The helium-cooled pebble bed (HCPB) blanket is one of the two concepts proposed as a driver blanket for the European Union Demonstration Fusion Power Reactor (EU DEMO). In contrast to past conceptual design studies, in the frame of the current Power Plant Physics and Technology of the EUROfusion Consortium, the ongoing EU DEMO preconceptual design activities have adopted a holistic and integrated (i.e., systems engineering) design approach. As a consequence of this new approach, many interfaces and requirements have been identified, some of them driving the design of the blankets. This paper shows the advancements in the HCPB breeding blanket and describes the lessons learned after implementing the new approach. This new set of requirements has led us to reconsider fundamental aspects of the HCPB blanket design, especially in the way of how the heat is extracted from the blanket. Among others, the requirement to achieve a mature balance of plant (BOP) system plays a central role as a key design driver and has forced us to reduce pressure drops in the breeding blanket. In this regard, the blanket has been redesigned, leading to an enhanced concept based on single-module segments with a hexagonal matrix of fuel-breeder pins. Both the fuel-breeder pins and the first wall (FW) are equipped with turbulence promoters (augmented wall roughness in the fuel-breeder pins and V-ribs in the FW), following a similar idea as in the past MAGNOX, Advanced Gas Reactor (AGR), and Gas Cooled Reactor (GCR) programs in fission. This has led to minimizing the pressure drops while maximizing the heat transfer. Also, the blanket outlet temperature has been extended to 520°C, following the same principle as in Generation IV’s GCRs of maximizing the temperature difference across the core to minimize the reactor mass flow rate and thus the circulating power. All these features have led to a remarkably low plant circulating power (80 to 90 MW) and the required power per helium blower (5 to 6 MW), which potentially solves the key long-standing problem of the BOP technology readiness level for an ≈2.4-GW(thermal) helium-cooled DEMO reactor.

The EU conducted the pre-conceptual design (PCD) phase of the demonstration reactor (DEMO) during 2014–2020 under the framework of the EUROfusion consortium. The current strategy of DEMO design is to bridge the breeding blanket (BB) technology gaps between ITER and a commercial fusion power plant (FPP) by playing the role of a “Component Test Facility” for the BB. Within this strategy, a so-called driver blanket, with nearly full in-vessel surface coverage, will aim at achieving high-level stakeholder requirements of tritium self-sufficiency and power extraction for net electricity production with rather conventional technology and/or operational parameters, while an advanced blanket (or several of them) will aim at demonstrating, with limited coverage, features that are deemed necessary for a commercial FPP. Currently, two driver blanket candidates are being investigated for the EU DEMO, namely the water-cooled lithium lead and the helium-cooled pebble bed breeding blanket concepts. The PCD phase has been characterized not only by the detailed design of the BB systems themselves, but also by their holistic integration in DEMO, prioritizing near-term solutions, in accordance with the idea of a driver blanket. This paper summarizes the status for both BB driver blanket candidates at the end of the PCD phase, including their corresponding tritium extraction and removal (TER) systems, underlining the main achievements and lessons learned, exposing outstanding key system design and R&D challenges and presenting identified opportunities to address those risks during the conceptual design (CD) phase that started in 2021.