In ITER, a retention of 10% of the tritium injected (T injection rate ~5¥1022 T s-1) during the 400s of burning phase would lead to the limit of 350g (nuclear licensing) in only 35 discharges. Thus the evaluation of fuel retention in present tokamaks is of high priority to establish a database for ITER, for which gas balance will be the dominant technique used to assess the fuel retention. In order to achieve high accuracy and to minimise the contribution from previous experiments (history), the overall particle balance has been studied in JET in a series of repetitive and identical discharges. The particle retention behaviour has been analysed for L and H-modes (type III and I ELMs). The resulting fuel retentions for these different plasma scenarios are reported in this paper and will be used as a reference for the comparison with the long term retention resulting from the use of different materials for plasma facing components for the future ITER-like Wall project of JET.