Models of grain-boundary cracking and diffusion growth of integrain bubbles in rapidly heated irradiated fuel

Models describing the behavior of nonequilibrium grain-boundary bubbles with gas in irradiated nuclear fuel undergoing rapid heating in the absence of irradiation are proposed. Two mechanisms for the decrease in excess volume in bubbles are examined: a decrease due to cracking of the matrix along grain boundaries and a decrease due to diffusion entry of vacancies. A criterion for cracking onset and an expression for the finite size of cracks, determined by the internal volume in bubbles and the thermomechanical stresses in a pellet, are obtained. The dynamics of growth of supercompressed bubbles, which is determined by the diffusion fluxes of vacancies and interstices, is described for the case where cracking does not occur. 1 figure, 11 references.