Radiative properties of uranium dioxide near its melting point

A new technique for measuring radiative properties of nuclear fuel material s 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 hea ting or cooling. Pulse diffuse optical probing, of the sample has been real ized for the first time in subsecond pyrometry of the open surface heated b y laser radiation. Such a procedure of sample irradiation during sample las er heating or cooling enables reflectivity and emissivity measurements near high temperature phase transitions to be performed in spite of possible sh arp changes of the reflection indicatrix at phase transitions in the invest igated material. With the method developed in this study, the spectral emis sivity 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 first-order phase transiti ons in uranium dioxide, such as solid-vapor-solid and liquid-vapor-liquid h ave been observed in uranium dioxide for the first lime during laser heatin g. Also, new data on the spectral emissivity of uranium dioxide at a wavele ngth of 0.644 mu m and in the temperature range of 2000 to 4200 K are prese nted.