Tritium Control in Helium-Cooled Blankets

As a part of the Blanket Comparison and Selection Study (BCSS), GA Technologies was responsible for the design of helium-cooled, solid- and liquid-metal breeder blankets. Conceptual blanket designs were developed, including the consideration of the generation, transport, and extraction of tritium. Evaluations were made of the inventory and leakage of tritium for helium-cooled Li₂O and LiAlO₂ and liquid lithium breeder blankets for tokamak and tandem mirror reactors. To facilitate the evaluation, a solid breeder tritium code TRIT4 was developed. In the helium-cooled solid-breeder blanket designs the bred tritium is extracted by a purge stream of helium flowing through the metal-clad solid breeder plates. For the Li₂O designs, the breeder tritium inventory was found to be dominated by the solubility of LiOT in Li₂O. For the LiAlO₂ designs, the breeder tritium inventory was found to be dominated by the slow bulk diffusion of tritium in the breeder. The total tritium inventories for these designs are in the range of 24 to 134 gm, which is quite acceptable. Tritium control of the Li designs consists of circulation of the liquid lithium and optional slipstream cleanup of the primary coolant. The tritium inventory, dominated by the efficiency of the tritium extraction system, can be kept to an acceptable 330 gm. Tritium losses by permeation through the steam generator were also calculated. The influx of tritium into the main coolant stream includes permeation through the breeder cladding and first wall. The leakage rates for all the helium-cooled blanket designs are less than 100 Ci/day. The results from this study indicate that tritium inventories and leakages are acceptable for the proposed helium-cooled blankets. An assumption made in the tritium leakage calculations was that tritium is released to the helium purge and coolant streams as T₂ and remains in that form. If oxidation to T₂O is possible, significant reduction in the tritium leakage will be possible. We conclude that more experimental data on breeder material properties and tritium permeation behavior are needed. However, we are certain that an adequate number of different techniques are available to control the breeder tritium inventory and leakage to an acceptable level in helium-cooled solid- and lithium-breeder blankets.