The first steps towards a conceptual design for a demonstration fusion power plant

How do you go about designing a power plant that uses a completely new method of generating energy, based on technologies for which so far only scientific experimental devices exist? This is basically the point at which EUROfusion’s Power Plant Physics & Technology (PPPT) Department begins the DEMO conceptual design.

Current fusion experiments were primarily designed to investigate plasma physics. However, DEMO must demonstrate the necessary technologies not only for controlling a more powerful plasma than has previously existed, but for safely generating electricity consistently, and for regular, rapid, and reliable maintenance of the plant. The design of such a plant must take account, not just of physics requirements, but also of engineering and technological limitations. Otherwise it may be, for instance, that the power exhausted from the plasma is so high that it is not possible to find a material with sufficient heat-resistance for the inner reactor wall. All relevant stakeholders like industry, safety regulators, utility, experts for public acceptance and others will be involved in setting what are called the DEMO high-level requirements: what the plant must demonstrate to the outside world to prove that fusion is a viable power source. Prior to such discussions, however, one must know the design space within which parameters can be changed in order to fulfil these demands.

Simulating a fusion power plant

An artist’s impression of a fusion power plant based on the European Power Plant Conceptual Study

An artist’s impression of a fusion power plant based on the European Power Plant Conceptual Study

Determining this design space is currently carried out with the help of a systems code that simulates the entire power plant. This code comprises sub-models for all plant components – the fusion plasma, systems for heating and current drive, the balance of plant (e.g. the electricity generation systems) and remote handling, the reactor wall blanket including its functions for energy harvesting and, vitally, tritium breeding, divertor or exhaust systems, magnets and others. The systems code is self-consistent, meaning that no sub-system is able to place demands that the other sub-systems cannot fulfil. If one sets, for instance, a certain plasma power, the system will issue an alert if this requirement violates the material heat stress limit set for certain areas of the reactor wall and attempt to find a new solution – a different size machine, for example – which can accommodate the thermal load.
The systems code is designed to execute very rapid calculations and therefore uses quite simple models for the sub-systems. The key aspect is to simulate their interaction. “With our calculations, we provide a starting point for others that model the isolated aspects and sub-systems in more detail”, says Richard Kemp from CCFE. He works with the PROCESS systems code, which has been used for the European Power Plant Conceptual Study and which is also used for the DEMO conceptual design.

Design considerations

Ultimately, DEMO design considerations begin with the goal set in the Fusion Roadmap: DEMO shall demonstrate production of electricity with a closed fuel cycle by 2050 – assuming that the ITER end of construction is not significantly delayed. “Establishing realistic plant performance requirements and project development schedules is expected to be a strong driver in the selection of the technical features of the device; favouring more conservative technology choices for near-term solutions” “, explains Gianfranco Federici, Head of the PPPT Department. ITER is the key facility in this strategy and DEMO design and R&D are expected to benefit greatly from the experience gained with the ITER design process in the past and with its construction and operation. The main performance requirements, i.e. pulse length and power generation, are combined with technology and physics boundary conditions, for instance for plasma power, divertor or magnets. On the basis of these conditions, the systems code calculates a set of parameters, i.e. for plasma volume temperature and density, magnetic fields and for the heating and current drive systems, and provide the input for more detailed models. Any input parameters and modules of the systems code which result in too much discrepancy in these more detailed calculations, are subsequently refined in an iterative process.

The design concept, which is currently under development, foresees a plant which provides 500 megawatts of electrical power to the grid and runs with a pulse length of 2 hours. The selection of these values is supported by systems code calculations that were carried out to explore the consequences of various combinations of electricity output and pulse lengths on electricity costs, capital investment and the inevitable uncertainty of machine performance.
Firstly, sensitivity studies are being carried out to determine which parameters’ changes have the strongest influence on the performance of DEMO. These will test the robustness of the operating point. Secondly, lower scale assessments – e.g. of more parameters at a time, or a more detailed examination of still uncertain physics like the plasma transport – are to be carried out in order to develop a better understanding of the different operating points considered and where technological or physics improvements would have the greatest benefits. Finally, the systems code is used to investigate a more daring DEMO concept, DEMO2. This study aims to show what would be possible if DEMO was given extended development time allowing the use of less mature technologies which would add some more unknown factors to the design.