reprinted with permission of the copyright holder.)

stage, and an alpha flux equal to that from a 5-/tCi source 1. Only the species that are produced in the water did not result in UO2 oxidation beyond the UO2 33 within one diffusion length from the UO2 surface can re- a stage. The presence of dissolved H2 in water at concen- act with UO2. trations ^1.6 X 10~4 mol-€~' reduced the oxidation and 2. The heterogeneous reaction could be substituted dissolution of UO due to alpha radiolysis at tempera- 2 with a homogeneous one, assuming that the UO reacts tures >100°C. In solutions, at temperatures >100°C and 2 as if one monomolecular layer of UO is dissolved within containing dissolved H , the oxidation did not increase 2 2 the range of the radicals. with increase in the alpha dose rate. It was concluded that alpha radiolysis caused by 500-yr-old CANDU 3. The rate of the heterogeneous reactions of U-species (Canada deuterium uranium) fuel (corresponding to an are similar to known rates of homogeneous reactions of alpha flux of 5 fid-cm'2) would not cause oxidation metal ions (such as iron) in water. beyond the UO2 33 stage and hence not cause increased dissolution.34 Based on these assumptions, Christensen and Bjerg- bakke set up a model, including reactions of water radiolysis products with each other. They tested the model against experiments by Gromov,21 and a fair agreement IV. MODELING was obtained between measured and calculated corrosion rates. l Th^itinetic model was further developed and opti- • IV.A. Gamma and Beta Radiolysis mized for the neutral solution conditions expected in ground waters.11-12 This refinement was based on a se- Christensen and Bjergbakke' have presented a mech- ries of electrochemical open-circuit corrosion experi- anistic model for UO oxidation in water solutions under- 3 410 2 ments, ' mainly using a gamma source; see Sec. II. going radiolysis. The model is based on three assumptions: A schematic diagram illustrating the general change in a the corrosion potential with time for neutral solutions, It has been shown that one can use a solubility-limited model and indicating the progression in the oxidation and dis- for calculating the dissolution rates of UO2 nuclear fuel if the uranium oxidation is

NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 109 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER to approximately UO2.33 and (b) a steady-state stage dur- tions are compared with results from electrochemical ex- ing which dissolution (as UVI) occurs at a constant rate periments in Tables I and V. from a surface layer of UO2.33 (5 to 8 nm thick). At suf- The initial concentration of UO2 is set at a value of ficiently large oxidant concentrations, reprecipitation of 5-10~4 mol-€~', which corresponds to the dissolution VI 23 U to yield a secondary phase (UO3 • 2H2O) can occur. of a monolayer of UO2 in a water layer of ~25-fim thick- We have commonly taken the time for the potential ness.1 As mentioned earlier, the thickness of the water to reach a value of -100 mV versus SCE as an approx- layer, effective in causing UO2 oxidation by the radiol- imate measure of the rate of formation of this UO2.33 ysis products, is assumed to be equal to the diffusion range layer.27 The appropriate corrosion (dissolution) rate is X of the reacting radicals. This range is estimated from then obtained once steady-state dissolution conditions the calculated lifetime of radiolytically generated spe- have been established, i.e., once the corrosion potential cies at a specific dose rate.43 (For the OH radicals, \ var- ! (ECORR)SS has been attained (Fig. 8). At this potential, ies from 16 fim at a dose rate of 280 Gy • h~ to 44 /im at 1 the current for the oxidative dissolution of UO2 is coun- 5 Gy-h" .) Since this dependence of diffusion range on terbalanced by an equal and opposite current for the ox- dose rate is only to the power —0.25 for the dose-rate idant reduction. It is this value for the ECORR that is range in our experiments, we have taken it to be constant used in the electrochemical relationship [Eq. (6)] to ob- at 25 fim. The diffusion range of molecular radiolysis 43 tain a value for the corrosion rate. For dissolution in products, i.e., O2 or H2O2 is much higher. aerated solutions, the rate-controlling step appears to be 39 Table V compares the times taken to reach 90% of the reduction of oxygen, a notoriously slow reaction. the steady-state concentration of U03H species (using the For radiolytic oxidants (OJ, OH, and H2O2), electron model given in Table IV) with the experimental times to transfer to the oxidant will be much faster, and the rate reach an open-circuit corrosion potential of —100 mV of the overall process is more likely to be controlled by versus SCE (Ref. 12). As stated in footnote a, the "oxi- the anodic dissolution step. dative dissolution" does not take place until the oxida- The reaction mechanism and rate constants used in our tion of the UO2 fuel surface to UO2 33 state has taken refined model are given in Table IV, and the G values are place. This takes place at a potential of -100 mV versus given in Table III. Some of the changes from the original SCE. Consequently, oxidative dissolution starts after the model1 to the refined model12 are discussed as follows: potential has reached a value of —100 mV. In the kinetic v model (given in Table IV), UO3 is formed by oxidation 1. The reversible formation of U species (denoted of UO3H. Thus, the final rate of corrosion will be ob- by UO3H in Table IV) can be assumed to represent the tained only after a steady state of UO3H is obtained. Com- reversible formation of the intermediate UO2.33 film. This parison of the time to reach a potential of —100 mV versus film reaches a steady-state thickness equivalent to a SCE in UO2, in corrosion experiments, with the time taken steady-state concentration of Uv in our model. to reach 90% of the steady-state concentration of UO3H 2. The formation of the UO2.33 surface film leads species, in model calculations, provides an independent to a decrease in the oxidation rate as it thickens. This is check on the model. brought about in the model by decreasing the concen- Table I compares the calculated and measured cor- tration of UO2 with time [reactions (59) and (60) in rosion (dissolution) rates for UO2. Experimental corro- Table IV]. sion rates were obtained from values of (ECORR)SS using the electrochemistry-based model.628 Calculated disso- 3. The rate constants for oxidation by H2O2 [reac- lution rates were obtained from concentration-time pro- tions (33) and (39)] were lowered relative to those for files predicted by the model. Figure 9 shows two such radicals in agreement with experimental observation. sets of calculated profiles for Ar-purged solutions, at ini- 4. Reaction with an arbitrary rate constant was added tial pH 9.5, undergoing gamma radiolysis at dose rates of 5 and 280 Gy • h~ \ respectively. Dissolution rates were to represent decomposition of H2O2 on the UO2 surface VI [i.e., reaction (50)]. obtained from the linear increase in dissolved U (denoted by UO3D in Table IV) over the period 20 to 30 h, 5. Reactions to represent the slow oxidation of UO2 as indicated in Fig. 9. by dissolved O2 in the absence of a radiation field were For high dose rates, the model predicts quite accu- added [reactions (57) and (58)]. rately the rate of oxidation of the UO2 surface to UO2.33, i.e., the predicted time to reach 90% of the steady-state 6. The experimental rates with which the predic- v concentration of U (UO H) is in good agreement tions of this model are to be compared were determined 3 with the measured time to reach —100 mV versus SCE, in an open system, making it necessary to include rate Table V. The largest discrepancy between measured and constants for the removal of gases (O and H ) from the 2 2 predicted rates in irradiated solutions is for measure- system [reactions (48) and (49)]. ments in N2O-purged solutions. In this case, the exper- Model calculations were carried out using the computer imental values are suspect, owing to unusual potential- 42 program MAKSIMA-CHEMIST, and these calcula- time curves.10

110 NUCLEAR TECHNOLOGY VOL.131 JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

TABLE IV Reaction Mechanism of UO2 Oxidation*

Number Reaction" Rate Constant11

c 1. OH + H2 = H + H2O k(l) = 3.400E+07 2. OH + H2O, = HO2 + H2O k{2) = 2.700E+07 3. OH + O2 + OH" Hi) = 9.000E+09 = o2 4. H + O2 = HO? k{A) = 1.800E+10 5. H + o2- = HOj fc(5) = 2.000E+10

6. E- + O2 = 02" *(6) = 1.900E+10 7. E" + H2Oo = OH + OH~ *(7) = 1.200E+10 8. E" + O2" = HO7 + OH" - H2O yfc(8) = 1.300E+10 9. E~ + H+ = H ifc(9) = 2.200E+10 10. E" + H2O = H + OH" ife(lO) = 2.000E+01 11. E" + HO2 = O" + OH" /t(ll) = 3.500E+09 12. OH + HO2 = H2O + O2 /t(12) = 7.900E+09 13. OH + OH = H2O2 Jfc(13) = 5.50OE+09 14. H + HO2 = H2O2 jt(14) = 2.000E+10 15. H + H2O2 = H2O + OH )t(15) = 6.000E+07

16. H + OH" = E" + H2O Jt(16) = 1.500E+07 17. HO + 02" + HOJ *(17) = 9.600E+07 2 = o2 18. HO2 + HO2 = H2O2 + O2 i(18) = 8.400E+05 + 19. H + O2" = HO, /t(19) = 4.500E+10 + 20. HO2 = H + O2 *(20) = 8.OOOE+O5 + 21. H + HO2 = H2O2 k(2\) = 2.000E+10 + 22. H2O2 = H + HO2 /t(22) = 3.560E-02 23. OH + OH" = H2O + 0" *r(23) = 1.200E+10 24. 0" + H2O = OH + OH" ifc(24) = 9.300E+07 + 25. H + OH" = H2O /t(25) = 1.430E+ll + 26. H2O = H + OH" *(26) = 2.574E-05 27. E" + OH = OH" fc(27) = 3.000E+10 28. H + OH = H2O fc(28) = 2.000E+10 29. H + H = H2 *(29) = 1.000E+10 30. E~ + H = H2 + OH" -H2O *(30) = 2.500E+10

31. OH + HO2 = HO2 + OH' /t(31) = 5.000E+09 32. UO2 + OH = UO3H k(32) = 4.000E+08 33. UO2 + H2O2 = UO3H + OH fc(33) = 2.000E-01 34. UO2 + HO2 = UO3H + H2O2 - H2O *(34) = 2.000E+08 35. UO2 + O2 = UO3H + HO2 - H2O jfc(35) = 2.000E+08

36. UO3H + UO3H = UO3 + UO2 + H2O *(36) = l.OOOE-01 37. UO3H + OH = UO3 + H2O *(37) = 8.000E+08 38. UO3H + E" = UO2 + OH~ /t(38) = 5.OOOE+O8 39. UO3H + H2O2 = UO3 + H2O + OH Jfc(39) = 2.000E-01 40. UO3H + o2- = UO3 + HO2 k(AO) = 4.000E+08

41. UO3H + HO2 = UO3 + H2O2 /fc(41) = 4.000E+08 42. UO3 + E" = UO3H + OH" -H2O ik(42) = 5.OOOE+O8 43. UO3 + O2" = UOJ + O2 *r(43) = 4.000E+07 44. UOJ + H2O = UO3H + OH~ fe(44) = 1.000E+01 45. UO3H + H = UO2 + H2O *(45) = 4.500E+06

46. UO3 + H = UO3H k(A6) = 4.500E+06 47. UO3 + HO2 = UO3H + O2 Jfe(47) = 4.000E+07 48. = O D Jt(48) = 2.100E-01 o2 2

See footnote at end of table. (Continued)

NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 111 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

TABLE IV (Continued)

Number Reaction8 Rate Constant1"

49. H2 = H2D jfc(49) = 3.5OOE-O1 50. H2O2 = H2O + O ifc(50) = 1.000E-03 51. O + O = o2 *(51) = 1.000E+09 52. N2O + E~ = N2 + OH + OH" - H2O Jt(52) = 6.000E+09 + 53. HCOOK + OH = CO2" + H2O + K *(53) = 2.000E+09 54. CO2" + O2 + CO2 *(54) = 6.000E+09 + 55. CO2 + H2O = HCOJ + H k(55) = 1.000E+03 56. UO3 = UO3D )t(56)=4.000E-04

57. UO2 + o2 = UO3H + HO2 - H2O fe(57) = 1.000E-03 58. UO3H + O2 = UO3 + HO2 Jt(58) = 1.000E-03 59. UO2 = UO2D *(59) = 7.000E-04 60. UO2D = UO2 *(60) = 3.500E-07 •Results are from Refs. 11, 12, 40, and 41. a UO3D represents UO3 diffusing out of the reaction layer near the UO2 surface [reaction (56)]. UO2D represents a dummy specie used to maintain supply of UO2 in the reaction layer [reactions (59) and (60)]; see Refs. 1 and 12. Reactions (48) and (49) are added to include rate constants for the removal of 62, and H2, respectively, from the solution system (as O2D and H2D). See text, Sec. IV.A, for details. bRates are in units of (mol • i~')~' • s~' for second-order reactions. Reaction rates are for room temperature. cRead as 3.400 X 107.

TABLE V

Comparison of Experimental and Calculated Times for the Formation of a UO2.33 Film on UO2*

Time (h) Dose Rateb Solution8 (Gyh"1) Calculated0 Experimental*1

Ar-purged 5 25 19 Ar-purged 280 2 2

N2O-purged 5 25 9 N2O-purged 280 0.2 0.5

O2-purged 5 20 1.6 O2-purged 280 0.2 0.15 O2-purged 0 17 3 6 H2O2 10" mol-€"' 0 > 40 8 4 1 H2O22Xl0- mol-€- 0 0.8 0.02

•Results from Refs. 6, 10, and 12. ! "Base solution is 0.1 mol-€~ NaC104, pH 9.5. bGamma radiation. Time to reach 90% steady-state concentration of UO3H species. dTime taken to reach a corrosion potential of —100 mV versus SCE.

The agreement between calculated and electrochem- IV.B. Alpha Radiolysis ically measured corrosion (dissolution) rates is generally good; see Table I. It should be noted that significant un- It was not possible to accurately predict the corro- certainty is associated with the extrapolation required to sion rate of UO2 in solutions undergoing alpha radiolysis calculate dissolution rates from electrochemical data. using the mechanistic model discussed earlier. The

112 NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

10- -e B- -a • o-

sa—• a a oo •• am UO2

10" 10"

H B—B

an o a^faa op am UO2

o ooo oo ooo UO3H ASn&AA^ UO3

H2O2

10 10* 10°

Fig. 9. Concentration of selected radiolysis products in solution with UO2 and undergoing gamma radiolysis: Ar-purged water, pH 9.5, dose rate (a) 5 Gy-h"1 and (b) 280 Gy-h"J. Arrows define the period over which the dissolution rates were measured. (From Ref. 12, reprinted with permission of the copyright holder.)

predicted corrosion rates were orders of magnitude higher values (*» DR1 8) (Ref. 38). Obviously, the model is lack- than the electrochemically measured values, see Table II. ing one or more parameters that are of decisive impor- In addition, the dose rate (DR) dependence is lower for tance in the alpha-radiolysis experiments. Additional the calculated values («* DR1'0) than for the measured work, both experimental and model development, is

NUCLEAR TECHNOLOGY VOL.131 JULY 2000 113 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER required to understand the effects of alpha radiolysis on fuel dissolution (see Sec. VII). \V -2 IV.C. Spent Fuel - 10 The dissolution rates of spent fuel, as a function of 28 \ e X dose rate, have been predicted using the empirical elec- _OXJ_DATn/E_piSSOC_N trochemical correlation given in Eq. (6). These dissolu- 10 THRESHOLD tion rates are based on the electrochemically measured corrosion potentials in solutions undergoing gamma and alpha radiolysis. < Predictions have been made for the corrosion rates (r 10" of both CANDU fuel and light water reactor (LWR) fuel in a geologic vault, e.g., see Figs. 10, 11, and 12. The O second x-axis (time axis) in Figs. 10, 11, and 12 gives to 10" the times for the dose rate shown in the first x-axis. V) (References giving relationships between the dose rate and its age, for the reference CANDU for the Canadian ,-10 Waste Management Program, are discussed in Sec. VII. 10" Such relationships for spent UO2 fuels with different 10- I02 10 10-1 burnups can be obtained using the method described in DOSE RATE (Rad, h"1) these references.) From these figures, it can be deduced 100 200 500 1000 00 TIME (a)

Fig. 11. Dissolution rates for UO2 as a function of the logarithm of beta dose rates. The second x axis (time axis) 1 — ™*— 1 1 I 1 1 «— shows the time since the discharge of the fuel from 1 "*••— the reactor, at which time such activity levels are achieved on the surface of the LWR fuel with a burnup id2 of 45 MW-d/kg U. Filled and open symbols indicate >. \ two independent sets of data (1 rad = 10 mGy). o \ 1 M ? \ OXIDATIVE DISSOL' N ^ ^ THRESHOLD Id4 - g . cm " =i i 0 LJJ H~ o that oxidative dissolution of CANDU fuel [burnup 685 or 6 \\ o ° id - GJ-kg-'U (~8 MWdkg-'U)] should not take place 2 o O o for fuel older than 500 yr. The corresponding time limit 1 \° o for LWR fuel (burnup 45 MWdka" U) is -2000 yr J o O 8

Z are higher (at longer times) than those given in Fig. 11. 10 IO 00 as they include the effects of alpha, beta, and gamma TIME (a) radiations, while those shown in Fig. 11 deal only with Fig. 10. Dissolution rates for UO2 as a function of the loga- the effects of the beta radiation. rithm of gamma dose rate: O, argon-purged solutions; •, aerated solutions. A is the range of dissolution rates measured in unirradiated but aerated solutions.22 The IV.O. General Aspects second x axis (time axis) represents the times at which such dose rates would be achieved at the surface of a The model, presented briefly earlier, has been pre- CANDU fuel bundle after being discharged from the sented in detail previously.12 It appears from Table I that reactor (1 rad = 10 mGy). the model gives an excellent agreement with results from

114 NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

Log (microCurie)

I I 100 1000 100000 Time / Years KAXIHUH PERIOD FOR WHICH OXIDATIVB DISSOLUTION

-*• IS PREDICTED TO BE IMPORTANT Fig. 12. Dissolution rates (in units of jag-cm~2-day^1) of UO2 as a function of alpha source strength in solutions undergoing alpha radiolysis (0.1 mol • t~' NaClO4, pH = 9.5). The second x axis (time axis) shows the time since the discharge of the fuel from the reactor, at which time such activity levels are achieved on the surface of the reference used fuel in the CNFWMP (Bruce CANDU reactor fuel, burnup 685 GJ/kg U). The solid line shows the maximum possible dissolution rate in such a solution, and the horizontal dashed line corresponds to the threshold below which the application of a kinetic model based on electrochemical principles is no longer necessary. (From Ref. 36, reprinted with permission of the copyright holder.).

gamma-radiolysis experiments. The agreement for the cor- V. EFFECT OF 02 AND H202 IN UNIRRADIATED rosion rate is excellent in unirradiated O2 purged and in SOLUTIONS H2O2 solutions and in almost all other cases within a factor of 2 (Table I). We think this is the most important and Although the effect of O2 and H2O2 in unirradiated useful part of the model. We recognize that there is a dis- solutions is not directly related to radiolysis, these sys- crepancy between model and experiments for the times tems can give information useful for evaluation of the shown in Table V, even in some cases in irradiated solu- effect of radiolysis. The effect of alpha radiolysis is some- tions. An explanation of this discrepancy is not easy to ex- 112 3840 times assumed to be the same as the effect of H2O2 plain here due to the lack of space' - and may not be (Refs. 24 and 37). of practical use although it is scientifically interesting. As shown in Sec. IV.B, the model fails to predict the V.A. Effects of Dissolved 0 results from alpha-radiolysis experiments. Possibly, new 2 experiments, planned and being carried out presently by Several workers have studied the effects of dis- other groups and using a different technique may solve solved oxygen on UO2 dissolution rates. Recent reviews this discrepancy (see Sec. VII). of these studies are given in Refs. 28 through 30 and 45.

NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 115 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

-8 -9 -8 -7 -6 -5 -4 -3 -2 Log( [02] /mo I /L) Fig. 13. Corrosion rates (in units of /xg-cm 2-day ') for UO2 as a function of dissolved oxygen concentration: O, Shoesmith 28 46 and Sunder ; A, Casas et al. The results of Shoesmith and Sunder were obtained from the ECORR measurements (see text), and those of Casas et al. are from the measurements of uranium concentrations in solutions.

Figure 13 shows the corrosion rates of UO2 in noncom- properties, instead of p-type semiconducting proper- plexing solutions at pH ~9 as a function of the dissolved ties,6'30-49) and several used-fuel dissolution studies have oxygen concentration, as reported by Shoesmith and Sun- reported a reaction order of <1.0 (Ref. 50 and refer- der.28 This figure also includes recent results of Casas ences therein). It should be noted here that it is difficult et al.46 for comparison purposes. Although the results of to determine reaction order with respect to oxygen from Shoesmith and Sunder were obtained using their electro- the studies with irradiated UO2 (used fuel) samples, as chemical model and those of Casas et al. were obtained one cannot separate the effects from the dissolved oxy- by directly measuring the amounts of dissolved uranium gen from those of the water-radiolysis products formed in the solution, there is a good agreement between these by the ionizing radiation associated with the used fuel. It results.b was shown earlier that the radiolysis of water increases The results shown in Fig. 13 suggest a first-order re- the rate of UO2 dissolution. lationship between the dissolution rates and dissolved- oxygen concentration. This conclusion is in agreement with the results of Grandstaff47 and of Thomas and Till.48 V.B. Effects of H202 However, there are several other studies that have re- Shoesmith and Sunder have studied effects of H2O2, ported values of less than one for the reaction order with 28 30 as a function of concentration, on UO dissolution. Fig- respect to oxygen concentration. A reaction order of 2 49 ure 14 shows their rates of dissolution of UO as a func- —0.5 was reported by Hocking et al. during their in- 2 tion of H O concentration in noncomplexing solutions, vestigations of O reduction on an electrode made of an 2 2 2 pH ~9. This figure also shows the results of Christensen unusual UO pellet (pellet with n-type semi-conducting 2 et al.51 and Gimenez et al.52 for comparison purposes. The results of Shoesmith and Sunder28 are from elec- b We have not included the results of Grambow et al.45 in Fig. 13 trochemical measurements while those of other workers as the units used in Fig. III.8 of Ref. 45 are unclear. were obtained by chemically measuring the amounts of

116 NUCLEAR TECHNOLOGY VOL.131 JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

-5 -4 -3 -2 -1 Log [H2O2] / mo I /L

2 Fig. 14. Corrosion rates (in units of fig-cm -day ') for UO2 as a function of hydrogen peroxide concentration: O, Shoesmith and Sunder28; I, Christensen et al.51; and *, Gimenez et al.52 The results of Shoesmith and Sunder were obtained from the ECORR measurements (see text) and those of Gimenez et al. are from measurements of uranium concentrations in solutions. In Christensen's experiment polished specimens of UO2 were exposed in H2O2 solutions. After exposure, uranium in solution and precipitated uranium species were added to the oxidized species on the surface (as determined by SIMS) to establish the corrosion rate.

dissolved uranium in the solution. There is a fair agree- The data of Shoesmith and Sunder, shown in Fig. 14, ment between the results of Shoesmith and Sunder with suggest three distinct regions of behavior of UO2 disso- 51 52 28 those of Christensen et al. and Gimenez et al. lution rates as a function of H2O2 concentration, i.e., A comparison of the results shown in Figs. 13 and 3 14° suggests that the dissolution rates are similar in so- 1. For [H2O2] > 5 X 10~ mol-€~\ the dissolution rate increases with the peroxide concentration with ap- lutions containing oxygen or H2O2 at comparable concentrations. This is despite the fact that the oxidation proximately first-order dependence. reaction 4 3 2. For 2 X 1(T < [H2O2] < 5 X 10" mol • Z~ \ the UO -» UO dissolution rate is independent of the peroxide concen- 2 2.33 tration. has been shown to be much faster (by a factor of ~200) 4 in H O than that in the oxygenated solutions.24'25'38 The 3. For[H2O2] < 2 x 1(T mol-T the dissolution 2 2 rate falls rapidly. similarity of the dissolution rates in H2O2 and oxygenated solutions, at comparable concentrations, suggests that It has been proposed that the hydrogen peroxide de- the dissolution reaction composition to oxygen and water in the intermediate concentration range competes with the oxidative dissolution UO233 reaction.24'25 Therefore, it is essential to include a per- is slow in both solutions. oxide decomposition step to model the effects of H2O2 on dissolution rates. Details of our attempts to include c We have not included the results of Grambow et al.45 in Fig. 14 the hydrogen peroxide decomposition reactions in our as the units used in Fig. III.8 of Ref. 45 are unclear. model to calculate radiolysis effects are given in Ref. 40.

NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 117 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

Sunder et al. have also studied the effects of pH on the Dose rates in water from alpha, beta, and gamma ra- UO2 oxidation by H2O2 (Ref. 25). They observed an diations were not given in the report by Loida et al., but increase in UO2 oxidation rate with a decrease in solu- some details have been given in a previous report by the tion pH. same group.55 Loida et al.53 used LWR fuel with a burnup >50 MWd/kg U. Inside a thin crack in 1 to 10-yr-old fuel of this burnup an alpha dose rate of ~3000 Gy/h VI. EFFECTS OF CARBONATE IN IRRADIATED SOLUTIONS and beta dose rates of 9000 to 80000 Gy/h may be expected.56 The alpha dose rates on an open surface may The presence of complexing anions increases the ox- be expected to be about half of the alpha-dose values in- idative dissolution rates of UO2 by increasing the rate of side the thin crack (irradiation from only one side). + removal of uranyl species (UO2 ) from the surface to Shoesmith et al. investigated the corrosion of used the solution. As carbonate ions form strong complexes nuclear fuel in aqueous perchlorate and carbonate solu- 57 with uranyl ions, the effects of carbonate on UO2 oxida- tions. The corrosion behavior of electrodes constructed tion have been studied by several researchers. Some re- from used fuel was compared to the corrosion behavior cent reviews of these investigations are given in Refs. 29, observed on unirradiated UO2 and SIMFUEL. Used-fuel 30, and 50. It has been shown that the UO2 dissolution electrodes exposed to aerated NaC104 solutions (pH ~ reaction order with respect to carbonate is dependent on 9.5) oxidized rapidly compared with unirradiated UO2 the carbonate concentration. Its value is generally much electrodes and achieved steady-state values of corrosion less than one, and values as low as 0.25 have been re- potential (ECORR) much more positive than expected when 50 ported. Further studies are required to model the ef- compared with values recorded on unirradiated UO2 elec- fects of the presence of carbonate on UO2 fuel oxidation trodes exposed to external gamma radiation fields. Such in carbonate-containing solutions undergoing radiolysis. positive steady-state corrosion potential values would sug- Radiolysis produces carbonate radicals (CO3) that can gest that rapid oxidative dissolution should be occurring react like OH radicals in oxidation reactions. Our initial on these electrodes. However, this was not borne out by attempts to model the effect of carbonate-containing so- visual or scanning electron microscopy inspection of the lutions undergoing radiolysis on fuel oxidation are sum- electrodes. Therefore, the authors57 suggested that the po- marized in Ref. 41. tentially fast oxidative dissolution process was being blocked by the accumulation of secondary phases formed on the electrode by precipitation of dissolved UVI spe- VII. RADIOLYSIS EFFECTS ON LEACHING OF SPENT cies. They also concluded that the accumulation of sec- FUEL AND APPLICATIONS FOR NUCLEAR FUEL ondary phases, formed during oxidation of the fuel, is WASTE MANAGEMENT localized at grain boundaries. Shoesmith et al.57 studied the differences in the cor- 53 Loida et al. measured gas generation rates from ra- rosion behaviors of unirradiated UO2, SIMFUEL, and diolysis during spent-fuel dissolution. In most cases, hy- used UO2 fuels. They concluded that the oxidation and drogen and oxygen were produced in a near stoichiometric dissolution behavior of different fuel specimens is deter- ratio. The gas production was not increased significantly mined predominantly by three factors: the strength of ra- when fuel powder (3-yam grain size) was used. The in- diation fields, the extent of burnup (which also determines terpretation of this result was that the radiolysis gases the degree of rare-earth doping), and the degree of non- were produced by gamma and not by alpha radiation. In stoichiometry. the presence of iron powder, the hydrogen production was It is essential to know the dose rates in water layers increased by about one order of magnitude and the mea- in contact with the used fuel58 in order to predict disso- sured oxygen production was close to zero. (Studies with lution rates of the used fuel in a disposal vault caused iron powder were carried out to simulate the effect of by water radiolysis. Sunder has calculated the alpha, beta, iron in the repository, e. g., as a constituent of the canis- and gamma dose rates, in contact with a reference used ter.) The fuel alteration rate was increased when fuel pow- CANDU fuel, as a function of time, and described a der was used and decreased in the presence of iron powder procedure to calculate the dose rates for fuels of other (10-/u,m grain size). In 95% saturated NaCl solution, the burnups.59'60 The dose rate information (as a function hydrogen production (HP) was higher and the fuel alter- of cooling time) combined with the knowledge of fuel ation (FA) lower than in deionized water: HP/FA were dissolution rates (as a function of dose rate) enables 15 and 3, respectively. (The units of HP and FA were one to predict the dissolution rates of used fuel as a func- mol H2 /(gy-day) and mol UO2/(gu-day), respective- tion of its cooling time. This information was used to ly.54 The authors conclude that the dissolution process is calculate the dissolution rates of used CANDU fuel in oxidative and caused by water radiolysis products.53 Fur- contact with groundwater in a defective copper contain- thermore, they concluded that the fuel dissolution rates er.3132'61 It was concluded that beta radiolysis of water were of the same order of magnitude as the radiolysis will be the main cause of oxidation of the used CANDU gas production rates. fuel in a failed container and that the use of a corrosion

118 NUCLEAR TECHNOLOGY VOL.131 JULY 2000 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER model is required to calculate the corrosion rates of the storage times (>1000 yr), the radiation field will be used fuel for the first —1000 yr of emplacement in the dominated by alpha radiation.3159'60 As stated earlier waste vault. (Sec. IV.B), the model failed to predict the results of With the kinetic model presented earlier (Sec. IV), it alpha-radiolysis experiments carried out at Whiteshell is possible to calculate the fuel dissolution rates as a func- Laboratories. However, if a diffusion resistance was in- tion of dose rate and groundwater chemistry. Combining cluded in the model, to represent the narrow gap be- this knowledge with the dose rate information (obtained tween the alpha source and the UO2 electrode in the using procedures similar to those described by Sun- experiments, better agreement was obtained.38 Conse- der,59- one should be able to predict the dissolution rate quently, we suggested another type of alpha-radiolysis of any used fuel as a function of time in the disposal vault. experiment using alpha-doped UO2 samples, thus avoid- Thus, the model was used to calculate the oxidation of ing the possible effects of the narrow gap.6263 Also, such UO2 and the gas production, simulating the experiments samples could simulate old fuel (>1000 yr). Experi- 53 of Loida et al., described earlier. The system was di- ments using alpha-doped UO2 have recently been car- vided into four phases, see Fig. 15, requiring three sep- ried out by Rondinella et al.64 and Cobos et al.65 They arate calculations: observed increased dissolution of uranium from alpha- doped samples in un-aerated solutions. Earlier, Gray 1. Phase 1: a 30-/im-thick surface layer adjacent to concluded from his preliminary experiments with alpha- the pellet, irradiated by alpha, beta, and gamma doped UO2 that alpha radiolysis had little effect on the radiation 66 release of UO2 in aerated solutions. 2. Phase 2: a 0.3-cm layer adjacent to phase 1, irradiated by beta and gamma radiation 3. Phase 3: the rest of the solution, irradiated by VIII. SUMMARY AND CONCLUSIONS gamma radiation 4. Phase 4: the gas phase, unirradiated. The direct disposal of used nuclear fuel requires a 54 knowledge of the effects of groundwater radiolysis on The conclusions of the calculations were that the UO2 used fuel oxidation. Although significant progress has corrosion was caused only by radiolysis in the thin sur- been made in understanding the effects of low-LET face layer adjacent to the fuel surface (phase 1), and the radiation (beta and gamma) on used-fuel corrosion, gas production was caused almost exclusively by gamma further studies, both experimental and theoretical, i.e., radiolysis in the bulk water (phase 3). The calculated fuel 8 model development, are required to fully understand the alteration rate was 2.2 X 10" mol UO2-(gU)~' -day~\ effects of high-LET radiation (alpha radiolysis). Results about three times higher than the experimental rate, 6.3 X 9 1 of such studies can be tested against used-fuel leaching 10~ molUO2-(gU)~' -day" . experiments being carried out in different countries. Long- It may not be surprising that the model gives a good term applicability of such models can be tested by ana- agreement with these experiments, as the radiation field 54 lyzing the behavior of uranium deposits, such as those was dominated by beta and alpha radiation. At longer found at Cigar Lake in Canada.67"69

APPENDIX

ELECTROCHEMICAL MODEL FOR U02 FUEL CORROSION RATES

Bulk solution, Gas phase only y-irradiated There is a large body of evidence demonstrating that Pliase 4 Phase 3 under oxidizing conditions, the dissolution of fuel must be considered as an electrochemical process.6'28"32 The dissolution reaction of UO2 is a coupled process under oxidizing conditions, involving the oxidative dissolution of the UO2 and the reduction of the available oxidant (Ox), i.e., (A.I) a, p, y-irradiated P, y-irradiated layer water layer. Phase 1 Phase 2 Ox + ne Red . (A.2) Fig. 15. Schematic representation used to model the used fuel Depending on the relative kinetics of these two half j experiments of Loida et al. (from Ref. 54 see text). reactions, the dissolving surface will adopt a potential,

NUCLEAR TECHNOLOGY VOL. 131 JULY 2000 119 Christensen and Sunder WATER RADIOLYSIS EFFECT ON CORROSION OF SNF IN WATER

known as the corrosion potential (ECORR). between the 4. S. SUNDER, D. W. SHOESMITH, H. CHRISTENSEN, equilibrium values for these two reactions (for further N. H. MILLER, and M. G. BAILEY, "Oxidation of UO2 Fuel explanation of this point, see Fig. 2 in Ref. 6 or Fig. 6 in by Radicals Formed During Radiolysis of Water," Scientific Ref. 28). Under these conditions, dissolution will occur, Basis for Nuclear Waste Management XIII, V. M. OVERSBY and P. W. BROWN, Eds., Mater. Res. Soc. Symp. Proc., 176, with both reactions away from equilibrium, and the fuel 457 (1990). can be said to be undergoing corrosion at a rate equal to that of either of the two reactions, (A. 1) or (A.2). 5. R. S. FORSYTH and L. O. WERME, "Spent Fuel Corro- According to the empirical electrochemistry-based sion and Dissolution," J. Nucl. Mater., 190, 3 (1992). model of Shoesmith and Sunder,28 the corrosion rate of UO2 in a solution can be obtained from the value of its 6. D. W. SHOESMITH and S. SUNDER, "The Prediction of steady-state corrosion potential in the solution using Nuclear Fuel (UO2) Dissolution Rates Under Waste Disposal Eq. (6) in the main text. This relationship between cor- Conditions," /. Nucl. Mater, 190, 20 (1992). rosion rate and corrosion potential was established by measuring anodic current as a function of applied poten- 7. W. J. GRAY, H. R. LEIDER, and S. A. STEWARD, "Para- tial.28 The relationship is applicable to UO2 fuel corro- metric Study of LWR Spent Fuel Dissolution Kinetics," J. Nucl. Mater., 190, 46 (1992). sion in noncomplexing solutions (i.e., with very little carbonate in the solution, <0.001 mol-€~' of carbonate) 8. J. BRUNO, I. CASAS, and A. SANDINO, "Static and Dy- at room temperature. The relationship has been used to namic SIMFUEL Dissolution Studies Under Oxic Condi- calculate the corrosion rate of UO2 in solutions giving tions," J. Nucl. Mater., 190, 61 (1992). steady-state corrosion potentials above —100 mV versus SCE (Ref. 28). For solutions giving corrosion potentials 9. K. OLLILA, "SIMFUEL, Dissolution Studies in Granitic below —100 mV versus SCE, one does not need a cor- Groundwater," J. Nucl. Mater., 190, 70 (1992). rosion model to calculate fuel dissolution (corrosion) rates, as one can use a solubility-limited model to obtain the 10. S. SUNDER, D. W. SHOESMITH, H. CHRISTENSEN, fuel dissolution rate in such a solution.27 and N. H. MILLER, "Oxidation of UO2 Fuel by the Products of Gamma Radiolysis of Water,"/ Nucl. Mater., 190,78 (1992).

11. H. CHRISTENSEN, S. SUNDER, and D. W. SHOE- ACKNOWLEDGMENTS SMITH, "Calculations of Radiolysis in Connection with UO2 Oxidation Studies. III. Adjustment of the Final Stage of Oxi- We thank J. C. Wren for a review of a draft of this paper. dation," Technical Note NS-90/99, Studsvik AB, Sweden This work was carried out under a European Union contract (1990). F14W-CT95-0004 awarded to H. Christensen. The Whiteshell work reviewed here was carried out as a part of the Canadian 12. H. CHRISTENSEN, S. SUNDER, and D. W. SHOE- Nuclear Fuel Waste Management Program, which was funded SMITH, "Oxidation of Nuclear Fuel (UO2) by the Products of jointly by Atomic Energy of Canada Limited and Ontario Water Radiolysis: Development of a Kinetic Model," J Alloys Hydro under the auspices of the CANDU Owners Group, as Compounds, 213, 93 (1994). indicated in the original publications referenced here. H. Chris- tensen also acknowledges support from the Swedish Nuclear 13. L. H. JOHNSON, D. M. LENEVEU, D. W. SHOESMITH, Fuel Management Company of Sweden during these studies. D. W. OSCARSON, M. N. GRAY, R. J. LEMIRE, and N. C. GARISTO, "The Disposal of Canada's Nuclear Fuel Waste: The Vault Model for Postclosure Assessment," AECL-10714, Atomic Energy of Canada Limited (1994). REFERENCES 14. L. H. JOHNSON, J. C. TAIT, D. W. SHOESMITH, J. J. 1. H. CHRISTENSEN and E. BJERGBAKKE, "Radiation In- CROSTHWAITE, and M. N. GRAY, "The Disposal of Cana- duced Dissolution of UO2," Scientific Basis for Nuclear Waste da's Nuclear Waste: Engineered Barriers Alternatives," AECL- Management X, J. K. BATES and W. B. SEEFELDT, Eds., 10718, Atomic Energy of Canada Limited (1994). Mater. Res. Soc. Symp. Proc, 84, 115 (1987). 15. B. GRAMBOW, A. LOIDA, P. DRESSLER, H. GECK- 2. L. H. JOHNSON, and D. W. SHOESMITH, "Spent Fuel," EIS, P. DIAZ, J. GAGO, I. CASSAS, I. J. de PABLO, J. GIME- Radioactive Waste Forms for the Future, p. 635, W. LUTZE NEZ, and M. E. TORRERO, "Long-Term Safety of Radioactive and R. C. EWING, Eds., Elsevier, Amsterdam (1988). Waste Disposal: Reaction of High Burnup Spent Fuel and UO2 in Saline Brines at Room Temperature," KfK 5377, Kernfor- 3. S. SUNDER, D. W. SHOESMITH, H. CHRISTENSEN, schungszentrum, Karlsruhe (1994). M. G. BAILEY, and N. H. MILLER, "Electrochemical and X-Ray Photoelectron Spectroscopic Studies of UO2 Fuel Ox- 16. G. A. PARKS and D. C. POHL, "Hydrothermal Solubility idation by Specific Radicals Formed During Radiolysis of of Uraninite," Geochim. Cosmochim. Acta, 52, 863 (1988). Groundwater," Scientific Basis for Nuclear Waste Manage- ment XII, W. LUTZE and R. C. EWING, Eds., Mater. Res. Soc. 17. R. J. LEMIRE and F. GARISTO, "The Solubility of U, Symp. Proc, 127, 317 (1989). Nb, Pu, Th and Tc in a Geological Disposal Vault for Used

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Nuclear Fuel," AECL-10009, Atomic Energy of Canada Lim- 31. L. H. JOHNSON, D. M. LENEVEU, F. KING, D. W. ited (1989). SHOESMITH, M. KOLAR, D. W. OSCARSON, S. SUNDER, C. ONOFREI, and J. L. CROSTHWAITE, "The Disposal of 18. M. GASCOYNE and D. C. KAMINENI, "Groundwater Canada's Nuclear Fuel Waste: A Study of Post-Closure Safety Chemistry and Fracture Mineralogy in the Whiteshell Re- of In-Room Emplacement of Used CANDU Fuel in Copper search Area. Supporting Data for the Geosphere and Bio- Containers in Permeable Plutonic Rock. Volume 2: Vault sphere Transport Models," TR-516, Atomic Energy of Canada Model," AECL-111494-2, Atomic Energy of Canada Limited Limited (1992). (1996).

19. A. O. ALLEN, The Radiation Chemistry of Water and Aque- 32. S. SUNDER, D. W. SHOESMITH, M. KOLAR, and D. M. ous Solutions, D. Van Nostrand Company, New York (1961). LENEVEU, "From Laboratory Experiments to a Geological Disposal Vault: Calculation of Used Nuclear Fuel Dissolution 20. J. W. T. SPINKS and R. J. WOODS, An Introduction to Rates," Scientific Basis for Nuclear Waste Management XXI. I. Radiation Chemistry, 3rd ed., Wiley-Interscience, New York G. MCKINLEY and C. MCCOMBIE, Eds., Mater. Res. Soc. (1990). Symp. Proc., 506, 273 (1998).

21. V. GROMOV, "Dissolution of Uranium Oxide in the 33. H. CHRISTENSEN, "Radiation Induced Dissolution of Gamma-Radiation Field," Radiat. Phys. Chem., 18, 135 (1981). UO2," Mater. Res. Soc. Symp. Proc., 212, 213 (1991). 22. D. W. SHOESMITH, S. SUNDER, M. G. BAILEY, and 34. S. SUNDER, G. D. BOYER, and N. H. MILLER, "XPS G. J. WALLACE, "The Corrosion of Nuclear Fuel (UO2) in Studies of UO2 Oxidation by Alpha Radiolysis of Water at Oxygenated Solutions," Corros. Sci., 29, 1115 (1989). 100°C," J. Nucl. Mater., 175, 163 (1990). 23. S. SUNDER, D. W. SHOESMITH, R. J. LEMIRE, M. G. BAILEY, and G. J. WALLACE, "The Effect of pH on the Cor- 35. S. SUNDER, D. W. SHOESMITH, L. H. JOHNSON, M. rosion of Nuclear Fuel in Oxygenated Solutions," Corros. Sci., G. BAILEY, and G. J. WALLACE, "Effect of Alpha Radiolysis 32, 373 (1991). of Water on CANDU™ Fuel Oxidation," Proc. 2nd Int. Conf. Radioactive Waste Management, p. 625, Canadian Nuclear So- 24. D. W. SHOESMITH, S. SUNDER, L. H. JOHNSON, and ciety (1986). M. G. BAILEY, "Oxidation of CANDU UO2 Fuel by the Alpha- Radiolysis Products of Water," Scientific Basis for Nuclear 36. S. SUNDER, D. W. SHOESMITH, and N. H. MILLER, Waste Management IX, L. O. WERME, Ed., Mater. Res. Soc. "Oxidation and Dissolution of Nuclear Fuel (UO2) by the Prod- Symp. Proc, 50, 309 (1985). ucts of the Alpha Radiolysis of Water," J. Nucl. Mater., 244, 66 (1997). 25. S. SUNDER, D. W. SHOESMITH, L. H. JOHNSON, M. G. BAILEY, G. J. WALLACE, and A. P. SNAGLEWSKI, "Ox- 37. M. G. BAILEY, L. H. JOHNSON, and D. W. SHOE- idation of CANDU™ Fuel by the Products of Alpha Radiol- SMITH, "The Effects of Alpha-Radiolysis of Water on the Cor- ysis of Groundwater," Scientific Basis for Nuclear Waste rosion of UO2," Corros. Sci., 25, 233 (1985). Management X, J. K. BATES and W. B. SEEFELDT, Eds., Mater. Res. Soc. Symp. Proc, 84, 103 (1987). 38. H. CHRISTENSEN, S. SUNDER, and D. W. SHOE- SMITH, "Calculation of Radiation Induced Dissolution of UO2. 26. S. SUNDER and H. CHRISTENSEN, "Gamma Radioly- Adjustment of the Model Based on a-Radiolysis Experi- sis of Water Solutions Relevant to the Nuclear Fuel Waste Man- ments," Studsvik M-92/20, Studsvik Material AB, Nykoping, agement Program," Nucl. Technol, 104, 403 (1993). Sweden (1992). 27. S. SUNDER, D. W. SHOESMITH, N. H. MILLER, and 39. W. H. HOCKING, J. S. BETTERIDGE, and D. W. SHOE- G. J. WALLACE, "Determination of Criteria for Selecting a SMITH, "The Cathodic Reduction of Dioxygen on Uranium UO2 Fuel Dissolution Model for Nuclear Waste Management Oxide in Dilute Alkaline Aqueous Solution," AECL-10402, Concept Assessment," Mater. Res. Soc. Symp. Proc., 257, 345 Atomic Energy of Canada Limited (1991). (1992). 40. H. CHRISTENSEN and S. SUNDER, "Calculation of Ra- 28. D. W. SHOESMITH and S. SUNDER, "An Electro- diation Induced Corrosion of UO2. Adjustment of Rate Con- chemistry-Based Model for the Dissolution of UO2," AECL- stants for Reaction of O2 and H2O2," M-94/120, Studsvik 10488, Atomic Energy of Canada Limited; also SKB Tech. Rep. Material AB (1994). 91-63, SKB, Stockholm (1991). 41. H. CHRISTENSEN and S. SUNDER, "Calculation of Ra- 29. S. SUNDER and D. W. SHOESMITH," Chemistry of UO2 diation Induced Corrosion of UO2 in Solutions Containing Car- Fuel Dissolution in Relation to the Disposal of Used Nuclear bonate," M-96/4, Studsvik Material AB (1996). Fuel," AECL-10395, Atomic Energy of Canada Limited (1991). 42. M. B. CARVER, D. V. HANLEY, and K. R. CHAPLIN, 30. D. W. SHOESMITH, S. SUNDER, and W. H. HOCKING, "MAKSIMA-CHEMIST, a Program for Mass Action Kinetics "Electrochemistry of UO2 Nuclear Fuel," Electrochemistry of Simulation by Automatic Chemical Equation Manipulation and Novel Materials, p. 297, J. LIPKOWSKI and P. N. ROSS, Eds., Integration Using Stiff Techniques," AECL-6413, Atomic En- VCH Publishers, New York (1994). ergy of Canada Limited (1979).

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43. H. CHRISTENSEN and S. SUNDER, "Calculations of Ra- Contact with Highly Burnt Up Nuclear Fuel," RM 91-23, ABB diolysis in Connection with UO2 Oxidation Studies," NS-89/ Atom (1991). 117, Studsvik Nuclear (1989). 57. D. W. SHOESMITH, S. SUNDER, M. G. BAILEY, and 44. H.CHRISTENSEN, "Calculation of Radiation Induced Dis- N. H. MILLER, "Corrosion of Used Nuclear Fuel in Aqueous solution of UO2," M-91/10, Studsvik Material AB (1991). Perchlorate and Carbonate Solutions," /. Nucl. Mater., 227,287 (1996). 45. B. GRAMBOW, A. LOIDA, P. DRESSLER, H. GECK- EIS, P. DIAZ, J. GAGO, I. CASAS, J. de PABLO, J. GIME- 58. H. CHRISTENSEN and S. SUNDER, "An Evaluation of NEZ, and M. E. TORRERO, "Long-Term Safety of Radioactive Water Layer Thickness Effective in the Oxidation of UO Fuel Waste Disposal: Reaction of High Burnup Spent Fuel and UO 2 2 Due to Radiolysis of Water," J. Nucl. Mater., 238, 70 (1996). in Saline Brines at Room Temperature," KfK 5377, Kernfor- schungszentrum Karlsruhe Report (1994). 59. S. SUNDER, "Alpha, Beta and Gamma Dose Rates in Wa- 46. I. CASAS, J. GIMENEZ, J. de PABLO, and M. E. TOR- ter in Contact with Used CANDU UO2 Fuel," AECL-11380, Atomic Energy of Canada Limited (1995). RERO, "The Dissolution of UO2 in Different Redox Condi- tions," Scientific Basis for Nuclear Waste Management XVI, C. G. INTERRANTE and R. T. PABALAN, Eds., Mater. Res. 60. S. SUNDER, "Calculation of Radiation Dose Rates in a Soc. Symp. Proc, 294, 67 (1993). Water Layer in Contact with Used CANDU UO2 Fuel," Nucl. Technol., 122, 211 (1997). 47. D. E. GRANDSTAFF, "A Kinetic Study of the Dissolu- tion of Uraninite," Econ. Geol., 71, 1493 (1976). 61. S. SUNDER, D. W. SHOESMITH, M. KOLAR, and D. M. LENEVEU, "Calculation of Used Nuclear Fuel Dissolution 48. G. F. THOMAS and G. TILL, "The Dissolution of Unirra- Rates Under Anticipated Canadian Waste Vault Conditions," diated UO2 Fuel Pellets Under Simulated Disposal Condi- J. Nucl. Mater., 250, 118(1997). tions," Nucl. Chem. Waste Manage., 5, 141 (1984). 62. H. CHRISTENSEN and S. SUNDER, Discussions (Sep. 49. W. H. HOCKING, J. S. BETTERIDGE, and D. W. SHOE- and Oct. 1994) and Personal Communication (Jan. 1995) to SMITH, "The Cathodic Reduction of Dioxygen on Uranium E. Y. VERNAZ, Marcoule, France; Personal Communication Oxide in Dilute Alkaline Aqueous Solution," AECL-10402, to J. L. PAUL, Commisariat a l'Energie Atomique (Mar. 22, Atomic Energy of Canada Limited (1991). 1995). 50. D. W. SHOESMITH, J. C. TAIT, S. SUNDER, W. J. GRAY, S. A. STEWARD, R. E. RUSSO, and J. D. RUDNICKI, "Fac- 63. S. SUNDER, "Experiments Using Alpha-Doped UO2," tors Affecting the Differences in Reactivity and Dissolution AECL Technical Memo # RCB-IMS-4-95-26, R-1005-1094, to L. H JOHNSON (May 11, 1995). Rates Between UO2 and Spent Nuclear Fuels," AECL-11515, Atomic Energy of Canada Limited (1996). 64. V. V. RONDINELLA, Hj. MATZKE, J. COBOS, and T. 51. H. CHRISTENSEN, R. FORSYTH, R. LUNDQVIST, and WISS, "a-Radiolysis and a-Radiation Damage Effects on UO2 L. O. WERME, "Radiation Induced Dissolution of UO2," NS- Dissolution Under Spent Fuel Storage Conditions," Mater. Res. 90/85, Studsvik Energiteknik AB (1990). Soc Symp. Proc. (In Press).

52. J. GIMENEZ, E. BARAJ, M. E. TORRERO, I. CASAS, 65. J. COBOS, Hj. MATZKE, V. V. RONDINELLA, T. WISS, and J. de PABLO, "Effect of H2O2, NaCIO and Fe on the Dis- and A. MARTINEZ-ESPARZA, Paper 168, presented at Int. 1 solution of Unirradiated UO2 in NaCl 5 mol-kg" . Compari- Conf. on Future Nuclear Systems (Global 1999), Jackson Hole, son with Spent Fuel Dissolution Experiments," J. Nucl. Mater., Wyoming, August 30-September 2, 1999. 238, 64 (1996). 66. W. J. GRAY, " Effect of Surface Oxidation, Alpha Radi- 53. A. LOIDA, H. GRAMBOW, H. BECKEIS, and P. olysis and Salt Brine Composition on Spent Fuel and UO2 DRESSLER, "Processes Controlling Radionuclide Release Leaching Performance," PNL/SRP-6689, Pacific Northwest from Spent Fuel," Scientific Basis for Nuclear Waste Manage- Laboratory (1988). ment XVIII, T. MURAKAMI and R. E. EWING, Eds., Mater. Res. Soc. Symp. Proc, 353, 577 (1995). 67. H. CHRISTENSEN, S. SUNDER, and D. W. SHOE- 54. H. CHRISTENSEN, "Calculations Simulating Spent-Fuel SMITH, "Radiolysis Modelling for the Cigar Lake Uranium Leaching Experiments," Nucl. Technol, 124, 165 (1998). Deposit," Studsvik M-92/56, Studsvik Material AB (1992). 55. B. GRAMBOW, A. LOIDA, P. DRESSLER, H. GECK- 68. H. CHRISTENSEN, "Effect of Gamma Radiolysis in the EIS, P. DIAZ, J. GAGO, I. CASAS, and J. de PABLO, "Reac- Cigar Lake Uranium Deposit," Studsvik M-92/99, Studsvik Ma- tion of High-Burnup Spent Fuel and UO2 in Salt Solutions," terial AB (1992). KfK 5377 Kernforschungszentrum Karlsruhe (1994). 69. S. SUNDER, J. J. CRAMER, and N. H. MILLER, "Geo- 56. T. INGEMANSSON and J. ELKERT, "Model for Calcu- chemistry of the Cigar Lake Uranium Deposit: XPS Studies," lation of Absorbed Alpha and Beta Radiation Dose to Water in Radiochim. Acta, 74, 303 (1996).

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Hilbert Christensen (MS, chemical engineering, Danish Institute of Tech- nology, Denmark, 1956; DSc, nuclear chemistry, Royal Institute of Technology, Sweden, 1971) is a senior scientist at Studsvik and an adjunct professor at the University of Linkoping. His major interest is radiation chemistry of aqueous solutions, including high-temperature pulse radiolysis. Another area of interest is radiolysis in connection with the storage of high-level radioactive waste, and this includes modeling using computer calculations. S. Sunder (PhD, chemistry, University of Alberta, Edmonton, Canada, 1972) is a scientist in the Fuel Safety Branch of Atomic Energy of Canada Limited (AECL) at Chalk River, Ontario. Before joining AECL in 1979, he carried out spectroscopic research at the National Research Council of Canada (Ottawa) and taught at the University of Heidelberg (Heidelberg, Germany). His research interests lie in studying the properties of nuclear fuel and fission products using a combination of spectroscopic and theoretical techniques. He has carried out research on nuclear fuels for the nuclear fuel storage and waste management pro- grams and on properties of fission products for the nuclear safety program. His current interests include high-temperature chemistry of nuclear fuel and fission products.

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