Redox Reactions of Uranium-Based Materials in Aqueous Systems and Under UHV Conditions. : Two models mimicking radiation-induced oxidation of spent nuclear fuel
Abstract: The demand for clean electricity is a primary concern in Europe. Nuclear power isconsidered as a greenhouse gas-free technology for generation of electricity. However,compared to other energy sources, it generates a highly toxic waste.In Sweden, the long-term solution for the waste disposal issue is called the KBS-3 method.It is based on protective natural and engineered barriers surrounding the spent nuclear fuelin order to isolate it from the biosphere, thereby ensuring protection of the environmentfrom increased levels of radioactivity. The spent nuclear fuel problem also concernscountries that no longer make use of nuclear power but still have large amounts of wasteto deal with.In recent years, the safety research concerning deep geological repositories for spentnuclear fuel have attracted significant attention. Scientists have investigated differentaspects of the worst-case scenario where the integrity of the barrier system is lost. Ifgroundwater actually comes into contact with the fuel, water radiolysis will be induced andthe products formed have the potential of oxidizing and dissolving the fuel matrix whichwill risk spreading radionuclides into the environment.Not surprisingly, actinide oxides are at the heart of many experimental studies. In manyinstances, their surfaces interaction with water radiolysis products are key to explain thematrix response in different environments. The main goal is to understand the fuelcorrosion/dissolution process, however many of the studies were performed on fairlycomplex systems which makes it difficult to draw reliable detailed mechanisticconclusions.In this thesis, the chemical processes involved in radiation induced oxidative dissolutionof UO2 were experimentally modeled in two different ways: The H2O2-induced oxidativedissolution of UO2 in aqueous solution and the reactive plasma interaction with uraniumoxide thin films under UHV conditions. H2O2 is one of the main oxidants generated inwater radiolysis. The impact of groundwater components on the mechanism of H2O2consumption and UO2 oxidation were investigated.The plasma of water gas successfully generated products similar to the ones formed by theradiolysis of liquid water. Three different types of uranium oxide (UO2, U2O5 and UO3)were probed with X-ray and Ultra-Violet Photoelectron Spectroscopy before and after theexposure to water plasma, pure or mixed with H2. A comprehensive examination was madeof redox events which mimic the radiation effects. Analysis of U5f emission was used todetermine the surface oxidation state. The nature of hydroxyl species adsorbed to thedifferent uranium oxides was also investigated in order to identify the conditions underwhich hydroxyl radical binding to uranium oxide surfaces is favored.
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