There are little data available for irradiated tungsten (see Chapter 1.04, Effect of Radiation on Strength and Ductility of Metals and Alloys). Based on results for other refractory alloys and limited data on tungsten, one would expect neutron irradiation to increase the strength and decrease the ductility of the tungsten armor largely through increases in the DBTT. To minimize the neutron-induced material

degradation, it is reasonable to limit the operational conditions for components in a neutron environment to temperatures above ^900 °C where recovery of tungsten takes place,107 as the ductility loss is more pronounced below about 0.3 Tm. This is possible in the region close to the plasma-facing surface, but it is impossible in the heat sink region as tungsten will be in contact with materials that cannot operate at this temperature and stress concentrations in these ‘cold’ areas have to be avoided.108 In the case of ITER, Cu will be employed in the heat sink while steel is more likely to be used in DEMO, which has a higher operating temperature.2 Hence, a greater under­standing of the irradiation response of tungsten at temperatures between 700 and 1000 °C is

needed.109,110 The effect of embrittlement is alle­viated when operating above 250 °C, although in the presence of He (produced by transmutation reactions) somewhat higher temperatures may be required.83

Although at intermediate temperatures (0.3-0.6 Tn), void swelling and irradiation creep are the dominant effects of irradiation, the amount of volumetric swelling associated with void formation in refractory alloys is generally within engineering design limits (<5%) even for high neutron fluences (^10dpa). Very little experimental data exist on irradiation creep of refractory alloys, but data for other bcc alloys suggest that the irradiation creep will produce negligi­ble deformation for near-term reactor applications.110