Speaker
Description
Abstract:
A crucial step within radioactive waste (RW) management is the conditioning of RWs inside a stable and durable waste form, capable of guaranteeing the long-term confinement of radionuclides. Geopolymers are promising alternatives to traditional cementitious materials for the conditioning of low-to-intermediate level waste, especially for challenging RWs, e.g. those containing high amounts of borates, sulphates, or organic compounds, which exhibit limited compatibility with cement. Alkali-activated geopolymers exhibit excellent cation retention capabilities, particularly for highly mobile fission products such as 137Cs+ which act as charge-balancing cations within the negatively charged aluminosilicate network. Conversely, phosphate-based geopolymers (PGPs), typically manufactured using phosphoric acid activator, possess a positively charged surface, hence they prove effective for the immobilisation of anionic species, e.g. 36Cl-, 129I-, or 99TcO4-. On the other hand, the confinement performance of PGPs for cations is significantly worse than their alkali counterparts, especially on the long-term.
The objective of this research carried out within the EURAD-2 partnership is to develop a PGP capable of confining both anionic and cationic species of interest for the nuclear industry, namely Cs and Co as cations representative of fission and activation products, and the pertechnetate anion. To this aim, the integration of a chabazite-rich zeolitite in metakaolin-based PGPs was investigated. Chabazite is a crystalline aluminosilicate with high cation-exchange capacity, exhibiting high selectivity especially for Cs+. Two PGP formulations were investigated, i.e. a metakaolin-based reference and a variant with partial substitution of metakaolin with the chabazite-rich zeolitite. Both formulations were spiked with 137Cs, 60Co, and 99Tc tracers and underwent a leaching test according to the ANSI/ANS-16.1-2019 protocol, as required by the Italian regulator. Similar tests were replicated using stable Cs and Co to evaluate whether substantial concentration variations affect the immobilisation of the contaminants. Notably, the leaching behaviour of Tc, introduced within the PGP matrix as 99TcO4-, was investigated for the first time. The leachates were measured by gamma spectrometry (137Cs and 60Co) and mass spectrometry (stable Cs, Co, and 99Tc) and the leaching rates were calculated. The studied matrices showed promising retention capacity with respect to the literature. In particular, the samples containing the zeolitite exhibited better Cs confinement. To better explain the outcomes of the leaching tests and discern possible degradation of chabazite due to the interaction with phosphoric acid activator, powder X-ray diffraction (XRD) analysis was employed to evaluate the main crystalline species in the PGP samples.
The findings from this work could be of interest in the development of PGP matrices capable of ensuring the long-term retention of radionuclides regardless of the ionic form in which they occur, paving the way for more effective and safer RW management strategies.
Acknowledgement: EURAD-2 is co-funded by the European Union under Grant Agreement No. 101166718