An Analytic Expression for the Tritium Burnup Fraction in Burning-Plasma Devices

The tritium burnup fraction f_burnup can strongly affect the design of a fusion reactor since it influences the size of the tritium reprocessing plant, the on-site tritium inventory, and hence, the licensing requirements and cost of the entire plant. In this paper a simple analytic expression for f_burnup is derived and then applied to typical parameters proposed for three possible fusion devices: ARIES-AT, FDF, and ITER. We find that for these parameters the burnup fraction is most strongly affected by the global recycling coefficient (through the global replacement time) and the fueling efficiency. The latter term may be the most easily influenced by plant design, such as by high-field-side pellet injection, for example. Because of the hotter edge plasmas in these devices compared to present-day tokamaks, the recycling coefficient will be lower, reducing the tritium burnup fraction. While this may not adversely affect ITER, which is limited to 400-s pulses for the inductive scenario, the tritium reprocessing for nearly continuous operation of devices such as ARIES-AT must be carefully considered in the overall plant design.