Transformation temperatures γU → δUZr2 Y γU →αU in U-Zr-Nb alloys

The international RERTR program has as primary objective the development of new fuels for research and test reactors to satisfy the requirement of low enrichment in 235U (lower than 20 %). It is known that the cubic-phase (γU) has an excellent behaviour under irradiation. In this context, in the Materials Department (GIDAT-GAEN-CNEA) U-Zr-Nb alloys are considered candidates for the development of a high-density monolithic-type nuclear fuel. It is necessary to evaluate the thermodynamic parameters that allow to obtain a range of concentrations of the U-Zr-Nb system in which this phase can be retained in the metastable condition with the required 235U density. In this work, eight U alloys with concentrations ranging from 13.9 to 43.7 wt.% Zr and from 0 to 6.4 wt.% Nb, were fabricated. Dynamical measurements of electrical resistivity, with a cooling rate of 4 oC/min, were performed and the results were analyzed. Considering this cooling rate, a Nb concentration of at least 17.8 wt. % would inhibit the transformation γU→ δUZr2 and a concentration of al least 23.3 wt % would inhibit the γU → αU transformation. (author)