M.V. Speight

An analysis of the diffusion of fission gas bubbles and its effect on the behaviour of reactor fuels

Abstract It is shown that small bubbles move in a way analogous to Brownian motion with an effective diffusion coefficient which is inversely proportional to their radius to the fourth power. This movement is relevant to the behaviour of reactor fuels insofar as it enables small bubbles to coalesce and an estimate is made of the rate of coalescence. The process influences the spacing of bubbles and enables a more satisfactory estimate to be made of swelling in fissile materials.

The role of bubbles in fission gas release from uranium dioxide

Abstract Experimental evidence for the mechanisms of bubble mobility in irradiated UO 2 is reviewed. Examination of the bubble distribution in irradiated material shows that significant directed bubble motion does not occur. This result is confirmed by calculation, and it is also predicted that Brownian movement is an unimportant mode of release for burn-ups of practical interest. A description is made of the theory of transport of gas to the grain boundaries involving the irradiation re-solution of intra- and inter-granular bubbles.

Bubble diffusion and coalescence during the heat treatment of materials containing irradiation-induced gases

Abstract The process of bubble diffusion and coalescence during the annealing of material containing insoluble irradiation-induced gases is analysed. An expression is obtained for the average bubble size in terms of annealing time and temperature which depends upon the influence of dislocations, and this is compared with experimental results. The analysis includes the case where solid precipitates in the material impede bubble movement.

A calculation on the in-pile diffusional release of fission products forming a general decay chain

Abstract The effect of precursors on the diffusional release of fission products has been calculated and the specific case of caesium examined in detail. Significantly increased fractional release of an isotope occurs when its rate of diffusion is slow relative to its precursors, particularly when these have half-lives comparable with the irradiation time. Under steady state conditions the release rate of a radioactive isotope depends on the balance between its rates of diffusion and decay, but the form of this dependence is influenced by precursors.

The migration of gas bubbles in material subject to a temperature gradient

Abstract It is shown that where gas bubbles are present in material subject to a temperature gradient, they may migrate by vapour transport of matrix material across the bubble. When the total pressure of gas and vapour within the bubble is balanced by the surface tension restraint of the surrounding matrix, the velocity of migration is proportional to the bubble radius. This mechanism is compared with bubble migration by surface diffusion of the matrix atoms surrounding them.

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.

Forces on bubbles and voids in a stress gradient

Abstract Forces on voids and gas bubbles in a solid under a stress gradient are evaluated. The bubbles are allowed to change volume to reach equilibrium with the local stress, and it is shown that the force originates solely with the discrepancy between surface energy and surface tension. The force on non-equilibrium voids is derived on the assumption that the temperature is sufficiently low to permit them to migrate at constant volume.

The effects of dislocation movement in enhancing swelling in α-uranium during irradiation

Abstract It is shown that there is a critical temperature range where dislocation movement may contribute to enhanced swelling during the irradiation of α-uranium. This range is one in which the temperature is sufficiently low for deformation to occur on account of internal stresses produced by irradiation growth, but still sufficiently high for fission gas bubble mobility to be adequate for bubbles to remain attached to dislocation lines whilst they are moving at speeds which enhance the rate of bubble coalescence. An analysis is presented which permits an assessment of the conditions under which swelling may become unacceptably large.

A model to describe the irradiation-induced dimensional changes in polycrystalline graphites and carbons

Abstract A model is presented that describes the irradiation-induced dimensional changes of polycrystalline graphites and carbons in terms of the preferred orientation and shape change of the individual crystallites. It is assumed that the shape change of each crystallite arises from the additive effects of uninhibited irradiation growth and plastic deformation induced by internal stresses. Plastic deformation is assumed to occur by steady state creep varying as the n th power of the internally generated stresses. The predictions of the model agree well with experimental observations.

On the origin of torsional deformation in irradiated uranium-magnox fuel elements

Abstract It is deduced that the torsion observed in fuel elements irradiated in current UK power-producing reactors develops from irradiationinduced effects occurring within the uranium bar. The observed rates of torsion can most satisfactorily be accounted for by postulating a slight preferred crystallographic orientation which takes the form of a helix, so that torsion results as a consequence of irradiation growth. The nature and extent of this texture are assessed.