Development of irradiation tolerant tungsten alloys for high temperature nuclear applications.

Development of refractory metals for application as plasma-facing armour material remains among priorities of fusion research programmes in Europe, China and Japan. Improving the resistance to high temperature recrystallization, enhancing material strength to sustain thermal fatigue cracking and tolerance to neutron irradiation are the key indicators used for the down selection of materials and manufacturing processes to be applied to deliver engineering materials. In this work we investigate the effect of neutron irradiation on mechanical properties and microstructure of several tungsten grades recently developed. Neutron irradiation campaign is arranged for screening purposes and therefore is limited to the fluence relevant for the ITER plasma facing components. At the same time, the neutron exposure covers a large span of irradiation temperatures from 600 up to 1000 degrees C. Four different grades are included in the study, namely: fine-grain tungsten strengthened by W-carbide (W-4wt.% W2C), fine-grain tungsten strengthened by Zr carbides (W-0.5% ZrC), W alloyed with 10 at.% chromium and 0.5 at.% yttrium (W-10Cr-0.5Y) and technologically pure W plate manufactured according to the ITER specification by Plansee (Austria). The strengthening by W2C and ZrC particles leads to an enhanced strength, moreover, the W-0.5ZrC material exhibits reduced DBTT (compared to ITER specification grade) and is available in the form of thick plate (i.e. high up-scaling potential). The W-10Cr-0.5Y grade is included as the material offering the self-passivation protection against the high temperature oxidation

Mock-ups fabrication by HRP technology with advanced W-alloy monoblocks for DEMO divertor target

Tungsten is the primary candidate armour material for the divertor target of the European demonstration fusion power plant. During operation at high temperature, pure tungsten is subject to fracture and recrystallization which results in a loss of strength and worsening of the thermal properties. Additionally, loss-of-coolant accidents with simultaneous air ingress can generate volatile and radioactive tungsten oxides. Advanced W-alloys were developed as alternative and upgrading armour materials of pure tungsten, such as potassium-doped tungsten laminates and self-passivating tungsten alloys. Three mock-ups were manufactured using potassium-doped tungsten laminates, W-10Cr-0.5Y and W-10Cr-0.5Y-0.5Zr as armour materials, each of them consisting of n degrees 4 blocks. The fabrication required optimization and upscaling of the ITER-like process which foresees oxygen-free high conductivity copper as interlayer joined to W-alloy armour block and CuCrZr ITER grade pipe welded to the Cu/W-alloy blocks by hot radial pressing. For quality control of the fabrication steps, non-destructive examination by ultrasonic testing was done on the monoblocks as received, after casting, after hot radial pressing and after high heat flux testing. The results demonstrated that these W-alloys can be used as armour materials of the European demonstration fusion power plant divertor target