Uptake of Np(IV) by C-S-H Phases and Cement Paste: An EXAFS Study

In many nuclear waste disposal concepts cementitious materials are foreseen to be used for the immobilization of long-lived intermediate level wastes (ILW). ILW may contain significant amounts of neptunium-237, which is expected as Np(IV) under the reducing conditions developing after the closure of the repository. Predicting the release of Np(IV) from this environment requires a sufficiently detailed understanding of its interaction with the main sorbing components of cement. In this study, the uptake of Np(IV) by calcium silicate hydrates (C-S-H) and hardened cement paste (HCP) has been investigated using extended X-ray absorption spectroscopy (EXAFS).

Evaluation of EXAFS data from Np(IV)-doped C-S-H and HCP indicates the predominant incorporation of Np in the C-S-H structure throughout the complete sequence of cement degradation. Two species were identified, corresponding to Np(IV) in C-S-H with Ca:Si 1.65 (fresh cement) and 0.75 (highly degraded cement). The coordination environment of Np(IV) was found to depend on the Ca:Si ratio but not on pH, consistently with the aqueous speciation of Np(IV) in the pH range 10 – 13.3 (with predominance of Np(OH)4(aq)). These results unequivocally show that C-S-H are responsible for Np(IV) immobilization in cementitious materials, whilst incorporation in the interlayer of the C-S-H structure is regarded as the predominant uptake mechanism.