Fusion reactor design with ceramics

Abstract

Ceramics are required to serve in a conventional role as electrical and thermal insulators and dielectrics in fusion power reactors. In addition, certain ceramic materials can play a unique role in fusion power reactors by virtue of their very low induced radioactivity from transmutation products produced by fusion neutron capture. The aspects of safety, long-term radioactive waste management, and personnel access for maintenance and repair can all be significantly improved by applying the low-activation ceramics to the first wall and blanket regions of a fusion reactor. This application imposes tensile, compressive, and shear structural loads and thermal stresses on the materials, and it is primarily in support of tensile stresses where problems in ceramic design lie. Silicon carbide, carbon, and graphite materials are three primary candidate structural ceramics. Electrical insulators and radio frequency electromagnetic wave windows commonly employ ceramics, such as Al 2O 3, MgO, SiO 2, Si 3N 4 and glasses. Material properties characteristic of the radiation damaged state must be used. The structural failure modes of low-ductility ceramics are by immediate fracture when a critical stress is applied or by slow crack growth eventually propagating to fracture. Both failure modes follow a statistical distribution with a finite probability of failure with low applied loads. Proof testing, however, can reduce this probability of failure to zero below some threshold stress by eliminating the weaker components, thus easing design problems. Design studies have been performed to develop conceptual designs of fusion power reactor components using low-activation ceramic and metallic materials. These components include limiters, first walls, blanket modules, shields, superconducting magnets, diagnostic instrumentation, electrical insulation, and radio frequency windows. Present day ceramics can fulfill all the functional requirements of these components without undue performance penalties. Improved ceramic materials, both monolithic and fiber composites, are being developed at a rapid pace, and these can readily be applied to improved fusion designs. It thus appears possible to design a fusion reactor using only low-activation ceramic materials, principally structural ceramics, in the high-neutron flux zones of the reactor. Presently operating fusion plasma devices employ graphite for limiters and armor, and ceramics for electrical insulators, providing a base for continued utilization when power-producing devices are built. The ultimate potential of fusion as an environmentally benign energy source with a high degree of safety and public acceptance is optimally achieved through the use of low-activation structural ceramics.