Physical modeling and numerical simulation of subcooled boiling in one-and three-dimensional representations of bundle geometry

Numerical simulation of subcooled boiling in one-dimensional geometry with the Homogeneous Equilibrium Model (HEM) may yield difficulties related to the very low sonic velocity associated with the HEM. These difficulties do not arise with subcritical flow. Possible solutions of the problem include introducing a relaxation of the vapor production rate. Three-dimensional simulations of subcooled boiling in bundle geometry typical of fast reactors can be performed by using two systems of conservation equations, one for the HEM and the other for a Separated Phases Model (SPM), with a smooth transition between the two models. The suitability of this approach for modeling subcooled boiling in three dimensional geometry has been proved by simulating numerically two selected experiments in 7- and 37-pin electrically heated bundles typical of fast reactor subassemblies. (author)