On the Use of RELAP5/Mod3.3 Numerical Tool to Support the Design of the WCLL BB and BoP on the Path for Exploiting Fusion Energy

EUROfusion, the European consortium for fusion research, coordinates and funds Research and Development (R&D) activities as part of a long-term road map toward achieving commercial fusion energy, primarily through projects like ITER and its successor, DEMO. A significant technological challenge in this endeavor is the development of the breeding blanket (BB), which is crucial for power generation, neutron shielding, and tritium production. Currently, two primary BB concepts are under study in Europe: the water-cooled lithium lead (WCLL) and the helium-cooled pebble bed. While ITER will not feature a fully operational BB, it will incorporate specialized test blanket modules (TBMs) designed to evaluate BB technologies under fusion-like conditions. These TBMs will simulate the functionalities of the BB to provide invaluable experimental data in view of the design and operation of EU-DEMO, which is going to be a prototype power plant equipped with a fully operational BB. In this context, ENEA and Sapienza University of Rome focus their joint R&D activities on the WCLL BB concept, conducting thermal-hydraulic (TH) analyses using the RELAP5/Mod3.3 system code. Their collaboration encompasses various aspects of BB design, component validation, and evaluation of balance of plant (BoP) systems, aiming to assess design resilience, optimize performances, and validate safety measures across steady-state and transient scenarios for both ITER and DEMO. Particular emphasis is placed on the challenges presented by the pulsed operational mode, which is unique to tokamak fusion reactors and imposes demanding dynamic conditions on components and systems due to rapid power transitions between pulse and dwell phases. To support this effort, ENEA R.C. Brasimone is designing and building the W-HYDRA experimental platform to support the validation and the design optimization of systems and components for both ITER and DEMO facilities. Its main objectives include reproducing the TH conditions of fusion reactor subsystems, facilitating component testing, and advancing safety and performance standards under realistic operational scenarios. This paper provides a comprehensive overview of the RELAP5/Mod3.3 TH analyses conducted to support the design and the operational strategies of the ITER WCLL TBM and its associated water cooling system, as well as the DEMO WCLL BB and BoP. Given the significance of experimental results in the adoption of well-proven technology for a fusion-like environment, this work also details the extensive TH analyses performed with the RELAP5/Mod3.3 to design and optimize the W-HYDRA platform.