Uranium Behavior in the Process of Primary Pitchblende Ores Alteration by the Post‐ore Hydrothermal Solutions: An Application to Assessment of Uranium Migration from Underground Spent Nuclear Fuel Repositories

Abstract

It has been shown that the main uranium ore mineral, pitchblende (uranium dioxide), is a natural analog of synthetic uraninite (also uranium dioxide), which constitutes 96% of spent nuclear fuel (SNF). Geochronological studies of the U-Pb isotope systems in unaltered pitchblende from the orebodies reveal that these systems remained completely closed over the entire period (approximately 135 Ma) since the formation of the deposits. The bulk of the primary uranium ores within the Streltsovskoye ore field was influenced to various degrees by post-ore hydrothermal solutions that led to pitchblende spherulites being replaced by pseudomorphs of an amorphous phase with a U-Si composition; this phase also re-precipitated in veinlets proximal to the pitchblende pseudomorphs. A technique specially developed by the authors was used to carry out quantitative counts of the abundance of uranium minerals by calculating the uranium mass balance in one of the orebodies subjected to hydrothermal alteration. The calculations reveal minimal uranium loss from the orebody. Uranium liberated in the process of the pseudomorphic replacement of pitchblende was immediately fixed, in situ, in the newly formed coffinite-like amorphous U-Si phase as a result of the development of an efficient geochemical barrier that prevented the long-distance migration of uranium. In assessing the long-term safety of underground SNF repositories, the results of the present study give us confidence that SNF uraninite, in terms of the preservation of its integrity as a mineral phase, provides for the reliable long-term isolation of uranium, transuranium elements, and fission products that are "sealed" in the uraninite matrix. In the case of the mineral transformation of the uraninite matrix by hydrothermal solutions, the liberated uranium would be efficiently immobilized by the newly formed amorphous U-Si phase.

Elevated uranium concentrations in Lake Baikal sediments: Burial and early diagenesis

“…Khlopkov, 2008;Moore et al, 2009). However, major U mining operations are located in the eastern transbaikalian region (Ischukova, 1997;Brunello et al, 2006;Laverov et al, 2009). One of the latter is the Khiagda ore field in the Vitim uranium district about 140 km north of Chita, whereby mineralization occurs within fluvial sediments in paleovalleys of relatively narrow tributaries (e.g.…”

Trivalent Cation-Controlled Phase Space of New U(IV) Fluorides, Na3MU6F30 (M = Al3+, Ga3+, Ti3+, V3+, Cr3+, Fe3+): Mild Hydrothermal Synthesis Including an in Situ Reduction Step, Structures, Optical, and Magnetic Properties

“…The chemistry of the actinide elements, specifically that of uranium, has been of topical interest primarily due to its relevance for long-term nuclear waste storage and for the development of advanced fuel rod assemblies. − In uranium chemistry, the two most common oxidation states are the reduced +4 and the fully oxidized +6, among which the latter is the most prevalent in uranium chemistry. Uranium shows distinct chemical behavior in its two oxidation states; for example, a change in reaction conditions may immobilize the soluble U(VI) through reduction to sparingly soluble U(IV) or, reversely, mobilize U(IV) through oxidation to U(VI), which has important environmental implications since uranium is a major component of highly radioactive spent nuclear fuel. − In spite of the interesting chemistry of U(IV)-containing materials, the chemistry of U(IV) has not been as extensively investigated as that of U(VI).…”