Engineering Problems of a Thermonuclear Fusion Reactor Günter Grieger, Garching (Max-Planck-Institut für Plasmaphysik)
Thermonuclear fusion is one of the It is worth reiterating that complica very few major options for covering the tions introduced by this class of property future energy needs. It is a challenge to are not generic to fusion in general. They Fig. 1 — Binding energy per nucleon vs. ato develop the necessary physics and tech are a consequence of the particular fu mic mass number. nology so that this source of energy be sion reaction selected, i.e. they are the We have now all the elements to de comes available for the benefit of man penalty one has to pay for taking advan fine a fusion power reactor based on the kind. As follows from Fig. 1, thermonu tage of the relatively large cross-section concept of toroidal magnetic confine clear fusion exploits the differences in of the DT reaction. ment. (In Europe all the preference is normalized masses between light ele One difficulty generic to fusion arises given to toroidal magnetic confinement ments and releases energy by fusing from the fact that even the large DT reac as compared to linear mirror confine light nuclei into heavier ones. Up to 1% tion cross-section is by orders of magni ment.) In a toroidal configuration, ma of the mass can be released - 10 times tude smaller than the hydrogen atomic gnetic fields primarily produced by cur that available from fission. dimensions being of the order of the pro rents in superconducting coils, confine a It would be of the highest benefit if a ton dimension. This unavoidably brings burning plasma of a temperature of fusion process could be utilized which the repulsive and long-range property of 10-15 keV and the highest reasonable involved only stable elements and would the electric charge of the nuclei into play, density (to achieve high power density). yield the reaction power via the kinetic and Fig. 2 gives an impression of how The plasma is surrounded by a blanket energy of the stable nuclei produced. much more frequent coulomb collisions for breeding the tritium fuel component Radioactivity would then be completely are with respect to fusion processes. and converting the neutron energy into absent. Such reactions do exist but, un From this figure, it becomes immediately heat. An illustration of the major engi fortunately, the one having by far the apparent that fusion by colliding beams neering difficulties connected with fu highest reaction cross-section does not of energetic particles has no chance sion reactors can best be achieved by in belong to this category. In view of the since the momentum of the beam par specting the INTOR concept 1, 2,3). immense difficulties, however, which ticles is lost too fast for a sufficient The INTOR concept is being develo have to be mastered before the harness number of fusion reactions to occur. ped by the INTOR Workshop which is an ing of fusion power becomes practica One rather has to use a medium in which international, collaborative activity com ble, we are obliged to utilize the largest the frequent coulomb collisions, at least prising Euratom, Japan, USA, and the available cross-section, at least for the on the average, do not lead to a loss of USSR, running under the auspices of the first generation of fusion reactors : energy of the energetic fusion candi IAEA, and concentrated on defining and D + T → 4He + n + 17.6 MeV dates. Such a medium is a plasma of ap designing the next step device in fusion There is enough deuterium available in propriate temperature, say 10 keV. programmes. INTOR is conceived to nature but no tritium because this iso Under typical conditions of magnetic demonstrate the availability of reactor tope of hydrogen decays by beta emis confinement, upon which method this relevant burning-plasma physics and to sion with a time constant of only 12.1 paper is concentrated, the integrated serve as a test bed for developing and years. In addition, one of the two reac path length of an average particle is of testing reactor technology. With these tion products is a neutron carrying 80% the order of the diameter of the Earth of the reaction energy. before it undergoes a fusion collision. Fig. 2 — <σ,v> vs. plasma temperature for Essentially, these are the two points This gives an impression on how large elastic Coulomb collisions la) and for DT fu from which most of the engineering dif the reflection coefficient has to be at the sion processes lb). ficulties start. Tritium has to be bred ends of linear devices, or how small the from lithium using the reactions : leakage of particles has to be out of 6Li + n → 4He + T + 4.8 MeV toroidal devices if the reacting plasma is 7Li + n → 4He + T + n - 2.5 MeV to be confined within devices of reaso In order to utilize the fusion neutrons nable dimensions. Under these condi for this purpose the breeding has to be tions the fusion power density is propor done in a blanket surrounding the reac tional to the square of the density of the tion chamber. At the same time, this reaction partners (D : T =1:1) times a blanket has the task of converting the function of their temperature. Magnetic kinetic energy of the fusion neutrons in confinement concepts are characterized to useful heat. The 6Li-reaction even by the maximum plasma pressure they contributes to the power output of a fu are able to confine, i.e. by the product of sion reactor, whereas the secondary plasma density times plasma tempera neutron of the 7Li-reaction makes it ture. If this product is kept constant, the possible in principle to achieve tritium particular energy dependence of the DT breeding ratios larger than one, without reaction yields the maximum fusion excessive use of extra neutron multi power density for plasma temperatures pliers. around 13 keV. 11 Thus the main damage of the first wall is arising from the high energy (14 MeV), high neutron flux which will be around 2-5 MW/m2 depending on concept. Structural parts which are exposed to this high irradiation should withstand a neutron fluence of at least 10-20 MWa/m2 before replacement becomes necessary. The materials available, stainless and ferritic steels, are develo ped for fission purposes and thus for other neutron spectra. The high energy neutrons present in fusion reactors cause much higher helium production rates than are known from typical fission applications so that the properties of these materials under fusion conditions have to be evaluated. Such programmes have been started but they lack a power ful 14 MeV neutron source. Due to the long lead-time especially tailored mate Fig. 3 — INTOR concept as of Phase II-A of the INTOR Workshop (artist's impression). The rials can only be developed in the long toroidal plasma is surrounded by a blanket and shield and these in turn by coils generating the run, whereas for the next step one has to toroidal magnetic field. Outside these coils indicate further sets of coils exciting the select from amongst those available. necessary poloidal magnetic field. At the right-hand side an RF antenna system is indicated As is also apparent from Fig. 3, three which has the task of heating the plasma to ignition temperatures. independent systems of coils are neces sary for generating the tokamak confin aims INTOR has to involve all essential walls by energetic particles and radia ing magnetic field. For reasons of low reactor components and is thus par tion leading to surface heating and sput power consumption all the coils are con ticularly suited to identify all of the tering. Under standard conditions the ceived as being superconducting. A set engineering problems. sputtering was seen to be so large that a of large planar coils (TF-coils) generate Fig. 3 is a display of the INTOR design sacrificial extra thickness of the wall of the toroidal magnetic field which provi concept at the end of Phase IIA, Part I of 1-2 cm had to be provided to achieve a des the largest fraction of the magnetic the INTOR Workshop. The innermost sufficient lifetime of the first wall — at field energy. The ohmic heating (OH) part of the device is the plasma dough least from the sputtering point of view. transformer coils excite a loop voltage nut which is to produce the fusion Such a thick wall, however, experienced and thus a plasma current whose ma power. The plasma is confined by a unacceptable thermal stresses arising gnetic field generates the necessary strong toroidally oriented magnetic field from the pulsed operation connected twist about the magnetic axis of the which is twisted around a "magnetic with tokamak devices. Moreover, the toroidal magnetic field. A third coil cir axis", the centre line of the plasma large amount of sputtered material had a cuit (PF-coils) provides a poloidal ma cross-section. This way nested toroidal significant probability of diffusing into gnetic field necessary to balance the magnetic surfaces are formed which the plasma core and quenching the burn hoop force of the plasma current and make particle and energy transport from by excessive heat losses via impurity generate the shape of the magnetic field surface to surface (i.e. from the inner radiation. This problem was recognized required for plasma equilibrium and sta part of the plasma towards its boundary) already very early and led to a screening bility. Thus the plasma current is an es difficult, such that a rather efficient con of all available materials for high sputter sential ingredient of the tokamak con finement of plasma density and energy ing resistance. But the potential of all cept. During the pulse it has to be raised is achieved. In order to maintain the bur these measures is only a gradual reduc from zero to the full burn value and then ning state of the plasma in steady state, tion of sputtering at best. the remaining and unavoidable power A much more efficient method would Table 1 : losses resulting from particle collisions, be to establish high density, low tempe INTOR major parameters, Phase IIA, Part I rature plasma edge conditions such that radiation, and convection have to be Major radius, R(m) 5.2 balanced by the 20% fraction of fusion the energy of the impinging particles Plasma radius, a(m) 1.2 power carried by 3.5 MeV alpha parti was below the sputtering threshold. Di Elongation, k 1.6 cles. The quantitative achievement of vertor discharges, which aim at connec Burn time, (s) 100/200* this balance determines the plasma di ting the outermost magnetic field lines Duty cycle, (%) 70/80* mensions, parameters, and the strength with remote divertor chambers, offer the Average beta, <β> (%) 5.6 of the magnetic field. Some key parame highest chance for establishing such Plasma current, I(MA) 6.4 ters of INTOR are listed in Table 1. conditions. As follows from divertor ex D, T-density, < ni> (m-3) 1.4 x 1020 One of the major engineering pro periments like ASDEX 4) and from the Ion temp.,
Comments on the Review Article of D.N. Stacey The uninformed reader may get the impression, from the review article by D.N. Stacey published in Europhysics News 16 (1985) 2 (February), that in the early work of C.C. and M.A. Bouchiat 1) the parity violation effects in heavy atoms were grossly overestimated : "The effects are much smaller than were at first predicted to be". In refe rence 1, a detailed evaluation was given for the case of atomic caesium only. The results of the Paris experiments can be expressed in terms of the ratio of parity Fig. 4 — Comparison between the Divertor Experiments ASDEX and ASDEX-Upgrade and violating 6S-7S electric dipole ampli INTOR which is relying on a functioning divertor. Note the big difference in scale. tude E1p.v. to the spin-independent tran sition dipole amplitude αEO induced by an electric field EO. The predicted ratio concerned but it requires a rather com to all engineering problems encounte was : plicated operation scenario and might be red, although admittedly some of these E1p.v./α ≈2.8 x 10-4 V/cm rejected finally for this reason. Perhaps, solutions are of a rather complicated which is to be compared to the experi then will come the time when the stella- nature. The construction and operation mental ratio (deduced from reference 2) rator's inherent potential of DC opera of an INTOR-like machine would there 1.56 ± 0.17 ± 0.12 x 10-4 V/cm. tion is given preference if also its other fore serve as an invaluable basis for the The parity violation effect was indeed properties have continued to develop fa development of second generation tech overestimated by a factor 1.8 but the vourably in the meantime. nologies aimed at further improving fu order of magnitude was clearly the cor One also has to remember that in all of sion reactor engineering. rect one. The p.v. electric dipole E1p.v is the world fusion programmes, the deve REFERENCES proportional to the weak charge of the lopment of fusion technologies was set 1. — INTOR-Group: International Tokamak nucleus Qw which depends upon the aside initially until sufficient confidence Reactor — Zero Phase, STI/PUB/556, weak mixing angle θw, the only free had been developed that all the physics (IAEA, Vienna) 1980. parameter in the Glashow-Weinberg- hurdles could be overcome. In view of 2. — INTOR-Group: International Tokamak Salam electroweak theory. In our 1974 the large extrapolations required this Reactor — Phase One, STI/PUB/619, (IAEA, work, we used a preliminary value of θw was a reasonable strategy, but it has Vienna) 1982. coming from the very early neutrino ex created a situation where one now can 3. — INTOR-Group: International Tokamak periments and got a value of Qw of only screen materials and technologies Reactor — Phase Two A, Part I, STI/PUB/ about - 100 while the most recent ex which have been developed for other 638, (IAEA, Vienna) 1983. perimental analysis, involving radiative purposes and check how they will per 4. — M. Keilhacker et al., Plasma Physics corrections, gives Qw = -68.6 ± 0.3. In form under fusion conditions. It is highly and Controlled Nuclear Fusion Research (Proc. 10th Int. Conf. London 1984), Vol. I, this way, a substantial part of the unlikely that the optimum materials and IAEA Vienna (1985), 71. discrepancy is accounted for. The re technologies are already among the fully 5. — Gormezano C., RF Driven Currents in maining 20% reduction factor is very developed ones, considering that the fu Fusion Devices, Europhysics News 15 likely to be associated with many-body sion conditions are rather unique. This is (1984) 8/9. effects not included in our early evalua not too bad a situation though, because tion. There exist now several calcula it now allows orienting the various tech tions of p.v. in atomic caesium, involving nology development paths with much D.N. Stacey writes : a treatment of many-body effects; all broader and deeper knowledge on the In my article, I referred to the Z3 law the results lie within the experimental required properties and achieving a derived by M. and Mme Bouchiat as a error bars 3). much higher cost-effectiveness than it major step in stimulating the programme would have been possible earlier. of research on parity nonconservation in 1. Bouchiat M.A. and Bouchiat C., Phys. Design work for INTOR-like devices atoms. In singling out this contribution Lett. 48B (1974) 111, and J. Phys. 35 will continue to identify the engineering for particular mention because of its im (1974) 899, and 36 (1975) 493. needs in greater and greater detail. The R portance I certainly did not intend to 2. Bouchiat M.A., Guéna J., Hunter L. and D programmes will provide answers associate M. and Mme Bouchiat with and Pottier L., Phys. Lett. 117B (1982) to these engineering needs, and one has early overestimates of PNC effects, and 358, and 134B (1984) 463, and Optics to be careful not to conclude that the indeed the article goes on to discuss Comm. 45 (1983) 35. state of the art reached up to now al these in the context of bismuth. I am 3. Dzuba V.A., Flambaum V.V., Silvestrov ready represented the full potential of fu therefore glad that the Editor has helped P.G. and Sushkov O.P., J. Phys. B18 sion technology. Nevertheless, in its deli to avoid any possibility that readers (1985) 597 and references therein. berations, the INTOR team has stated might be misled on this point by printing that solutions have already been found the above comment. M.A. and C. Bouchiat 14