title: Uptake of Np(IV) by C-S-H phases and cement paste: an EXAFS study.
abstract: Nuclear waste disposal concepts developed worldwide foresee the use of cementitious materials for the immobilization of long-lived intermediate level waste (ILW). This waste form may contain significant amounts of neptunium-237, which is expected to be present as Np(IV) under the reducing conditions encountered after the closure of the repository. Predicting the release of Np(IV) from the cementitious near field of an ILW repository requires a sufficiently detailed understanding of its interaction with the main sorbing components of hardened cement paste (HCP). In this study, the uptake of Np(IV) by calcium silicate hydrates (C-S-H) and HCP has been investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS studies on Np(IV)-doped C-S-H and HCP samples reveal that Np(IV) is predominantly incorporated in the structure of C-S-H phases having different Ca:Si ratios. The two main species identified correspond to Np(IV) in C-S-H with a Ca:Si mol ratio of 1.65 as in fresh cement and with a Ca:Si mol ratio of 0.75 as in highly degraded cement. The local structure of Np(IV) changes with the Ca:Si mol ratio and does not depend on pH. Furthermore, Np(IV) shows the same coordination environment in C-S-H and HCP samples. This study shows that C-S-H phases are responsible for the Np(IV) uptake by cementitious materials and further that incorporation in the interlayer of the C-S-H structure is the dominant uptake mechanism.
authors: Laboratory for Waste Management, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland. xavier.gaona@kit.edu
authors: Xavier
authors: X
authors: Gaona
authors: Rainer
authors: R
authors: Dähn
authors: Jan
authors: J
authors: Tits
authors: Andreas C
authors: AC
authors: Scheinost
authors: Erich
authors: E
authors: Wieland
mesh_terms: D003252:Construction Materials / Q000032:analysis*; D004784:Environmental Monitoring; D009405:Neptunium / Q000032:analysis*; D011850:Radioactive Waste / Q000032:analysis*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D011850:Radioactive Waste; D009405:Neptunium