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%0 Thesis
%A Wirtz, Oliver Marius
%T Thermal shock behaviour of different tungsten grades under varying conditions
%C Aachen
%I Publikationsserver der RWTH Aachen University
%M RWTH-CONV-124387
%P XIV, 130 S. : Ill., graph. Darst.
%D 2012
%Z Zsfassung in dt. und engl. Sprache
%Z Aachen, Techn. Hochsch., Diss., 2012
%X Thermonuclear fusion power plants are a promising option to ensure the energy supply for future generations, but in many fields of research enormous challenges have to be faced. A major step on the way to the prototype fusion reactor DEMO will be ITER which is build in Cadarache, southern France. One of the most critical issues is the field of in-vessel materials and components, in particular the plasma facing materials (PFM). PFMs that will be used in a device like ITER have to withstand severe environmental conditions in terms of steady state and transient thermal loads as well as high particle fluxes such as hydrogen, helium and neutrons. Candidate wall materials are beryllium, tungsten and carbon based materials like CFC (carbon fibre composite). Tungsten is the most promising material for an application in the divertor region with very severe loading conditions and it will most probably also be used as PFM for DEMO. Hence, this work focuses on the investigation of the thermal shock response of different tungsten grades in order to understand the damage mechanisms and to identify material parameters which influence this behaviour under ITER and DEMO relevant operation conditions. Therefore the microstructure and the mechanical and thermal properties of five industrially manufactured tungsten grades were characterised. All five tungsten grades were exposed to transient thermal events with very high power densities of up to 1.27 GW/m² at varying base temperatures between RT and 600°C in the electron beam device JUDITH 1. The pulse numbers were limited to a maximum of 1000 in order to avoid immoderate workload on the test facility and to have enough time to cover a wide range of loading conditions. The results of this damage mapping enable to define different damage and cracking thresholds for the investigated tungsten grades and to identify certain material parameters which influence the location of these thresholds and the distinction of the induced damages. Furthermore the grain structure and the recrystallisation of the material have a significant influence on the thermal shock damage, especially the cracking pattern and surface roughening. Beside the thermal shock damage mapping tungsten was also successively exposed to steady state high flux hydrogen-plasma and to cyclic thermal shock events simulated with an electron beam. The induced damages were investigated to determine if the exposure to hydrogen-plasma has an influence on the thermal shock response of tungsten. Special attention was paid to the thermal shock crack parameters such as distance, width and depth. The investigations showed that there is a significant influence on the damage behaviour of tungsten, especially if the tungsten targets are pre-loaded with hydrogen plasma. Beside the sequence of the exposure also the surface temperature during the plasma loading shows a clear influence on the thermal shock behaviour. One of the most obvious changes is that due to the pre-exposure with hydrogen the thermal shock cracks are not limited to the electron beam loaded area anymore. They propagate through the only plasma loaded surface and even penetrate into the unexposed area at the edges of the tungsten targets. Additionally, all crack parameters decrease if the targets are pre-loaded with hydrogen plasma. The combination of these results shows that the thermal shock response of tungsten as a PFM is very complex and depends on a wide range of parameters. It can be very well characterised by electron beam tests, but it has to be taken into account that these tests only simulate thermal loads without any influence of particle exposure. The influence of high flux hydrogen-plasma exposure on the thermal shock behaviour of tungsten can be estimated based on the results obtained from successive loading experiments. They give a good overview of the expected damages on tungsten as a PFM. These damages on the surface of the materials which are induced by high power densities and in combination with particle fluxes are detrimental for the operation of next step fusion devices such as ITER and DEMO and will have significant impact on the operational limits for large pulse numbers of 1,000,000 or more as expected in ITER.
%K Wolfram (SWD)
%K Sinterwerkstoff (SWD)
%K Material (SWD)
%K Thermische Belastung (SWD)
%K ITER (SWD)
%K Metall-Wasserstoff-System (SWD)
%K Elektronenstrahl (SWD)
%K Fusionsplasma (SWD)
%K Kernfusion (SWD)
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%U https://publications.rwth-aachen.de/record/62897