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Inexhaustible, safe and respective for the environment

Fusion has the potential to release humanity from the burden of greenhouse gases and acid rain, and it does not contribute to the deterioration of the protective ozone layer around the earth. The fuels for the first generation of fusion reactors will be deuterium and tritium. Deuterium can be found in water in large quantities: each ton of seawater contains 33grams of it. Tritium, on the other hand, can be bred inside the tokamak relying on the energetic fusion-produced neutrons bombarding a blanket surrounding the machine's inner vessel that will be composed out of the light metal lithium. Lithium can be extracted from mines but is also found in water. The vastness of the oceans guarantees an almost limitless availability of the two primary fuels for the fusion process. Much less than is the case both for fossil and nuclear fission fuels, the supply of fuel will thus be an issue in a future society relying on fusion for producing its energy. Due to the fact that reconfiguring an atomic nucleus releases much larger energies than those associated with the reshuffling of electrons orbiting around nuclei (roughly speaking the energy levels associated with nuclear and chemical reactions are a factor of a million apart), the quantities of required fuels are much more modest for fusion power stations than for power stations burning coal or gas: a typical inhabitant of Western Europe needs a mere 10 grams of deuterium and 15 grams of tritium to guarantee a lifelong electricity supply while tens of tons of coal would be needed. Fusion therefore promises mankind to resolve the energy problem for hundreds of millions of years. The end product of the reaction, helium, is chemically inert ("noble") and non-radioactive; it is a much-desired substance in industrial applications.

Comparison of the radiotoxicity of fusion, fission and coal power plants in years after switch-off of the reactor. Click on the image to enlarge.

Once in operation, fusion power plants would provide large amounts of base load electric energy, burning a deuterium-tritium fuel. As tritium would be produced from lithium within a closed cycle in the machine and on the site of the power plant, no radioactive material would have to be transported. In a fusion power plant based on magnetic confinement, only small quantities of fuel are injected in the plasma. Even in large-size tokamaks the total weight of the plasma is only a few grams at any given moment. If the fuel supply is interrupted but magnetic confinement is untouched, the fusion reactions cease to take place in less than a minute. An abrupt change in confinement can halt the fusion processes in a fraction of a second. Consequently, a fusion machine is inherently safe. Tritium is a source of limited radiological hazard that can be mitigated by careful design of the plant facilities. The most important design feature to achieve this is radioactivity confinement using multiple confinement barriers (the vacuum vessel, the cryostat and the buildings) that prevent any tritium and dust escaping to the outside world.

In contrast to fission, D-T fusion is not intrinsically poised by radioactivity as the helium nucleus nor the neutron produced in the D-T reaction are radioactive. But the activation by neutrons produced in the fusion reaction gives rise indirectly to radioactivity in the metal structures that surround the plasma. The radiological characteristics of the machine will depend on the choice of the materials adopted to build the facility. Research is being carried out to identify materials with the most advanced performances in terms of reduced activation and mechanical and physical properties. Two major candidates exist: vanadium alloys (that are mainly promoted by our USA colleagues) and doped stainless steel (the solution which Europe finds most promising). The potential for the future exploitation of these materials in commercial plants is being studied. The vast majority (80% or more) of the ITER tokamak is expected to be cleared out of regulatory control after 100 years after the decommissioning of the experimental fusion power plant, while the remaining 20% or less will require storage for longer times. In a commercial reactor, the use of low activation materials should ensure that after a period of the order of 100 years no long-term storage at all would be required. In this way - and in contrast to what is the case for waste from fission reactors - every generation would take care of its own radioactive waste once and for all. Material development is in progress to reach this goal. Reducing the amount of long-term radioactive waste from 20% to zero may seem trivial but is actually quite a challenge. A dedicated fusion materials test facility such as IFMIF (International Fusion Material Irradiation Facility) must be built to allow the acquirement of the required know-how. Aside from that, further technological progress might ultimately allow relying on burning deuterium and helium-3 (3He is a lighter isotope of the common helium, 4He) in the second generation of fusion powered electricity production stations. Burning these two fuels does not yield neutrons but protons aside from helium and thus could help getting rid of activation altogether. Sustaining the 100 million degrees needed for the D-T reaction is already at the limit of the possibilities of present-day machines. Unfortunately, the required bulk temperature for the D-3He reaction is roughly even a factor of 5 larger than that needed for the D-T reaction. Moreover the reactivity of D-3He fusion process is an order of magnitude smaller than that of burning deuterium and tritium.