Radiative Properties of Uranium Dioxide Near Its Melting Point

A new technique for measuring radiative properties of nuclear fuel materials at high temperatures has been developed. The technique is based on pulse diffuse optical probing of the sample surface and on pyroreflectometry used in measuring radiative properties of refractory materials during laser heating or cooling. Pulse diffuse optical probing of the sample has been realized for the first time in subsecond pyrometry of the open surface heated by laser radiation. Such a procedure of sample irradiation during sample laser heating or cooling enables reflectivity and emissivity measurements near high temperature phase transitions to be performed in spite of possible sharp changes of the reflection indicatrix at phase transitions in the investigated material. With the method developed in this study, the spectral emissivity and reflectivity of uranium dioxide near its premelting and melting points have been measured. It has been found that condensation of the vapor plume formed above the sample during laser heating influenced the melting and boiling temperatures of uranium dioxide. The firstorder phase transitions in uranium dioxide, such as solid-vapor-solid and liquid-vapor-liquid, have been observed in uranium dioxide for the first time during laser heating. Also, new data on the spectral emissivity of uranium dioxide at a wavelength of 0.644 /«n and in the temperature range of 2000 to 4200 K are preseInted.