Nuclear waste viewed in a new light; a synchrotron study of uranium encapsulated in grout

Abstract

How do you characterise the contents of a sealed nuclear waste package without breaking it open? This question is important when the contained corrosion products are potentially reactive with air and radioactive. Synchrotron X-rays have been used to perform micro-scale in-situ observation and characterisation of uranium encapsulated in grout; a simulation for a typical intermediate level waste storage packet. X-ray tomography and X-ray powder diffraction generated both qualitative and quantitative data from a grout-encapsulated uranium sample before, and after, deliberately constrained H2 corrosion. Tomographic reconstructions provided a means of assessing the extent, rates and character of the corrosion reactions by comparing the relative densities between the materials and the volume of reaction products. The oxidation of uranium in grout was found to follow the anoxic U + H2O oxidation regime, and the pore network within the grout was observed to influence the growth of uranium hydride sites across the metal surface. Powder diffraction analysis identified the corrosion products as UO2 and UH3, and permitted measurement of corrosion-induced strain. Together, X-ray tomography and diffraction provide means of accurately determining the types and extent of uranium corrosion occurring, thereby offering a future tool for isolating and studying the reactions occurring in real full-scale waste package systems

Evaluating laser-driven Bremsstrahlung radiation sources for imaging and analysis of nuclear waste packages

“…As an alternative approach to more accurately identify the waste chemistry and arising hazardous corrosion products, recent investigations have explored the use of synchrotron x-rays [14] capable of analysis at micron length scales. Synchrotron x-ray powder diffraction and tomographic analysis were used to explore the rate and mechanisms of metal corrosion, successfully characterising the developing morphology of the arising corrosion products and residual metal to a 1µm per pixel resolution.…”

“…Important corrosion characteristics of uranium metal were determined using this method, for example, formation of UH 3 on uranium encapsulated in grout was observed to initiate and propagate in large, protruding, blisters instead of forming a continuous layer across the metal surface (which is the typical case with unenclosed uranium metal). Furthermore, it was shown that these blisters persisted whilst submersed in water for at least 10 months [14,15]. However, due to uranium's high density (18.95g.cm −3 ), the x-ray attenuation was too great to analyse samples thicker than 1mm, even utilising the highest beam energy achievable at the synchrotron (130keV).…”

“…Thus radiographs are expected to retain better absorption transmission contrast in these marginal areas with evidence of this observed in figure 4, which displays the entirety of the sample. Further experimentation is required to determine if additional low density corrosion products can also be identified using this method, however previous lower energy x-ray studies (115keV) have shown that this should be easily achievable [14].…”

Engineering Science at the I12 Beamline at Diamond Light Source

“…This pyrophoric compound reacts vigorously with oxygen to make UO 2 and, as such, is thought to exist only fleetingly. Stitt et al [26,27] investigated the risk posed by an accumulation of pyrophoric uranium hydride. For these experiments, UH 3 was stored underwater after being artificially formed on a uranium rod and encased in the grout used at nuclear waste facilities.…”

The effect of work-hardening and thermal annealing on the early stages of the uranium-hydrogen corrosion reaction

“…However, the nucleation number density is finite for certain conditions, since the solubility of hydrogen in uranium is solely controlled by temperature and metal microstructure [17]. Temperature may also affect the location of hydriding initiation and growth to a certain extent [22,28,29]. Working on early stage hydriding of a wellannealed depleted uranium sample at the high-temperature regime (≥240°C), Scott et al [22] found that hydrides nucleated mainly across high misorientation angle grain and twin boundaries, forming chains instead of distinct spots [22].…”