Properties of Neutron Irradiated Tungsten Material: Recent Lessons

Within European material’s programme, the portfolio of baseline materials for DEMO contains the following items: (i) EUROFER(97), a 9Cr Reduced Activation Ferritic Martensitic (RAFM) steel, as structural material for the breeding blanket, (ii) commercially pure tungsten as plasma facing component armor material, and (iii) copper chromium zirconium (CuCrZr) alloy as heat sink material for the divertor coolant interface. This contribution reviews the efforts done towards the assessment of the irradiation effects and operational temperature window performed over the last several years in the frame of the European fusion programme focusing on the armour and heat sink materials.Based on the already available knowledge gained in FP6 and FP7 programmes, the operational conditions for the baseline in-vessel materials are tentatively determined. For each of the baseline material, the lower temperature bound is defined by the embrittlement (fracture without plastic deformation), while the upper temperature bound is determined by softening of the material (reduction of the yield point). Accordingly, the main challenges in the formulated irradiation programmes were linked to: (I) assessment of the ductile-to-brittle transition temperature (DBTT) of baseline tungsten and advanced tungsten alloys; (ii) investigation of baseline tungsten under irradiation at very high temperature, reflecting operational conditions in divertor; (iii) assessment of the mechanical properties of reinforced CuCrZr alloys. The screening exposure to perform early down selection of the materials was also performed. The choice of the advanced materials is driven naturally by the need to extend the operation temperature/fluence window to extend the design space. Although the fusion neutron spectrum implies an important difference in the transmutation reactions compared to fission spectrum, the current R&D programme utilizes available Material Test Reactors (MTRs).Driven by the technological priorities, the irradiation tests campaigns were arranged in two waves. The first one involved baseline materials (EUROFER97: 20 dpa; tungsten: 1 dpa; CuCrZr: 1 dpa) focusing on delivery of the engineering design data and the second one targeted screening irradiation of the advanced materials (advanced/optimized EUOFER97 specification steels: 2.5 dpa, tungsten alloys and composites: 1 dpa; reinforced CuCrZr: 2.5 dpa). Execution of the programmes was realized in Europe (LVR-15 and BR2 reactors) and USA (HFIR reactor).Extraction of the properties of the neutron exposed materials involved massive post irradiation examination (PIE) campaigns. The performance of the advanced materials for divertor (W and CuCrZr) is assessed and presented, which already at this stage allows drawing some important conclusions.