CFD Investigation of MHTGR Natural Circulation and Decay Heat Removal in P-LOFC Accident

Chengqi Wang,Xiaodong Sun,Piyush Sabharwall

doi:10.3389/fenrg.2020.00129

Abstract

One of the key postulated accidents in a high-temperature gas-cooled reactor (HTGR) is the pressurized loss of forced circulation (P-LOFC) of the primary loop, which can be triggered by its primary helium circulator trip or turbine trip. If the reactor shutdown cooling system (SCS) fails during a P-LOFC accident, part of the reactor decay heat is absorbed by the reactor core materials and the rest removed by the reactor cavity cooling system (RCCS). In the extended period of P-LOFC accident compound with SCS failure, the core decay heat is supposed to be removed by conduction, natural circulation, convection, and radiation. A three-dimensional (3-D) computational fluid dynamics (CFD) simulation was performed in this research to study the long-term heat removal mechanisms in the General Atomics’ Modular High Temperature Gas-cooled Reactor (MHTGR) design in a P-LOFC accident. The reactor core temperature distribution and flow field were obtained at different decay power levels. The sensitivity of the natural circulation flow to the bypass gap width was investigated. The natural circulation flow intensity is relatively weak but very sensitive to the width of the bypass gaps.

Highlights

The high-temperature gas-cooled reactors (HTGRs) have received great attention due to their potential to provide high-temperature process heat in addition to their high thermal-to-electric power conversion efficiency and inherent safety features (Alonso et al, 2014; Fang et al, 2017; Wang et al, 2019)
To obtain more accurate flow and temperature distributions in an HTGR core under a pressurized loss of forced cooling (P-LOFC) accident, it is desirable to include in the model all the heat transfer paths, including: the fuel columns, graphite reflectors, helium flow, and reactor pressure vessel (RPV) wall
Since this study focuses on the decay heat removal after a natural circulation flow through the reactor core has been established, the reactor core is cooled down slowly, which could be reasonably regarded as a quasi-steady state condition (Oh et al, 2012)

Summary

INTRODUCTION

The high-temperature gas-cooled reactors (HTGRs) have received great attention due to their potential to provide high-temperature process heat in addition to their high thermal-to-electric power conversion efficiency and inherent safety features (Alonso et al, 2014; Fang et al, 2017; Wang et al, 2019). During a P-LOFC accident, core decay heat will be removed by a combination of heat conduction, natural circulation/convection, and thermal radiation to the reactor cavity cooling system (RCCS), so the maximum fuel temperature does not exceed the design limit. To obtain more accurate flow and temperature distributions in an HTGR core under a P-LOFC accident, it is desirable to include in the model all the heat transfer paths, including: the fuel columns, graphite reflectors, helium flow, and reactor pressure vessel (RPV) wall. The meshes of the fuel columns, hot plenum, and bypass gaps are depicted in Figure 3A with a detailed view of the FIGURE 1 | MHTGR design: (A) cut away view of the RPV and (B) cross-sectional view of the reactor core (Nuclear Energy Agency, 2018).

Input Results

64 Re ρ u21

RESULTS AND DISCUSSION

CONCLUSION