Computer simulation of the processing of tritium in exhaust gas from the LHD

A new type of tritium cleanup system that differs from conventional systems in its application of a decomposition-processing vessel has been investigated by means of computer simulation. In the decomposition-processing vessel, tritiated compounds are decomposed so that it is possible to extract the tritium in the form of hydrogen molecules while other elements such as carbon are fixed within the vessel. In our computer simulation of the removal of tritium, a single run of processing was assumed to handle 1 N m3 of a gaseous mixture that contained 1 GBq tritium and was 90% hydrogen, 5% methane and 5% helium. As a result, three pressure curves, one for hydrogen, a ‘both-sum’ (hydrogen+methane) curve and a ‘total-sum’ (hydrogen+methane+helium) curve, showed strong and marked changes in trend at almost the same elapsed time. This time corresponded to the completion of a single cycle of processing. The changes in trend suggest that the three curves had been dominated in the first cycle by the hydrogen initially contained in the exhaust gas and that, after one cycle, they had come to be dominated by the remaining methane and by the hydrogen generated by methane decomposition. It took about 8.7 h for the concentration of tritium in the gas being processed to fall below the relevant limit.