The most common fuel used in pressurized water reactors is the uranium dioxide (UO_2). However, there are other modern conceptions of nuclear fuels that have advantages over UO_2. Gadolinium is used as a burnable poison which enables compensation of reactivity and the power distribution adjustment in the reactor core. The utilization of (U,Gd)O_2 fuels instead of separate burnable poison rods is considered in pressurized water reactors for the purpose of extended fuel cycles and higher target burnups. The thermal conductivity of the fuel material is one of the most important properties to evaluate their thermal performance under irradiation. Pellets of (U,Gd)O_2 were produced by uniaxial pressing microspheres and powder of UO_2 mixed with 6wt% of Gd_2O_3 powder, as well as pressing microspheres of uranium and gadolinium mixed oxide (U,Gd)O_2, with concentrations of Gd ranging between 2wt% and 10wt%. The purpose of this study was to determine the thermophysical properties of these (U,Gd)O_2 pellets and compare with the thermal conductivity of a pellet produced by pressing microspheres of UO_2, and taken as a reference, to investigate the effect of gadolinium oxide content on the thermal diffusivity and thermal conductivity of the fuel. The pellets thermophysical properties were measured at the LMPT - Laboratorio de Medicao de Propriedades Termofisicas de Combustiveis Nucleares e Materiais of CDTN - Centro de Desenvolvimento da Tecnologia Nuclear, applying the laser flash method. The thermophysical properties, density, specific heat, thermal diffusivity and thermal conductivity are reported. Values of thermal conductivity varying, in function of the gadolinium content, in the range from 5.57 W·m~(-1)·K~(-1) to 7.74 W·m~(-1)·K~(-1), thermal diffusivity from 1.74×10~(-6) m~2·s~(-1) to 2.36×10~(-6) m~2·s~(-1), and specific heat from 279.26 J·kg~(-1)·K~(-1) to 363.99 J·kg~(-1)·K~(-1), were obtained. The results showed that the addition of gadolinium to UO_2 tends to decrease the thermal conductivity and the specific heat of the fuel.
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