In fusion devices a part of the hydrogen retained by implantation has been demonstrated to be accessible during plasma operation and can be removed from the wall by He or isotope exchange with plasma. Starting from an empty D2 and H2 wall after two weeks of He campaign series of change over experiments from He to H2 have been carried out in JET. Hydrogen plasma concentration up to 85-90% has been reached after only 7 pure H pulses for a total of ~210s of plasma operation whilst an accessible wall inventory of ~2.0x1023D (~1g of T) has been evaluated. Extrapolation to ITER shows that accessible T content would be 21.3g, 2.63g and 1.63g respectively for C, C/Be/W and Be/W as materials facing the plasma in ITER. Finally, these quantities are "small" compared to retention by co-deposition which will dominate the retention with time since this is a cumulative process.