High pulse number thermal shock testing of tungsten alloys produced by powder injection molding

The investigation of plasma facing materials (PFM) subjected to a large number (≥10,000) of thermal shocks is of interest to determine long term morphological changes which might influence component lifetime in and plasma performance of a fusion reactor. The electron beam facility JUDITH 2 was used to simulate these conditions experimentally. In this study eight different tungsten grades produced by powder injection molding (PIM) were investigated: Two pure tungsten grades, one with 2 wt% Y₂O₃, three with 1, 2 and 3 wt% TiC, and two with 0.5 and 1 wt% TaC. Samples of 10 × 10 × 4 mm³ were brazed to a copper cooling structure and subjected to 10⁵ thermal shocks of 0.5 ms duration and an intensity of L$_{abs}$=0.55 GW/m² (F$_{HF}$=12 MWs½/m2) at a base temperature of T$_{base}$ = 700 °C. The PIM grades showed damages in general comparable with a sintered and forged pure tungsten reference grade (>99.97 wt% W) that complies with the ITER specifications. One exception was the 2 wt% TiC doped material which failed early during the experiment by delamination of a large part of the surface. The Y₂O₃ doped material showed a comparatively good performance with respect to crack width (<15 μm) and roughening (R$_{a}$ = 0.75 μm), but showed melt droplets of ∼3–4 μm diameter, while the 1 wt% TiC doped material showed wide cracks (up to 50 μm) and strong roughening (R$_{a}$ = 2.5 μm). The paper discusses the post-mortem analysis of all grades, comparing them with respect to roughness (from laser profilometry), crack network characteristics and local melt droplet formation or other special morphological features (from SEM images) as well as crack depth (from metallographic cross sections)

Impact of Microstructure on the Plasma Performance of Industrial and High-End Tungsten Grades

Tungsten and tungsten alloys are actually the primary choice as plasma facing materials for future fusion reactors. Thereby, the material’s response to the different loading conditions occurring in a tokamak is strongly depending on the material properties and therefore the material’s microstructure. This is on the one hand controlled via the manufacturing process and/or the material’s composition and on the other hand by the operational conditions causing recrystallization and melting, and subsequently not only a modified microstructure but also locally a modified composition. The influence of the variation in microstructure is addressed and the pros and cons for using the respective materials and tungsten in general in a fusion environment with steady state and transient thermal loads are outlined. While roughening and the related cracking can hardly be avoided, melting will thwart all efforts to establish a high end microstructure with defined directional properties

Neutron irradiation effects on the low cycle thermal fatigue performance of first wall mock-ups

For the first wall (FW) of ITER, beryllium will be the plasma facing material and has to sustain high thermal, particle and neutron loads. In order to assess the synergistic effects of thermal and neutron loads in total ten Be-armored FW normal heat flux (NHF) mock-ups were produced consisting of two Be flat tiles each joined via hot isostatic pressing (HIP) to a CuCrZr heat sink and a steel support structure by different manufacturing routes to assess the most promising ones. Five of them were neutron irradiated, two at Centrum Výzkumu Řež in Czech Republic and three in the High Flux Reactor at NRG in the Netherlands. The remaining mock-ups were kept as reference. All flat-tile mock-ups had two Be-tiles and were exposed to cyclic steady state heat loads in the electron beam facilities JUDITH 1 and 2 up to a maximum power density of 3.75 MW/m2, in order to find the damaging threshold. A screening step using 1 MW/m2 was performed after finishing each loading step and after failure for direct comparison and detection of any deterioration caused by the cycling. Despite being still in the early development phase of Be joining, all mock-ups sustained the cycling up to at least 2.75 MW/m2 and clear differences in the performance of irradiated vs. non-irradiated mock-ups were observed

TEM analysis of recrystallized double forged tungsten after exposure in JUDITH 1 and JUDITH 2

Five samples of recrystallized pure tungsten were exposed to transient heat loads using the electron beam of the JUDITH 1 and JUDITH 2 installations of Forschungszentrum Jülich. The heat flux and base temperature were the same for all samples; only the number of pulses and exposure device differed. Transmission electron microscopy was applied to determine the first defects that are introduced during exposure and to compare the effects of the two machines. With increasing number of pulses, first dislocations are formed near the grain boundaries, and then line dislocations and clusters of dislocations appear within the grains. Upon prolonged exposure, the dislocations migrate and cluster in dislocation pile-ups. Comparing exposure in JUDITH 1 to JUDITH 2, the amount of defects is much higher in the samples exposed in JUDITH 1

Performance Estimation of Beryllium Under ITER Relevant Transient Thermal Loads

The plasma facing first wall in ITER will be armored with beryllium. During operation, the armor has to sustain direct plasma contact during the start-up and ramp-down of the plasma. On top, transient thermal loads originating from a variety of plasma instabilities or mitigation systems are impacting the 8–10 mm thick beryllium tiles. In this work, possible armor thickness losses caused by the expected transient heat loads are reviewed. Applying conservative assumptions, vertical displacement events can cause locally a melt layer with a thickness of up to 3 mm. However, cracks after solidification/cool down are confined to the melt layer and the connection between melt layer and bulk remains strong. Radiative cooling mechanisms can be applied to significantly decrease the melt and evaporation layer thickness. To mitigate the critical damage potential of plasma disruptions, massive gas injections or shattered pellet injections can be deployed to transform the stored plasma energy into radiation, which implies a much more homogeneous distribution of energy to the plasma facing components. For a full power plasma discharge in ITER, these radiative loads can cause temperatures exceeding the melting temperature of beryllium. Experiments have demonstrated that a thickness of 340 µm at the entire first wall armor can be affected by these mechanisms over the lifetime of ITER. Edge localized modes with expected characteristics obtained by fluid model simulations caused fatigue cracks with a depth of up to 350 µm in experimental simulations. The critical heat flux factor FHF above which inflicted damage accumulates with each subsequent pulse has been determined to be in the range of FHF ≈ 9–12 MW m−2 s0.5. The damage from thermal loads below this threshold saturates between 104 and 106 pulses. Neutron irradiation has a deteriorating effect on the thermomechanical properties of beryllium, which strongly influence its resistance against thermally induced damages. The rather low neutron fluence over the lifetime of ITER is expected to reduce the material strength and thermal conductivity by a few tens of percents. If the thickness losses are affected to a similar extent, a sufficient margin of armor thickness will remain. Overall, the damage imposed by radiative loads from massive gas injections or shattered pellet injections is expected to be the dominant force influencing the condition of the first wall armor, at least if all disruptions can be successfully mitigated and the number of vertical displacement events can be constrained to a few occurrences over the service time of ITER

Transient heat load challenges for plasma-facing materials during long-term operation

The study summarizes the experimental results on fusion relevant pure heat load exposures of different tungsten products in the electron beam devices JUDITH 1 and 2. Besides steady state heat loading, up to 106 transient ELM-like pulses were applied. A detailed postmortem analysis reveals a wide and complex range of thermally-induced surface modifications and damages, such as roughening due to plastic deformation, cracking, and melting of parts of the material surface. Different industrially available tungsten products with varying thermal and mechanical properties were investigated in order to examine their influence on the thermal shock response. Furthermore, recrystallisation of the material, which will take place during long term operation, will additionally deteriorate the mechanical strength of the plasma facing material. The results show that the mechanical strength of the material has a significant influence on the formation and evolution of damage. Especially, recrystallisation and melting/resolidification will make the material more prone to thermal shock and fatigue, accelerating the evolution of damages. The combination of different material modifications/damages accompanied by the degradation of mechanical properties will have a strong impact on the plasma performance and lifetime of plasma facing materials/components

High Pulse Number transient heat loads on beryllium

The experimental fusion reactor ITER will apply beryllium as first wall armor material. In present fusion experiments, e.g. ASDEX Upgrade, it has been detected that up to 25% of the plasma energy loss is deposited in non divertor regions during edge localized mode (ELM) events. Therefore, the impact of transient heating events on beryllium needs to be investigated to reliably predict the performance of the beryllium armor tiles under ITER operational conditions. In the present experiments, the electron beam facility JUDITH 2 was used to exert transient heat pulses with power densities of 0.14–1.0 GW m−2, as they can be expected for mitigated Type 1 ELMs in ITER, pulse durations in the range of 0.08–1.0 ms, and a number of pulses in the range of 103–107 on S-65 beryllium specimens that were brazed on an actively cooled copper structure. Thereby, a strong drop of the melting threshold was discovered from a heat flux factor FHF = 22–25 MW m−2s0.5 for 102 pulses to FHF < 12 MW m−2s0.5 for 103 pulses. However, a saturation of the thermally induced damage was observed for FHF ≤ 9 MW m−2s0.5 after 105 pulses. This result indicated a promising performance of beryllium under a high number of transient heat pulses in ITER. Nevertheless, the synergistic effects between thermal loads, particle loads, and neutron irradiation might affect the saturation threshold and need to be investigated in future studies