The main coolants available are pressurized water, liquid metals, and helium. Water can be used only for near-term experiments. Reactors will probably need helium gas at a high temperature. The structural materials would be the same as those considered for the first wall: ferritic steels, vanadium alloys, or silicon carbide composites. The lithium can be in the form of solid pebbles of lithium ceramic, a liquid mixture of lead and lithium, or a molten salt called FLiBe [3]. Figure 9.12 shows how a TBM will be inserted into one of the ports in ITER.

The helium-cooled ceramic breeder (HCCB) uses solid material, with the beryllium multiplier and the lithium breeder in separate compartments. Figure 9.13 shows

the parts of an HCCB module. The slabs containing the beryllium and the lithium ceramic are shown in red and blue. Between the slabs are cooling channels through which helium is pumped under 80 atmospheres of pressure [3]. The temperature of the helium can reach 500°C, and the breeder material can reach 900°C. Note that the front of the blanket is part of the first wall. In a reactor, a blanket module can be assembled from submodules, as shown in Fig. 9.14. The thickness of the blanket is about 50 cm and its width about 3 m.

The solid breeding material consists of ceramic pebbles of lithium orthosilicate (Li4SiO4), lithium metatitanate (Li2TiO3), or other similar materials. Techniques have been developed to manufacture identical spherical pebbles which can distribute themselves uniformly. The size should be small, less than 1 mm in diameter, to minimize the temperature difference across the radius so that the brittle spheres do not crack [7]. To extract the tritium, a flow of helium containing some deuterium (D2) or hydrogen (H2) is passed through the pebble bed, and the tritium (T2) is carried out in the flow. The gases are then frozen and separated by distillation, since each has a different boiling point. The important thermal properties of a pebble bed have been measured [8].

A helium-cooled lithium lead (HCLL) blanket uses a molten alloy of lithium and lead called a eutectic. Meaning easily melted in Greek, a eutectic melts at a lower temperature than its constituents. The preferred eutectic is Pb-17Li, contain­ing 17% lithium enriched to 90% Li6. This melts at 234°C, compared with 328°C for lead and 181°C for lithium. In a blanket, the eutectic is heated from 400 to 660°C by the neutrons [3]. Since lead is a neutron multiplier like beryllium (Fig. 9.11), the multiplying and breeding are done in the same liquid. The submodules

Unit backplate

Fig. 9.15 Helium cooling arrangement in an HCLL blanket submodule [ ] in Fig. 9.14 will have circulating paths for the Pb-Li interspersed with channels for the helium coolant. The helium part is shown in Fig. 9.15, and the Pb-Li will go between the cooling plates. The tritium generated in the Pb-Li can be recovered by one of the two methods: permeation or bubbling. Hydrogen has a tendency to diffuse through walls, and tritium is just another form of hydrogen. Inside the blanket, tritium permeation into the helium coolant or other places where it does not belong is to be avoided. Outside the blanket, however, permeation windows can be made to allow hydrogen to go through and mix with a helium flow headed for a tritium separation facility. Alternatively, the Pb-Li can be formed into bubble columns where bubbles of helium capture the tritium in the liquid Pb-Li and carry it to the processing plant.

In earlier work, a molten salt called FLiBe, containing beryllium fluoride (BeF2) and one or two parts of lithium fluoride (LiF) was proposed as a breeder fluid, but now Pb-Li is preferred. The work on FLiBe uncovered the problem of magnetohy­drodynamic flow [9], which also applies to Pb-Li [10]. Both are electrically con­ducting fluids, and when these move inside a magnetic field, electric currents are generated in the fluid; and these currents react back on the magnetic field to pro­duce a drag on the fluid motion. Considering how strong the magnetic fields are in a tokamak, this drag is a serious problem that increases the required pumping power. The drag is less if the flow goes along the magnetic field lines, but eventu­ally the fluid has to cross the field lines to get out of the breeding region.

A dual-cooled lithium lead (DCLL) blanket uses both helium and the Pb-Li itself as coolants. This concept is shown in Fig. 9.16. Since Pb-Li is a liquid, it can be sent to its own heat exchanger and act as its own coolant. Helium is used to cool

Pb-47Li

EUROFER structure
(FW + grids)

Fig. 9.16 Schematic of a dual-cooled Lithium lead blanket module [34]. ODS, EUROFER, and SiC/SiC refer to high-temperature materials described under The First Wall and Other Materials the first wall separately. The flow in the Pb-Li channels is shown in Fig. 9.17 for a case in which the magnetic field direction is into the paper. Computer models have been developed to describe the flow of the conducting liquid, including the buoyancy effect when the temperatures at the top and bottom are different. The eddies in the flow, as calculated, are shown in the inset. Since each module in a tokamak will be oriented at a different angle to the magnetic field, the structure of the flow, and hence the pressure drop, will be different at each location in the machine.

In advanced designs, the helium is eliminated, resulting in a self-cooled lithium lead breeding blanket, in which Pb-Li does all the cooling. It may take a lot of power to pump Pb-Li fast against the drag by the magnetic field. The possibility also depends on the development of the wonder-material SiC/SiC, which can operate at 1,000°C and contain a higher temperature fluid than other materials.

These blanket designs do not show all the auxiliary equipment necessary to operate the blanket. The roomful of pipes, heat exchangers, shields, and instru­ments for a single TBM in ITER is shown in Fig. 9.18. The blanket module itself is only the curved unit at the left, which forms part of the first wall.

Blankets for a full-scale reactor would have to satisfy many other requirements besides cooling and breeding. Maintenance and operation presents serious problems for a reactor designed to operate for over 25 years. The blanket material will have to be replaced many times during the life of the reactor. Solid breeders such as the

TBM system integrated inside port cell 16 of ITER

(systems for both TBMs are shown)

Equatorial floor
only poet 16 shown

Fig. 9.18 Diagram of a proposed test blanket installation in ITER [6]

pebble-bed HCCB have to be physically removed to change the pebbles. In liquid blankets, the Pb-Li can be circulated outside the blanket and renewed without removing the blanket. Eventually, however, blankets will have to be replaced, requir­ing a shutdown. For easier replacement, banana-shaped blankets fitting the contour of the D-shaped plasma have been proposed. These would be lowered from the top of the tokamak during a shutdown, and all the connections to the blanket would have to come from the top. All this has to be done with remote handling, since there will be too much radioactivity for humans to work on the reactor.

Since the blankets are located inside the vacuum, they must be leak proof. Welds must be secure. Inside the blanket there are many interfaces between breeders and coolants, and a leak there would be impossible to fix without removing the blanket. There are also numerous joints in the pipes connecting the blanket to the world outside the vacuum tank. In 2008-2009, the Large Hadron Collider in Geneva suffered from a single leak in the liquid helium system which delayed the startup of the machine for over a year. In 2003, a single piece of loose foam brought down the shuttle Columbia, killing seven astronauts. Accidents happen, and extreme care must be taken in a tokamak reactor, where there are a million places where a leak can occur.

There are also safety issues in the case of an accident, including decay heat and radiotoxicity after shutdown [11]. Recycling and treatment of waste have also to be considered. However, these are not specific to blankets and will be covered in another section.