Experiments were done to investigate the behavior of non-gaseous fission products in nitric-acid-leach process in which the conditions of the leaching and postirradiation heating were changed for the irradiated graphite-base fuel element that most of the fission products had been introduced into the graphite matrix by fission recoil process.
The temperature and time of the postirradiation heating were found to be most effective to drive off the fission products from the graphite. A fast and a slow migration step were observed for the migration of the fission products through the graphite crystal. The former was thought to represent the transfer process of all the fission products that may migrate through the graphite crystal in the temperature region below the heating temperature, and the latter the one of the fission products which may migrate only at the temperature of the postirradiation heating. The migration behavior was independent of particle size of graphite.

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The chloride-volatility process was applied to the processing of irradiated UO₂-graphite fuel elements, in which most of fission products were introduced into the graphite matrix by fission recoil.
In proper conditions, 99% of UO₂ in the sample was chlorinated with CCl₄ vapor. However, the major objective in the present study was to investigate the behavior of the fission products during the chlorination. It was elucidated that besides the postirradiation heating itself the existence of the chlorinating agent was additionally effective in the migration of fission products in the graphite matrix.
The possibility of low temperature chlorination with chlorine gas was also studied.

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