Exposure and Characterization of Candidate Materials for Use in Supercritical Water Reactors
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Authors
Karmiol, Zachary
Issue Date
2022
Type
Dissertation
Language
Keywords
Supercritical Water
Alternative Title
Abstract
The use of supercritical water as the heat transfer medium between a power source and the steam generating system lead to an increase in thermodynamic efficiency. Supercritical water systems have already been employed in fossil fuel power generation, and a proposed Gen IV nuclear reactor is designed to do the same which would result in an increase in efficiency by ~40% compared to current light water reactors. Achieving these conditions requires that the heat transfer medium of water transition from the subcritical state to the higher enthalpy supercritical state above the critical point of water, 22.09 MPA and 647.3K (374.14°C). This region of transition is often referred to as the pseudocritical region and has been observed to be the region with the greatest variation in thermophysical properties, with reports of a local maximum in corrosion rate. Austenitic stainless steels and nickel based superalloys are candidate materials for the structural components of energy generation that makes use of supercritical water, owing to the high strength and corrosion resistance required for operation under these conditions. The supercritical water loop at The University of Nevada, Reno allows for the study of the effects on materials exposed to the pseudocritical region and compare them to effects observed when exposed to water in either subcritical or supercritical states. Morphological differences were observed on samples of stainless steel 316 that were exposed to the near-critical, transition, and near-supercritical region. Characterization of the surface oxides on the samples showed the presence of magnetite (Fe3O4) on all samples. The samples exposed to subcritical water in the near-critical region also showed the presence of a nickel-iron chromium mixed spinel Ni(Fe2-xCrx)O4. The variation in oxide morphology observed based on exposure temperature was attributed to an increased nucleation and growth rate, and an increased generation of Fe2+ on samples exposed to the transition region and supercritical water.
This dissertation also investigated the oxidation of a nickel-based superalloy, namely Alloy X, in water at elevated temperatures: subcritical water at 261°C and 27 MPa, the transition between subcritical and supercritical water at 374°C and 27 MPa, and supercritical water at 380°C and 27 MPa for 100 h. The morphology of the sample surfaces were studied using scanning electron microscopy coupled with focused ion beam milling, and the surface chemistry was investigated using X-ray diffraction, Raman spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy before and after exposure studies. Surfaces of samples showed that a ferrite spinel containing aluminum formed under all conditions.