Density, porosity and grain size, (II) irradiated ROX fuels to burn-up of 28%FIMA

The irradiation of ROX fuels to a burn-up of 28%FIMA in JRR-3M was made. Ob tained results from post-irradiation examinations are: (1) The fuel bulk density of the ROX-SZR decreased by irradiation from 4.6g /cc (82%TD) to 3.4g/cc (61%TD), while that of the ROX-ThO2 also decreased f rom 5.2 g/cc (83%TD) to 3.4 g/cc (55%TD). (2) The porosity of the ROX-SZR increased from 18 to 39%, while that of the ROX-ThO2 also increased from 17 to 46%. The increase of porosity, that is, the unconstrained gas bubble swelling in the present case might be attribu ted to the agglomeration of pores, where Al2O3 and PuAl11O18 of ROX compone nts took an important role. (3) The rate of unconstrained gas bubble swelling was estimated to be 0.8%D elta V/V per 1%FIMA for the ROX-SZR and 1%Delta V/V per 1%FIMA for the ROX- ThO2, respectively. For UO2, it was about 1%Delta V/V per 1%FIMA. Hence, wi th respect to the rate of unconstrained gas bubble swelling between the ROX fuels and UO2, the difference was insignificant. (4) The average grain size of the ROX-SZR increased from 2 to 6 mu m, while that of the ROX-ThO2 increased from 2 to 4 mu m A grain growth occurred sl ightly.