On the evaluation of the DEMO WCLL Breeding Blanket sector thermal-hydraulic performances at a system level

In the last years, an intense research activity has been carried out within the framework of the EUROfusion Consortium to optimize the layout of the EU-DEMO Breeding Blanket (BB). Two main concepts are currently under investigation: the Helium-Cooled Pebble Bed and the Water-Cooled Lead-Lithium, that is the option object of this work. This layout relies on pressurized water as coolant, liquid metal (i.e., lead-lithium) as breeder, neutron multiplier and tritium carrier, and reduced-activation ferritic/martensitic steel as structural material. In addition, a thin tungsten armor is used to protect the First Wall component. The overall BB is constituted by sixteen toroidal sectors. Each sector is composed by inboard (two) and outboard (three) segments, considering their radial position with respect to the plasma chamber. At its time, the segments consist in a stack of vertically-piled breeding cells, that are the system elementary units. Their layout differs according to the poloidal position and the segment they belong.

To obtain an affordable BB design, one of the key issues is assessing its thermal-hydraulic performances during operational and accidental conditions. For this, an analysis of the component transient behavior is needed to improve and refine the project. To achieve this goal, during the last year, the Nuclear Engineering Research Group of Sapienza University of Rome, in collaboration with ENEA, has developed a detailed model of the DEMO WCLL BB sector, including both the inboard and outboard segments. The rationale has been dividing the overall stack of cells belonging to each segment in poloidal sections. The number of these sections has been the object of a sensitivity study. The preliminary objective of this work is to have a dedicated BB sector model to evaluate its thermal-hydraulic performances at a system level during the DEMO normal operations, considering the pulsed plasma regime and the spatial distributions of the various loads insisting on this component. In addition, the developed input deck allowed to have insights into the BB behavior during accidental conditions, demonstrating the capability of the current design to effectively withstand such scenarios without exceeding safety margins. The level of detail adopted to prepare the BB model enabled to preliminary exclude the occurrence of local damaging phenomena, such as thermal crisis.