Stability of Cs-Ionsiv in Portland cement blends for radioactive waste disposal
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Stability of Cs-Ionsiv in Portland cement blends for radioactive waste disposal Neil C Hyatt, Andreas Jenni and Martin C. Stennett Immobilisation Science Laboratory, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield, S1 3JD.UK. ABSTRACT The suitability of Portland cement blends for encapsulation of Cs-Ionsiv in a monolithic wasteform was investigated. No evidence of reaction or dissolution of the Cs-Ionsiv in the cementitious environment was found by scanning electron microscopy and X-ray diffraction. However, a small fraction (≤1.6 wt%) of the Cs inventory was released from the encapsulated Ionsiv during leaching experiments carried out on hydrated samples. Cs release was enhanced by exchange of K and Na present in the cementitious pore water. Cement systems lower in K and Na, such as slag based blends, showed lower Cs release than the fly ash based analogues. INTRODUCTION Several microporous materials are known for their capability to selectively incorporate Cs. The natural zeolite clinoptilolite is currently used for Cs removal from aqueous nuclear waste streams, such as that arising from legacy waste ponds at the Sellafield site. Alternatively, the commercial product Ionsiv (UOP LLC, Des Plaines, Illinois, USA) has been developed, and is used for Cs sequestration in UK fuel storage ponds. The principal phase in Ionsiv is a crystalline silicotitanite (CST), Na2Ti2SiO7·2H2O, which incorporates Cs more selectively than clinoptilolite [1, 2]. Na present in CST can be fully exchanged for H by ion exchange in dilute HCl, the product, H2Ti2SiO7·1.5H2O, has a higher Cs selectivity than the Na form [3]. Ion exchange occurs on three sites, one situated in the framework, the other two in channels formed in (001) direction (also containing H2O). Cs exchanges for H partially on the channel sites, but not within the framework. Due to steric constraints not all channel sites can be occupied by Cs and a maximum of 20-25 mol% H can be exchanged [3,4]. In the commercial product used in this study, Ti was substituted with 30 mol% Nb for enhanced selectivity, and amorphous Zr(OH)4 served as a binder. In the UK, Cs-exchanged Ionsiv is considered an intermediate-level waste (ILW) and requires a sentencing and disposal route. Although cementitious materials have proven to be suitable encapsulants for many ILW types, the use of cement based matrices for encapsulation of Ionsiv (and related ion exchange materials) may be anticipated to have several potential drawbacks, including: • Decomposition of CST framework in high pH cement pore water and release of Cs, as reported in case of clinoptilolite [5]. The Cs retention by ordinary Portland cement (OPC) was shown to be too low to allow for safe disposal [6]. • Ion exchange of Cs with competing ions from the cement and pore water and subsequent Cs release. • Desorption of Cs from Ionsiv due to elevated temperatures [2], which can occur during the exothermic cement hydration. The present study investigated the interactions between Cs-Io
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