Thermal interaction between molten fuel and coolant plays an important role in nuclear reactor safety. A molten fuel-coolant interaction model for water was developed and tested by calculating the propagation of pressure waves observed in some of the in-pile SPERT experiments. Pressure buildup in the interaction zone results from a high heat flux to the coolant, assuming direct contact for the initial phase. Both interaction and acceleration zones are modeled by the same set of equations that accounts for compressibility and inertia of the coolant. Phase changes of the water are controlled by a finite mass transfer rate. Calculations with this one-dimensional model BLAFCI show that particle size distribution and fragmentation time are the most sensitive parameters. Calculated peak pressures and some of the pressure time curves up to 10 msec agree well with the experimental data.