Burn-up swelling of high density oxide, HTR fuel elements

BISO- or TRISO-coated high-density UO2 kernels with varying degrees of enrichment and with mean kernel diameters between 220 and 820 μm have been irradiated at 900-13000C to burn-ups between 0.5 and 52.5% fima. From microradiographic measurements, burn-up-induced increases in kernel diameters were deduced: these ranged up to 12.8% (corresponding to 43.5% volume increase). In the low burn-up region, kernel diameters increase approximately linearly with burn-up. This increase is more pronounced for small kernels than for larger ones, it is higher for TRISO-coated kernels than for BISO-coated ones. In the high burn-up region, the dimensional increase seems to become independent of burn-up. (U, Th)O2 kernels show more swelling than pure UO2 kernels with identical diameters. All these results can be explained, at least qualitatively, taking into account results from fission-gas release measurements. (Auth.)