The Critic-I Irradiation of Li₂O - Tritium Release and Measurement^a

Results from the CRITIC-I, vented capsule irradiation of Li₂O are presented. A total lithium burnup of 0.74% has been achieved and 1500 curies^b of tritium have been collected over the first 15 months of irradiation. The temperature has been varied between 400 and 850°C, and the sweep gas composition changed progressively from pure He to He-1% H₂. The amount of tritium recovered in the reduced form (HT) has increased from an initial value of approximately 50% with pure He sweep gas to a current value of 99% with He-1% H₂. The increasing H₂ concentration in the sweep gas has also reduced the time constants for tritium release (tritium residence time in the Li₂O). Although the results indicate tritium release is controlled by surface desorption, simple first-order desorption theories do not explain all the observations. Most noticeably, for temperature increase tests, tritium release peak maxima can be delayed as long as 6 h and inventory changes depend not only on the initial temperature but also on the time at the initial temperature. An explanation is given based on the buildup of free oxygen in the ceramic from lithium burnup which leads to tritium trapping, perhaps as LiOH(T). Dissociation of LiOH(T) then occurs following an increase in the ceramic temperature, in addition to the simple first-order desorption process of isotopic exchange with H₂ in the sweep gas.