The solvation energy of hard spherical ions immersed in dipolar hard sphere solvents is investigated as a function of ion diameter. We apply both the mean spherical and linearized hypernetted-chain approximations and show that for ions of physically realistic size both theories give qualitatively similar results. The ion solvation energy is obtained as the sum of two competing terms: UID, the direct ion–solvent interaction energy, which is negative, and UDD, the change in the solvent–solvent interaction energy per ion at infinite dilution. UDD is found to be positive and to make an important contribution to the solvation energy for all ion diameters.

Alpha-radiolysis / 238 Pu-doped UO 2 / Electrochemistry / Corrosion potential / Dissolution rate Summary. The long-term stability of spent nuclear fuel under deep geologic repository conditions will be determined mostly by the influence of α-radiolysis, since the dose-rate for α-radiolysis will exceed that for γ/β-radiolysis beyond a fuel age of ∼ 100 years and will persist for more than 10 000 years. Dissolution rates derived from studies with currently available spent fuel include radiolysis effects from γ/β-as well as α-radiolysis. The use of external α-sources and chemically added H 2 O 2 in UO 2 studies has provided much information on the basic chemistry/electrochemistry of α-radiolysis effects on UO 2 fuel dissolution. Here, alpha-radiolysis effects were measured directly by performing electrochemical experiments with α-doped UO 2 . The 238 Pu-doped UO 2 pellets, produced by milling, pressing and sintering, had activities of approximately 0.01 and 0.1 Ci/g. Discs cut from these pellets were fabricated into electrodes. Corrosion potentials (E corr ) were measured for these electrodes, in glass cells containing deaerated 0.1 M NaClO 4 (pH 9.5) for periods up to 1600 h. The effects of carbonate and Fe(II) on E corr values were also investigated. Preliminary results showed that E corr values correlated with doping levels, and were slightly higher than E corr values obtained with UO 2 electrodes in the presence of external α-sources. The presence of carbonate reduced E corr values considerably. Addition of Fe(II) caused generally small reductions in E corr values, suggesting that Fe(II) affects the dissolution process, either by scavenging radiolytic oxidants, reducing oxidized surface states, or reducing dissolved U(VI). These effects were often transient due to depletion of the added Fe(II). Accurate dissolution rates could not be determined from measured solution and surface concentrations because of material entering the solutions during the electroreduction procedure, probably due to the dissolution of fines present on the electrode surface after polishing. Experiments are continuing in order to confirm the preliminary findings, with greater attention being paid to determining accurate dissolution rates.

A mixed-potential model to predict the corrosion behaviour of nuclear fuel inside a failed carbon steel-lined copper waste container in a granitic repository is briefly described. A number of experiments underway to improve the mechanistic form of the model and to provide the necessary input data are discussed. A primary emphasis is placed on the consequences of the accumulation of corrosion product deposits on the fuel surface on the development of aggressive local chemistries, the cathodic reduction of H 2 O 2 and potential for scavenging of H 2 O 2 by the products of carbon steel corrosion (in particular H 2 ).

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