Abstract
The aim of this work was to study the separate effect of fission fragment damage on the dissolution of simulant UK advanced gas-cooled reactor nuclear fuel in water. Plain UO2 and UO2 samples, doped with inactive fission products to simulate 43 GWd/tU of burn-up, were fabricated. A set of these samples were then irradiated with 92 MeV 129Xe23+ ions to a fluence of 4.8 × 1015 ions/cm2 to simulate the fission damage that occurs within nuclear fuels. The primary effect of the irradiation on the UO2 samples, observed by scanning electron microscopy, was to induce a smoothening of the surface features and formation of hollow blisters, which was attributed to multiple overlap of ion tracks. Dissolution experiments were conducted in single-pass flow-through (SPFT) mode under anoxic conditions (<0.1 O2 ppm in Ar) to study the effect of the induced irradiation damage on the dissolution of the UO2 matrix with data collection capturing six minute intervals for several hours. These time-resolved data showed that the irradiated samples showed a higher initial release of uranium than unirradiated samples, but that the uranium concentrations converged towards ∼10−9 mol/l after a few hours. Apart from the initial spike in uranium concentration, attributed to irradiation induced surficial micro-structural changes, no noticeable difference in uranium chemistry as measured by X-ray electron spectroscopy or ‘effective solubility’ was observed between the irradiated, doped and undoped samples in this work. Some secondary phase formation was observed on the surface of UO2 samples after the dissolution experiment.
Highlights
Some countries have decided or are positioning themselves to decide in favour of complete or partial geological disposal of spent nuclear fuel (SNF) [1]
scanning electron microscopy (SEM) (Fig. 1a) and atomic force microscopy (AFM) characterisation showed that the unirradiated cut disks have a high surface roughness with height variation up to
Since the initial electronic stopping power of 92 MeV Xe ions is 24.6 keV/nm, which should be sufficient to cause visible ion tracks with a diameter about 2 nm [21], we suggest that the observed microstructure is a result of multiple overlap (~150 times) of ion tracks with the centreline temperature exceeding the melting point of UO2 (3150 K) [43]
Summary
Some countries have decided or are positioning themselves to decide in favour of complete or partial geological disposal of spent nuclear fuel (SNF) [1]. About 80% (~170 MeV) of the energy liberated in a nuclear fission event is given to fission fragments as kinetic energy which is transformed into heat by their interaction with the crystal electrons and the crystal atoms in the fuel matrix [15]. This results in fission damage which is manifested in lattice parameter increase and lattice strain, surface fission tracks, high burn-up structure, enhanced diffusion, and creep, leading to the degradation of the fuel's properties [16e19]
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