Breeding blankets for fusion reactors are categorized into solid breeder and liquid breeder concepts. The liquid breeder blankets have certain advantages over the solid breeder blankets such as continuous chemi­cal control of the breeding material including isoto­pic control of Li, impurity control and tritium recovery, and immunity to irradiation effects. How­ever, some issues such as compatibility of the breeder with structural materials are more serious for liquid breeder blankets4. In addition, blanket structure

can be simplified significantly if the liquid breeder functions as the coolant as well (self-cooled liquid breeder blanket). At present, the major candidate liquid breeder materials are Li, Li-Pb, and molten salt Flibe (LiF-BeF2).

In the cases of self-cooled liquid Li and Li-Pb blankets in magnetic confinement fusion systems, the high-speed flow of these materials perpendicular to the strong magnetic field causes an electric current, which then produces an electromagnetic force as a result of interaction with the magnetic field. This force changes the velocity profile in the cooling ducts and acts to retard the coolant flow, leading to what is called a magnetohydrodynamic (MHD) pres­sure drop. This process is schematically shown in Figure 1(a). The MHD pressure drop may result in loss of flow control and mechanical stresses exceeding the allowable limits of the structural materials. The problems arising from the MHD pressure drop are critical feasibility issues for self-cooled liquid metal breeder blanket concepts with metallic structures.

The quantification of the MHD pressure drop requires a rather complex numerical analysis. How­ever, in simple cases such as straight and constant area cross-section flow in conductive ducts with a uniform magnetic field in a traverse direction, the pressure gradient along the flowing direction, dp/dx, is given as follows5:

dp/dx = ksUB2

where s, U, and B are the electrical conductivity, flow velocity of the liquid metal, and magnetic flux den­sity, respectively, and k is a positive function of elec­trical conductivity of the wall. The equation implies that the MHD pressure drop is an issue in the case of high magnetic field and high velocity flow of conduc­tive liquid metals. In the case of a low flow rate such as would occur in a helium-cooled Li-Pb blanket, the MHD pressure drop will not be an issue.

Magnetic field

*

iquid Li flow

Insulator

coating

Figure 1 Schematic illustration of magnetohydrodynamic pressure drop (left) and the role of insulator coating (right).

To reduce the MHD pressure drop, optimization of the coolant flow path by enhancing the flow fraction parallel to the magnetic field may have some effect. However, a more effective way to reduce the MHD pressure drop would be to electrically insulate the coolant flow from the surrounding walls.6 The reduc­tion of MHD pressure drop by an insulator coating is schematically illustrated in Figure 1(b). The require­ments for the coating can be summarized as follows:

1. compatibility with liquid breeder under flowing conditions with a temperature gradient,

2. high electrical resistivity under irradiation,

3. robustness and/or an effective self-healing capability,

4. potential for covering large and complex surfaces, and

5. fundamental requirements for in-vessel materials such as radiation resistance, low activation proper­ties, and low tritium inventories in blanket conditions.

Quantitative evaluation of the required electrical resistance and an allowable crack fraction are subject to overall blanket design including flow channel structures. A recent model calculation showed that the ratio of electrical resistivity of the insulator to the wall needs to be ^106 and crack areal fraction to be ;S 10~6 to maintain the pressure drop within tolerable range, assuming Li wets cracks.7,8

For the Li-Pb blankets, the insulator coating should be a critical issue if a self-cooled Li-Pb blanket with metallic structural materials is to be designed. How­ever, current blanket design options with Li-Pb are (1) helium cooled with slow-flowing Li-Pb, (2) dual­coolant Li-Pb with fast-flowing Li-Pb but electrically insulated from the wall by a SiC/SiC flow channel insert (FCI), or (3) self-cooled Li-Pb using SiC/SiC as the structural material. None of these concepts needs the insulator coating. However, development of a ceramic coating, necessary mostly for tritium perme­ation reduction and possibly for corrosion protection, is still a critical issue.9