AbstractThe Helium Cooled Pebble Bed (HCPB) blanket is one of the two concepts to be further developed for a European DEMO power reactor. Tritium leakage due to the permeation through the main structural materials constitutes one of the main problems of the fusion technology. In the present work, the tritium permeation through and inventory in the first wall between the plasma and the first wall coolant has been evaluated. The influence of temperature gradient, surface conditions, isotopic effect, and trapping in ion- and neutron-induced defects on the hydrogen isotope permeation and inventory has been considered.