Gas balance and fuel retention in fusion devices

The evaluation of hydrogenic retention in present tokamaks is of a crucial importance to estimate the expected tritium (T) vessel inventory in ITER, limited for safety considerations to 350g. In the frame of the European Task Force on Plasma Wall Interaction (EU TF PWI), efforts are underway to investigate the gas balance and fuel retention during discharges and compare with that from post-mortem tile analysis integrated over experimental campaigns. The aim is to assess the dominant processes that determine the fuel retention in order to extrapolate to ITER. This paper summarizes the principal findings from coordinated studies on gas balance and fuel retention from a number of European tokamaks e.g. ASDEX-Upgrade (AUG), JET, TEXTOR and Tore Supra. Gas balance is one of the few possibilities to evaluate the fuel retention in ITER in the non-activated phase. This contribution will also discuss the work that is required to improve the fuel inventory evaluation from gas balance and tile analysis. However, from the database available from tokamaks with carbon as main PFCs, the important conclusion is that T inventory limit could be reached in ITER within only ∼100 discharges and could therefore seriously impact the device operation unless efficient T removal processes are developed. (author)