CFD Analysis of Core Bypass Phenomena

Abstract

EXECUTIVE SUMMARYThe U.S. Department of Energy is exploring the potential for the very high temperature gas-cooled reactor (VHTR), which will be either a prismatic or pebble-bed type reactor. One important design consideration for the reactor core of a prismatic VHTR is coolant bypass flow, which occurs in the interstitial regions between fuel blocks. Such gaps are an inherent presence in the reactor core because of tolerances in manufacturing the blocks and the inexact nature of their installation. Furthermore, the geometry of graphite blocks changes over the lifetime of the reactor because of thermal expansion and irradiation damage. The existence of the gaps induces a flow bias in the fuel blocks and results in unexpected increases in maximum fuel temperatures.Traditionally, simplified methods such as flow network calculations employing experimental correlations have been used to estimate flow and temperature distributions in the core design. However, the distribution of temperature in the fuel pins and graphite blocks as well as coolant outlet temperatures are strongly coupled with the local heat generation rate within fuel blocks which is not uniformly distributed in the core. Hence, it is crucial to establish mechanistic based methods that can be applied to the reactor core thermal hydraulic design and safety analysis.Computational fluid dynamics (CFD) codes, which have a local physics based simulation capability, are widely used in various industrial fields. This study investigates core bypass flow phenomena with the assistance of commercial CFD codes and establishes a baseline for evaluation methods. A one-twelfth sector of the hexagonal block surface is modeled and extruded down to a whole core length of 10.704 m. The computational domain is divided vertically with an upper reflector, a fuel section, and a lower reflector. Each side of the sector grid can be set as a symmetry boundary. The geometry used in the present study is shown in Figure E A steady Reynolds-averaged Navier-Stokes approach is employed using the commercial code FLUENT, which employs the finite volume method. Prior to the calculations, simple validation and grid sensitivity studies are conducted in order to evaluate the turbulence models and reduce numerical errors. Comparative studies are conducted varying several parameters including gap width, heat generation vii profile, turbulence model, and irradiation shrinkage to investigate the sensitivity of the flow and temperatures in the core to each parameter. In addition, a study wherein 20% bypass flow is achieved, thought to have occurred in earlier gas-cooled reactors, is performed to determine approximate gap widths that would be present and to visualize flow and temperature distributions.Results of a study that varied the gap width from 0 to 5 mm show that the bypass flow provides significant cooling to the graphite block, and that the maximum fuel temperature increases with gap width if the bypass flow is "robbed" from the coolant channels. Bypass flow induces a steep overall tem...