Engineering Problems of a Thermonuclear Fusion Reactor

Thermonuclear fusion is one of the very few major options for covering the future energy needs. It is a challenge to develop the necessary physics and tech­ nology so that this source of energy be­ comes available for the benefit of man­ kind. As follows from Fig. 1, thermonu­ clear fusion exploits the differences in normalized masses between light ele­ ments and releases energy by fusing light nuclei into heavier ones. Up to 1% of the mass can be released 10 times that available from fission. It would be of the highest benefit if a fusion process could be utilized which involved only stable elements and would yield the reaction power via the kinetic energy of the stable nuclei produced. Radioactivity would then be completely absent. Such reactions do exist but, un­ fortunately, the one having by far the highest reaction cross-section does not belong to this category. In view of the immense difficulties, however, which have to be mastered before the harness­ ing of fusion power becomes practica­ ble, we are obliged to utilize the largest available cross-section, at least for the first generation of fusion reactors : D + T → 4He + n + 17.6 MeV There is enough deuterium available in nature but no tritium because this iso­ tope of hydrogen decays by beta emis­ sion with a time constant of only 12.1 years. In addition, one of the two reac­ tion products is a neutron carrying 80% of the reaction energy. Essentially, these are the two points from which most of the engineering dif­ ficulties start. Tritium has to be bred from lithium using the reactions : 6Li + n → 4He + T + 4.8 MeV 7Li + n → 4He + T + n 2.5 MeV In order to utilize the fusion neutrons for this purpose the breeding has to be done in a blanket surrounding the reac­ tion chamber. At the same time, this blanket has the task of converting the kinetic energy of the fusion neutrons in­ to useful heat. The 6Li-reaction even contributes to the power output of a fu­ sion reactor, whereas the secondary neutron of the 7Li-reaction makes it possible in principle to achieve tritium breeding ratios larger than one, without excessive use of extra neutron multi­ pliers. It is worth reiterating that complica­ tions introduced by this class of property are not generic to fusion in general. They are a consequence of the particular fu­ sion reaction selected, i.e. they are the penalty one has to pay for taking advan­ tage of the relatively large cross-section of the DT reaction. One difficulty generic to fusion arises from the fact that even the large DT reac­ tion cross-section is by orders of magni­ tude smaller than the hydrogen atomic dimensions being of the order of the pro­ ton dimension. This unavoidably brings the repulsive and long-range property of the electric charge of the nuclei into play, and Fig. 2 gives an impression of how much more frequent coulomb collisions are with respect to fusion processes. From this figure, it becomes immediately apparent that fusion by colliding beams of energetic particles has no chance since the momentum of the beam par­ ticles is lost too fast for a sufficient number of fusion reactions to occur. One rather has to use a medium in which the frequent coulomb collisions, at least on the average, do not lead to a loss of energy of the energetic fusion candi­ dates. Such a medium is a plasma of ap­ propriate temperature, say 10 keV. Under typical conditions of magnetic confinement, upon which method this paper is concentrated, the integrated path length of an average particle is of the order of the diameter of the Earth before it undergoes a fusion collision. This gives an impression on how large the reflection coefficient has to be at the ends of linear devices, or how small the leakage of particles has to be out of toroidal devices if the reacting plasma is to be confined within devices of reaso­ nable dimensions. Under these condi­ tions the fusion power density is propor­ tional to the square of the density of the reaction partners (D : T =1:1) times a function of their temperature. Magnetic confinement concepts are characterized by the maximum plasma pressure they are able to confine, i.e. by the product of plasma density times plasma tempera­ ture. If this product is kept constant, the particular energy dependence of the DT reaction yields the maximum fusion power density for plasma temperatures around 13 keV. Fig. 1 — Binding energy per nucleon vs. ato­ mic mass number.