Mechanical and microstructural investigations of tungsten and doped tungsten materials produced via powder injection molding

The physical properties of tungsten such as the high melting point of 3420°C, the high strength and thermal conductivity, the low thermal expansion and low erosion rate make this material attractive as a plasma facing material. However, the manufacturing of such tungsten parts by mechanical machining such as milling and turning is extremely costly and time intensive because this material is very hard and brittle. Powder Injection Molding (PIM) as special process allows the mass production of components, the joining of different materials without brazing and the creation of composite and prototype materials, and is an ideal tool for scientific investigations. This contribution describes the characterization and analyses of prototype materials produced via PIM. The investigation of the pure tungsten and oxide or carbide doped tungsten materials comprises the microstructure examination, element allocation, texture analyses, and mechanical testing via four-point bend (4-PB). Furthermore, the different materials were characterized by high heat flux (HHF) tests applying transient thermal loads at different base temperatures to address thermal shock and thermal fatigue performance. Additionally, HHF investigations provide information about the thermo-mechanical behavior to extreme steady state thermal loading and measurements of the thermal conductivity as well as oxidation tests were done. Post mortem analyses are performed quantifying and qualifying the occurring damage with respect to reference tungsten grades by metallographic and microscopical means

Impact of materials technology on the breeding blanket design – Recent progress and case studies in materials technology

A major part in the EUROfusion materials research program is dedicated to characterize and quantify nuclear fusion specific neutron damage in structural materials. While the majority of irradiation data gives a relatively clear view on the displacement damage, the effect of transmutation – i.e. especially hydrogen and helium production in steels – is not yet explored very well. However, few available results indicate that EUROFER-type steels will reach their operating limit as soon as the formation of helium bubbles reaches a critical amount or size. At that point, the material would fail due to embrittlement at the considered load. This paper presents a strategy for the mitigation of the before-mentioned problem using the following facts: • the neutron dose and related transmutation rate decreases quickly inside the first wall, that is, only a plasma-near area is extremely loaded • nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium and hydrogen, which would suppress the formation of large helium bubbles • compared to conventional steels, ODS steels show improved irradiation tensile ductility and creep strength In summary, producing the plasma facing, highly neutron and heat loaded part of blankets by an ODS steel, while using EUROFER97 for everything else, would allow a higher heat flux as well as a longer operating period. Consequently, we (1) developed and produced 14 % Cr ferritic ODS steel plates. (2) We fabricated a mockup with 5 cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. And finally, (3) we performed high heat flux tests in the HELOKA facility (Helium Loop Karlsruhe at KIT) applying short and up to 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e., 550 °C) was finally exceeded by 100 K. Thereafter, microstructure and defect analyses did not reveal defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. This demonstrates that the use of ODS steel could make a decisive difference in the future design and performance of breeding blankets

Fabrication routes for advanced first wall design alternatives

In future nuclear fusion reactors, plasma facing components have to sustain specific neutron damage. While the majority of irradiation data provides a relatively clear picture of the displacement damage, the effect of helium transmutation is not yet explored in detail. Nevertheless, available results from simulation experiments indicate that 9%-chromium steels will reach their operating limit as soon as the growing helium bubbles extent a critical size. At that point, the material would most probably fail due to grain boundary embrittlement. In this contribution, we present a strategy for the mitigation of the before-mentioned problem using the following facts. (1) The neutron dose and related transmutation rate decreases quickly inside the first wall of the breeding blankets, that is, only a plasma-near area is extremely loaded. (2) Nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium, which would suppress the formation of large helium bubbles for a much longer period. (3) Compared to conventional steels, ODS steels also provide improved irradiation tensile ductility and creep strength. Therefore, a design, based on the fabrication of the plasma facing and highly neutron and heat loaded parts of blankets by an ODS steel, while using EUROFER97 for everything else, would extend the operating time and enable a higher heat flux. Consequently, we (i) developed and produced 14%Cr ferritic ODS steel plates and (ii) optimized and demonstrated a scalable industrial production route. (iii) We fabricated a mock-up with five cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. (iv) Finally, we performed high heat flux tests in the Helium Loop Karlsruhe, applying a few hundred short and a few 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e. 550 ◦C) was finally exceeded by 100 K. (v) Thereafter, microstructure and defect analyses did not reveal critical defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. However, a solution to prohibit the formation of such heat affected zones is given. These research contributions demonstrate that the use of ODS steel is not only feasible and affordable but could make a decisive difference in the future design and performance of breeding blankets

Exposures of EU W-CFC combined targets with QSPA Kh-50 plasma streams simulating ITER ELMs

Repeated load tests of special combined W-CFC samples were performed with QSPA plasma streams either resulting in strong melting of W surface layer or below the melting, but above the cracking threshold. Experiments show that in result of target exposures with heat load of 0.4 MJ/m2 (no melting) only cracks formation was found on both tungsten and CFC surfaces. It is obtained that enhanced evaporation of CFC results in additional shielding of tungsten surface by C cloud and protects W surface from evaporation even for essentially increased energy density in impacting plasma. Exposures of combined target with heat loads of 0.82 MJ/m2 resulted in strong melting of tungsten. Meshes of macro-cracks and micro-cracks as well as ripple structures are appeared on the resolidified surface.Проведено циклічні іспити спеціальних комбінованих зразків W-CFC з використанням плазмових потоків КСПП з варійованими енергетичними навантаженнями, які приводять до розвитого плавлення поверхневого шару W, або знаходяться нижче порога плавлення, але вище порога розтріскування. Експериментально показано, що в результаті опромінення мішеней з густиною енергії 0,4 MДж/м2 (відсутність плавлення) було зареєстровано тільки розтріскування поверхонь CFC і вольфраму. Зі збільшенням густини енергії розвинуте паротворення CFC приводить до додаткового екранування поверхні вольфраму шаром вуглецевої плазми і захищає поверхню W від паротворення навіть при істотно збільшеній густині енергії в плазмі, що налітає. Опромінення мішеней з тепловими навантаженнями 0,82 MДж/м2 приводить до інтенсивного плавлення вольфраму. Сітки макро-тріщин і мікро-тріщин, а також хвильова структура з'являються на повторно затверділій поверхні.Проведены циклические испытания специальных комбинированных образцов W-CFC с использованием плазменных потоков КСПУ с варьируемыми энергетическими нагрузками, которые приводят к развитому плавлению поверхностного слоя W, либо находятся ниже порога плавления, но выше порога растрескивания. Экспериментально показано, что в результате облучения мишеней плазменными потоками с плотностью энергии 0,4 MДж/м2 (отсутствие плавления) было зарегистрировано лишь растрескивание поверхностей CFC и вольфрама. С увеличением плотности энергии в плазменном потоке развитое парообразование CFC приводит к дополнительной экранировке поверхности вольфрама облаком углеродной плазмы и предохраняет поверхность W от испарения даже при существенно возросшей плотности энергии налетающей плазмы. Облучение мишеней с тепловыми нагрузками 0,82 MДж/м2 приводит к интенсивному плавлению вольфрама. Сетки макро-трещин и микро-трещин, а также волновые структуры появляются на повторно затвердевающей поверхности