Received September 28, 1981 Accepted for Publication February 26, 1982 Issues related to the potential for a large-scale sodium vapor explosion in a carbide-fueled liquid-metal fast breeder reactor accident were addressed in the AX1 test in the Transient Reactor Test Facility. Test design and operating conditions were selected to meet the spontaneous nucleation temperature criterion for an energetic, explosive molten fuel-sodium interaction. Although that criterion appears to have been achieved, thermal and mechanical analyses of the test data indicate that the interaction was not especially energetic. Comparison to similar tests on oxide fuel indicates that, under the particular test conditions employed, the conversion of thermal energy to mechanical work is similar for the two fuel types. Transient bulk fuel motion was extensive, with axial fuel motion generally coinciding in time and space with the ejection of the coolant from the original fuel region. Posttest examination of the hardware revealed that nearly all of the fuel had mixed on a microscopic scale with the stainless steel cladding. Relative proportions of iron, chromium, and nickel in the mixture varied widely. The melting point of the mixture was apparently much lower than that of stainless steel.