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BARRIERS TO SUCCESSFUL NUCLEAR FUSION
hen nuclear fusion AllJ research is spoken VV of today, it is the large projects which attract attention. However, experiments such as Aadex and Wcndeistctn in the Max Planck Institute for Plasma physics in Garchtng or the European Community (EC) project JET in Culham. Great Britain are only trigonometric points in a rich research landscape. in which solutions to the most difficult problems arc being sought. In this area, the Nuclear Research Center in Karlsruhe is playing an important role, because here, about a third of the fusion technology programme of the EC is heing carried out.
Nearly all national and international fusion programmes have as their goal the construction of a so-called tokamak reactor. The main components .of a tokamak arc a tyre-like container, the torus, and a system of magnets which surround this torus. Inside the torus. an electrically charged plasma, consisting of a reactive mixture of the hydrogen isotopes deuterium and tritium, is produced: the temperature of such a plasma is around 100 million degrees Celsius. The most important function of the magnet system is to prevent plasma particles, which arc extremely destructive at these temperatures, from touching the torus wail. This is achieved by superpost-Honing two magnetic fields which are produced using different coil systems.
Recently, with JET. a plasma condition close to the desired ignition range was achieved. This result has given wings to the attempts to construct a successor reactor which is capable of ignition. In fact, two very similar reactors are being considered: NET (Next European Torus) is a project sponsored by the EC. whereas, the whole international community is involved in the development of ITER ( International Thermonuclear Experimental Reactor). Which of these reactors will be built has not been decided.
Nearly all the components must standards which go way beyond the current stale of technology. The magnitude of the problems is illustrated by the fact that while many critics of the fusion programme believe in the possible production of a plasma capable of ig-nition, they harbor serious doubts about overcoming these technical problems.
Most of the problems arc caused by the neutrons produced by the fusion process.
Each time a deuterium nucleus fuses with a tritium nucleus to form a helium nucleus, a neutron possessing the energy released tn the fusion process is produced. The neutrons, which, because they arc uncharged. cannot be affected by magnetic fields, collide with . the container walls at high velocities. Especially the first wall, which contains various components for heating and purifying the plasma, must be constructed from materials which do not become embrittled as a result of the neutron bombardment. To find materials which do not fatigue too quickly under such stress, extensive tests under simulated operating conditions arc being carried out in Karlsruhe. Here, special steels and ceramics, i.e. silicon carbides arc being used. Tile experience gained from working with current experiment reactors is of little use. since their operating conditions arc different form those of a future NET or ITER.
Once the neutrons have 1 penetrated the first wall, they must be captured by the blanket situated behind it. Here, the neutron energy is absorbed and converted lo a slate in which it can be used-- 1. c. via intermediate stages, to steam. In addition, the neutrons must breed one of the fuel compo-1 nents.
Deuterium cun be pro-i duced externally from water and fed into the torus. However, the second compo-nent- tritium, is produced by splitting the light metal lithium in the reactor blanket itself. How this is best accomplished is the object of extensive research in various European laboratories. Il is also necessary lo Integrate beryllium layers, in which a fast neutron is converted lo two slow ones, into the blanket to ensure that enough tritium' is bred.
In view of this, two concepts arc being followed at the Nuclear Research Center in Karlsruhe. The first is based on an alloy of lead and lithium, which is liquid under operating conditions, and which is pumped through a lightly meshed channel system. In the second, fine small spheres of lithium ceramic serve as fertile material. while the cixiling Js taken over by a helium circulation system. The blankets have very complex formations, and that on a large scale. NET will have an external diameter of approximately 20 meters and a height of around 20 meters. This means that its blanket inside the torus can .have a liciglil of several meters and a total length of more than 30 meters. At present, around 20 percent of all research and development work of the Center in Karlsruhe is dedicated to fusion technology. This corresponds to financing of the order of several hundred million DM per year. In the whole of Europe around a. thousand million DM is invested in fusion research each year.
— Dietrich Zi m me rma n n (O RS).