First wall material issues and related activities at JET

To close the JET tritium material balance a comprehensive programme to determine the tritium inventory in a selected poloidal set of JET tiles was initiated. First results strongly suggest that a significant fraction of the tritium remaining in the machine is immobilised in flakes accumulated in the sub-divertor region. Measurements performed with tiles and flakes retrieved from JET after the DTE1 campaign revealed a chronic tritium release at room temperature, possibly caused by a radiation-induced mechanism. Deposited films of the order of 5 μm thick are found at the vertical surfaces of the inner divertor. Recent analysis of the distribution of beryllium across the divertor shows factors of ≈10 higher beryllium concentrations (relative to carbon) in these deposits than found in the main camber walls. Very little beryllium is found on the outer divertor. Also no beryllium is detectable in the thick deposits accumulated in the shadowed regions of the inner divertor. These results suggest that in ITER migration of beryllium from the main chamber to the divertor surfaces could substantially reduce the chemical erosion and consequently the retention of tritium.