Plastische vervorming van wolfraam onder blootstelling aan een fusie-plasma-omgeving

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 11-06-2018Using Nuclear Fusion to produce price-competitive electricity is an important element in the European Research and Development Agenda. The next big step is to create ‘first plasma’ in ITER – the key facility for development of commercial fusion power. While this fusion test-bed is under construction, a number of technical and scientific challenges remain problems that must be solved to enable the efficient and safe operation of ITER. Ensuring proper functioning of the divertor is one of those challenges. The field lines of a diverted plasma deflect particles, in particular impurities, in the scrape-off layer (the region from the vessel region up to the magnetic separatrix, which is the boundary of the confined plasma region) towards the divertor, where they are to be neutralized and removed through a pumping system. The divertor exhausts the heat produced by the fusion reactions and enables the removal of helium and other impurities by apumping system, hence it is a vital component of the whole system...Het gebruik van kernfusie om prijsconcurrentiële elektriciteit te produceren is eenbelangrijk element in de Europese Onderzoeks- en Ontwikkelingsagenda. De volgendegrote stap is het creëren van het eerste plasma in ITER - de belangrijkste faciliteit voorde ontwikkeling van commerciële fusiecentrales. Terwijl deze fusie testreactor inaanbouw is, blijven er een aantal technische en wetenschappelijke uitdagingen,problemen die moeten worden opgelost om de efficiënte en veilige werking van ITERmogelijk te maken.Het verzekeren van een goede werking van de divertor is een van die uitdagingen. Develdlijnen van een afgeleid plasma leiden de deeltjes om, in het bijzonderonzuiverheden, in de afscheidingslaag (het gebied in het vacuümvat tot aan demagnetische separatrix, die de grens van het afgesloten plasmagebied vormt) naar dedivertor, waar ze worden geneutraliseerd en verwijderd via een pompsysteem. Dedivertor vormt de uitlaat voor de warmte die door de fusie reacties wordt veroorzaakten maakt het mogelijk om He en andere onzuiverheden te verwijderen doo..Fac. de Ciencias FísicasTRUEunpu

Microstructural modifications in tungsten induced by high flux plasma exposure : TEM examination

We have performed microstructural characterization using transmission electron microscopy (TEM) techniques to reveal nanometric features in the sub-surface region of tungsten samples exposed to high flux, low energy deuterium plasma. TEM examination revealed formation of a dense dislocation network and dislocation tangles, overall resulting in a strong increase in the dislocation density by at least one order of magnitude as compared to the initial one. Plasma-induced dislocation microstructure vanishes beyond a depth of about 10 mu m from the top of the exposed surface where the dislocation density and its morphology becomes comparable to the reference microstructure. Interstitial edge dislocation loops with Burgers vector a(0)/2 and a(0) were regularly observed within 6 mu m of the sub-surface region of the exposed samples, but absent in the reference material. The presence of these loops points to a co-existence of nanometric D bubbles, growing by loop punching mechanism, and sub-micron deuterium flakes, resulting in the formation of surface blisters, also observed here by scanning electron microscopy

Depth-Sensing Hardness Measurements to Probe Hardening Behaviour and Dynamic Strain Ageing Effects of Iron during Tensile Pre-Deformation

This work reports results from quasi-static nanoindentation measurements of iron, in the un-strained state and subjected to 15% tensile pre-straining at room temperature, 125 °C and 300 °C, in order to extract room temperature hardness and elastic modulus as a function of indentation depth. The material is found to exhibit increased disposition for pile-up formation due to the pre-straining, affecting the evaluation of the mechanical properties of the material. Nanoindentation data obtained with and without pre-straining are compared with bulk tensile properties derived from the tensile pre-straining tests at various temperatures. A significant mismatch between the hardness of the material and the tensile test results is observed and attributed to increased pile-up behaviour of the material after pre-straining, as evidenced by atomic force microscopy. The observations can be quantitatively reconciled by an elastic modulus correction applied to the nanoindentation data, and the remaining discrepancies explained by taking into account that strain hardening behaviour and nano-hardness results are closely affected by dynamic strain ageing caused by carbon interstitial impurities, which is clearly manifested at the intermediate temperature of 125 °C

Effect of statistically stored dislocations in tungsten on the irradiation induced nano-hardening analyzed by different methods

Tungsten self-ion irradiation was performed at 800 °C up to 0.01-1 dpa on two different W grades with essentially different dislocation density. Nanoindentation was applied to characterize the radiation hardening in two W grades with different microstructure. Different methods to analyze the indentation curves were applied to extract the bulk equivalent radiation hardening. It was shown that depending on the applied method, different outcomes may occur. The most satisfactory procedure was established and a consistent set of parameters was found. The bulk equivalent radiation hardening was found to saturate above 0.1 dpa. The characteristic distance between irradiation induced defects acting as dislocation pinning points was found to decrease up to 0.1 dpa, and then saturate/increase with irradiation dose. No essential difference in radiation hardening was observed between the studied W grades with essentially different initial dislocation density

Assessment of mechanical properties of SPS-produced tungsten including effect of neutron irradiation

Production and supply of tungsten for the first wall fusion application is becoming an important aspect given the progress of ITER construction. Exploration of advanced routes alternative to the conventional powder metallurgy is currently undertaken. In this work we have assessed a potential of the spark plasma sintering (SPS) production route to deliver well controlled microstructure, chemistry and mechanical properties of bulk tungsten as a first step. SPS-produced tungsten was sintered at 2000 °C and was characterized in terms of mechanical properties, namely: tensile, three point bending and fracture toughness data in the temperaure range of 250–600 °C. Then, neutron irradiation was performed at 600 °C and the change of the fracture toughness was measured after irradiation together with the characterization of the fracture surface. The results are compared with those obtained for the commerically produced swaged tungsten irradiated and tested in equivalent conditions. The obtained results show that SPS technology offers the production of bulk tungsten with a good potential for further optimization (by e.g. swaging/rolling). Neutron irradiation causes the reduction of the fracture toughness comparable to the one induced in the commercially produced tungsten

Effect of high flux plasma exposure on the micro-structural and mechanical properties of ITER specification tungsten

We have performed a combined study using transmission electron microscopy (TEM), nuclear reaction analysis (NRA) and nano-indentation (NI) techniques to reveal the impact of high flux plasma exposure on the properties of a sub-surface region of the commercially available pure tungsten fabricated following the ITER specification. TEM examination revealed the formation of a dense dislocation network and dislocation tangles, resulting in a strong increase in the dislocation density by at least one order of magnitude as compared to the bulk density. The plasma-induced dislocation microstructure vanishes within a depth of about 10-15 mu m from the top of the exposed surface. Surface hardness after the plasma exposure was characterized by NI and was found to increase significantly in the sub-surface region of 1.5-3 mu m. That was attributed to the resistance of the plasma-induced dislocation networks and deuterium-induced defects, whose presence within a depth of similar to 1 mu m was unambiguously detected by the NRA measurements as well. (C) 2016 Elsevier B.V. All rights reserved

Thermal desorption spectroscopy of deformed and undeformed tungsten after exposure to a high-intensity plasma flow

As a result of the exposure of tungsten to a high-intensity plasma flow, it is established that the exposure of recrystallized and plastically deformed samples leads to fundamentally different mechanisms of confinement of plasma particles and associated deformation of the surface. The surface of the exposed deformed samples contains micrometer-sized ruptured blisters: an indication of the formation of subsurface bubbles on a grid of dislocations forming during deformation. Desorption spectra of both types of sample are decomposed into three peaks, corresponding to the detachment of plasma-gas particles from dislocations, deuterium-vacancy clusters, and pores. Plastic deformation, which leads to an increase in the dislocation density, does not change the position of the three peaks in the desorption spectra but increases their amplitude in comparison with the recrystallized material