In-pile tritium release rate limiting process study

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Diffusion and desorption are normally taken into consideration as the main rate limiting processes for tritium release from solid breeder materials in thermo-nuclear fusion devices. The importance of the correct identification of the process, in order to determine correct process parameters, is universally recognized. Because the two processes are temperature dependent, generally following an Arrhenius law, it is very important to determine, for a certain material microstructure, the temperature ranges in which they are effective, or the existence of a transition zone. The process identification in in-pile experiments is normally very difficult and generally requires several measurements on the same materials with different grain sizes. Even so it is almost impossible to assure that materials, different in grain sizes, have identical all the other microstructural parameters. This again induces data interpretation problems. In this paper the theoretical possibility to discriminate between diffusion and desorption, especially for a particular but interesting in-pile case, is pointed out. For the particular case of big stepwise temperature decrease, the process, at the final temperature, can be identified by a single step. The identification is possible, in full analogy to the out-of-pile case, by observing the slope of the early tritium release, at small times compared with the residence time, in a log-log plot.