Simultaneous crust growth in a fluid and the melting of an adjacent colder structure surface in contact with it are examined with emphasis on fast breeder reactor safety applications. The calculations were made subject to the following assumptions: 1. As long as a crust exists, melted structure adjacent to it stays in place until it melts. 2. When a crust melts, melted structure is ablated. The ablation process does not influence the temperature profile in the remaining intact structure. The dependence of the Fourier number at which complete structure melting occurs on fluid-to-crust heat flux is obtained for uranium oxide and thorium oxide crusts forming on steel surfaces. The crust behavior is also investigated as the heat flux is varied. The dependence of the results on internal heat generation and the Biot number on the other side of the structure is determined.