Degradation and defects in plasma facing components for future fusion devices

摘要

The main function of the first wall and the divertor are to remove the power generated by the plasma and to shield from neutrons. The plasma facing components (PFCs) are optimised the high heat flux energy removal. PFCs are composed of a thick armour joined to an actively cooled heat sink to provide the necessary transfer of the incident power to the cooling system. During normal operation these components have to dissipate a heat flux up to 5 MW/m² in the divertor and 0.5 MW/m² on the first wall. During short time off-normal events these loads can locally rise up to 20 MW/m². Consequently, only materials with excellent thermal properties and sufficient thermal shock resistance are tolerated in these regions. Three materials, beryllium, carbon fiber composites (CFCs) and tungsten, are selected as candidates for armour materials in the fusion facility ITER. For basic studies of PFC heat transfer properties inspection and quality control an infrared inspection facility (IRINA) has been installed. The impact of local differences in emissivity of the armour materials on temperature measurements was studied. To compensate local temperature variations originating from emissivity inhomogeneities of the most specimens a temperature correction method was applied successfully. On different components defect zones with reduced heat transfer properties could be detected. In the combination with FE-calculations a correlations between defects within the component and the measured temperature field was found. On this basis the minimum temperature difference between intact and defect zones for detection by IR analysis could be given. The heat transfer in defect areas of plasma facing components has been tested in the electron beam facility JUDITH under cyclic loads with different configurations. Special emphasis has been given on the thermal fatigue behavior of CFC flat tile divertor modules. Two regimes of surface temperature increase rate were detected. It was found that slow temperature increase characterizes small structure imperfections growing with thermal fatigue. A strong surface temperature increase indicated catastrophic crack propagation leading to armour detachment. The heat transfer reduction of beryllium armoured modules during cyclic loading has in general been not detected. The tested first wall modules did not shown degradation of heat transfer rate during 1000 cycles at 1.5 MW/m². It was shown that complete failure of beryllium tiles progressed with heat flux 2 MW/m² during a few seconds. The reasons of failure were found to be joint damages including cracks, the formation of intermetallic phases and high thermal stresses. Additionally, the neutron induced heat transfer degradation of tungsten and carbon-based modules was investigated. It was found that neutron irradiation did not reduced the heat transfer ability of tungsten armoured plasma facing components under static loads remarkable. But the heat transfer reduction of irradiated CFC modules was significant. It is caused by a strong decrease of thermal diffusivity of C-based materials after neutron irradiation. During cycling at loads of 10 MW/m² the surface temperature of irradiated CFC modules slightly decreased with time. It indicates an improvement of the that heat transfer properties due to annealing effects of armour material. The heat transfer degradation of irradiated modules due to thermal fatigue were observed at lower loads compared to non-irradiated reference samples.