C. Corbel

Alpha-radiolysis effects on UO2 alteration in water

The behaviour of UO2/water interface under irradiation has been investigated as a function of alpha flux using an alpha beam provided by a cyclotron. The effects of alpha-radiolysis on UO2 alteration in aerated deionized water were studied by characterizing both the chemistry of irradiated aqueous solutions and the UO2 surface. Uranium (as uranyl ion UO22+) and hydrogen peroxide concentration (H2O2) increased whereas pH decreased in the irradiated solutions when alpha-beam flux increased. The formation of hydrated uranium peroxide (metastudtite UO4·2H2O) on UO2 leached surface was identified by X-ray diffractometry. The production of metastudtite can be considered as a direct effect of water radiolysis due to the production of radiolytic species H2O2, since its formation is known to occur out of irradiation via a precipitation reaction between hydrogen peroxide and uranyl ion. From both these observations and literature about hydrated uranium peroxide occurrence, the possibility of metastudtite formation on nuclear spent fuel in storage conditions is discussed.

Effect of alpha irradiation on UO2 surface reactivity in aqueous media

Abstract The option of direct disposal of spent nuclear fuel in a deep geological formation raises the need to investigate the long-term behavior of the UO2 matrix in aqueous media subjected to α-β-γ radiation. The β-γ emitters account for most of the activity of spent fuel at the moment it is removed from the reactor, but diminish within a millennial time frame by over three orders of magnitude to less than the long-term activity. The latter persists over much longer time periods and must therefore be taken into account over a geological disposal time scale. Leaching experiments with solution renewal were carried out on UO2 pellets doped with alpha emitters (238Pu and 239Pu) to quantify the impact of alpha irradiation on UO2 matrix alteration. Three batches of doped UO2 pellets with different alpha flux levels (3.30×104, 3.30×105, and 3.2×106 α cm-2 s-1) were studied. The results obtained in aerated and deaerated media immediately after sample annealing or interim storage in air provide a better understanding of the UO2 matrix alteration mechanisms under alpha irradiation. Interim storage in air of UO2 pellets doped with alpha emitters results in variations of the UO2 surface reactivity, which depends on the alpha particle flux at the interface and on the interim storage duration. The variation in the surface reactivity and the greater uranium release following interim storage cannot be attributed to the effect of alpha radiolysis in aerated media since the uranium release tends toward the same value after several leaching cycles for the doped UO2 pellet batches and spent fuel. Oxygen diffusion enhanced by alpha irradiation of the extreme surface layer and/or radiolysis of the air could account for the oxidation of the surface UO2 to UO2+x. However, leaching experiments performed in deaerated media after annealing the samples and preleaching the surface suggest that alpha radiolysis does indeed affect the dissolution, which varies with the flux at the UO2/water interface.

Increase of the uranium release at an UO2/H2O interface under He2+ ion beam irradiation

Abstract The release of uranium in aerated deionized water at a uranium oxide/water interface under He2+ irradiation is investigated as a function of the fluence by using an external ion beam. A high-energy He2+ beam delivered by a cyclotron (CERI–CNRS) goes through the thin oxide and emerges in the water with 20 MeV energy. First results are reported here showing that the release of uranium increases by three orders of magnitude in aerated deionized water under high flux (⩾ 3.3×10 10 α cm −2 s −1 ).

Leaching of zirconolite ceramics under H+ and He2+irradiation

Zirconolite is a candidate host material for conditioning minor tri- and tetra-valent actinides arising from enhanced nuclear spent fuel reprocessing and partitioning, which can be disposed in a geological repository for nuclear waste. Its chemical durability has been studied here under charged particle-induced radiolysis (He2+ and proton external beams) to identify possible effects on dissolution rates and mechanisms in pure water. Two geometries of experiments have been used to evaluate the influence of the following parameters: solid irradiation, Linear Energy Transfer (LET) at the interface and total deposited energy. Preliminary results on the elemental releases due to the enhanced dissolution of the zirconolite surface during charged particle-induced irradiation are first presented. Then, we focus on H2O2 production which is one of the major molecular species, created under water radiolysis, and likely to interact with the zirconolite surface. In presence of zirconolite, first results indicate an apparent consumption of the radiolytic hydrogen peroxide or its precursors compared to the production in pure water calculated from the primary yield GH2O2. The measured H2O2 concentration varies linearly with the total deposited energy in water over the irradiation duration (between 1 h and 6 h) and in the conditions of our experiments. Moreover, the H2O2 concentration decreases when the local density of the deposited energy close to the interface increases. Thus, we suggest that the mechanism(s) leading to the consumption of H2O2 or its precursors involve zirconolite surface reactions.

Enhancement of Zirconolite Dissolution Due to Water Radiolysis

Zirconolite is a candidate host material for conditioning minor tri- and tetra-valent actinides arising from enhanced nuclear spent fuel reprocessing and partitioning, in the case of disposal of the nuclear waste. Its chemical durability has been studied here under charged particle-induced radiolysis (He{sup 2+} and proton external beams) to identify the possible effects of water radiolysis on the dissolution rates in pure water and to describe the alteration mechanisms. Two experimental geometries have been used in order to evaluate the influence of the following parameters: solid irradiation, water radiolysis. In the first geometry the beam gets through the sample before stopping at the surface/water interface. In the second one the beam stops before the surface/water interface. Results on the elemental releases due to the enhanced dissolution of the zirconolite surface during charged particle-induced irradiation of water are presented. Under radiolysis, an increase of one order of magnitude is observed in the Ti, Zr and Nd elemental releases. No difference in the total elemental releases can be noticed when the solid is also irradiated. (authors)

Effect of Spent Fuel Burnup and Composition on Alteration of the U(Pu)O2 Matrix

For a potential performance assessment of direct disposal of spent fuel in a nuclear waste repository, the chemical reactions between the fuel and possible intruding water must be understood and the resulting radionuclide release must be quantified. Leaching experiments were performed with five spent fuel samples from French power reactors (four UO 2 fuel samples with burnup ratings of 22, 37, 47 and 60 GWd·t HM −1 and a MOX fuel sample irradiated to 47 GWd·t HM −1 ) to determine the release kinetics of the matrix containing most (over 95%) of the radionuclides. The experiments were carried out with granitic groundwater on previously leached sections of clad fuel rods in static mode, in an aerated medium at room temperature (25°C) in a hot cell. After 1000 or 2000 days of leaching, the Sr/U congruence ratios for all the UO 2 fuel samples ranged from 1 to 2; allowing for the experimental uncertainty, strontium can thus be considered as a satisfactory matrix alteration tracer. No significant burnup effect was observed on the alteration of the UO 2 fuel matrix. The daily strontium release factor was approximately 1 a 10 −7 d −1 for UO 2 fuel, and five to six times higher for MOX fuel. Several alteration mechanisms (radiolysis, solubility, precipitation/clogging) are examined to account for the experimental findings.

Leaching of a zirconolite ceramic waste-form under proton and He2+ irradiation

Abstract In the hypothesis of a nuclear waste geological disposal, zirconolite is a candidate host material for minor tri- and tetra-valent actinides arising from enhanced nuclear spent fuel reprocessing and partitioning. Its chemical durability has been studied here under charged particle-induced radiolysis (He2+ and proton external beams) to identify possible effects on dissolution rates and mechanisms in pure water. Two experimental geometries have been used to evaluate the influence of the following parameters: solid irradiation and total deposited energy. Results on the evolution of the elemental releases due to the enhanced dissolution of the zirconolite surface during charged particle-induced irradiation of water are presented. Under radiolysis, elemental releases are first kinetically controlled. When the titanium and the zirconium releases reach (or exceed) their corresponding hydroxide solubility limits, the zirconolite dissolution becomes thermodynamically controlled.