Balance of plant design and optimization for Tokamak fusion power plants: applications to ARC-like and EU-DEMO reactors

This doctoral research is part of a collaboration between the Department of Astronautical, Electrical, and Energy Engineering (DIAEE) – Nuclear Engineering Research Group (NERG) – at Sapienza University of Rome and two research institutions: the MAgnetic Fusion Energy (MAFE) group at ENI S.p.A. and the Experimental Engineering Division – Nuclear Department – at ENEA. These partnerships are dedicated to the study and design of two advanced demonstration fusion reactors for electricity generation: ARC and DEMO. The ARC (Affordable, Robust, Compact) project, led by the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center and developed by Commonwealth Fusion Systems (CFS) in collaboration with ENI, proposes an innovative approach to fusion reactors design. It differs from traditional facilities by proposing a compact, modular design made possible by the use of high-temperature superconducting materials, which enable for the generation of stronger magnetic fields, leading to a more efficient plasma confinement. ARC also introduces advanced technologies, including a liquid Breeding Blanket (BB) system in which the entire Vacuum Vessel (VV) is submerged. This design offers several advantages, including an enhancement in thermal recovery and a simplification in fuel management. The project is based on insights that will be gained through the realisation of the SPARC experimental reactor, and aims to demonstrate the feasibility of generating fusion energy through scalable devices that can be easily integrated on an industrial scale. The EU-DEMO (EUropean DEMOnstration power plant) project, promoted by the EUROfusion consortium, represents the next step toward the realisation of a commercial fusion reactor. One of its key research areas is the study and development of the BB, particularly in the Water-Cooled Lead-Lithium (WCLL) configuration. The BB plays a fundamental role in a fusion reactor, being responsible of absorbing energy generated by fusion reaction, producing tritium needed to sustain the reactions, and providing neutron shielding, protecting reactor components from radiation damage. The WCLL configuration employs pressurized water as coolant and a eutectic Lead-Lithium (PbLi) mixture for tritium breeding and neutron shielding. Designing components for tokamak fusion reactors presents significant engineering challenges, particularly due to their pulsed operation, which alternates between active (pulse) and pause (dwell) phases. These cycles induce thermal and mechanical stresses on reactor materials and structures, requiring a careful design to ensure resistance against pressure and temperature fluctuations. For the ARC facility, the research investigates different BoP configurations, focusing on power cycle optimization and heat exchanger design. Three thermodynamic cycles for energy conversion — supercritical Rankine, supercritical CO₂ Brayton, and supercritical He Brayton — are analysed and compared, adapting them to ARC operational conditions starting from pre-existing cycles. The GateCycle software is employed for the optimization process, with the objective of maximizing power generation efficiency. The analysis identifies the Rankine cycle as the most promising solution for an ARC-like facility BoP, given to both its high efficiency and technological maturity compared to the other two candidates. Additionally, a preliminary design of Double-Wall Heat eXchangers (DWHX) is developed, and its performance is assessed under both steady-state and transient conditions (e.g. pulse-dwell transient). Simulations conducted with the RELAP5-3D system code offers an evaluation of thermal and flow variations affecting component integrity. A transient power analysis is also performed, in order to identify strategies for mitigating temperature fluctuations in the secondary fluid sent to the turbine. A reduction in these fluctuations is fundamental to minimize stresses on which the components are subjected. Results indicate that a secondary fluid mass flow trend following dwell-phase power fluctuations reduces secondary temperature oscillations but increases fluctuations in the power delivered to the turbine. Conversely, a constant secondary mass flow results in higher temperature fluctuations but lower power reduction during the dwell phase. For the DEMO reactor, the research focuses on two main areas, the intermediate circuit and the BB. Considering an indirect BoP configuration, a Helical Coil Steam Generator (HCSG) is designed to couple the intermediate and secondary circuits. This heat exchanger is designed to be integrated into a more complete intermediate circuit model, for which the design scheme and transient pulse-dwell behaviour are defined. A control strategy is also developed to regulate temperature and power fluctuations within the system. Regarding the DEMO BB, a detailed model of one of 16 sectors is developed. Each sector is divided into five main segments (LOB, COB, ROB, LIB, RIB), each comprising approximately 100 piled cells, with separate cooling systems for the First Wall and the Breeding Zone. Starting from the equatorial cell of the central Outboard segment, the cooling system and heat transfer are modelled for the entire sector, which is divided into seven poloidal zones. The behaviour of this cooling systems is analysed under steady-state and transient conditions, including accident scenarios. In the steady-state analysis, an orificing procedure on the coolant channels in both cooling systems is conducted to achieve uniform outlet temperatures. The transient scenarios investigated involve events that cause a rapid coolant flow variation, followed by a plasma power reduction due to the reactor shutdown. The objective of these analyses is to observe the system response on these phenomena and to identify potential vulnerabilities. Results indicate that while hot-spots appear in the initial seconds of the accident scenarios, the component overall thermal-hydraulic behaviour can be considered acceptable in an initial evaluation. This research contributes to the analysis and design of BoP solutions for tokamak fusion reactors, investigating their performance under both normal and accidental conditions through modelling and simulation processes. It provides an initial characterization of key BoP components, highlighting critical aspects and establishing a base for further improvements. Future efforts should focus on improving thermodynamic cycle efficiency, exploring heat recovery systems integration, and refining control strategies for managing thermal and power fluctuations. The next phase will involve the integration of the analysed components into larger systems to evaluate overall BoP performance and identify any potential actions to optimize reliability and improve plant safety and efficiency.