An integrated thermal-hydraulic system CFD model of a prismatic block HTR core using Flownex

Abstract

The active core of a prismatic high temperature reactor (HTR) consists of standard fuel, reserve shutdown and control blocks which must be maintained within a safe operating envelope. The fuel blocks are coupled to the fluid behaviour by heat transfer and neutronic interactions. The evaluation of the heat transfer and fluid behaviour for prismatic HTRs is complex and have many facets. The primary objective of this study is the philosophical development of a thermal-hydraulic Flownex network based model for a prismatic block-type HTR that can reasonably predict the temperature distribution for a standard fuel block. The application of the methodology is illustrated through Flownex models of segments and assemblies of the standard fuel block in steady state and transient conditions. Verification of the 1/6th three-dimensional (3-D) representation of the fuel bock model were done with the two-dimensional (2-D) previously developed Flownex and STAR-CCM+ integrated prismatic block models. The results were in good agreement. Validation of steady state models was also done by comparing temperature distribution results for various cases with those obtained using KAERI’s Core Reliable Optimization and thermo-fluid Network Analysis (CORONA) code, which is a hybrid between 3-D CFD and two-dimensional (1-D) fluid system, developed for prismatic HTRs. These cases include a single fuel rod cell, a 1/6th of a prismatic fuel block, a 1/6th fuel block assembly, a standard fuel block, a fluid flow model and a three-fuel block assembly. Flownex results and CORONA results were found to be in good agreement. A coolant channel blockage incident was also investigated in a 1/6th fuel block assembly and also on a single standard fuel block to see the effect of blocked coolant channels in the tangential heat transfer. In the case of blocked coolant channels, Flownex tends to under-predict the fuel compact temperatures and it becomes more evident as the number of blocked coolant channels increases.