Simulation of Intersubchannel Mixing in a Triangular Nuclear Fuel Bundle Geometry

Abstract

Predicting temperature distributions in fuel rod bundles is an important component of safety analysis of nuclear reactors. Intersubchannel mixing acts to homogenize coolant temperatures thus reducing the likelihood of localized regions of high fuel temperature. Previous research has shown that intersubchannel mixing in nuclear fuel rod bundles is enhanced by a large-scale quasi-periodic energetic fluid motion, which transports fluid on the cross-plane between the narrow gaps connecting subchannels. This phenomenon has also been observed in laminar flows. Unsteady laminar flow simulations were performed in a simplified bundle of three rods with a pipe. The objective was to study the dynamics of this flow phenomenon, the interactions between subchannels, and the impact on thermal mixing. Three similar geometries of varying gap width were examined. Thermal mixing was driven by the advection of energy between subchannels by the cross-plane flow. Flow through the rod-to-wall gaps in the wall subchannels alternated with a dominant frequency, particularly when rod-to-wall gaps were smaller than rod-to-rod gaps. Cross-plane flow through the rod-to-rod gaps in the triangular subchannel was irregular in each case. This was due to the fundamental irregularity of the triangular subchannel geometry. Vortices were continually broken up by cross-plane flow from other gaps due to the odd number of fluid pathways within the central subchannel. Significant phase-linking between rod-to-wall gaps was observed such that a peripheral circulation occurred through each gap simultaneously. Cross-plane flow in subchannel geometries is highly interconnected between gaps. The flow structure through the gaps is more complicated than has been previously reported and depends very much on the subchannel geometry.