CFD investigation of Flexblue hull

Flexblue® is an underwater nuclear power station operating up to 100 m depth several km away from the shore. The immersion is beneficial for safety because it protects the reactor against severe external aggressions such as tsunamis, extreme weather conditions and malevolent actions. It also provides an infinite heat sink to cool down the reactor in any situation. In case of an accident, the safety systems take advantage of this natural source of cooling to bring and maintain the reactor in a safe state without human intervention for an indefinite period of time. The metallic containment itself is naturally cooled on its external side by the ocean. The present work investigates seawater natural convection fluid dynamics and heat transfer features, to evaluate the capabilities of the containment to reject the decay power to the exterior. A preliminary lumped parameters approach is adopted, revealing that the large diameter of the hull (14 m) is such that the ranges of validity of empirical correlations for natural convection heat transfer are always exceeded and that conditions for their correct application are not satisfied. Hence, a 2D, unsteady CFD analysis is performed to simulate the natural convection flow in the ocean, and to obtain predictions for heat flux distribution, hull superficial temperature profile and heat transfer coefficient. Both CFD sensitivity and parametric analyses are carried out, albeit limited to a 2D approach to limit the computational burden. The results show that the heat transfer process is globally satisfactory and enables all decay heat removal. A 3D approach and an experimental campaign aiming at validating the CFD results is planned for the next stage.