Safety at the heart of Gauss Fusion’s HEXA™ breeding blanket design

Gauss Fusion and the Karlsruhe Institute of Technology's Institute for Neutron Physics and Reactor Technology (KIT-INR) have kicked-off an initial safety analysis of HEXA™, Gauss Fusion's Tritium Breeding Blanket concept, and its ancillary systems.

The work, which is part of the BMFTR-funded Synergy Alliance for Fuel Cycle and Tritium Technologies (SyrVBreTT) project, will examine Gauss Fusion’s HEXA™ design at an early stage, using the company’s GIGA™ power plant platform as a reference case. This approach will allow safety considerations to inform HEXA's continued development and industrialisation.

We spoke to Salvatore d'Amico, Project Lead at KIT-INR, about what the collaboration is intended to achieve.

Q. What will the initial safety-analysis work examine in practice, and what role will thermal-hydraulic modelling and dose calculations play?

A. In practice, we are looking at what would happen if a coolant boundary of the HEXA™ ancillary systems were to fail. The initial work focuses on a small number of representative loss-of-coolant accidents, selected jointly with Gauss Fusion: a leak inside the vacuum vessel, which would pressurise the vessel, and a break in the primary heat transfer system outside it, which would affect the surrounding building rooms.

Thermal-hydraulic system modelling shows how such an accident develops: how pressures, temperatures and coolant flows evolve, how much tritium and other radioactive material could be mobilised, and how close components and confinement barriers come to their limits. The dose calculations then use those results to estimate the potential radiological consequences for the public and the environment, under both typical and deliberately conservative conditions. Linking the two gives a consistent picture from the initiating event through to its possible consequences. On this basis, the activities will also identify possible mitigation actions and safety systems to reduce those consequences, supporting a safety-informed design of HEXA™ and GIGA™ from an early stage.

Q. Why is this type of modelling and analysis useful while a breeding blanket concept such as HEXA™ is still being developed?

A. Early in the development of a breeding blanket, there is still real flexibility in the design. Choices that affect accident behaviour, such as the layout of the cooling loops, pipe sizes, or the volume available to accommodate a pressure build-up, are much easier to adjust now than once the design is fixed. Sensitivity studies on parameters like these show which ones matter most for safety, so the design team can focus its efforts where they have the greatest effect.

Building on these results, the analysis will inform several aspects of the safety design at once: how each accident scenario should be managed, from detection through to a safe, stable state; which mitigation measures and safety systems need to be deployed, such as pressure suppression, leak detection and isolation; and what safety-related instrumentation must be in place, together with the setpoints that would trigger safety actions. It will also provide a preliminary assessment of the safety functions that the breeding blanket and its ancillary systems must fulfil, such as confinement of tritium and other radioactive materials. Defining these early means they can be integrated into the plant design from the outset rather than added later.

The models can be then updated as the HEXA™ design matures, and early findings will be shared as they emerge, so the results feed directly into ongoing engineering decisions. The outcome is a baseline of safety margins that can be refined over time. It is not yet a licensing-grade safety case, but it lays the foundation on which one can be built.

Q. What experience and capabilities does KIT-INR bring to this work, and what are you hoping the collaboration will contribute to HEXA's subsequent development?

A. The KIT-INR team brings decades of experience in simulating accident behaviour in nuclear systems, from design-basis transients through to beyond-design-basis and severe accident sequences. In fission, this covers both standard Generation III/III+ plants and innovative reactor concepts, including small modular reactors, using deterministic and probabilistic approaches. A distinctive part of this expertise is the development, verification and validation of system and safety analysis codes, not only their application, which gives us a detailed understanding of their capabilities and limitations.

For more than ten years, the team has applied and adapted this expertise to fusion, contributing to the safety studies of ITER, the fusion device now under construction, and of the European DEMO reactor under development. This has built specific know-how in fusion accident analysis, tritium and activated-material source terms, and confinement assessment, which can now be applied to an industrial blanket concept such as HEXA™.

Looking ahead, we hope the collaboration will give Gauss Fusion a solid, quantitative basis for the next design iterations of HEXA™: clearer safety margins, concrete design recommendations, and an early definition of the safety functions, systems and instrumentation the blanket will need. More broadly, it is a good example of how research institutes and industry can work together to bring fusion closer to commercial deployment.