Blanket

The title of this article is ambiguous. Further meanings are listed under Blanket (disambiguation) .

The Blanket (English for ceiling, shell ) of a nuclear fusion reactor is located inside the vacuum vessel to the plasma . It protects the external superconducting coils and other parts from heat and radiation. In future fusion power plants, it will also convert the neutron energy gained into thermal energy (and is therefore part of the first cooling circuit) and also breed the fuel tritium .

Modular structure, materials

The blanket must consist of separable sections ( modules ) that fit through openings in the vacuum vessel for replacement. In the case of power reactors, this exchange will have to be carried out remotely by manipulators because of the radioactive components .

On the plasma side, blankets are coated with tungsten or beryllium - metals with a very high melting temperature. In some constructions, a first wall coated in this way can be exchanged separately.

Blankets for power reactors must contain components for incubating tritium and a rinsing circuit for extracting the tritium and must consist of materials with low levels of activation (see nuclear fusion reactor requirements ); in particular, nickel cannot be used as a steel component, since it forms the relatively long-lived radioactive cobalt-60 when irradiated with fast neutrons.

Energy conversion

In the blanket, the fast neutrons released during the nuclear fusion reaction of tritium and deuterium transfer their kinetic energy to atomic nuclei through elastic and inelastic collisions. This useful energy is increased by around 20% through the energy gain from the brood reaction (see below). The heat is dissipated by a coolant and z. B. used in a conventional steam cycle with turbine and generator to generate electrical energy.

Neutron balance, tritium breeding

The second task of the blanket is to incubate tritium from lithium . The breeder reactor concept is suitable for fusion reactors because the fusion fuel tritium ( ³ H) is only available in very small quantities as a natural resource. However, it can be obtained from the common element lithium by means of neutrons, and neutrons are available as “waste” in the fusion reactor after their energy has been given off. This breeding process is absolutely necessary for the generation of fusion energy on a large scale, because there are hardly any other methods of economically generating the necessary amounts of tritium.

Breeding reactions occur on both lithium isotopes , the rarer ⁶ Li (7.5%) and the more common ⁷ Li (92.5%):

${\displaystyle ^{6}\mathrm {Li} +\mathrm {n} \ \rightarrow \ ^{4}\mathrm {He} +{}^{3}\mathrm {H} +4{,}8\;\mathrm {MeV} }$

${\displaystyle ^{7}\mathrm {Li} +\mathrm {n} \ \rightarrow \ ^{4}\mathrm {He} +{}^{3}\mathrm {H} +\mathrm {n} '-2{,}5\;\mathrm {MeV} }$

The endothermic (see nuclear reactions ) reaction at ⁷ Li has the advantage that the neutron is not consumed, but is released again with reduced energy (because of its reduced energy, it is designated in the formula with n ', in contrast to the original neutron n) . It is therefore basically still available for a second reaction at ⁶ Li. The disadvantage of the ⁷ Li reaction, however, is its high energy threshold . The consequence of this is that the ⁷ Li breeding reaction can only play a minor role in the more modern designs because of the neutron spectrum in the blanket . Therefore, the use of lithium is envisaged, which is enriched up to 90% ⁶ Li. The exothermic and therefore possible with slow neutrons breeding reaction at ⁶ Li has the side effect of a considerable energy gain of 4.8  MeV , which is added to the energy yield of the fusion reaction. The tritium breeding ratio can then be adjusted and readjusted via the degree of enrichment of the lithium.

Neutron multiplication

Tritium breeding with excess is not possible with the fusion neutrons alone, because the fusion reaction delivers only one neutron per consumed fusion pair (one tritium and one deuterium atom). Some of the neutrons are lost through absorption in the structural material and through leakage to the outside. The tritium produced cannot be extracted completely, either, and a small portion decays radioactively.

Commercial fusion reactors must be designed in such a way that, despite the losses mentioned, a slight overproduction of tritium is possible. Therefore the neutrons in the blanket have to be increased by about 30% to 50%. The (n, 2n) nuclear reaction on beryllium or lead is suitable for this , as it has relatively low energy thresholds on these materials. For example, there is the (n, 2n) nuclear reaction on beryllium

${\displaystyle \mathrm {\,^{9}Be+n\rightarrow 2\,\,^{4}He+2\,n-1,57\,MeV} }$.

Both released neutrons have much lower energies than the fusion neutrons (the light nuclide beryllium also acts as a moderator ), but can generate tritium by reacting with ⁶ Li.

Energy balance

The exact useful energy gain per fusion reaction depends somewhat on the blanket construction. The initial energy of the fusion neutron averages 14.1 MeV. In addition, 4.8 MeV comes from the ⁶ Li breeding reaction. For a large part of the fusion neutrons, the energy loss of the neutron multiplication reaction (around 2 MeV) as well as amounts of energy that are lost in other, unavoidable neutron absorptions, insofar as they cannot technically be used as heat, must be deducted.

The detailed analysis of a DEMO design with a helium-cooled lithium-lead blanket (see below) resulted in an energy multiplication of 1.17, i.e. an average usable energy of around 16.5 MeV per individual DT reaction.

shielding

The blank parts used for tritium incubation, which are located directly behind the “first wall” facing the plasma, weaken the total neutron flux by only around a factor of 10; however, the energy spectrum of the neutrons behind it is “softer” than in the first wall, since most of the neutron energy is given off in this part. Sufficient shielding of the parts behind it is the third task of the blanket, because remaining fast neutrons generate dislocation damage through elastic scattering and radioactive nuclides through nuclear reactions as a result of neutron activation , which must also be kept as low as possible. The most important thing is to minimize radiation damage in the magnet coils, both in the superconductor fibers and in the copper that surrounds them for stabilization .

Technical blanket concepts

In the oldest concepts for fusion power plants, up to around 1980, the blanket was a tank filled with pure lithium. The cooling (energy dissipation) could be carried out by a separate coolant guided in pipes, or the lithium, as liquid metal, could also be a coolant itself by being circulated through heat exchangers by means of pumps.

Metallic lithium, however, has a corrosive effect on other metals, especially as a hot melt, and represents a safety risk because it reacts violently chemically with air or water, similar to the coolant sodium in fission breeder reactors . The amount of lithium in the fusion reactor would be much larger than the amount of sodium in a fission breeder reactor of the same power. Therefore, in the more realistic blanket concepts

• either a ceramic lithium compound (oxide, carbonate or silicate) provided as breeding material, together with beryllium parts for neutron multiplication and helium gas as a coolant,
• or a liquid lead-lithium alloy that acts as a neutron multiplier and breeding material at the same time and is chemically much less aggressive than pure lithium. This liquid metal could also act as a coolant; however, this leads to difficulties in the magnetic field of the fusion reactor because of the magnetohydrodynamic braking of the flow. Concepts with unmoved lead-lithium and a separate cooling circuit with e.g. B. water or helium.

Blanket modules for ITER

The ITER pilot plant will not yet be a power reactor . Use of the energy obtained and incubation of tritium are omitted here, but experimental designs of power reactor blanks will be tried out. These blanket modules for ITER are structurally made of chrome-nickel stainless steel as well as copper for better heat conduction. Deep slots in the poloidal direction reduce eddy currents and the associated mechanical loads.

literature

• AA Harms, KF Schoepf, GH Miley, DR Kingdon: Principles of Fusion Energy . World Scientific, Singapore 2000, ISBN 981-02-4335-9
• WM Stacey: Fusion . 2nd Edition. Wiley, Weinheim 2010, ISBN 978-3-527-40967-9 . ( limited preview in Google Book search)
• G. McCracken, P. Stott: Fusion - the Energy of the Universe. 2nd Edition. Elsevier, Munich 2012, ISBN 978-0-12-384656-3 . (An overview that is understandable even for laypeople)

Design for a blanket with ceramic breeding material:

• M. Dalle Donne (Ed.): European DEMO BOT solid breeder blanket . Nuclear Research Center Karlsruhe, 1994, DNB 944269257 . (Report KfK-5429)
• U. Fischer, H. Tsige-Tamirat: Activation characteristics of a solid breeder blanket for a fusion power demonstration reactor. Journal of Nuclear Materials , 307-311 (2002), pp. 798-802.

Drafts for blankets with lead-lithium as breeding material:

• P. Norajitra, L. Bühler, U. Fischer et al: The EU advanced dual coolant blanket concept. Fusion Engineering and Design , Vol. 61-62 (2002), pp. 449-453.
• A. Li Puma et al .: Breeding blanket design and systems integration for a helium-cooled lithium-lead fusion power plant. Fusion Engineering and Design Vol. 81 (2006), pp. 469-476.

Individual evidence

1. ^ The three tasks of the blanket ( Memento from March 4, 2016 in the Internet Archive )
2. ↑ ^{a b} U. Fischer, P. Pereslavtsev, D. Grosse u. A .: Nuclear design analyzes of the helium cooled lithium lead blanket for a fusion power demonstration reactor. Fusion Engineering and Design Vol. 85 (2010) pp. 1133-1138
3. ^ Raffaele Albanese et al .: Electromagnetic Disruption Loads on ITER Blanket Modules. IEEE Trans. Magn. 46, 2010, pp. 2935-38, doi: 10.1109 / TMAG.2010.2048560 ( online ).