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This paper describes the progress of the DEMO design and R&D activities in Europe. The focus is on a systems engineering and design integration approach, which is recognized to be essential from an early stage to identify and address the engineering and operational challenges, and the requirements for technology and physics R&D. We present some of the preliminary design choices/sensitivity studies to explore and narrow down the design space and identify/select attractive design points. We also discuss some of the initial results of work being executed in the EUROfusion Consortium by a geographically distributed project team involving many EU laboratories, universities, and industries in Europe. (C) 2016 The Authors. Published by Elsevier B.V

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Tritium breeding requirements• Tritium self-sufficiency must be guaranteed for DEMO:Net Tritium Breeding Ratio (TBR) ≥ 1.0 mandatory  Global TBR with margin in excess of unity required to account for Tritium losses and uncertainties: TBR ≥ 1.0 +  Uncertainties to be considered/to be accounted for:• Nuclear data uncertainties: 1-2 % for liquid metal blankets utilizing Pb-Li , 5 -10 % for solid breeder blankets with Beryllium • Blanket mock-up experiments: Measured Tritium production perfectly reproduced for HCLL, and underestimated by 5 to 10 % for HCPB blanket mock-up.  No need for TBR margin related to nuclear data uncertainties• Statistical uncertainties of Monte Carlo calculations: negligible • Uncertainties due to modelling assumptions: depend on expert judgement and can be reduced to insignificant level ≤ 0.5 % • Uncertainties due to specific engineering design assumptions: margin of 2 to 3% assumed; not mandatory -might be neglected if one can be sure the design is technically mature. • Effect of 6 Li burn-up on TBR: negligible for Pb-Li based blankets; 1 to 2 % TBR reduction for HCPB type blankets @ assumed DEMO blanket lifetime (2 to 5 fpy) and 6 Li enrichment (30 to 60%).• Effect of blanket ports on TBR: very important and significant, larger for Pb-Li than for HCPB type solid breeder/Be blankets.• Tritium losses in fuel cycle: dominated by Tritium decay (5%/year).TBR design target: TBR = 1.10 calculated by 3D Monte Carlo calculation not considering blanket ports and burn-up. Rationale (pragmatic approach): Assume a conservative margin of 5% for Tritium losses in fuel cycle, neglect nuclear data uncertainties and burn-up, assign 5% margin for port effect (limits the total port area to around 3 %). Context Objective• Specification of neutronic requirements for DEMO.• Assessment of TBR uncertainties, specification of design margins and definition of TBR design target (mandatory).• Radiation shielding requirements: tolerable radiation induced damage to vessel, limitation to the gas production in steel, and the radiation loads to the super-conducting magnets.• Performance of current DEMO models based on HCPB ("helium cooled pebble bed", HCLL ("helium cooled lithium lead") and WCLL ("water cooled lithium lead") blanket concepts.• European Power Plant Physics and Technology (PPPT) programme • Launched initially by EFDA , now conducted within the EUROfusion Consortium organisation. • Conceptual design of DEMO power plant within "Horizon 2020" and the embedded roadmap to fusion.• Key neutronics requirements must be fulfilled for DEMO to operate reliable and safe.• Sufficient Tritium breeding and shielding performance to assured. Void portsPlugged (steel) Port surface area 16 x 1 m x 2 m = 32 m 2 % of first wall area 2.95 TBR reduction HCPB DEMO 10% 4 % HCLL DEMO 15% 6% Shielding requirements• Sufficient protection of the super-conducting magnets from radiation penetrating the blanket/vessel/shield system  Limits for the radiation loads on the Toroidal Field Coils (TFC) to prevent degradation of the supercon...