A computer model has been developed to evaluate tritium permeation to coolant and in vanadium tubes inventory in Safety and Environmental Assessment of Fusion Power (SEAFP) blanket. The mean tritium partial pressure in gaseous breeder phase are in the range from 0.5 to 5 Pa for helium purge gas velocity from 0.1 to 0.4 m/s; in these conditions the tritium permeation to coolant changes from 32.8 to 16.4 g/day and the tritium inventory in vanadium tubes from 4000 to 2000 g. The H/T ratio involves a relevant tritium permeation variation: with 0.2 m/s helium purge gas velocity varying the H/T ratio from 100 to 50 the tritium permeation to coolant ranges from 23.2 to 32.7 g/day. The analysis shows that defects free thin permeation barriers (SiC and Al2O3) are very effective to making negligible the tritium permeation to coolant and the tritium inventory in tubes.