Simulated fission radiation effects in UO2 and simfuel

Radiation effects due to fission events were simulated in UO 2 and in simulated high burnup UO 2 , so-called SIMFUEL, by irradiation with a typical fission product of fission energy, i.e. 72 MeV iodine ions obtained from the TASCC accelerator at Chalk River. Specimens pre-implanted with radioactive Kr 85 were used to measure fission-enhanced gas release, formation of Kr-microbubbles in the fission spikes and, indirectly, re-solution of gas from bubbles. These processes were known to exist but could now be quantified in out-of-pile experiments. As an example, for the ion current used, the radiation-enhanced diffusion coefficient of Kr was shown to be athermal and of the order of 10 -15 cm 2 s -1 . X-ray diffraction was used to measure lattice parameter increases and internal strain as a function of fluence and temperature. Surface modifications were observed by scanning electron microscopy, and swelling was monitored by laser profilometry and atomic force microscopy. Small modifications were found for fluences up to 10 16 ions/cm 2 , and no measurable swelling was observed. At the highest dose of 10 17 ions/cm 2 , subgrain formation and porosity development similar to that known from the rim structure of high burnup LWR fuels were observed.