Analysis of different options for the cooling of in-vessel components in DEMO fusion reactor

The present work investigates the time to cool down a Water-Cooled Lithium-Lead (WCLL) Breeding Blanket (BB) sector of the EU DEMO fusion reactor, starting from the after-shutdown condition of 300°C towards the prescribed temperature for Remote Maintenance (RM) operations, 150°C. First part of the analysis foresees air in natural convection as cooling medium. Weighted solid properties and decay heat load of WCLL have been evaluated and implemented in a 3D transient conjugate heat transfer model, already developed for HCPB BB concept. Comparison between the two concept is carried out. Air in forced convection is the second investigated cooling strategy. The previous 3D model has been properly modified to take into account the presence of turbulence, which significantly rises the complexity of the problem. Two simpler 2D model problems have been solved and adopted to make the right choice on turbulence model and meshing strategy for the complex 3D setup. The first 2D model studies turbulence inside a narrow rectangular duct, which has the same width of the small gaps between adjacent BB segments. Nusselt number found numerically has been compared with correlations for turbulent flow available in literature, Reynolds number ranging from Re=12000 to Re=51000. Once assessed the agreement between the two, a second model problem has been developed to study the abrupt area contraction experienced by air which enters small gaps coming from the equatorial port, the prescribed air inlet. Nusselt number has been evaluated to ensure agreement with the previous cases whereas localized contraction coefficient has been compared with literature values to assess the goodness of the model in this critical location. The selected meshing strategy and turbulence model have been adopted in the full 3D model, even if some adjustments have been necessary to reduce the cells count, which, In the worst case, goes up to ~30 millions. The results in forced convection are then compared against those obtained in natural convection.