Microhydration of caesium metaborate: structural and thermochemical properties of CsBO₂ + n H₂O (n = 1-4) aggregates

The structures and thermodynamic properties of microhydrates of caesium metaborate (CsBO₂) of nuclear safety interest are reported in this work. CsBO₂ + n H₂O (n = 1-4) molecular complexes were identified on the potential energy surface. The structures were optimized using the ωB97XD DFT method and the aug-cc-pVTZ basis set. Single-point energies were calculated at the CCSD(T)-F12a/awCVTZ and the ωB97XD/aug-cc-pVQZ levels of theory. The standard reaction enthalpies and the standard Gibbs free reaction energies were reported for all molecular complexes. The temperature dependence of Δ_rG°(T) was evaluated for all studied structures over the temperature range 300-2000 K. Total hydration reactions were investigated. The results showed that the mono-hydrated form of CsBO₂ exists only at temperatures lower than 720 K under standard conditions. The influence on the thermodynamic properties of the number of water molecules in the clusters was described, with successive dehydration from 720 to 480 K. In nuclear severe accident conditions, gaseous CsBO₂ will remain unhydrated in the reactor coolant system.