Discrete element modeling of conduction cool down in pebble-bed reactors under loss of forced convection

The most common way of conducting thermal analysis on a Pebble Bed Reactor (PBR) geometry has always been to treat the reactor core as homogeneous or semi-homogeneous domain, through which heat conducts at a rate defined by an effective thermal conductivity. Unfortunately, for this method to be effective at predicting the temperature distribution within the reactor core, the combined effects of numerous complicated physical phenomenon (e.g. point thermal contacts, thermal radiation, packing geometry, and heat generation) must be accurately and fully encapsulated in this effective thermal conductivity. Recently, computational power has increased to where it has become possible to perform some CFD simulations on spatially-resolved PBR core geometries. However, validation of these models is challenging, especially for accident conditions and at locations near the bed wall. In this work, a simple Discrete Element Method (DEM) code was written and used to create several different geometries of randomly packed spheres and cylinders. Heat transfer within these geometries was then modeled using both a numerical Finite Element Method (FEM) and an idealized homogeneous method. After validating the DEM code and analysis methods using temperature data obtained from cooling experiments featuring cylindrical geometry, the methods were extended to a PBR type geometry. A conduction cool down scenario was modeled and the results from the FEM model were compared to best possible results obtainable from a more traditional, homogeneous 1–D approximation. It was found that even the best possible homogeneous model was unable to match the FEM results under post-accident conditions.