R. Duwe

Overview of the EU Small Scale Mock-up Tests for ITER High Heat Flux Components

Abstract This task within the EU R&D for ITER was aimed at the development of basic manufacturing solutions for the high heat flux plasma facing components such as the divertor targets, the baffles and limiters. More than 50 representative small-scale mock-ups have been manufactured with beryllium, carbon and tungsten armour using various joining technologies. High heat flux testing of 20 of these mock-ups showed the carbon mono-blocks to be the most robust solution, surviving 2000 cycles at absorbed heat fluxes of up to 24 MW m−2. With flat armour tiles rapid joint failures occurred at 5–16 MW m−2 depending on joining technology and armour material. These test results serve as a basis for the selection of manufacturing options and materials for the prototypes now being ordered.

Performance of beryllium, carbon, and tungsten under intense thermal fluxes

Abstract Transient heat loads on a millisecond timescale with deposited energy densities beyond 1 MJ m −2 have been simulated in a plasma accelerator facility (VIKA) and in two high power electron beam teststands (JUDITH, JEBIS). Main objective of these experiments was to study and to compare the behaviour of different plasma facing materials (Be, CFC, W) under heat loads which occur during disruptions in future thermonuclear fusion reactors such as ITER. In these tests special attention was paid to the thermal shock resistance, the processes during melt layer formation, and the resulting material erosion. To perform these tests specific loading techniques and diagnostics have been developed and applied. Among these are high heat flux loading experiments at elevated temperatures ( T > DBTT ) of the test coupons, fast surface pyrometry, and reliable techniques for the quantification of the absorbed energy.

European Development of the ITER Divertor Target

Abstract The main European contribution to the ITER divertor project was the development of the divertor target with severe operating requirements such as peak heat loads of up to 20 MW/m2. This development involving EU laboratories and industry included R&D on armour materials, thermo-hydraulics testing, component manufacture, high heat flux testing, design and manufacture of prototypes for later testing. The 4-year EU R&D effort achieved the demonstration of the feasibility of a robust divertor target design based on carbon and tungsten armour. This EU solution has eventually been adopted for the ITER reference design and could be valid also for other ITER high heat flux components such as limiters or baffles.

Material damage to beryllium, carbon, and tungsten under severe thermal shocks

Abstract Material damage and erosion of plasma facing material and components due to severe thermal shocks which occur during disruptions and vertical displacement events have been investigated experimentally using electron beam devices. The deposited energy densities were ⩽12 MJ m−2 (disruption simulation) and 60 MJ m−2 (VDE simulation); effective pulse durations were 5 ms and 1 s, respectively. The resulting material degradation was determined by weight loss measurements, profilometry, metallography and scanning electron microscopy.

Plasma-sprayed boron carbide coatings for first-wall protection

Abstract Plasma-sprayed boron carbide coatings have been manufactured by different suppliers onto substrates of type 316L stainless steel. The coating thickness ranges from 0.3 to 2.0 mm. The larger thicknesses could only be achieved by application of an adaptive or gradient bond-layer between substrate and the boron carbide top coating. Measurements of the thermal diffusivity of coating materials are reported. Several high heat flux facilities have been used to study the thermal shock and erosion behaviour of the coated samples. A supporting numerical analysis of the thermal behaviour of the coating under normal and off-normal heat loads is presented, focussing on the differences between electron beam and laser beam tests due to volumetric energy deposition. Some aspects of the applicability of plasma sprayed B 4 C coatings for first-wall protection in a next step device are discussed.

Thermomechanical behavior of actively cooled, brazed divertor components under cyclic high heat flux loads

Actively cooled divertor mock-ups consisting of various low-Z armor tiles brazed to refractory metal heat sinks were tested in the electron beam test facility at Julich. Screening and thermal cycling tests were perfomed on the mock-ups to estimate the overall thermal performance under cyclic high heat flux (HHF) loadings. By detecting the temperature of the armor surface and the braze layer, it was possible to assess the heat removal capability and the accumulation of interfacial damage. Microstructures were investigated to elucidate the degradation of the joints. Finite element analyses are carried out for the simulated HHF test conditions. Temperature fields and thermal stresses are calculated for a typical divertor module. The nature of thermomechanical behavior of the divertor mock-ups under cyclic HHF loadings is discussed.

Thermal Fatigue Tests with Actively Cooled Divertor Mock-ups for ITER

Abstract Mock-ups for high heat flux components with beryllium and CFC armour materials have been tested by means of the electron beam facility JUDITH. The experiments concerned screening tests to evaluate heat removal efficiency and thermal fatigue tests. CFC monoblocks attached to DS-Cu (Glidcop Al25) and CuCrZr tubes by active metal casting and Ti brazing showed the best thermal fatigue behaviour. They survived more than 1000 cycles at heat loads up to 25 MW m−2 without any indication of failure. Operational limits are given only by the surface temperature on the CFC tiles. Most of the beryllium mock-ups were of the flat tile type. Joining techniques were brazing, hot isostatic pressing (HIP) and diffusion bonding. HIPed and diffusion bonded Be/Cu modules have not yet reached the standards for application in high heat flux components. The limit of this production method is reached for heat loads of approximately 5 MW m−2. Brazing with and without silver seems to be a more robust solution. A flat tile mock-up with CuMnSnCe braze was loaded at 5.4 MW m−2 for 1000 cycles without damage. The first test with a beryllium monoblock joined to a CuCrZr tube by means of Incusil brazing shows promising results; it survived 1000 cycles at 4.5 MW m−2 without failure.

Thermal Fatigue Testing of an ITER Primary Wall Small Scale Mock up

Abstract A first primary wall small scale mock up with beryllium as the armor material was manufactured by hot isostatic pressing (HIPing) by CEA and tested at the electron beam test facility JUDITH. The mock up consisted of a 9.4 mm thick beryllium armor joint onto a 20 mm thick dispersion strengthened copper (DS-Cu) alloy plate. Type 316L Stainless Steel tubes, 10/12 mm in diameter were embedded in the DS-Cu, which was subsequently joined onto a 30 mm thick 316LN stainless steel plate. The mock up was tested under a surface heat flux of 2.5 MW m −2 for 100 preparatory cycles and 900 cycles of 30 s heating and 30 s cooling time. At the end of the 1000 cycles, the surface and the Be/DS-Cu joint of the mock up did not show any damage due to the fatigue test.

Boron carbide-based coatings on graphite for plasma facing components

In the effort to evaluate boron-rich coatings as plasma facing surfaces in fusion devices, a new process for applying boron carbide (B{sub 4}C) coatings to graphite was developed. The process entails eutectic melting of the carbon (C) substrate surface with a precursor layer of B{sub 4}C particles. Adherent coatings were achieved which consisted of two layers: a surface layer and a graded penetration zone in the outer portion of the substrate. The surface-layer microstructure was multiphase and ranged from reaction-sintered structures of sintered B{sub 4}C particles in an eutectic-formed matrix to that of hypereutectic carbon particles in a B{sub 4}C-C eutectic matrix. Because of high surface energy, the coating generally developed a nonuniform thickness. Quantitative evaluations of the coating were performed with limiters in the TEXTOR fusion device and with coupons in electron beam tests. Test results revealed the following: good adherence of the coating even after remelting; and, during remelting, diagnostics detected a corresponding interaction of boron with the plasma.

Testing of actively cooled high heat flux mock-ups

Abstract Several un-irradiated CFC monoblock mock-ups have been loaded in thermal fatigue tests up to 1000 cycles at power densities 2 . No indication of failure was observed for these loading conditions. Two of the mock-ups were inspected by ultra-sonic methods before thermal cycling. It could be proved that the voids found in the post-mortem metallography existed before and had no effect on the integrity of the mock-up. For the first time, neutron-irradiated CFC monoblock mock-ups have been tested in the electron beam facility JUDITH. These mock-ups had been irradiated before in the High Flux Reactor at Petten up to 0.3 dpa at 320°C and 770°C. All samples showed a significant increase of surface temperature, due to the irradiation induced decrease in thermal conductivity of the CFC materials.