KIT’s Fusion Programme is developing innovative breeding blanket designs and advanced materials for fusion energy, addressing the challenges of material selection and performance in extreme environments to facilitate the transition to safe and efficient fusion power.
At the Karlsruhe Institute of Technology (KIT), the Fusion Programme is navigating the complexities of extreme environments and utilising cutting-edge research to develop innovative breeding blanket designs and advanced materials essential for the successful implementation of fusion energy.
KIT’s Fusion Programme drives the technological foundations for future fusion power plants by combining research excellence with system-level integration. It unites plant engineering, fuel cycle development, advanced materials, and plasma operation technologies within one coherent strategy. By leveraging unique infrastructures and strong cross-disciplinary expertise, KIT bridges fundamental research and industrial application. The programme supports the transition from experimental devices to safe, efficient, and industrially relevant fusion energy. At the same time, it trains the next generation of experts for research, industry, and regulatory bodies.
How does KIT select materials for use in fusion? What properties are the most important, and what challenges do you face when using specific materials in the extreme environments found in fusion reactors?
At the heart of every future fusion power plant lies the challenge of developing materials capable of performing reliably in one of the most extreme environments engineered by mankind. For structural materials, reliability under relevant operating conditions is the ultimate benchmark. But what does ‘relevant’ mean in fusion? The combination of high temperatures, mechanical stresses, and 14 MeV neutrons is unique in fusion reactors. In particular, 14 MeV neutrons are currently not available at the required fluences, which makes dedicated irradiation facilities such as IFMIF-DONES indispensable. Then, material selection follows strict principles: low activation, which already narrows the options for alloying elements significantly, resistance to neutron-induced damage, and long-term reliability. When scalability, industrial manufacturability, and processability are added as further criteria, only very few candidate materials – such as the European baseline material EUROFER developed at KIT – remain. Finally, systematic irradiation testing and qualification are a central element of fusion materials development. This requires dedicated hot labs such as the fusion materials laboratory (FML) at KIT, an outstanding characterisation facility for toxic and radioactive materials. In the future, we are looking forward to also testing in reactor-relevant conditions.
Structural materials face a wide spectrum of challenges, ranging from classical irradiation damage to the formation of transmutation products such as helium; these effects are strongly temperature-dependent and directly constrain component design, as excessively high temperatures lead to unacceptable softening while comparatively low temperatures – still in the range of several hundred degrees Celsius – cause embrittlement, which, for example, prevents conventional ferritic–martensitic steels from being operated under water cooling as they otherwise shatter like glass, with implications for the overall power plant design.
Similar considerations apply to functional materials, such as ceramic breeder materials and neutron multipliers. They must withstand the same extreme fusion environment, including high temperatures and intense neutron irradiation, while maintaining long-term stability and compatibility within the blanket system. However, functional materials face an additional key requirement: performance. In particular, they must ensure a sufficiently high tritium breeding ratio of the breeding blanket.
What are breeding blankets, and why is it an important technology for fusion? How does the KIT Fusion Programme approach the design and optimisation of breeding blankets, and what challenges do you encounter in this area?
Breeding blankets are one of the most critical systems of a fusion power plant. Their primary function is to generate tritium from lithium under neutron irradiation, enabling a self-sufficient breeding reaction. In that sense, the breeding blanket produces the fuel of a fusion power plant. At the same time, breeding blankets must efficiently extract heat and operate reliably under extreme neutron and thermal loads.
KIT’s approach is holistic. In addition to engineering design, materials development and testing, the proper manufacturing technologies and finally remote maintenance options are integral parts of the fusion programme. All relevant disciplines – from multi-physics system analysis to functional and structural materials research – are combined at one location, allowing interdisciplinary perspectives on breeding blanket optimisation and validation.
But how do we ensure that a breeding blanket actually works? Fortunately, qualifications can be divided into two parts. Non-nuclear qualification – such as demonstrating efficient heat removal and the interaction of material systems at high temperatures – can be carried out in dedicated infrastructures like KIT’s helium loop facility HELOKA. Nuclear qualification, however, which ultimately must guarantee a sufficient tritium breeding ratio, requires entirely different infrastructures. Worldwide, and also at KIT, strategies are being developed to achieve efficient and reliable nuclear qualification under reactor-relevant conditions.
How does the KIT Fusion Programme address the challenges related to fuel cycle technologies, and what innovations are being implemented in this field?
The KIT Fusion Programme addresses fuel cycle challenges through an integrated approach that combines technology development, system modelling, and dedicated large-scale infrastructures. The goal is to enable safe, reliable, and economically viable tritium operation, including shutdown and decommissioning, with minimised tritium inventories.
A central asset is the Tritium Laboratory Karlsruhe (TLK), which for many years has processed significant quantities of tritium in fuel-cycle-like processes and provides unique expertise in tritium handling, analytics, safety, and accountancy. Complementing TLK, our new Direct Internal Recycling Development Platform Karlsruhe (DIPAK) is currently under construction. It enables integrated testing of key components such as pellet injection, gas distribution, isotope separation, and vacuum pumping at the relevant scale.
Can you explain the role of gyrotrons in your research and development efforts, as well as in the overall functioning of fusion reactors?
Gyrotrons play a central role in the field of electron cyclotron resonance heating (ECRH), which is one of the key technologies for plasma heating and control in magnetic confinement fusion devices. Already in current – or currently built – experimental facilities, such as W7-X and ITER, reliable and efficient plasma heating is essential to achieve and sustain the high temperatures required. Gyrotrons generate high-power millimetre-wave radiation that is injected into the plasma to heat and control it with high precision.
Within KIT’s Fusion Programme, gyrotron development is embedded in a broader strategy to advance technologies for plasma operation. KIT is involved in the design and validation of gyrotrons for W7-X and ITER. A major asset is the Fusion Long Pulse Gyrotron Test Facility (FULGOR), which provides world-leading capabilities for testing gyrotrons under relevant long-pulse conditions.
Current development efforts focus on increasing efficiency, reliability, and manufacturability of high-power gyrotrons and – at the same time – increasing output power and tuning frequencies. This includes advancing coaxial-cavity concepts for high-field devices and establishing next-generation systems with significantly improved operability.
Gyrotrons must not be considered in isolation. They are part of a complete launcher system that includes transmission lines and, in particular, large diamond torus windows. These components are themselves highly specialised functional materials. On the one hand, they are safety-relevant elements forming part of the confinement system and must withstand mechanical stress and accident scenarios. On the other hand, they must fulfil the highest optical requirements to ensure efficient and reliable transmission of high-power millimetre-wave radiation into the plasma. Their qualification is therefore an essential element of the overall ECRH system development.
Could you discuss other components or processes that are integral to fusion? How are you working to advance these technologies at the KIT Fusion Programme?
Beyond breeding blankets, neutron-resistant materials, fuel cycle systems, and plasma heating technologies, several additional components and processes are integral to fusion. Combining all systems in a cross-cutting plant engineering approach plays a central role in ensuring safety. Neutronics, thermal-hydraulics, and remote maintenance throughout the plant life cycle are other aspects to be mentioned. At KIT, these disciplines are addressed, for instance, through validated safety analyses, radiological protection programmes, and advanced multi-physics modelling workflows. Automated neutronics tools, balance-of-plant concepts, and remote handling strategies are developed to enhance reliability and availability, and thereby increase economic competitiveness. By integrating system engineering with safety culture and licensing considerations from the outset, the KIT Fusion Programme advances fusion technologies toward safe and industrially viable power plants.
What do you consider to be the biggest priority in the development of new fusion technologies?
Before responding to my short list of the biggest priorities, it is important to recognise that the fusion ecosystem is moving at a pace never seen before. We are witnessing a transition from a predominantly government-driven research landscape to an increasingly industry-driven environment.
For decades, brilliant technological concepts were developed, yet only a limited number were pursued with the necessary technological depth, as the global fusion ecosystem was significantly constrained by available resources. Today, a wide variety of concepts are advancing simultaneously. This diversity goes far beyond the distinction between magnetic confinement and inertial fusion. It includes numerous tokamak and stellarator designs. But other technological show-stoppers receive way less attention – breeding blankets, for instance. This rapid diversification fundamentally changes the role of fundamental research. On the one hand, we as scientists must make our knowledge and infrastructures broadly accessible. On the other hand, we must clearly identify and communicate the remaining research needs – and contribute to solving them.
From a technological perspective, two issues stand out as potential showstoppers: tritium self-sufficiency and neutron-resistant materials. Reliable breeding blankets that satisfy tritium self-sufficiency are essential. This can be a technological show stopper, while component durability combined with short maintenance times will ultimately determine economic competitiveness – not meeting them would be an economic show stopper. Both areas are currently not the main focus of the emerging industry, which makes sustained public research efforts indispensable.
Equally important is the development of a qualified workforce. Fusion is inherently interdisciplinary, and the current transition from plasma-physics facilities such as JET, ITER, and W7-X toward real fusion power plants requires substantial human resources. At KIT, we are in a unique position to contribute. Our dual role – both as a national laboratory within the Helmholtz Association following a structured fusion programme and as a university in Baden-Württemberg with access to highly motivated young talent – enables us to play a decisive role in building the necessary expertise. To embrace the inherently interdisciplinary spirit of fusion, we are not creating a standalone ‘fusion degree,’ but embedding dedicated fusion specialisations within our established engineering programmes – such as Mechanical Engineering and Materials Science and Engineering – empowering students to apply cutting-edge fusion challenges directly within their core disciplines. In line with our commitment to lifelong learning, we also provide training for doctoral researchers and are preparing postgraduate programmes for professionals entering fusion from other fields. In the coming years, we also aim to strengthen vocational education with fusion-specific know-how.
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