AbstractA fuel/coolant interaction (FCI) is a phenomenon that may occur during a light water reactor severe accident when molten fuel comes into contact with residual water in-vessel or in the reactor containment. A new one-dimensional multifluid transient model, which currently has the capability of examining nonexplosive FCI behavior, is described. A unique dynamic fuel fragmentation model based on relative velocities was incorporated into this model along with other constitutive relations. The model was compared to the limited FCI data on mixing and nonexplosive FCI events and shows relatively good agreement. In addition, a set of FCI parametric calculations for in-vessel fuel-coolant mixing was performed to gain insight into the important variable affecting mixing (i.e., pressure, water depth, multiple jets) and the initial conditions for the explosion.