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Title: The corrosion of uranium in sealed environments containing oxygen and water vapour
Author: Harker, N. J.
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2012
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The corrosion of uranium is not simply a topic for fundamental academic dispute, but is is a real problem with serious implications if allowed to occur unchecked. A significant quantity of this material already exists as 'legacy waste' from its use in civilian and military applications. If a renaissance in nuclear power is experienced then more material will be produced that will require effective stewardship [ ]. It is therefore imperative we fully understand how this waste will behave in confined storage environments in order to predict the associated risks it may pose over extended periods of time. The kinetics of uranium corrosion in environments containing water vapour and oxygen have been well investigated and reviewed, however the mechanism by which it occurs remains still unresolved [ , - , ]. For this study, a specially made corrosion rig was constructed that allowed the reaction to be followed by mass spectrometry of the corrosive gas phase along side continuous temperature and pressure measurements of the reaction volume. This allowed the change in environmental conditions surrounding the corroding metal to be observed over time, thereby providing an accurate simulation of the sealed storage conditions uranium may experience. The corrosion product was also analysed using Secondary Ion Mass Spectrometry (SIMS) to determine the primary corroding species for the reaction. This comprehensive analysis has allowed a mechanism to be proposed that accounts for the observed changes and that can be corroborated with results already published in the literature. Discrepancies also exist in the described reaction kinetics and pressure dependence in the reaction between uranium and water vapour [ , " ]. Here this problem is addressed at a microscale, by examining the influence of impurities in the form of carbide inclusions on the reaction [ ]. Samples of uranium contained 600 ppm carbon were imaged during and after exposure to water vapour. The results of this study indicate that carbide particles on the surface of uranium readily react with water ,vapour to form voluminous U03, H20 growths at rates significantly faster than that of the metal. This observation may have implications for previous experimental studies of uranium-water interactions, where the presence of differing levels of undetected carbide may partly account for the discrepancies observed between datasets. Upon completion of the U + H20 reaction within a sealed volume, the metal is left in a H2 environment, where following reaction may then occur: 2 U + 3 H2 ---+ 2 UH3 This reaction can convert the total bulk metal into a pyrophoric powder, providing the supply of H2 is sufficient [ " ]. The inhibition of this reaction is therefore highly desirable. Surface features that could provide preferential initiation sites for the instigation of this destructive reaction were analysed both before and after limited U + H2 reaction. The samples used containing 600ppm carbon; one set prepared by mechanically polishing to a fine grade and the other received a subsequent electropolishing. 81M8 analysis showed that the additional electropolishing resulted in oxide development along the inclusion-metal interface that was not present after only mechanical polishing. Both sets of samples then underwent hydrogen exposure for a limited period under conditions expected to result in UH3 formation. For uranium prepared by only mechanical polishing, the hydride growths were observed to occur in significant numbers and almost exclusively around exposed inclusions. Conversely, preparation involving electropolishing resulted in extremely limited numbers of hydride growth sites not obviously associated with inclusions. These differences in UH3 formation behaviour are attributed to the presence of oxide formed along the inclusion-metal interface resulting from electropolishing, and highlighted how the observed hydride-forming behaviours exhibited by uranium can be significantly altered by the method of surface preparation. In summary, the current PhD has examined in detail the mechanisms and principle parameters controlling the rate of uranium corrosion in scenarios applicable to dry storage. This has been achieved using bespoke analysis systems combined with cutting edge analytical techniques allowing a variety of aspects of this complex reaction to be tested and observed simultaneously.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available