Densifications of UO2-PuO2 fuel

The in-reactor densification of UO2 - PuO2 fuel is studied by means of the P17 and P20 programmes carried out in the BR2 reactor at Mol, Belgium. The twelve P17 rods are typical of BWR's and consist of chamfered pellets enriched with 3.4 % plutonium fissile and 0.6 % U235; eight rods have been loaded with 94 % TD fuel and four rods with 91 % TD fuel. The eleven P20 fuel rods contain fuel typical of PWR's : 91 % TD dished pellets enriched with 0.9 % plutonium fissile and 0.2 % U235. The in-reactor fuel axial length changes are deduced from the photodensitometric analysis of the fuel rods neutronographies carried out after successive BR2 irradiation cycles. The analysis of the post-irradiation examinations of the first eight P17 fuel rods indicates fuel shortening rates decreasing with irradiation time; the low density fuel shows greater shrinkage than the high density fuel, the maximum shortening occurring between 7 and 8 GWd/tM. Lower shrinkages are noticed in the P20 fuel rods, which show sometimes high densification rates. (Auth.)