# Neutronic Activity for Development of the Promising Alternative Water-Cooled DEMO Concepts
###### Abstract  
An emerging breeding blanket that fulfills performance criteria, meets the safety requirements, and is reliable enough to meet the plant availability is a challenging issue that assumes complex studies involving numerous neutronic analyses based on the Monte Carlo simulations with MCNP code. Two different concepts are now candidates to be implemented as a driver blanket for DEMO fusion reactor: WCLL (Water-Cooled Lithium Lead) and HCPB (Helium-Cooled Pebble Bed). The current R&D work within the EUROfusion DEMO project is concentrated on a search for the new water-cooled blanket layouts: a deep upgrade of the WCLL blanket to ensure a sufficient tritium breeding capability and an elaboration of the hybrid concept coupling technological advantages of water coolant, lead neutron multiplier, and ceramic breeder. To this end, very detailed, fully heterogeneous MCNP geometry models were developed for the newest designs of the WCLL-db (WCLL-double bundle) and WLCB (Water-cooled liquid Lead Ceramic Breeder) DEMO blankets to verify the new engineering solutions. This makes rigorous calculations possible to find an optimal breeder blanket layout. The basic response, tritium breeding ratio (TBR), was assessed for both concepts, and it appeared to be TBR = 1.16 for the WCLL-db and TBR \(\leq\) 1.13 for the WLCB DEMOs, respectively. Several geometry layouts of the WLCB breeder blanket were investigated to reach the TBR sufficient for a sustainable tritium fuel cycle. Two promising novel solutions were suggested to enhance the tritium breeding performance of the WLCB blanket and to achieve TBR \(\geq\) 1.16: heavy water coolant and an advanced breeder ceramic. Various nuclear safety aspects of the technologies utilized in both blanket concepts are addressed in this work to facilitate engineering decisions aimed at the consolidated blanket design for the upcoming DEMO reactor. D 2023 202021 21 June 2023 21 June 2023 21 June 2023 21 June 2023 21 June 2023  
source. Regarding safety issues, the blanket design should ensure the secure integrity of its structure during routine operation and transition processes and also prevent infiltration of radioactive substances (tritium, radioactive products, etc.) in a primary cooling system. As for reliability, the blanket should be simple enough in order to be built with the minimum number of elements, including welds acting against leakage of pressurized fluids, to minimize its failure rate. As accepted in the EUROfusion DEMO project, the breeder blanket design development follows a strategy that aims at achieving the maximum TBR for a geometry layout under elaboration. The achievement of the TBR \(\geq\) TBRreq is a prerequisite to ensure the tritium sustainability of the DEMO fuel cycle that, in the end, could make the whole project more economically and publicly acceptant. This assumes intensive optimization work of the design to adjust the blanket performances, such as TBR, material temperatures, and thermal stresses in the constructions, to the project limitations and requirements. Within this frame, the modifications that can be applied to the geometry layout become the most important to find an optimal blanket configuration. The optimization procedure may also include a variation of the materials used for the design. These two options are the main tools to achieve the robust blanket geometry layout that satisfies the system requirements. Two different concepts are now in progress and selected to be a driver blanket for the DEMO fusion reactor: WCLL (Water-Cooled Lithium Lead) [3] and HCPB (Helium-Cooled Pebble Bed) [4]. Because of the different reactor technologies and materials utilized, these two concepts have their own cons and pros affecting EUROfusion R&D activity, respectively. Pressurized water (155 bar) assumed in the cooling system of the WCLL DEMO, even in spite of complex chemistry, seems to have some advantages compared to helium gas coolant: the technology is cheap, well-established, and robust.