29th Symposium on Fusion Technology (SOFT 2016)

P3.107 Helium ion irradiation of tungsten carbide neutron shields

7 Sep 2016, 11:00

1h 20m

Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Board: 107

Poster F. Plasma Facing Components P3 Poster session

Speaker

Samuel A. Humphry-Baker (Department of Materials)

Description

High-field spherical tokamaks may be a viable technology for relatively compact fusion power devices (Costley et al Nucl. Fus. 2015). However, such reactors leave little space for shielding of the central column, which must protect the inner superconducting magnets from high energy neutrons. Tungsten carbide cermets are promising candidate materials for such shields: They have high thermal conductivity, satisfactory oxidation properties, and can be manufactured at relatively low temperatures in complicated geometries. Furthermore, the neutronics properties of such cermets are very encouraging (Windsor et al Nucl. Fus. 2015). However a particular concern is the production of helium ions via (n,α) transmutation reactions under high-energy neutron irradiation, which could lead to stabilisation of void embryos and bubble formation resulting in potential structural degradation. In this study we simulate the production of helium gas by implanting helium ions into cermet thin foils. The foils are made from tungsten carbide cermet with an iron-chromium metallic binder. Their microstructural evolution under 6 keV helium irradiation is tracked in-situ through transmission electron microscopy. A range of fluences and temperatures were investigated: from 0 to the order 10¹⁷17 ions/cm²2, and from room temperature to 750 ^oo C. Implantation led to the formation of nanoscale bubbles in the foils, both in the major carbide phase and in the minor metallic binder phase. In general, bubbles in the WC phase were very small, e.g. on the order of 1-2 nm, while in the binder such defects were typically much larger. Interestingly, at carbide-metal phase boundaries we observe bubble coalescence, which is particularly prominent at low temperatures and very high fluences. Such bubble coalescence has not yet been reported and may adversely affect bulk mechanical properties. Our systematic work in quantifying these effects as a function of irradiation conditions is therefore particularly needed.