A safety comparison was made for two 1200-MW(electric) liquid-metal fast breeder reactor cores with homogeneous and heterogeneous fuel arrangements, respectively. The two cores were conceptually designed to be identical except for those parameters affected by different fuel arrangements. The comparison was limited to the issue of initiating phase energetics in the hypothetical core disruptive accident. Both cores were assumed to be at end-of-equilibrium cycle and subject to unprotected loss-of-flow transients. The SAS3D code was used for analyses with four sets of phenomenological assumptions at different degrees of conservatism. Results of the four corresponding cases showed that the heterogeneous core consistently behaved more mildly than the homogeneous core due to its relatively much lower “effective” sodium voiding reactivity worth.