Abstract
The nanoporous oxide 12CaO·7Al2O3 (C12A7) can capture large concentrations of extra-framework species inside its nanopores, while maintaining its thermodynamical stability. Here we use atomistic simulation to predict the efficacy of C12A7 to encapsulate volatile fission products, in its stoichiometric and much more effective electride forms. In the stoichiometric form, while Xe, Kr and Cs are not captured, Br, I and Te exhibit strong encapsulation energies while Rb is only weakly encapsulated from atoms. The high electronegativities of Br, I and Te stabilize their encapsulation as anions. The electride form of C12A7 shows a significant enhancement in the encapsulation of Br, I and Te with all three stable as anions from their atom and dimer reference states. Successive encapsulation of multiple Br, I and Te as single anions in adjacent cages is also energetically favourable. Conversely, Xe, Kr, Rb and Cs are unbound. Encapsulation of homonuclear dimers (Br2, I2 and Te2) and heteronuclear dimers (CsBr and CsI) in a single cage is also unfavourable. Thus, C12A7 offers the desirable prospect of species selectivity.
Highlights
Volatile radioactive species generated during the processing of spent nuclear fuel must be disposed of carefully to prevent contamination of the biosphere
The positive charge of the framework is compensated by negative ions occupying some of the cages; two O2− ions in stoichiometric complex inorganic oxide 12CaO·7Al2O3 (C12A7) (C12A7:O2−)[10,11], four OH− ions in fully hydrated C12A7: OH− 14,15, and four electrons in the electride form, C12A7 (C12A7:e−)[16,17]
The extra-framework electrons can be replaced with other anions such as F− 18, Cl− 18, H− 15, S2–19, O2− 20 and OH− 14,15. Transition metals such as Au21 and Ru22,23 have been incorporated in order to form catalysts
Summary
Volatile radioactive species generated during the processing of spent nuclear fuel must be disposed of carefully to prevent contamination of the biosphere. Both Xe and Kr demonstrate positive encapsulation energies with unmodified outer electronic configurations (refer to the Bader charge in Table 1) meaning that they are unstable inside the cages. The encapsulation energy calculated using the dimer as the reference indicates that even Br2 molecules favourably occupy (separate) cage sites as Br− ions (i.e. with an energy gain) despite the molecular dissociation energy penalty.
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