Numerical Modeling of Flow and Heat Transfer During Quenching for a Postulated Loss-of-Coolant Accident

During a loss-of-coolant accident in a boiling water reactor, the liquid inventory drains from the reactor vessel and eventually the fuel bundle begins a rapid heatup when the liquid level drops below the top of the fuel and the only available cooling mechanism is a small steam flow created through coolant evaporation. Emergency core cooling liquid is either sprayed from the top of fuel bundle downward or injected from fuel bottom in an effort to remove the residual heat following the reactor shutdown and to maintain the fuel temperatures at an acceptable level as the whole system cools. Subcooled liquid injected from the top of the bundle forms a falling film that flows down the fuel rods; however this liquid is quickly evaporated by heat transfer due to the high fuel cladding temperature; the high fuel temperatures will also obstruct the liquid film from penetrating further down into the fuel bundle. The ability to predict to the film propagation is critical in determining cooling capacity and core operating limits for BWRs that rely on this cooling mode. Most thermal-hydraulic system codes used throughout the nuclear industry for accident analysis utilize a one-dimensional 2-fluid model that tracks single homogeneous liquid and vapor phases and therefore must rely on simplified models or correlations to model the complex heat transfer characteristics at the film quench front. This analysis develops an improved quenching model by incorporating a local quench front propagation model into an existing 3-field model. The new model utilizes the physical mechanism associated with the liquid film, dispersed droplets, steam flow and multidimensional conduction in the fuel rod to predict film propagation. Finally, the numerical implementation is described and compared against experimental data