Experimental and Numerical Analysis of the Flow in Rod Bundles with Spacer Grids

Abstract

Pressurized Water Reactors (PWRs) are the most common types of electricity generating reactors. Inside of a PWR core, pressurized liquid water is used as a coolant to remove the fission heat generated in the cylindrical fuel rods. The fuel rods are arranged in the reactor using spacer grids, which keeps the rods in the designed lattice and provides structural support. In addition, the spacer grids are used to enhance the flow mixture and the heat exchange between the coolant and the fuel. The presence of mixing-vanes or channel structures in the spacer grids increase the complexity of the flow of the coolant. The turbulent mixing directly impacts the heat exchange characteristics in the reactor core, which affects the safety margins and operating temperatures. Thermal-hydraulics plays an important role in the design and licensing of nuclear reactors. While an in-depth understanding of the flow characteristics can lead to better fuel assembly geometries, most of the computational codes used for design and licensing rely on reduced order models. One example is the use of thermal-hydraulics subchannel codes. In the first portion of this work, the cross-flow planes in a fuel assembly typical of a PWR were investigated with optical measurement techniques. The objective was to study the mass transfer associated with the mean flow and with turbulence between adjacent subchannels. The data aims to improve how the mixing between subchannels is accounted in reduced-order models. In addition, a comprehensive uncertainty quantification procedure for PIV was developed.

The use of mixing-vane spacer grids in PWR is commonly related to the appearance of flow-induced vibrations and fretting. This can be potentially mitigated with the use of other spacer grid designs, such as channel-type spacer grids. In the second portion of this study, a channel-type spacer grid designed at Texas A&M was 3D-printed and the flow profiles were obtained in flow visualization experiments. The objective was to provide data to the validation of computational models for flow calculation. In addition, the spacer grid design is non-proprietary, facilitating the data exchange between different groups. The results from this experimental campaign were used as validation data for another study presented in this dissertation, which consists of a turbulence modelling sensitivity study. The objective was to understand what the implications are of applying high level of turbulence modelling (RANS), and to understand if the PANS model is applicable to this flow.

In another portion of this work, the experimental techniques developed for flow visualization in PWR fuel assemblies were applied to the study of the flow in two fuel assemblies being developed for the next generation of nuclear reactors (GEN IV). Several techniques, such as 3D-printing, PIV setup, data processing and post-processing, and uncertainty quantifications were directly applied to this portion of the work.