Investigations of the ternary uranium-zirconium-oxygen system

The boundaries of the phase fields in the U-Zr-O system was determined by means of specimens prepared under a powder metallurgical method at 1,000, 1,500 and 2,0000C. It appears that the UO2-Zr section corresponding to the contact in LWR's between fuel and cladding is not quasi-binary but that UO2, α-Zr(O) and the (γ-U, β-Zr) solid solution occur as equilibrium phases at lower temperatures and (U, Zr)O2 and melt, respectively, at elevated temperatures. UO2, α-Zr(O) and 30 at.% oxygen are in equilibrium. The eutectic temperature in the α-Zr(O)-UO2 quasi-binary system is 1,800 +- 200C, the distectic temperature at 2,4000 +- 300C. The beginning of melting in the UO2-ZrO2-U-Zr subsystem of the U-Zr-O system was observed with an apparatus used for evaluation of the melting point. With the aid of high-temperature X-ray diffractometry of Zr-O specimens and considering all literature data an improved Zr-O equilibrium diagram is proposed. According to the investigation, the U-Zr-O system is decisive for understanding the interaction between UO2 (fuel) and Zry (cladding). In the course of LWR fuel pin meltdown first liquid phases are already formed below 1,5000C. (orig.)