Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

1 Publication

Uptake of niobium by cement systems relevant for nuclear waste disposal: impact of ISA and chloride

Cevirim-Papaioannou, N.; Jo, Y.; Franke, K.; Fuss, M.; de Blochouse, B.; Altmaier, M.; Gaona, X.


The isotope 94 of niobium (t1/2= 2.04·104 a) is produced during the operation of nuclear reactors due to the neutron activation of 93Nb, which is present in some structural components in nuclear reactor vessels. Related waste streams will be disposed of in repositories for low and intermediate level wastes (L/ILW), where cement is widely used for the stabilization of the waste and for construction purposes.
The retention of niobium by young cement was investigated in a series of sorption experiments using a combination of active (95Nb + 91mNb) and inactive (93Nb) niobium isotopes. Sorption experiments assessed also the impact of iso-saccharinic acid (ISA, main degradation product of cellulose) and chloride on Nb retention, both expected in specific L/ILW.
A pyrochlore-structure Ca-Nb(V) oxide was found to control the solubility of Nb(V) in cement pore water at pH= 13.5, defining [Nb(V)]aq = 2∙10-6 - 7·10–8 M. Sorption experiments revealed a strong uptake of Nb(V) by cement (5 ≤ log Rd ≤ 7, Rd in L·kg–1), in spite of the predominance of anionic hydrolysis species in the cement pore water. Sorption isotherms were found to be linear within 10–14 M ≤ [Nb(V)]aq ≤ 10–9 M. Calcium silicate hydrate (C-S-H) phases are defined as main sink of Nb(V) in cement, with Ca2+ playing a key role in bridging the negatively charged Nb(V) species to C-S-H. ISA significantly decreases the uptake of Nb(V) at [ISA]aq ≥ 10–4 M, reflecting the formation of stable (Ca-)Nb(V)-ISA complexes in the aqueous phase. Sorption is not affected by chloride within the investigated concentration range (10–5 M ≤ [Cl] ≤ 1.0 M).
This work represents the first experimental evidence on the uptake of niobium by young cement in the absence and presence of ISA and Cl, thus providing key inputs for the assessment of 94Nb retention in the context of the Safety Case for repositories for nuclear waste.

Keywords: niobium; cement; ISA; chloride; sorption

Related publications


Years: 2023 2022 2021 2020 2019 2018 2017 2016 2015