High heat flux test results for a thermal break DEMO divertor target and subsequent design and manufacture development

Abstract

This paper focuses on the development of the water-cooled divertor target concept known as Thermal Break, which was carried out in two phases. In Phase 1, six small scale mock-ups were fabricated and subjected to high heat flux (HHF) testing of up to 25 MW/m 2 and thermal cycling of up to 500 cycles at 20 MW/m 2. All six mock-ups survived the campaign and maintained 20 MW/m 2 heat exhaust capability. Detailed examination of mock-ups was carried out to understand the damage mechanisms. One mock-up, which was tested beyond its design intent at 500 cycles, shows signs of progressive damage. Potential damage modes were identified and influenced subsequent Phase 2 mock-up design. Although there are signs of tungsten surface cracking, the predominant damage mode is not by "deep cracking" but substantial permanent deformation in the interlayer features. Therefore, in Phase 2 the manufacturing procedure was updated, the interlayer grooves were given stress-relieving radii which have significantly reduced the interlayer plastic strain range. Interlayer design parameters were selected following the use of response surface-based design search and optimization. Mock-ups of the Phase 2 design have been manufactured and HHF testing is planned within 2018.

Small-angle neutron scattering from CuCrZr coupons and components

“…The brazing cycle for the MUPs is shown in Fig. 1(b) -the difference between MUPs A and B is that the CuCrZr pipe in 'B' has undergone this cycle twice, to improve braze flow and adhesion after removal of the segregated Zr at the surface according to prior experience at the UK Atomic Energy Authority with brazing this alloy (Fursdon et al, 2017;Lukenskas et al, 2019).…”

“…Manufacturing mock-ups. For each MUP sample, a single rolled tungsten block, with dimensions 12 Â 23 Â 30 mm and a central bore of 19 mm, containing a 1 mm cast Cu interlayer (ALMT, Japan) was vacuum brazed using an Au-Cu braze to a 15 mm-diameter, 1.5 mm-thick CuCrZr pipe as previously described (Lukenskas et al, 2019). The brazing cycle for the MUPs is shown in Fig.…”

“…High-heat-flux-exposed pipes. Thermal break divertor mock-ups containing four tungsten tiles were manufactured and subsequently exposed to high-heat-flux conditions in the GLADIS test facility (Greuner et al, 2007), while being actively cooled with pressurized water at 403 K (Lukenskas et al, 2019;Greuner et al, 2019). The heat flux in this facility is delivered by a hydrogen ion beam.…”

“…Small-angle neutron scattering (SANS), on the other hand, is able to provide a bulk statistical measurement on millimetre-scale samples, while maintaining the sensitivity for the precipitate dimensions (Schmidt, 1991;Vogel, 2013). Here, we compare SANS data for different ageing conditions of a CuCrZr alloy in small (centimetre-scale) samples as well as in bulk components ($10 cm) exposed to different temperature cycles as part of the EU-DEMO 'thermal break' divertor exhaust mock-up development process (Barrett et al, 2015;You et al, 2018;Fursdon et al, 2017;Lukenskas et al, 2019).…”

Structural integrity of DEMO divertor target assessed by neutron tomography

“…For this study three sample types of 2 nd phase thermal break mockups [2,[11][12][13][14] of the DEMO divertor target design were used: as manufactured (labelled as CCFE11), with a single cycle at 25 MWm − 2 plus 100 cycles at 10 MWm − 2 (labelled as CCFE9) performed at the IPP test facility GLADIS [15], and before HHF testing (labelled as CCFE10) as shown in Fig. 2 from left to right.…”