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Abstract
The fusion power plant operation with a fuel cycle including a direct internal recycling aims to increase the fuel burnup fraction and to reduce the tritium inventory. The expected accumulation of protium and fuel imbalance requires control of plasma fueling and particle exhaust. The formation of protium and tritium in the reactor chamber in concomitant to DT fusion reactions and the release of tritium from the first wall can lead both to an accumulation of protium followed by the fuel dilution and the fuel imbalance in the reactor chamber followed by the power dip. During relatively long reactor operation, its first wall could be the main source of neutral gas feeding the plasma. Moreover, in these cases the tokamak wall contains the amount of hydrogen isotops, which is significantly larger in many cases by orders of magnitude than that in the plasma.
In this work, we estimate the accumulation of protium in the reactor chamber due to fuel recirculation, when the exhaust gas mixture returns after purification back to the reactor chamber, depending on the separation fraction of the metal foil pumps and the duration of the burning pulse. It has been found that the acceptable concentration of protium is limited by the requirements for a self-sustaining reaction to occur, and strongly depends on wall conditioning. It is shown that in order to achieve maximum burn, in the case of fuel imbalance, a lower tritium concentration will be required in the fuel supplied from an external source to the reactor chamber.