The steam generator in a sodium-cooled fast reactor is an important safety barrier between sodium and water. If a heat-transfer tube ruptures, high-pressure water enters liquid sodium and triggers a violent sodium–water reaction. This reaction causes a localized temperature rise and can induce adjacent-tube failure. Reducing sodium–water reactivity on the sodium coolant side can therefore improve the accident-mitigation capability of the steam generator. Experiments have shown that titanium nanoparticles reduce the sodium–water reaction rate. However, the microscopic mechanism by which titanium nanoparticles modulate this reaction remains unclear, and particle-size selection lacks a theoretical basis for engineering applications.
To support the design of sodium-based nanofluid accident-mitigation materials, this study constructed Na–Ti_n–H₂O model systems (n = 1-13) and combined density functional theory(DFT) with transition-state theory(TST) to examine how Ti_n cluster affects sodium–water reactivity and the electronic-structure modulation mechanism governing the reaction. Binding energies calculated at the CCSD(T)/cc-pVQZ level of theory served as reference values for benchmark calculations. These calculations identified revPBE-D3BJ/def2-TZVPP as a computational scheme that balances accuracy and computational efficiency. Using the selected scheme, stable-structure searches for Ti_n and Na–Ti_n, transition-state verification, intrinsic reaction coordinate analysis, and reaction Gibbs free-energy calculations were performed.
Na preferentially adsorbed at high-coordination adsorption sites on Ti_n cluster surfaces without disrupting the overall cluster structure, indicating that Ti_n clusters retain good structural stability under reaction conditions. Ti_n clusters improve the Gibbs free-energy barrier of the sodium–water reaction and exhibit a pronounced, nonmonotonic size dependence. Atomic Dipole Moment Corrected Hirshfeld(ADCH) charge analysis, electron-density-difference(EDD) analysis, and quantum theory of atoms in molecules(QTAIM) analysis show that multicenter electron-delocalization interactions occur between Ti clusters and Na, as well as the O and H atoms in H₂O. These multicenter electron-delocalization interactions weaken electron transfer from Na to the water-activation site. Consequently, the Na-O interaction and O-H bond polarization are weakened, water activation is suppressed, and the sodium–water reaction rate decreases.
 

Sodium-cooled fast reactor(SFR),Sodium-water reaction,Density functional theory(DFT),Gibbs free-energy,Electronic structure analysis