Abstract: Techniques are described for delivering a metered flow of tritium gas to a fusion power system at a constant (or substantially constant) flow without feedback control being necessary, and while allowing all (or almost all) of the tritium in a reservoir to be delivered to the system. A constant pressure (isobaric) tritium injection system is described comprising a process chamber, at least part of which is flexible, and a regulating chamber arranged adjacent to the process chamber. Tritium in the process chamber may be pushed out of the injection system by managing the pressure of a regulating gas in the regulating chamber. As the pressure of the regulating gas increases, this causes the process chamber to be compressed due to the flexible portion(s) of the process chamber, thereby increasing the pressure of the tritium gas.