The extent of penetration of flowing molten ceramic fuel in steel channels before solidification is a problem that arises in the analysis of hypothetical core disruptive accidents. Considerations of fuel crust behavior indicate that fuel freezing in steel channels can occur in two distinct ways that can be identified as conduction-limited freezing (fuel crust growth) and bulk freezing (fuel crust removal). Fuel crust removal can arise from two sources: (a) mechanical breakup and (b) melting heat transfer. Explicit formulas providing rough estimates of critical fuel crust removal conditions are presented. If the conditions in the fuel flow are such to prevent fuel crust growth then the steel wall melting can become severe. It is proposed here that steel ablation rapidly leads to fuel freezing in a bulk manner via turbulent mixing between the relatively “cold” molten steel and hot molten fuel. This steel ablation-induced freezing concept is used to obtain a simple expression for molten fuel penetration into steel channels.