The paper describes a new version of a general 2D-multiphase flow code (ESE). The ESE code has been developed to model the interaction of molten core debris with water. It takes into account the steam generation when melt pours into water, the interaction between the steam, water, air and melt phases, and the associated fragmentation and cooling of the melt. In case of a steam explosion, a trigger produces locally enhanced heat transfer and pressurisation that can propagate through the coarse mixture. This propagation phase of the interaction is not modelled by the code, but ESE predicts how the initial conditions for such an event will evolve in time, giving an indication of the amount of melt that is well-mixed at the time of the trigger.

The ESE code solves the conservation equations for mass, momentum, and internal energy for the four fluids (melt, water, vapor and air) in two-dimensional cylindrical coordinate system. Each fluid is characterised by its phase presence probability in each computational node and its own velocity and temperature field. A common pressure field is assumed.

The main improvement of the new version ESE2.0 is in the more accurate numerical treatment of the probabilistic multiphase flow equations. The transport equations is solved in conservative form on a staggered grid using a high-resolution finite difference method. High-resolution methods are at least second order accurate on smooth solutions and yet give well resolved, nonoscillatory discontinuities. To achieve also second order temporal accuracy the high-resolution method is based on Lax-Wendroff scheme. The Jacobian matrix appearing in the non-linear formulation of the Lax-Wendroff scheme is evaluated numerically. To satisfy the total variation diminishing (TVD) condition for the high-resolution scheme the smooth van Leer flux-limiter function is chosen. The pressure equation is solved using Alternating Direction Implicit (ADI) scheme.

To examine the accuracy of the improved code ESE2.0 the premixing experiment “MIXA06” with corresponding geometry and initial conditions was simulated. The results of the simulation were compared with experimental data and presented in form of graphs, depicting the velocity, temperature and phase presence probability fields for all four phases (melt, water, vapor and air), the depth of melt penetration and the steam flow rate for different times.