Presently ENEA programs in the field of nuclear energy are focused principally on new concepts of advanced reactors, characterized by inherent and passive safety features. Among the several plants proposed or designed by organizations or industries in the world, great interest was aroused by the PRISM reactor, designed by General Electric, as a relatively small sodium-cooled fast reactor able to withstand severe uncontrolled accidents without serious core damages. U-Pu-Zr metal is the reference fuel for the PRISM core. However, an oxide core has been designed as an alternative within the same core envelope, aimed at achieving comparable transient response characteristics to the reference metal fuel. In 1989, General Electric and ENEA agreed on a joint activity to verify, compare and, if possible, optimize the alternative oxide core steady-state and transient characteristics, taking into account safety and economic requirements. The parallel utilization of USA (by GE) and European (by ENEA) methodologies allow a complete comparison of the results. The initial methodology sensitivity studies are summarized in this paper.