Radiation Transmission Imaging Applications for Nuclear Reactor Systems

Abstract

Computational advancements in the past couple of decades have propelled the capabilities to design, model, and benchmark nuclear reactor systems. However, the lack of high-resolution two-phase flow data has hampered the development and validation of high-fidelity models for computational fluid dynamics (CFD) and subchannel codes, in particular pertaining to Light Water Reactor (LWR) systems. Radiation based methods have gained traction and are being widely deployed for the study of two-phase flows. These methods present inherit advantages over conventional instrumentation due to their non-intrusiveness as well as the capability to perform measurements through complex and opaque geometries. The present dissertation is focused on the development of in-house gamma tomography and high-speed x-ray radiography systems and their application to high-resolution two-phase flow measurements. The two measurement systems were also applied to non-destructive measurements of sodium fast reactor mock-up fuel assemblies under severe accident scenarios. A major contribution of this dissertation consisted in supporting the development and testing of the High-Resolution Gamma Tomography System (HRGTS). This is a computed tomography (CT) imaging device designed and assembled in-house at the Experimental and Computational Multiphase Flow Laboratory (ECMFL) with the purpose of performing high-spatial resolution void fraction measurements. This CT system is deployed at the Michigan Adiabatic Rod Bundle Flow Experiment (MARBLE) facility which consists of an 8 x 8 modular assembly designed to simulate scaled PWR and BWR assemblies. The present work discusses the design, construction, and operation of the MARBLE facility. This facility has been instrumented to establish a high-spatial resolution experimental database of two-phase flows inside reactor assembly geometries and investigate the effect of spacer grids and mixing vanes on void drift across subchannels. The present work discusses the design, construction, modeling, and initial measurements performed with the HRGTS at the MARBLE facility. Additionally, the high-speed x-ray radiography imaging system was assembled and deployed at the post-CHF Heat Transfer (PCHT) facility. This experiment is designed to achieve post Critical Heat Flux (CHF) conditions to investigate the following two-phase flow regimes: Inverted Annular Film Boiling (IAFB), Inverted Slug Film Boiling (ISFB) ranging to Dispersed Flow Film Boiling (DFFB). However, radiation-based measurements of boiling experiments bear several challenges due to the high temperature conditions which result in the following: mismatch of calibration to experimental conditions, x-ray beam hardening, thermal expansion, material and working fluid density. The present research focuses on developing methods to overcome these challenges and obtain meaningful quantitative results. These were validated through rigorous modeling and testing in which the aforementioned challenges are recreated. Preliminary pool boiling measurements are analyzed with the developed post-processing strategies at hand.