Suppression of rhenium and osmium production in tungsten by selective isotopic enrichment

Tungsten (W) based materials are primary candidates for the plasma facing components (PFCs) in fusion power stations. A side effect of the use of W in neutron irradiation environments such as fusion is the transmutation to Rhenium (Re) and Osmium (Os), which are known to degrade the mechanical and thermal properties. In this study, neutron transport and nuclear inventory simulations were used to investigate a strategy of selective isotopic enrichment and/or depletion of W isotopes to supress the formation of Re and Os in a representative first-wall monoblock design. It was found that Re and Os production can be reduced by up to three orders of magnitude compared to natural W by depleting W-186 isotope to 100 %, as ¹⁸⁶W(n,γ) ¹⁸⁷W followed by beta decay is the main pathway for Re production. Two design methodologies were investigated that considered 1) an approach with two discrete, depleted in W-186 regions, with a range of depletion degree investigated and 2) a graded approach with layers of different levels of depletion to emulate a functionally graded material.