Effect of compaction pressure on the thermal conductivity of UO₂ BeO Gd₂O₃ pellets

The (U,Gd)O₂ fuels are used in pressurized water reactors (PWR) to control the neutron population in the reactor during the early life with the purpose of extended fuel cycles and higher target burnups. Nevertheless, the incorporation of Gd₂O₃ in the UO₂ fuel decreases the thermal conductivity, leading to premature fuel degradation. This is the reason for the addition of beryllium oxide (BeO) which has a high thermal conductivity and is chemically compatible with UO₂. Pellets with BeO contents of 2 and 3 wt% and 6wt% of Gd₂O₃ solid solution were compacted at pressures of 400 MPa, 500 MPa, 600 MPa and 700 MPa based on the conventional UO₂ pellet manufacturing process with steps of the mixing of powders, pressing, and sintering under reducing atmosphere. The purpose of this study was investigate the effect of BeO, Gd₂O₃ and compaction pressure on the thermal conductivity of the UO₂ pellets. The thermal diffusivity and conductivity of the pellets were determined from 298 K to 773 K and the results obtained were compared to fuel pellets of UO₂. The thermal diffusivity was determined by Laser Flash and Thermal Quadrupole methods and the thermal conductivity was calculated from the product of thermal diffusivity, the specific heat capacity and density. The sintered density of the pellets was determined by xylol penetration and immersion method. The results showed an increase in the thermal conductivity of the pellets with additions of BeO and with the increase of the compaction pressure as compared to the values obtained with UO₂ pellets. (author)