Fuel retention in the JET-ITER like wall (JET-ILW) has decreased by a factor of 20 compared to operation with a carbon wall [1]. The reduction in retention is due largely to the decrease in chemical erosion of the beryllium main chamber. A reduction in co-deposition of fuel with beryllium has previously been reported in the divertor [2]. However, 0.2% of fuel remains as long-term retention in beryllium tiles [1]. The study of plasma retention and its mechanism continues for application to the ITER tritium safety case and removal procedures.

This contribution studies the Be limiter tiles exposed in JET with the aim of understanding how the microstructure influences fuel retention. Dump plate tiles, Inner, and Outer wall limiters tiles have been investigated, encompassing deposited, eroded and melted regions of the vessel. A wide range of techniques have been used to study these phenomena at different length and depth scales. Focused Ion Beam (FIB) ‘serial milling’ studies have been used for compositional understanding of 20μm3 Beryllium cut outs. Further TEM studies have been undertaken of 20 μm2 lift-outs. SEM and EDX studies have also been undertaken of the surface morphology and composition at different energies. Finally, Raman Spectroscopy has been applied for the first time to JET tiles, to investigate the chemical bonding of surface layers up to 50nm depth.

Raman investigations have uncovered the presence of BeO bonded to deuterium in BeOxDy, for the first time, on melted surface regions in upper dump plate tiles. It is proposed that this bonding is important in the retention mechanisms present for beryllium in JET. Preliminary Density Functional Theory (DFT) modelling has been undertaken, which confirms the Raman band for the wurtzite BeO crystal structure. A review of the literature has supported the formation of BeO bonding even under Ultra High Vacuum (UHV) conditions above temperatures of ~630 °C [3-4]. SEM-EDX studies of the samples support the presence of oxide island formation, with both oxide island size and number densities decreasing between co-deposited wing tiles and eroded central regions. A thorough investigation of thermal desorption has been undertaken toroidally across the midplane of outer and inner limiter and dump plates tiles. The initial trends seen in the TDS data support the presence of a different desorption peak behaviour in the central eroded regions of tiles, which suggests a different retention trap behaviour. Some toroidal asymmetry is also apparent on a first review of the TDS data, showing peak behaviour does differ between the left and right wing co-deposited positions. TMAP7 analysis will be applied to these TDS spectra in greater detail, utilizing the microstructural features found with the techniques above in the code.