Within the fusion community, there is an acute awareness of the necessity to construct a suitable irradiation testing facility for materials, which will enable both testing and development of materials for future fusion reactor devices with a fusion-like neutron spectrum. Within this context, both concep­tual and engineering design activities were under­taken during the 1990s within the IEA framework with the view of providing such a facility, the IFMIF (International Fusion Materials Irradiation Facility).46-50 This work has been recently renewed under the EU-Japan Broader Approach (BA) activ­ities with the EVEDA (Engineering Validation and Engineering Design Activities) tasks.51,52 However, at the present time no entirely suitable irradiation testing facility exists, and as a consequence experi­ments have been performed in nuclear fission reac­tors and particle accelerators, as well as g — and X-ray sources, in an attempt to simulate the real operating conditions of the insulating materials and compo­nents. The experiments required must simulate the neutron and g radiation field, that is, the displace­ment and ionization damage rates, the radiation envi­ronment, that is, vacuum and temperature, and also the operating conditions such as applied voltage, or mechanical stress. As will be seen, for the insulator physical properties, it is furthermore essential that in situ testing is carried out to determine whether or not the required physical properties of the material or component are maintained during irradiation. Examples of this include the electrical conductivity, which can increase many orders of magnitude due to the ionizing radiation, or optical windows, which may emit intense RL.

Experimental nuclear fission reactors clearly have the advantage of producing a radiation field consist­ing of both neutrons and g-rays, although in most cases the actual neutron energy spectrum and the dpa to ionization and He ratios are not those which will be experienced in a fusion reactor.50 However, it is worthwhile noting that to date experimental fission reactors have mainly been used for irradiations in the metals programs where the emphasis is on the neutron flux and little consideration is given to the g field. As a result, the irradiation channels have in general been designed and installed with this criterion. However, it should be possible to select positions within the reactors which, together with suitable neutron absorber materials and neutron to g converters, provide acceptable radiation fields. The main difficulties with in-reactor experiments come from the inaccessibility of the radiation volume and are concerned with the problem of carrying out in situ measurements and achieving the correct irra­diation environment. While considerable success has been attained in the in situ measurement require­ment, with parameters such as electrical conductivity, optical absorption and emission, and even radiofre­quency dielectric loss being determined, the problem of irradiating in vacuum still remains, with most experiments being performed in a controlled He environment. Irradiation in a controlled atmosphere such as He causes an immediate problem for in situ electrical and dielectric measurements because of the radiation-enhanced electrical conductivity of the gas,53 and even in the case of irradiation in vacuum at about 10~3mbar spurious leakage currents will occur.54 Furthermore, many in-reactor experiments rely on nuclear heating to reach the required temperature, and hence have difficulty maintaining a controlled temperature, in part because of the changes in the reactor power, and also because of the problem of calculating the final sample or component tempera­ture. These aspects will be further discussed later. One additional difficulty comes from the nuclear activation of the sample or component, which gener­ally means that postirradiation examination (PIE) has either to be carried out in a hot cell or postponed until the material can be safely handled.

Particle accelerators, on the other hand, are ideal for carrying out in situ experiments in high vacuum and at well-controlled temperatures because of the easy access and the very localized radiation field. High levels of displacement damage and ionization can be achieved with little or no nuclear activation. It is however in the nonnuclear aspect ofthe radiation field where their disadvantage is evident, and great care has to be taken to ensure that appropriate dis­placement rates are deduced to enable reliable com­parison with the expected fusion damage. A further serious disadvantage is due to the limited irradiation volume and particle penetration depth. This in gen­eral means that only small thin material samples or components can be tested.

The present-day situation of materials and com­ponent radiation testing for fusion applications takes full advantage not only of fission reactors and particle accelerators, but also 60Co g irradiation facilities and even X-ray sources. The use of such widely different radiation sources can be justified as long as the influ­ence of the type of radiation on the physical parame­ter of interest is known. This, in certain cases, is true for radiation-induced electrical conductivity and RL for example, where for low total fluences it is the ionizing component of the radiation field which is important. In situ measurements can now be made during irradiation of the important electrical, dielec­tric, and optical properties. In addition other aspects such as mechanical strength and tritium diffusion are being assessed during irradiation. Undoubtedly, suc­cessful modeling could be of help to address this diverse use of irradiation sources; however, general modeling for the insulators has hardly got off the ground because of the difficulties associated with describing radiation effects in polyatomic band — structured materials. As a result, in contrast to the extended activity for metallic structural materials, to date there has been no coordinated activity for the insulators, with only specific models for aspects such as electrical and thermal conductivity being developed.