Transmutation, Sigma Phase Formation and Neutron Embrittlement in Candidate High-Z Plasma-facing Fusion Materials

G. Cottrell, EURATOM/UKAEA Fusion Association, Oxfordshire (UK) 
In a fusion power plant, 14 MeV D-T neutrons striking the wall will transmute the atoms of the plasma facing material (PFM); this can give rise to significant changes in chemical composition. Such effects may lead to important thermodynamic phase and property changes. In this paper, we consider the evolution of the chemical composition of some candidate high-Z refactory PFMs at the locations of the first wall and divertor of a typical tokamak power plant design. In view of its high cohesive energy and resulting high resistance to sputtering by impinging low-Z plasma particles, tungsten and some of its alloys are currently considered as main candidates for the first-wall and divertor armour. After a service life of five years as a first wall PFM in a power plant with a neutron wall loading of 2 MW per square metre, pure tungsten will have transmuted into an alloy of about 75 W, 13 Os and 12 Re (atomic percent). This composition is close to the homogeneous sigma field of the phase diagram. Since sigma is extremely brittle, it is important to know whether the alpha (bcc) to sigma transformation will occur. In the presence of a superthermally high concentration of irradiation-produced vacancies, indirect kinetic arguments suggest that the sigma phase will form, and that we should expect failure of the tungsten armour. We discuss the possible use of tantalum as an alternative to tungsten that will avoid this issue.

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