E. R. Hodgson

4.22.1 Introduction

It is envisaged that early in the twenty-first century ITER (International Thermonuclear Experimental Reactor) will come into operation, and it is expected that this intermediate ‘technology’ machine will help to bridge the gap between the present-day large ‘physics’ machines and the precommercial DEMO power reactor, thus paving the way for commercial fusion reactors to become available by the end of the century. Although this ‘next-step’ device will undoubtedly help to solve many of the problems, which still remain in the field of plasma confinement, it will also present additional operational and experi­mental difficulties not found in present-day machines. These problems are related to the expected radia­tion damage effects as a result of the intense radiation field from the ‘burning’ plasma. This ignited plasma will give rise to high-energy neutron and gamma fluxes, penetrating well beyond the first wall, from which one foresees a serious materials problem that has to be solved. In the initial physics phase of opera­tion of such a machine, it is the radiation flux, which will be of concern, whereas in the later technology phase, both flux and fluence will play important roles as fluence (dose)-dependent radiation damage builds up in the materials. For structural metallic materials, radiation damage in ITER is expected to be severe, although tolerable, only near to the first wall. How­ever, the problem facing the numerous insulating components is far more serious because of the neces­sity to maintain not only the mechanical, but also the far more sensitive physical properties intact. An addi­tional concern arises from the need to carry out inspection, maintenance, and repair remotely because of the neutron-induced activation of the machine. This ‘remote handling’ activity will employ machin­ery, which requires the use of numerous standard components ranging from simple wires, connectors, and motors, to optical components such as windows, lenses, and fibers, as well as electronic devices such as cameras and various sophisticated sensors. All these components use insulating materials. It is clear, there­fore, that we face a situation in which insulating materials will be required to operate under a radiation field, in a number of key systems from plasma heating and current drive (H&CD), to diagnostics, as well as remote handling maintenance systems. All these sys­tems directly affect not only the operation, but also the safety, control, and long-term reliability of the machine. Even for ITER, the performance of some potential insulating materials appears marginal. In the long term, beyond ITER, the solution of the materials problem will determine the viability of fusion power.

The radiation field will modify to some degree all of the important material physical and mechanical properties. Some of the induced changes will be flux dependent, while others will be modified by the total fluence. Clearly, the former flux-dependent pro­cesses will be of concern from the onset of operation of future next-step devices. The fluence-dependent effects on the other hand are the important para­meters affecting the component or material lifetime. The properties of concern which need to be consid­ered for the many applications include electrical resistance, dielectric loss, optical absorption, and emission, as well as thermal and mechanical proper­ties. Numerous papers have been published discuss­ing both general, and more recently, specific aspects of radiation damage in insulating materials for fusion applications, and those most relevant to the present chapter are included.1-26

In recent years, because of the acute lack of data for insulators and the recognition of their high sensi­tivity to radiation, most work has concentrated on the immediate needs for ITER. A comprehensive cera­mics irradiation program was established to investi­gate radiation effects on a wide range of materials for essentially all components foreseen for H&CD and diagnostics in ITER, and to find solutions for the problems which have been identified. A large number of relevant components and candidate materials have been, and are being, studied systematically at gradu­ally increasing radiation dose rates and doses, under increasingly realistic conditions. A considerable vol­ume of the work discussed here was carried out within the ITER framework during the CDA, EDA, and EDA extension (Conceptual and Engineering Design Activities 1992-2002) as specific tasks assigned to the various Home Teams (T26/28 and T246; EU, JA, RF, US; T252/445 and T492; EU, JA, RF).27,28 Since these last ITER tasks, no new coordinated tasks related to insulators have been formulated. However, despite the lack of an official framework in which to develop and assign further common tasks following the end of the ITER-EDA extension, col­laborative work has continued between the EU, JA, RF, and US Home Teams on both basic and applied aspects of radiation damage in insulator materials. This has resulted in considerable progress being made on the understanding of the pertinent effects of radiation on in-vessel components and materials in particular for diagnostic applications. Problems which have been addressed and for which irradiation testing has been performed include comparison of absorption and luminescence for different optical fibers and win­dow materials, RIEMF (radiation-induced electro­motive force) and related effects for MI (mineral insulated) cables and coils, alternative bolometers to the reference JET type gold on mica, hot filament pressure gauges, and electric field effects in aluminas.

One must however remember that ITER is only an intermediate ‘technology’ machine on the road to a precommercial power reactor. Such power reactors will face the same radiation flux problems as antici­pated in ITER, but the fluence problems will be far more severe. It is also important to note that the radiation flux and fluence levels will be different from one type of device to another depending on the design (e. g., in ITER and the Fusion Ignition Research Experiment (FIRE)26), and also on the spe­cific location within that device. Because of the gen­eral uncertainty in defining radiation levels, most radiation effects studies have taken this into account by providing where possible data as a function of dose rate (flux), dose (fluence), and irradiation tempera­ture. Although the task ahead is difficult, important advances are being made not only in the identifica­tion of potential problems and operational limita­tions, but also in the understanding of the relevant radiation effects, as well as materials selection and design accommodation to enable the materials lim­itations to be tolerated.

Following a brief introduction to the problem of radiation damage in both metals and insulators, the important aspect of simulating the operating envi­ronment for the component or material under exam­ination will be presented, with reference to present experimental procedures. The chapter will then con­centrate on the problems facing the use of insulators, with the radiation effects on the main physical prop­erties being discussed, concentrating in particular on the dielectric properties.