Behaviour of Mo(VI), Eu(III) and U(VI) in calcium-silicate-hydrate phases: Immobilisation and dynamic remobilisation under repository-relevant conditions

Abstract

Calcium-silicate-hydrate (C–S–H) phases play an essential role as a geotechnical barrier in high-level nuclear waste (HLW) repositories. To guarantee the long-term safety of a HLW disposal site, it is important to know how and to which degree radionuclides are retained by C–S–H phases. To this date, only little to no data on the remobilisation dynamics of the repository-relevant elements Mo(VI), Eu(III) and U(VI) under realistic conditions are available. In this study, C–S–H phases incorporating these elements were synthesised, the element immobilisation was quantified via ICP-QQQ and their structure was investigated via X-ray diffractometry. All C–S–H phases had a calcium-silicon ratio (C/S) of 1.066 ± 0.003 and no relevant structural influences of the studied elements could be observed. In all cases, quantitative immobilisation was observed for Eu(III) and U(VI), whereas Mo(VI) was retained/incorporated to only about 50 %. Afterwards, their leaching behaviour with ultrapure water was studied in batch and mini-column experiments (MCE). Mo(VI) was remobilised quickly, while Eu(III) and U(VI) were retained nearly quantitatively. Due to the more realistic and therefore more representative conditions, further experiments to investigate the effects of three repository-relevant parameters were conducted only using MCE. A higher ionic strength significantly enhanced U(VI) remobilisation, while Eu(III) and Mo(VI) remained nearly unaffected. The displacement agents Fe(III) and Tb(III) only affected Mo(VI)'s remobilisation by retarding it. The addition of a complexing agent (2-phosphonobutane-1,2,4-tricarboxylic acid; PBTC) led to a delayed remobilisation of Mo(VI), while Eu(III) and U(VI) were remobilised to some degree after sufficient PBTC eluation. Overall, this work delivers important information on key processes relevant to nuclear waste disposal.