A new method for measuring radiative properties of nuclear fuel materials at high temperatures has been developed. The method is based on the pulsed-diffuse optical probing of the flat surface heated by a powerful laser beam and on multiwavelength pyroreflectometry.
Such an approach enables one to measure spectral reflectivity and emissivity of the nuclear fuel material with diffuse, specular or mixed specular-diffuse reflection surface at high temperatures. Moreover, this method permits reflectivity and emissivity measurements to be performed near the melting point of the material being investigated with a high resolution, despite the sharp change in the reflection indicatrix at melting temperature. The described technique was successfully applied up to 4000 K in the spectral range 0.4-1.1 μm at different wavelengths. The reflectivity and emissivity measurement uncertainty does not exceed 3%. The potential of the method developed has been demonstrated in measuring radiative properties of uranium dioxide at high temperatures. The experimental data on radiative properties of uranium dioxide in the spectral range 0.4-1.1 μm and in the temperature interval 2000–3500 K are presented in the paper. Direct measurements of the spectral reflectivity near the premelting point and investigation of the complex behavior of the emissivity during solidification of uranium dioxide have been performed for the first time.