Nuclear fusion: the big picture

As an engineer and a physicist, I have to deal most of the time with the details of fusion: how to optimize a particular component of one of the heating systems, how to understand the distribution function of fast ions during some MHD event. Busy with the intricacies of the day-to-day work, I often forget the long term purpose of nuclear fusion: to produce energy for the grid in an effective and rentable way.

A short reminder of the big picture is thus sometimes welcome. I will summarize in this post some guidelines, the reader interested in the topic should refer to the presentations and articles of David Maisonnier from the European Commission or Harmut Zohm from The Max Planck Institute for Plasma Physics who have put a lof of effort in “vulgarizing” fusion research: Power Plant Conceptual Studies in Europe, Overview of Reactor Studies, on the minimum size of DEMO.

The purpose of fusion research is to develop a Fusion Power Plant (FPP).

I will not tackle here the subject of the reason why we need fusion as a source of energy: it si a very controversial issue where emotional and political inclinations play an important role. If you are interested in the subject and want to  make your opinion, please visit the site of the International Energy Agency, where the World Energy Outlook 2008 is free for downloading. A very good basic physics approach is also given by David JC MacKay. If I have time later, I will try to enter this debate more in detail but, for the moment, I consider the three basic assumptions for nuclear fusion. You can agree with them or not, but this is the start of the studies on Fusion Power Plants:

  • Fusion is a relatively clean source of energy
  • Fusion is a safe source of energy
  • Fusion fuels are available for everybody (energy independence)

The EU Fusion Programme is reactor oriented, one of its purposes is to clarify the nature of a FPP (Fusion Power Plant), to identify the technological and conceptual gaps between our present knowledge on fusion and what is required for a FPP and to establish a plant to bridge these gaps.

The European Power Plant Conceptual Study (PPCS), finished in April 2005, identified 5 types of power plants, all based on tokamaks in steady-state mode, ranging from limited to advanced extrapolations in physics and technology.

A typical Fusion Power Plant has to meet the following main requirements:

  • concerning safety and environmental aspects, there shall be no need for emergency evacuation, no active system for safe shutdown and no structure melting down following an accident, minimum wastes to repository . This requirement is the main difference with a fission reactor: an accident of a fusion reactor does not have to impact the  population. The last specification on wastes is, I must admit, vague and it is a weakness of fusion power plants working with Tritium: materials are activated by fast neutrons. But the amount of radioactive wastes will depend on the architecture of the power plant and of the nature of materials used. The purpose is to reduce it as much as possible.
  • The operation of the plant shall make it possible to produce 1GWe as base load with an availability of 75% to 80%, and a few unplanned shutdowns per year.
  • Economically, a fusion power plant shall be more expansive than other “acceptable” energy sources.
  • Last point, the solution should be accepted by the public.

The standard solutions are based on the ITER design with some minor extrapolations IPB98y2 scaling law (this scale “predicts” the confinement of energy in function of the machine parameters).  More advanced solutions  bet on improvements in performance: better confinement, strong shaping for a better current profile control and minimization of divertor loads.

The technology is based on the same coolant for the different of the tokamaks (either water, heliium of even LiPb):  on the divertor with fluxes between 5 and 15MW/m2 (depending on the solution adopted), and on the first wall: 0.5MW/m2 on average 1MW/m2 peak.

The blanket structural material is EUROFER, a low activation ferritic-martensitic steel (550deg max. temperature), with a lifetime of 150dpa (displacement par atom). For an average nuclear load of 2MWa/m2, it corresponds to 5 full-power years.

The vacuum vessel is in stainless steel AISI 316LN, water-cooled and must be re-weldable to allow major repair operation.

The last critical technology concerns the magnets: they are assumed to be like those on ITER: low temperature superconductors Ni3Sn or NiTi cooled with liquid helium. The lifetime is set by the irradiation limit of epoxy insulation.

These are all the basic assumptions used to model Fusion Power Plants: it does not that they define the future design. These models are developed as guidelines, to get order of magnitudes of achievable efficiencies with the present knowledge. In other words, they make it possible a parametric studies of fusion power plants: playing with these models show the weight of the different parameters and help the designer choose the most sensitive.

To give an idea of the results, the most conservative design gives an efficiency of 30%, the most advanced 60%. The problem with the low efficiency of conservative design is the amount of power used for active Helium cooling and for additional heating.

I stop here for today, this should give a small overview of the first steps needed to build not an experimental reactor but a real fusion power plant and how difficulties and technology gaps are evaluated.


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