The study of intense radiation effects in metals has been closely associated with the development of nuclear fission reactors, and as a result at the begin­ning of the 1980s when the urgent need to consider radiation damage aspects of materials to be employed in future fusion reactors was fully realized, a consid­erable amount of knowledge and expertise already existed for metallic materials.29 This was not the case for the insulating materials, mainly because of the fact that the required use of insulators in fission — type reactors is in general limited to low radiation regions, well protected from the reactor core. How­ever, despite the late start and the reduced number of specialists working in related fields at the time, together with the complexity of the mechanisms involved in radiation damage processes in insulators, considerable progress has been made not only in assessing the possible problem areas, but also in finding viable solutions. Several general reviews give a good introduction to the specific problem of radiation damage in insulators.30-36

The materials employed in the next-step fusion machine will be subjected to fluxes of neutrons and gammas originating in the ignited plasma. The radiation intensity will depend not only on the dis­tance from the plasma, but also in a complex way on the actual position within the machine because of the radiation streaming along the numerous pene­trations required for cooling systems, blanket struc­tures, heating systems, and diagnostic and inspection channels, as well as the radiation coming from the water in the outgoing cooling channels due to the 16O(n, p)16N nuclear reaction. However one-, two-, and even three-dimensional models are now available, which enable the neutron and gamma fluxes to be calculated with confidence at most, if not all, machine positions.37-40

Radiation damage is generally divided into two components: displacement damage and ionization effects. In a fusion environment, displacement dam­age, which affects both metals and insulators, will result from the direct knock-on of atoms/ions from their lattice sites by the neutrons, giving rise to vacancies and interstitials. Those primary knock-on atoms (PKAs) with sufficient energy may go on to produce further displacements, so-called cascades. The numerous point defects thus produced may either recombine, in which case no net damage results, or they may stabilize and even aggregate producing more stable extended defects. These sec­ondary processes which determine the fate of the vacancies and interstitials are governed by their mobilities. These mobilities are highly temperature dependent, and in the case of insulators even depend on the ionizing radiation level (radiation-enhanced diffusion). Displacement damage is measured in ‘dpa’ (displacements per atom) where 1 dpa is equivalent to displacing all the atoms once from their lattice sites. At the first wall of ITER, the primary displacement dose rate will be of the order of 10~6dpas~

In contrast, ionizing radiation although absorbed by both metals and insulators, in general, only produces heating in metals. However, certain aspects of radia­tion damage in metals, such as radiation-enhanced corrosion and grain boundary modification are related to ionization. The effects of ionization on insulators are in comparison quite marked because of the exci­tation of electrons from the valence to the conduction band giving rise to charge transfer effects. Ionizing radiation is measured in absorbed dose Gy (Gray) where 1 Gy = 1J kg-1. At the first wall of ITER, the dose rate will be of the order of 104Gy s-1.

The response of insulators to both displacement and ionizing radiation is far more complex than in the case of metals. Apart from a few specific cases (diamond for example), insulating materials are polyatomic in nature. This leads to the following:

(i) We have in general two or more sublattices which may not tolerate mixing.

(ii) This gives rise to more types of defects than can exist in metals.

(iii) Because of the electrically insulating nature, the defects may have different charge states, and hence different mobilities.

(iv) The displacement rates and thresholds, as well as the mobilities, may be different on each sublattice.

(v) We may have interaction between the defects on different sublattices.

(vi) Defects can be produced in some cases by purely electronic processes (radiolysis); however, in the insulating materials of interest for fusion, this is generally not the case.

As a consequence of these factors, while radiation damage affects all materials, the insulators are far more sensitive to radiation damage than metals. While stainless steel, for example, can withstand sev­eral dpa and GGy with no problem, some properties of insulating materials can be noticeably modified by as little as 10-5 dpa or a few kGy. Because of this, the present ongoing programs of radiation testing for diagnostics are concentrating mainly on the insulat­ing components of the systems. The results of these radiation damage processes are flux — and fluence — dependent changes in the physical and mechanical properties ofthe materials, which may be particularly severe for the insulators. The properties of concern which suffer modification are the electrical and thermal conductivity, dielectric loss and permittivity, optical properties, and to a lesser extent the mechan­ical strength and volume. The effects of such changes are that the insulators may suffer Joule heating because of the increased electrical conductivity or lower thermal conductivity, and absorption in windows and fibers can increase from the microwave to the optical region and they emit strong lumines­cence (radioluminescence, RL); in addition, the materials may become more brittle and may suffer swelling. Clearly, some materials are more radiation resistant than others. The organic insulators, which are widely used in multiple applications in general, degrade under purely ionizing radiation and are not suitable for use at temperatures above about 200 °C; as a result their use will be limited to superconducting magnet insulation and remote handling applications during reactor shutdown. Inorganic insulators of the alkali halide class have been widely studied and are used as optical windows; however, they are suscepti­ble to radiolysis (displacement damage induced by electronic excitation) and in general become opaque at low radiation fluences. Of the numerous insulating materials, it is the refractory oxides and nitrides, which in general show the highest radia­tion resistance, and of these the ones which have received specific attention within the fusion program include MgO, Al2O3, MgAl2O4, BeO, AlN, and Si3N4. In addition, different forms of SiO2 and materials such as diamond and silicon have been examined for various window and optical transmis­sion applications.

One other aspect of radiation damage that should be mentioned is nuclear transmutation. The high — energy neutrons will produce nuclear reactions in all the materials giving rise to transmutation pro — ducts.1 These will build up with time and represent impurities in the materials, which may modify their properties. The physical properties of insulators are particularly sensitive to impurities. Furthermore, some of these transmutation products may be radio­active and give rise to the need for remote handling and hot cell manipulation in the case of component removal, repair, or replacement. For the structural materials, in the present concepts mainly steel alloys, considerable work has been carried out on the devel­opment of so-called low or reduced activation mate­rials (LAM, RAFM — reduced activation ferritic/ martensitic) for possible use in DEMO and future commercial fusion reactors.41-45 This work with the aim of reducing the amount of nuclear waste has studied not only the substitution ofradiological prob­lem alloying elements such as Mo and Nb in steels, but also the viability of other materials such as vana­dium and SiC/SiC composites. In the case of the insulating materials, no equivalent study or development has been carried out, in part because of the small fraction of the total material volume repre­sented by the insulators, and also because the impor­tant physical properties of these materials are expected to be degraded before the transmutation products become of concern. Certainly, for a next-step machine such as ITER, transmutation products, with the possi­ble exception of hydrogen and helium, are not expected to present a serious problem.