Investigation of transient processes of tritium release from biphasic lithium ceramics Li4SiO4-Li2TiO3 at negative neutron flux pulse

Biphasic lithium ceramics based on lithium orthosilicate Li4SiO4 and lithium metatitanate Li2TiO3 is one of the most promising materials for breeder blankets of future fusion reactors. One of the important issues of biphasic lithium ceramics application in the fusion reactor blanket is to determine the parameters and mechanisms of tritium transfer within and from the ceramics. This paper continues the analysis of irradiation experiments carried out at the WWR-K reactor (Almaty, Kazakhstan) with a sample of biphasic lithium ceramics Li4SiO4-Li2TiO3 (pebbles of lithium orthosilicate with 35 mol% lithium metatitanate with diameter 250 -1250 μm). The section of the experiment in which the reactor was temporarily shutdown for 1.5 hours was investigated in detail. During this period of time the sample temperature rapidly decreased from 665°C to 100°C, generation of tritium and helium in the lithium ceramic sample ceased, but the desorption of previously generated gases from the ceramic surface continued. The experiments were carried out by the vacuum extraction method. The nature of tritium-containing molecules and helium release for that specified time interval was analyzed. The kinetics of tritium release from ceramics in the experiment during reactor shutdown was simulated and the expression for the effective diffusion coefficient D=5e-11(m2/s)∙exp(-20(kJ/mole)/RT) was determined. It was suggested that one the most realistic mechanisms for tritium release is the mechanism associated with both diffusion and desorption of tritium from the pebbles surface and release from the open pores of the pebble. This mode of the experiment made it possible to estimate the parameters of tritium release immediately after irradiation, which imitates the conditions of breeding blanket operation in the fusion reactor.