The presence of an Internal Transport Barrier (ITB) in the plasma can dramatically affect the transport of fuel and impurity particles. However, effective control of the plasma density, fuel concentration and deuterium-tritium mixture is required to maintain fusion power. Dedicated experiments have been performed in JET to address these issues, using trace amount of tritium injected transiently to make it possible to determine separately the T diffusion and convection. Three fuelling methods were compared in otherwise similar ITB plasma conditions: T gas puffing, neutral T beam injection and T recycled from the wall.