ABSTRACT
At the present, the international goal is to develop and qualify a very high U density fuel for high flux research reactors. The selected U-alloy must present an isotropic crystalline structure at the end of the fuel fabrication process. These types of structures exhibit stable behavior under irradiation. In particular, it is wanted to retain as metastable the γU phase (or some phase derivated of this). Several investigation lines are being developed in Argentina, regarding both dispersed and monolithic fuels. One of this lines is focused on development of U(Mo)/Zry-4 monolithic fuel. Simultaneously, the fabrication of U(Zr,Nb)/Zry-4 miniplates was started in 2005.
As part of this last monolithic fuel development, 10 alloys with U densities between 7-10 gU/cm3 were fabricated. These alloys were homogenized in γU(Zr,Nb) phase by annealing at 850 ºC for 1 and 24 h and finished by quenching.
In this work the morphological and crystallographic characterization of this set of alloys is presented. The major phases identified were: γU (Zr, Nb) (or the transition phase γS derived from it), δUZr2 and α” (transition phase derived from the equilibrium αU phase). In addition, four types of precipitates, rich in Zr, with different morphologies were identified. The presence of each of them was associated to the stage in which they were formed: during the casting process of the alloy, the heat treatment and / or the annealing process. This result allows us to evaluate which parameters need to be controlled to obtain a homogeneous alloy at the time of making a nuclear fuel.
Keywords
Nuclear fuel; uranium; phase transformations
