On Microwave Experiments in Dielectric Media to Simulate Nuclear Volumetric Heating

Abstract

The use of microwave experiments in normal fluids is proposed for the approximation of the volumetric heating distribution in cryogenic flow of radiation-heated deuterium in the advanced neutron source (ANS) cold neutron source (CNS) planned for construction at Oak Ridge National Laboratory. The potential of such experiments is investigated by solving Maxwell's equations for microwave propgation and absorption in several noncryogenic model fluids. Included are an analytical Mie series solution for an idealized ANS CNS geometry and a solution in a more complex and realistic geometry by the computational finite difference time domain (FDTD) technique. Though data and anecdotal evidence suggest difficulty with specifying a given volumetric heating distribution in a boiling liquid in a microwave cavity, the computational results suggest that CNS-like heating distributions can be obtained by using microwave irradiation. Two aspects of microwave heating are examined. The first is scale dependence of heating across various fluid particle sizes in a potentially complex multiphase flow, and the second is detailed heating distribution across a realistic three-dimensional ANS CNS geometry. By using a Mie series solution in spherical geometries to indicate dependence of microwave heating on fluid particle size for flow scales relevant to ANS CNS flows, several fluids, including n-propanol and n-butanol, are found to show < 20% variation in heating on scales from 0.01 down to 10 -7 m. By using FDTD computations, the expected ANS heating distribution in liquid deuterium is compared with heating distribution under microwave irradiation for several different model fluids. Good qualitative agreement is found between expected ANS heating distribution and microwave heating in the n-propanol and n-butanol fluids including the heating asymmetry expected in ANS CNS flows. By using this simulated heating distribution, volume-heated flow can be investigated. Expected results from such an investigation include flow regime determination, effects of nucleation phenomena, and other physical characteristics such as heating distribution, container shape, and fluid properties