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Abstract: Out-of-pile tritium release experiments under different water uptake contents and purge gas chemistry were performed on Li4SiO4. Water measurement was performed on samples under different experimental procedures. It was found that water was adsorbed on the sample during its transferring and storage process. A strong dependence of tritium release behavior on water uptake was determined. By doping H2 in the sweep gas, the formation of water in orthosilicate was observed in addition to the isotope exchange reaction with H2 gas. Thermal desorption peaks of the water formation reaction and H2 isotope exchange reaction appeared at 668°C and 463°C, respectively, at ramping rate of 5°C/min. [Copyright &y& Elsevier]

Abstract: Out-of-pile tritium release examinations of irradiated Li4SiO4 pebbles were performed in TRINPC-I experiments for evaluating material performance and verifying the system design. To generate tritium the specimens were irradiated with neutrons. Li4SiO4 pebbles were made by a freeze-drying method. In the experiments, concentrations of tritium in the form of tritium gas (HT+T2) and tritiated water (HTO+T2O) in the outlet streams of a reactor tube were measured separately with an ionization chamber and a liquid scintillation radiometer. The results show that the percentage of tritium gas (HT+T2) and tritiated water trapped by the breeder pebbles were about 72% and 19% of totally released tritium, respectively. Thus, more tritium was released in the form of tritium gas in this work. In addition to tritium trapped by the breeder pebbles, the amount of free tritium was also measured by breaking on-line a quartz capsule containing Li4SiO4 pebbles, the percentage of which was 9% of totally released tritium. The temperature peaks of tritium gas mainly appeared at about 477°C and 654°C, while the temperature peak of tritiated water appeared at about 402°C, under which most of tritiated water released. [Copyright &y& Elsevier]

The correlation between water desorption and tritium release from Li 4 SiO 4 pebbles was studied by temperature programmed desorption. The released water and tritium from irradiated samples were monitored simultaneously. The main peak for tritium release from the irradiated samples that were exposed to air for more than a month, was shifted from 500 to about 250 °C, as compared to that from the unexposed samples. The peak temperatures for water desorption and tritium release overlapped very well, suggesting a strong correlation between the two processes. Accordingly, a two-step mechanism, involving isotope exchange between the tritium trapped on the grain surface and the surface hydroxyls (–OH), and subsequent desorption of tritiated water through recombination of the –OH/–OT groups, was proposed to explain the tritium release behavior for the air-exposed samples. It is believed that the formation and desorption of surface hydroxyl groups at 200–300 °C can affect the behavior of tritium release from Li 4 SiO 4 significantly. [ABSTRACT FROM AUTHOR]

Abstract: Tritium release kinetics in lithium orthosilicate (Li4SiO4) ceramic pebbles irradiated with low thermal-neutron fluence was studied by out-of-pile annealing experiments. It was found that the tritium produced in Li4SiO4 pebbles was mainly released as tritiated water vapor (HTO). The apparent desorption activation energy of tritium on the pebble surface was consistent with the diffusion activation energy of tritium in the crystal grains, indicating that tritium release was mainly controlled by diffusion process. The diffusion coefficients of tritium in the crystal grains at temperatures ranging from 450K to 600K were obtained by isothermal annealing tests, and the Arrhenius relation was determined to be D =1×10−7.0 exp(−40.3×103/RT) cm2 s−1. [Copyright &y& Elsevier]