Secondary phases of spent nuclear fuel: synthesis, Raman Spectra, and solubility in High-pH repository conditions

Abstract

The long-term safety of deep geological repositories for spent nuclear fuel depends in part on the chemical stability of radionuclides under alkaline conditions. In particular, the formation of secondary solid phases may either immobilize or increase the mobility of radionuclides such as uranium and key fission products. This thesis investigates the synthesis, Raman characterization, and solubility of six such phases: Na2U2O7, CaUO4, CaU2O7, CaRuO3, Cs2MoO4, and Cs2U2O7. The phases were synthesized via either high-temperature calcination or alkaline crystallization, and analyzed using X-ray diffraction (XRD) and Raman spectroscopy. While high-purity samples were successfully obtained for Na2U2O7, CaRuO3, Cs2MoO4, and Cs2U2O7 through calcination, CaUO4 was only synthesized in pure form through crystallization with Ca(OH)2. CaU2O7 could not be obtained as a single phase in any of these attempted methods.

The solubility of the synthesized materials was investigated in static dissolution experiments at pH 11–13, with or without bicarbonate or hydrogen peroxide, using both NaOH and tetramethylammonium hydroxide (TMAH) as base. Concentrations of dissolved U, Cs, Mo, and Ru were measured by inductively coupled plasma mass spectrometry (ICP-MS). Cs2MoO4 displayed the highest solubility across all conditions, while CaRuO3 remained below detection limits, indicating strong immobilization. Uranium solubility generally increased with pH, particularly in TMAH, but was strongly influenced by base identity and partially by presence of the above mentioned additives. Notably, Cs2U2O7 in NaOH exhibited reduced U release and increased Cs release, suggesting in-situ formation of Na2U2O7. Overall, the results demonstrate that pH, additive speciation, and alkali metal cations affect phase stability and solubility, with implications for radionuclide mobility in deep geological repositories.

Incoming