The formation and behaviour of gas bubbles in a non-uniform temperature environment
Review articleOpen access
Abstract:

AbstractIrradiation of uranium- and plutonium-based fuels results in the accumulation of fission products of which about 10% are the inert gases xenon and krypton. These gas atoms are insoluble and precipitate in the matrix to eventually form gas bubbles. This paper considers in detail the nucleation and subsequent behaviour of the gas bubbles in a non-uniform temperature environment. It is therefore particularly relevant to ceramic fuels in which steep temperature gradients are typically present during operation. A homogeneous nucleation model is used to describe the formation of the gas atom clusters during irradiation. It is shown that the scale of nucleation is a function of temperature, and thus in a temperature gradient the distribution of nuclei is non-uniform. The subsequent growth of the nuclei to form gas bubbles leads to the development of non-uniform swelling stresses.The gas bubbles are subjected to forces resulting from the temperature and stress gradients, which tend to drive them up the gradients. They are also subjected to restraining forces applied by structural heterogeneities. The present paper considers these forces, and in particular the stress gradient driving force, and the interaction between a bubble and a dislocation are treated in detail. Consideration is also given to the energies involved in the process of coalescence between bubbles that collide. It is concluded that during irradiation when a large supersaturation of vacancies is present, the energy gained by the matrix from the vacancies absorbed into the bubble provides the dominant driving force for growth.Finally, the theory developed in the paper has been applied to a uranium carbide pellet operating under a specific set of conditions typical of those existing in a fast reactor environment. Up to 10% burn-up the predicted swelling is consistent with the values estimated from experiments.

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