Alteration mechanisms of spent nuclear fuel and characterization of potential uranium secondary phases.

Thesis submitted in 2019 and examined in Feb 2020.This research thesis progresses along two pathways, alteration of spent nuclear fuel and identification of said alteration. The relative abundance of uranium as an energy resource, coupled with the high costs of spent nuclear fuel reprocessing and the associated risks of nuclear proliferation make a strong case for direct disposal of SNF in deep underground geological disposal facilities. The escape of radionuclides from underground spent nuclear fuel disposal facilities will likely result from anoxic dissolution of spent nuclear fuel by intruding groundwater with potential high alpha radioactivity even after hundreds of years. Considering the lack of oxygen at repository depths of 500 m to few km below the Earth, anoxic dissolution experiments with uranium dioxide in various solid forms was conducted to investigate secondary phases formation, the escape of radioactivity in the form of dissolved uranium and electrochemistry evolution to understand the redox changes happening in the surface and solution of our experiments due to the interaction of water and spent nuclear fuel. The other research thrust in this thesis is the analysis of potential secondary phases via non-destructive scientific techniques such as scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), electron- backscattered diffraction (EBSD), X Ray Diffraction (XRD), Raman spectroscopy with a chapter dedicated on 17O NMR. For the latter, given the long alteration timeline for uranium dioxide, it is challenging to achieve sufficient alteration products for analysis within a short 4-year PhD program. Some known uranium compounds were synthesized and enriched with 17O, an NMR active isotope with spin 5/2 for investigation of their properties. The many advantages of NMR over conventional x- ray diffraction technique render this an important chapter, especially when alteration products may not be single- phase and crystalline.Singapore Nuclear Research and Safety Initiative Cambridge Philosophical Society Wolfson College Scholarshi

An Atomic-Scale Understanding of UO2 Surface Evolution During Anoxic Dissolution

Our present understanding of surface dissolution of nuclear fuels such as uranium dioxide (UO2) is limited by the use of non-local characterization techniques. Here we discuss the use of state-of-the-art scanning transmission electron microscopy (STEM) to reveal atomic–scale changes occurring to a UO2 thin film subjected to anoxic dissolution in deionised water. No amorphisation of the UO2 film surface during dissolution is observed, and dissolution occurs preferentially at surface reactive sites that present as surface pits which increase in size as the dissolution proceeds. Using a combination of STEM imaging modes, energy-dispersive X-ray spectroscopy (STEM-EDS), and electron energy loss spectroscopy (STEM-EELS), we investigate structural defects and oxygen passivation of the surface that originates from the filling of the octahedral interstitial site in the centre of the unit cells and its associated lattice contraction. Taken together, our results reveal complex pathways for both the dissolution and infiltration of solutions into UO2 surfaces

Surface and electrochemical controls on UO2 dissolution under anoxic conditions

The escape of radionuclides from underground spent nuclear fuel disposal facilities will likely result from anoxic dissolution of spent nuclear fuel by intruding groundwater. Anoxic dissolution of various forms of uranium dioxide (UO2), namely bulk pellet, powder and thin film, has been investigated. Long-duration static batch dissolution experiments were designed to investigate the release of uranium ions in deionized water and any surface chemistry that may occur on the UO2 surface. The dissolved uranium concentration for anoxic dissolution of nearly stoichiometric UO2 was found to be of the order of 10-9 mol/l for the three different sample types. Further, clusters (~500 nm) of homogeneous uranium-containing precipitates of ~20-100 nm grains were observed in thin film dissolution experiments. Such a low solubility of UO2 across sample types and the observation of secondary phases in deionized water suggest that anoxic UO2 dissolution does not only occur through a U(IV)(solid) to U(VI)(aqueous) process. Thus, we propose that dissolution of uranium under anoxic repository conditions may also proceed via U(IV)(solid) to U(IV)(aqueous), with subsequent U(IV)(precipitates) in a less defective form. Quantitative analysis of surface-sensitive EBSD diffractograms was conducted to elucidate lattice-mismatch induced cracks observed in UO2 thin film studies. Variable temperature anoxic dissolution was conducted, and no increased uranium concentration was observed in elevated temperatures

Surface alteration evidence for a mechanism of anoxic dissolution of UO2

A secondary phase has been observed to nucleate on the surface of UO2 in a solution with uranium concentration values of ~10-9 mol/l. The UO2 was in the form of a 100 nm single crystalline film of UO2 epitaxially deposited on the (001) surface of a single crystalline silicon substrate. An extended (140 days) dissolution experiment with UO2 in contact with a solution in deoxygenated, deionised water, under an Ar atmosphere (~0.1 O2 ppm) at ambient temperature (~25 °C) suggests that uranium dioxide should dissolve and precipitate while remaining in the U4+ oxidation state to enable nucleation of a low solubility secondary phase. A mechanism for the anoxic dissolution of UO2 in deionised water is proposed that involves U4+ dissolution at defect sites that subsequently nucleate and precipitate in a less defective form.B.T. Tan acknowledges funding from the Singapore Nuclear and Research Safety Initiative (SNRSI)

Surface alteration evidence for a mechanism of anoxic dissolution of UO2

A secondary phase has been observed to nucleate on the surface of UO2 in a solution with uranium concentration values of ∼10−9 mol/l. The UO2 was in the form of a 100 nm single crystalline film of UO2 epitaxially deposited on the (0 0 1) surface of a single crystalline silicon substrate. An extended (140 days) dissolution experiment with UO2 in contact with a solution in deoxygenated, deionised water, under an Ar atmosphere (∼0.1 O2 ppm) at ambient temperature (∼25 °C) suggests that uranium dioxide should dissolve and precipitate while remaining in the U4+ oxidation state to enable nucleation of a low solubility secondary phase. A mechanism for the anoxic dissolution of UO2 in deionised water is proposed that involves U4+ dissolution at defect sites that subsequently nucleate and precipitate in a less defective form.JRC.G.I.5-Advanced Nuclear Knowledg

Surface alteration evidence for a mechanism of anoxic dissolution of UO2

10.1016/j.apsusc.2018.09.094Applied Surface Science464376-37

Surface and electrochemical controls on UO2 dissolution under anoxic conditions

The escape of radionuclides from underground spent nuclear fuel disposal facilities will likely result from anoxic dissolution of spent nuclear fuel by intruding groundwater. Anoxic dissolution of various forms of uranium dioxide (UO2), namely bulk pellet, powder and thin film, has been investigated. Long-duration static batch dissolution experiments were designed to investigate the release of uranium ions in deionized water and any surface chemistry that may occur on the UO2 surface. The dissolved uranium concentration for anoxic dissolution of nearly stoichiometric UO2 was found to be of the order of 10−9 mol/l for the three different sample types. Further, clusters (∼500 nm) of homogenous uranium-containing precipitates of ∼20–100 nm grains were observed in thin film dissolution experiments. Such a low solubility of UO2 across sample types and the observation of secondary phases in deionized water suggest that anoxic UO2 dissolution does not only occur through a U(IV)(solid) to U(VI)(aqueous) process. Thus, we propose that dissolution of uranium under anoxic repository conditions may also proceed via U(IV)(solid) to U(IV)(aqueous), with subsequent U(IV) (precipitates) in a less defective form. Quantitative analysis of surface-sensitive EBSD diffractograms was conducted to elucidate lattice-mismatch induced cracks observed in UO2 thin film studies. Variable temperature anoxic dissolution was conducted, and no increased uranium concentration was observed in elevated temperatures

Surface alteration evidence for a mechanism of anoxic dissolution of UO2

A secondary phase has been observed to nucleate on the surface of UO2 in a solution with uranium concentration values of ∼10−9 mol/l. The UO2 was in the form of a 100 nm single crystalline film of UO2 epitaxially deposited on the (0 0 1) surface of a single crystalline silicon substrate. An extended (140 days) dissolution experiment with UO2 in contact with a solution in deoxygenated, deionised water, under an Ar atmosphere (∼0.1 O2 ppm) at ambient temperature (∼25 °C) suggests that uranium dioxide should dissolve and precipitate while remaining in the U4+ oxidation state to enable nucleation of a low solubility secondary phase. A mechanism for the anoxic dissolution of UO2 in deionised water is proposed that involves U4+ dissolution at defect sites that subsequently nucleate and precipitate in a less defective form

An Atomic-Scale Understanding of UO2 Surface Evolution During Anoxic Dissolution

Our present understanding of surface dissolution of nuclear fuels such as uranium dioxide (UO2) is limited by the use of nonlocal characterization techniques. Here we discuss the use of state-of-the-art scanning transmission electron microscopy (STEM) to reveal atomic-scale changes occurring to a UO2 thin film subjected to anoxic dissolution in deionized water. No amorphization of the UO2 film surface during dissolution is observed, and dissolution occurs preferentially at surface reactive sites that present as surface pits which increase in size as the dissolution proceeds. Using a combination of STEM imaging modes, energy-dispersive X-ray spectroscopy (STEM-EDS), and electron energy loss spectroscopy (STEM-EELS), we investigate structural defects and oxygen passivation of the surface that originates from the filling of the octahedral interstitial site in the center of the unit cells and its associated lattice contraction. Taken together, our results reveal complex pathways for both the dissolution and infiltration of solutions into UO2 surfaces.JRC.G.I.5-Advanced Nuclear Knowledg

An Atomic-Scale Understanding of UO2 Surface Evolution during Anoxic Dissolution.

Our present understanding of surface dissolution of nuclear fuels such as uranium dioxide (UO2) is limited by the use of nonlocal characterization techniques. Here we discuss the use of state-of-the-art scanning transmission electron microscopy (STEM) to reveal atomic-scale changes occurring to a UO2 thin film subjected to anoxic dissolution in deionized water. No amorphization of the UO2 film surface during dissolution is observed, and dissolution occurs preferentially at surface reactive sites that present as surface pits which increase in size as the dissolution proceeds. Using a combination of STEM imaging modes, energy-dispersive X-ray spectroscopy (STEM-EDS), and electron energy loss spectroscopy (STEM-EELS), we investigate structural defects and oxygen passivation of the surface that originates from the filling of the octahedral interstitial site in the center of the unit cells and its associated lattice contraction. Taken together, our results reveal complex pathways for both the dissolution and infiltration of solutions into UO2 surfaces.EPSRC Atlantic consortium (UKRI not explicitly acknowledged