Power plant operating parameters

Target factory

32 The fusion targets, which must be manufactured at rates of up to ten per second, have a simple structure. The fuel capsule is a spherical shell that contains the D-T fuel and that doubles as an ablator and pusher for the implosion process. In indirect-drive targets this fuel capsule is surrounded by a high-atomic-number hohlraum wall to contain the x rays. The gain at low drive energy depends upon the surface finish of the capsule. Surface finishes with no features larger than 1000 Å are required for high gain. However, the hohlraum shell does not have high-precision requirements and can simply be stamped out.

33 Several techniques have been proposed for making the high-precision capsules. Drop towers are used to make today's targets at rates of several hundred per second, although only for very short times, because we do not need many targets today. In drop towers, the shell material is forced through a collection of small openings. A succes­sion of droplets falls through the heated tower, and the droplets cure into nearly perfectly spherical shells of a predetermined size as they fall. The yield of this technique for thin targets has been 10-25% with surface finishes better than 1000 A. In current research only the best targets are used. Microencapsulation is an emulsion technique that may be better for the larger and thicker capsules needed in the future. This technique has been used in the batch mode to date but appears adaptable to continuous operation. Machinery costs for a full-sized target factory, with redundancy, are estimated at only a few million dollars.

34 A power plant using several 400-MJ targets per second requires a tritium throughput of 1-2 kg per day. The fuel can be loaded into the capsule by diffusion through the capsule wall or through small holes that are later sealed. Diffusion filling requires a high-pressure chamber containing many times the target-fuel mass. A uniform layer of fuel can be made in several ways. For ex­ample, if liquid D-T in a spherical container is allowed to sit, then heating from beta decay of the tritium is greater in the thicker regions than in the thinner ones, and the D-T sublimes and migrates to make the shell uniform—the so-called beta-layering technique. Alternately the fuel can be frozen in a tailored thermal environment, or liquid fuel can be supported by a low-density, low-atomic-number foam shell.

35 The total amount of tritium contained in the target factory depends upon the methods of filling and of establishing a uniform spherical shell of fuel. If diffusion filling and beta layering are used, then the tritium inventory in the target factory may be as much as 8 kg. For some alternative methods the inventory could be less than 1 kg. The cost of handling the 1-8 kg of tritium may be significantly higher than the cost of the machinery to manufacture the targets themselves. However, research is needed on a variety of specific fabrication techniques before we can choose the best ones for a reactor.