The mobility of intragranular bubbles in uranium dioxide during irradiation

A survey of the literature will show that opinion is still divided as to the role of bubble mobility in the kinetics of fission gas release. It must be conceded that the columnar grain structure observed at high temperature and high rating results from the migration of pores up the temperature gradient and as such must be the significant if not dominant gas release process. However, in the cooler regions where equiaxed-grains are found, although there is plenty of evidence of both inter- and intragranular gas bubbles, there is no indication whether or not they are mobile. If a bubble has a diffusion coefficient Dsub(b), then even in the absence of a driving force it will change its position, moving a distance (x2(mean))sup(1/2) (the root mean square migration distance) in a time t, where in three dimensions (x2(mean)=6Dsub(b)t, and in the two dimensions of a projection on a plane (x2(mean)=4Dsub(b)t. It is therefore argued by some that this random motion and subsequent collision and agglomeration is a major contribution in controlling the intragranular bubble population and hence the escape of fission gas from solution. (Auth.)