Shielding materials in the compact spherical tokamak

Abstract

Neutron shielding materials are a critical area of development for nuclear fusion technology. In the compact spherical tokamak, shielding efficiency improvements are particularly needed because of severe space constraints. The most spatially restricted component is the central column shield. It must protect the superconducting magnets from excessive radiation-induced degradation, but also from associated heating, so that energy consumption of the cryogenic systems is kept to an acceptable level. Recent simulations show that tungsten carbide and its composites form an attractive class of neutron-attenuating materials. In this paper, the key structure-property relationships of these materials are assessed, as they relate to generic materials challenges for plasma-facing materials. We first consider some fundamental materials properties of monolithic tungsten carbide including thermal transport, mechanical properties and plasma interaction. WC is found to have generally favourable properties compared to metallic tungsten shields. We then report progress on the development of a new candidate cermet material, WC-FeCr. Recent results on its accident safety, thermo-mechanical properties, and irradiation behaviour are presented. This review also highlights the need for further study, particularly in the areas of irradiation damage and hydrogen trapping.This article is part of a discussion meeting issue 'Fusion energy using tokamaks: can development be accelerated?'.

Can the development of fusion energy be accelerated? An introduction to the proceedings

“…George Smith [39] gave us some answers to these questions. There are new materials such as the cemented tungsten carbides-familiar from tool steel cutting heads.…”

Ablation resistance of tungsten carbide cermets under extreme conditions

“…In all of the above concepts, WC will activate strongly under neutron irradiation [13], therefore accident safety is a concern. W will form radioactive Re, Os, and Ta and C will form He and Be via the (n, a) reaction [14] (some 14 C will also form, although the amount will be much smaller than from N in steels, e.g. Eurofer'97 [15]).…”

“…The performance of WC-based materials under a LOCA is still relatively poorly understood. While fully-dense monolithic WC appears to show slightly improved oxidation resistance compared to metallic tungsten at 1000°C, WC-cermets show accelerated damage [14,23,24]. A question that remains to be addressed is how the release of hazardous tungsten oxides is affected by rapid air flow rates.…”

“…An Fe-based binder was selected instead of a more typical Co-based one for several reasons: Co is not considered viable for fusion because of the formation of long-lived 60 Co under neutron irradiation; also Co powders are now regarded as carcinogenic; and Co is expensive due to its limited supply chain [12]. Fe by contrast forms relatively benign activation products, mainly Mn and Cr from gas emission [14]. An Fe-based binder might be alloyed either with small additions of Cr, which improves oxidation and irradiation resistance [11], or Ni, which suppresses the formation of the deleterious h-phase [8].…”

Thermal shock of tungsten carbide in plasma-facing conditions

“…The UHTC tungsten carbide could be a promising plasma-facing material because of its combination of the above properties. The properties relating to the generic materials challenges for fusion, such as heat flux, irradiation, and hydrogen interaction, were recently reviewed [13]. The performance of WC in a simulated plasma facing environment remains unexplored.…”