MIGRATION OF GAS-BUBBLES IN SOLIDS TOWARDS VACANCY SOURCES

It is well known that overpressurized gas bubbles in solids will grow and equilibrate by acquisition of vacancies from nearby free surfaces, grain boundaries and other vacancy sources. Recently, Evans (1994, J. nucl. Mater., 210, 21) has suggested that the transfer of intragranul ar gas to grain boundaries in UO2 may be substantially enhanced by gas bubble diffusion biased towards the grain boundaries by the flow of t hermal vacancies from the boundaries into the vacancy-depleted grain i nteriors. To address this possibility, upper and lower bounds on the v elocity of a gas bubble towards a vacancy source are determined by cal culating the vacancy flux through an immobile bubble fixed with respec t to the lattice, and the flux through a bubble moving with the host m aterial respectively.