Hydrogen inventories in the vacuum pumping systems of ITER

The ITER in-vessel hydrogen inventory is highly dynamic and determined by a broad range of parameters. Moreover, it is of prime importance to allocate the inventory as accurately as possible. Two essential control issues have to be met. The basic safety approach is to limit the amount of hydrogen such that the resulting pressure in case of an explosion accident is compatible to the design criteria of the vacuum vessel. This requirement drives the operational philosophy and the design of the cryogenic pumps which are connected to the vessel. The administrative limitation of the tritium inventory is another essential requirement on reactor operation. This paper illustrates how hydrogen inventory control issues are reflected by the operational schemes of the vacuum pumping systems. Latest results include the release characteristics of hydrogens and tritium during normal regeneration of the cryopumps and under off-normal events. The different semi-permanent contributions to the overall tritium inventory are discussed. The forepumping systems reduce the in-vessel inventory and act as interface to the tritium plant. A typical example of an integral pump-out curve is discussed. It is shown that sufficient safety margin is included in the ITER vacuum pumping design.