Cold sprayed superhydrophilic porous metallic coating for enhancing the critical heat flux of the pressurized water-cooled reactor vessel in nuclear power plants

In the nuclear power plants, external reactor vessel cooling (ERVC) by water is an effective strategy to ensure in-vessel retention (IVR) of the molten fuel in the pressurized water-cooled reactors (PWRs) to avoid the release of nuclear fission products under severe accidents. The effectiveness of IVR - ERVC, namely the heat taken away by the ERVC highly depends on the critical heat flux (CHF) of the outer wall of lower hemisphere the Reactor Vessel. Although porous coating has been shown significantly enhanced CHF experimentally and theoretically, flexible coating process is still in great demand for practical applications. In this work, cold spray (CS) is employed to deposit porous coatings by controlling the particle impact velocities of Ti-based alloy powder sprayed on a SA508 Gr3 steel substrate, which is one of the frequently used material for fabricating the PWRs. Effect of the spraying parameters on porosity, adhesion strength and wetting ability of the coating was examined. The boiling heat transfer CHF of the coating was correlated with microstructure features of the coatings. The microstructural observations show that the porosity of the coating can reach up to 30% by the controlling the impact velocity of the spraying particle. The porous microstructure makes the coating surface superhydrophilic and leads to a remarkable increment of up to 50.3% (499 kW/m²) in the CHF as compared to the bare SA508 Gr3 steel surface. Although the adhesion strength of the coating decreases with the porosity of the coating, cracking, delamination or peeling-off were not observed on the coating with highest porosity even after 4 cycles of CHF measurement and 100 cycles of water quenching thermal shock from 300 °C suggesting its high mechanical robustness.