Numerical Modeling for Characterization of Pebble Bed Reactor Bypass Flows

Abstract

High-Temperature Gas-Cooled Reactor (HTGR) concepts are Gen-IV reactor designs distinguished by a high level of technology readiness, walk-away safety, and high coolant outlet temperatures. All developed HTGR designs can be categorized into two concepts, the prismatic reactor and pebble bed reactor (PBR). The prismatic reactor core is comprised of hexagonal graphite blocks with cylindrical borings that contain cylindrical fuel compacts or coolant flow; alternatively, the pebble bed concept fuel is encapsulated by tennis-ball sized spherical graphite pebbles that move through the reactor bed under gravity. Encompassing both core designs are graphite reflector blocks that are distributed with small radial and axial gaps to mitigate any additional stresses from thermal expansion. These gaps form secondary coolant flow paths that may allow a significant amount of the coolant to bypass the core increasing the core peak fuel temperature. For this reason, a relevant full core analysis must take into account all the bypass flow, including secondary and leakage flow paths.

Higher order numerical analysis of flow patterns in HTGR concepts can be performed using Computational Fluid Dynamics (CFD), however, considering the complexity of the geometry and the duration of some safety related transients, CFD modeling would require a prohibitive amount of computational resources. A computationally efficient approach would be using a lower-fidelity thermal-hydraulics code such as Pronghorn. Pronghorn is a coarse-mesh, multi-dimensional, thermal-hydraulic (T/H) simulation tool developed by Idaho National Laboratory (INL) using the Multiphysics Object-Oriented Simulation Environment (MOOSE) framework. T/H codes such as Pronghorn can be an effective solution, however, they lack the ability to account for local phenomena and important characteristics of bypass flow. In order to establish these capabilities in Pronghorn, CFD steady state simulations can be utilized to create correlations for implementation in Pronghorn. This two-step approach uses higher-fidelity CFD methodology to inform Pronghorn, resulting in a computationally efficient simulation tool able to capture the relevant phenomena. Consequently, Pronghorn will be able to run safety related transients maintaining a good level of accuracy but drastically reducing the computational time.

The proposed research aims to employ a two-step approach with CFD models of representative PBR geometries used to derive an ad-hoc approach for Pronghorn in order to correctly predict the mass redistribution within the reactor with a model capable of running on a workstation within minutes. The present investigation sets out to develop a representative two-dimensional PBR model based on the HTR-10, a 10 MWt prototype pebble bed reactor. The representative two-dimensional, axisymmetric model includes secondary flow paths and employs the commercial finite volume CFD software, STAR-CCM+, to form a training dataset for a Pronghorn model of the same geometry. Multiple gap sizes and flow conditions have been investigated to generalize the approach that will be utilized in Pronghorn to demonstrate feasibility of computing bypass flow. Ultimately, a PBR bypass flow investigation is conducted using Pronghorn to explore the effects of bypass flow on PBR phenomena. This research sets the groundwork for future studies aimed at high accuracy validation efforts, which will further enhance the utility of contributions made in the present study regarding PBR bypass flow phenomena.