Patent Number: 046631100
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to the production of fissile fuel for fission reactors, and in particular to the breeding of such fuels using the neutrons generated by a fusion reactor Fertile materials such as thorium 232 (.sup.232 TH) and uranium 238 (.sup.238 U) may be bombarded by neutrons to produce fissile materials such as uranium 233 (.sup.233 U) and plutonium 239 (.sup.239 Pu), respectively. A "fissile" material is one which is fissionable by slow neutron capture, and a "fertile" material is one which can be rendered fissile by neutron absorption. The source of the neutrons may be a fusion reactor. The fusion breeder concept is attractive because the subsequent burning of a bred fissile atom releases 200 MeV as compared to the 20 MeV typically produced by a single fusion reaction. Generally, the fusion neutron enriched fuel must be chemically reprocessed so that the concentration of fissile fuel is sufficient for fabrication of the fuel elements (blocks, control rods, pins, etc) which are to be inserted into a fission reactor. Reprocessing is subject to certain dangers which are the focus of resistance to the development of nuclear reactors. Not the least objectionable aspect of reprocessing are the opportunities provided for diversion of materials which could be used to manufacture a nuclear explosive device. Reprocessing is also expensive, adding up to $50 per gram or more to the cost of the bred fuel. In part to circumvent such problems, it has been proposed to bypass reprocessing by breeding fuel within assembled fuel elements, pins and rods inserted into the blanket of a fusion reactor. The problems introduced by this approach include thermal and radiation damage to structural material of the fuel element Additionally, since the neutron flux is not generally uniform throughout the fusion blanket, the eventual fuel elements contain variably enriched fuel. The relatively long residence time required adversely affects the materials and economics involved. For example, in one approach the calculated residence time is on the order of 2.6 years. Furthermore, approximately 11-12% of the .sup.233 U bred in the blanket is burned up before 4% enrichment is achieved. [R. W. Conn, S. I. Abdel-Khalik, G. A. Moses, G. L. Kulcinski, E. Larsen, C. W. Maynard, M. M. H. Ragheb, I. N. Sviatoslavsky, W. F. Vogelsang, W. G. Wolfer, M. Ortman, R. Watson and M. Z. Youssef, "Fusion-Fission Hybrid Design with Analysis of Direct Enrichment and Non-Proliferation Features (The Solase-H Study)", Nuclear Engineering and Design, 63 (1981) pp. 357-374.] Accordingly, it is an object of the present invention to provide a method for enriching fertile material using a fusion reactor, which method does not require chemical reprocessing, provides for uniformity of enrichment, and does not damage the structural components of the fuel elements. It is also an object of the invention to provide for a shortened residence time of the fission fuel in the fusion blanket. SUMMARY OF THE INVENTION A fusion blanket and method permit the breeding of fissile material which may be fabricated for use as fuel in a fission reactor. The blanket includes a chamber wall for isolating the fusion chamber from the rest of the blanket, a neutron multiplication section, an enrichment section and a reflector, in radially outward succession, respectively. The neutron multiplication section includes a material which may produce about two neutrons upon bombardment by one neutron of high energy. The reflector reflects neutrons back through the enrichment section to increase the efficiency of the enrichment process. The enrichment section includes fertile material, which is relatively dilute so as to limit fissioning and undue competition among fertile atoms for enriching neutrons. The enrichment section provides material capable of absorbing thermal neutrons in order to repress thermal fissioning of the bred fissile material. The amount of thermal neutron absorbing material is preferably selected to limit the thermal neutron flux without excessively competing with the fertile material for enriching neutrons. In a first preferred embodiment, the particles are packed into chambers extending through the enrichment section of the fusion blanket. In a second preferred embodiment, the particles are suspended in a slurry flowing through pipes in the enrichment section. The slurry includes a carrier which is preferably a good thermal conductor and thermal neutron absorber. Such a carrier promotes effective heat transfer while suppressing thermal fissioning of the bred fissile atoms. In accordance with the present method, particles of fertile material are included in the blanket of a fusion reactor. The particles may be packed in chambers or suspended in a slurry which flows through pipes extending through the blanket The particles are exposed to neutrons produced directly and indirectly through the fusion process until the desired level of enrichment is achieved. The enriched particles are removed from the blanket, any carrier being removed at this point. The particles are then mixed to compensate for nonuniformities, such as those introduced as a function of particle location within the blanket. The mixing may be random or systematic, provided the enrichment profile of the blanket is known. If the enrichment profile of the blanket is known, the particles may then be collected and grouped according to location within the reactor so that the groups formed have different average enrichments. The particles within each group may then be mixed to obtain within-group uniformity. This grouping method is applicable where more than one level of enrichment is desired. For example, fuel may be bred for two different types of fission reactors, or for different elements of a single reactor, such as fuel blocks and control rods in a high-temperature gas-cooled reactor (HTGR). The enriched material is then inserted and/or fabricated into fuel elements, such as fuel blocks, fuel pins, or control rods, as required by the fission reactors to be supported by the fusion reactor. The fuel elements then may be used in the fission reactors to generate power. Since the fuel elements' structural members (cladding, hexblocks, graphite matrices, etc.) are not included in the blanket, irradiation damage is minimized. Furthermore, the power swing in the present design is limited by the low concentration of fertile fuel and the relatively short fertile residence time. The limited power swing reduces materials demands in the blanket, lowering the probability of blanket failure. No separate reprocessing step is required, thus the opportunities for diversion are limited. Should reprocessing be deemed desirable, recovery of residual fissile material from the spent fuel may be added, thereby greatly increasing the number of fission reactors which may be supplied by a fusion reactor of equal power.