Influence of defects on rare-gas diffusion in solids

Abstract

The diffusion of 133Xe in CsI was studied as an aid to understanding rare gas diffusion in more complex systems, such as nuclear reactor fuels. Diffusion at low gas concentrations and negligible radiation damage levels was measured by growing single crystals of CsI containing radioactive 133I which decayed to 133Xe (T12 = 20.8 h). Both the time rate of release and temperature dependence of the rare gas diffusion coefficient were consistent with classical diffusion solutions, giving: D = D0 exp (−Q/kT), D0 = (0.57+2.30−0.43) cm2/sec, Q = (1.01 ± 0.04) eV.Crystals containing high concentrations of defects exhibited trapping of the rare gas and anomalous diffusion kinetics. Trapped gas atoms tended to stabilize the defects and prevent their annealing during heating. Diffusion of fission gas recoiled into CsI specimen surface layers from an external fissionable source obeyed classical diffusion solutions at low fission recoil concentrations, while at high concentrations, radiation damage created traps which decreased gas diffusion rates. These traps differed significantly from natural defects in trap concentration, gas atom binding energies, and annealing characteristics. Increasing the gas concentration independent of radiation damage also lowered rare gas diffusion coefficients, showing that formation of small gas atom clusters also produced trapping. The results showed that classical rare gas diffusion could be obtained under ideal conditions and that distinctive trapping behavior with different characteristics could be associated with the presence of natural defects, radiation damage, and high gas concentration.