The Effect of Grain Size on FGR and PCMI in High Burnup Fuel (IFA-534.14)

The fission gas release results indicate that rod 19 (grain size 8.5 μm), has experienced a higher degree of fission gas diffusion to grain boundaries during the irradiation which is released to the free volume of the rods during the planned power change (ramps) in the Hal-den reactor than rod 18 (grain size 22 μm). Delayed (observation of) fission gas release is observed. At the end of the experiment, at a burnup of about 55.1 MWd/kgUO2, the fission gas release is ∼3 % in rod 18 and ∼9 % in rod 19. The conclusion reached from the comparison of experimental data and code predictions is that the characteristics of the fission gas release reflects the difference in rod design, e.g. grain size. At the end of the experiment the fit between measured and predicted fission gas release is very good. The increases in cladding elongation for rods 20 and 21, with grain sizes of 22 and 38 μm respectively, are similar to that expected from a solid fission product fuel swelling rate of 0.75 % ΔVN per 10 MWd/kgUO2. Some relaxation takes place during the first part of the irradiation. A permanent elongation of 0.20-0.30 mm at the end of the irradiation is mainly due to cladding creep induced by the strong PCMI, with some small contribution from irradiation induced cladding growth. The IFA-534.14 experiment indicates that increased UO2 grain size is an effective way of suppressing fission gas release in LWR fuel at the burnup covered by the investigations. (author)