Three-dimensional simulation of the coupled convective, conductive, and radiative heat transfer during decay heat removal in an HTR

A three-dimensional model has been constructed to simulate the passive heat removal in a modular prismatic-block high temperature reactor during a loss of active cooling accident. This model, developed using the STAR-CD general computational fluid dynamics code, solves the combined conductive, convective, and radiative heat transfer within a 30° section of the core and reactor vessel. To accommodate the different spatial scales, it uses homogeneous equivalent media to represent the coolant flow and the prismatic fuel blocks. A customized procedure that manages solving alternatively the dynamic and thermal fields permits the computation of very long transients, which typically are performed for 100 or more hours of simulated time. The global methodology and specific modeling procedures are explained, and key points of the CFD analysis are highlighted. Next, the results of several calculations are presented, and the physical phenomena represented are described. Two commonly investigated loss of active cooling scenarios are considered: depressurized conduction cooldown and pressurized conduction cooldown. The results for these two scenarios are compared to assess the effect of heat transfer via internal natural convection – which is negligible during the depressurized event – on the thermal behavior of the system. In addition, the evolution of the natural convection flow through the core and in the annular spaces is examined and discussed.