J.A. Turnbull

The diffusion coefficients of gaseous and volatile species during the irradiation of uranium dioxide

Abstract Measurements of release for rare gas fission products from single and polycrystalline uranium dioxide during irradiation are described. The fuel samples were enriched to 20% in 235U to avoid uncertainties in the fission rate due to plutonium production. The surface area of the single crystals was determined optically to provide a reference of known surface-to-volume ratio for the determination of diffusion coefficients for the rare gases and their halogen precursors. The initial part of the experiment was isothermal, during which the time dependence of release was observed and the polycrystalline material developed extensively interlinked grain-boundary porosity. Subsequently the temperature dependence of the release process was studied. It is concluded that the diffusion of rare gas atoms in UO2 during irradiation is a complex process involving more than one rate-controlling mechanism and bears a close similarity to cation self-diffusion. In both cases, the low-temperature kinetics are radically affected by irradiation damage. The present results are well represented by a composite diffusion coefficient containing three terms — one representing high-temperature intrinsic behaviour whilst the other two represent the effect of irradiation enhancement. This equation should be modified to take account of intragranular bubbles at all temperatures for stable gas release, and at high temperatures in the case of unstable species.

The distribution of intragranular fission gas bubbles in UO2 during irradiation

Abstract A model is presented describing the nucleation and growth of intragranular fission gas bubbles during the irradiation of uranium dioxide. The bubbles are considered to be heterogeneously nucleated in the wake of energetic fission fragments where they grow by collecting gas by atomic diffusion for a length of time controlled by a re-solution process. The model satisfactorily explains the variation of size and concentration of bubbles with temperature as observed by electron microscopy of irradiated material.

The effect of grain size on the swelling and gas release properties of uo2 during irradiation

Abstract Uranium dioxide specimens of two different grain sizes have been irradiated at 1750°C up to 0.4% FIMA and measurements made of fission-gas release and dimensional changes. The results show a significant reduction of both these effects with increasing grain size. Also, at swelling values around 10%, the major contribution to swelling was due to grain-edge tunnels which were extensively interlinked to the free surface. The consequence of this is discussed regarding models for both gas release and swelling.

The re-solution of gas atoms from bubbles during the irradiation of UO2

Samples of UO2 containing fission gas bubbles of radius 50–75 A have been irradiated to doses between 1017 and 1019 fissions/cm3 at 200 °C. Electron microscope examination has been used to investigate the stability of these bubbles after irradiation with the view to determining the dose required to disperse bubbles of this size. The results show that the bubbles are dispersed by a dose ≲ 3 × 1018 fissions/cm3 and the gas atoms are knocked to small distances (< 600 A) by the re-solution process.

The release of fission products from nuclear fuel during irradiation by both lattice and grain boundary diffusion

Abstract An analysis has been made to describe the steady state release of radioactive fission products from nuclear fuel during irradiation, by both lattice and grain-boundary diffusion. The analysis is appropriate for calculating the release of volatile fission products from regions of fuel pins prior to the development of gross interlinked porosity. In general terms, the dependence of fractional release on decay constant, temperature and solute atom-grain boundary interaction energy has been investigated. The analysis is applied to a specific irradiation experiment where the release of the rare gases, 133 I, 131 I, and 132 Te were measured from both small and very large grain UO 2 samples.

The relation between microstructure and the release of unstable fission products during high temperature irradiation of uranium dioxide

Abstract Observations are presented of unstable fission-product release as a function of irradiation time at high temperature. Changes in the microstructure of the fuel are reflected in the changes in release rates of the fission products, in particular the onset of grain-edge porosity interlinkage is accompanied by a large increase in release rate. For the quasi-steady state period observed subsequent to interlinkage, a model is developed describing the release process, taking into account release direct to free surfaces and release into periodically venting tunnel networks. The model is fitted to the results, spanning many orders of magnitude of half-life, and is shown to be in good agreement with the observations.

The characteristics of fission gas release from uranium dioxide during irradiation

Abstract Release rates for the unstable rare gases 88 Kr, 87 Kr, 85m Kr, 133 Xe, 135 Xe, 138 Xe were measured for samples of polycrystalline and large-grained uranium dioxide in the temperature range 700–1550°C. Interpretation is in terms of simple diffusion of gas atoms and their halogen precursors. Derived diffusion coefficients for iodine are compared with those obtained from direct measurements of 133 I, 131 I release from the same experiment and show good agreement. Diffusion coefficients calculated for krypton, xenon and iodine are similar in their magnitudes and temperature dependences, but magnitudes for bromine are much (≈ 200 times) higher.

The characteristics of fission gas release from monocrystalline uranium dioxide during irradiation

Abstract Release rates for 85m Kr, 87 Kr, 88 Kr, 133 Xe, 135 xe and 138 Xe were measured in the temperature range 700–1550°C. The data were analysed in terms of diffusion of the rare gases and their halogen precursors. The diffusion coefficients for xenon and iodine were found to be similar whilst krypton also had a similar mobility at ~1200°C but otherwise diffused more slowly. Bromine had a high mobility compared with the rare gases (X 200).

The nucleation of bubbles and re-solution effects in uranium dioxide irradiated at elevated temperatures

Abstract Uranium dioxide samples have been irradiated at 1200 °C to a variety of doses up to 3.2 × 10 25 flssions/m 3 . Examination of these samples by transmission electron microscopy has shown that there is a critical dose below which intragranular fission-gas bubbles are not observed to form. Above this critical dose these bubbles are shown to nucleate heterogeneously upon fission-fragment tracks. Irradiation-induced re-solution has been shown to be effective in the removal of gas atoms from within the sintering pores of this material.

Enhanced fission-product release by grain-boundary diffusion

Abstract An analysis is presented to describe the effect of simultaneous lattice and grain boundary diffusion on the steady state release rate of active fission products from fuel during irradiation. In fuel containing little interconnected intergranular porosity through which volatile and gaseous fission products can escape internally, grain boundary diffusion has been observed to contribute significantly to the overall release rate of these isotopes at the free surface. Under these conditions the calculation describes the details of the diffusion process and the ensuing total release rate of any particular isotope.