Experimental validation of a method for performance monitoring of the Impurity Processing stage in the TEP system of ITER

The Tokamak Exhaust Processing (TEP) system within the Tritium Plant of ITER needs to be designed such that tritium is recovered from all exhaust gases produced during different modes and operational conditions of the vacuum vessel. The reference process for the TEP system of ITER is called CAPER and comprises three different, consecutive steps to recover hydrogen isotopes at highest purity for direct transfer to the cryogenic Isotope Separation System. The second step ('' impurity processing '', IP) of the CAPER process developed at TLK is carried out in a closed loop involving heterogeneously catalyzed cracking or conversion reactions to liberate tritium from tritiated hydrocarbons or tritiated water combined with permeation of hydrogen isotopes through a Pd/Ag permeator. This combination shifts chemical equilibria towards dehydrogenation and therefore enables detritiation factors higher than 1000 in the IP stage. A closed loop with a catalyst reactor and a permeator can in principle be operated in two different modes, the batch mode or the continuous mode. In the batch mode the loop is filled with the gas to be detritiated, its cycled for a certain time until the tritium level is sufficiently lowered, and then the loop is emptied for the next batch. In the continuous mode the loop is continuously fed with gas and simultaneously gas is withdrawn from the loop. Both modes are sensitive to different extents to the actual permeability of the Pd/Ag membrane, which unavoidably becomes deteriorated by the decomposition of methane and coverage with carbon. Therefore, the permeator needs to be repeatedly regenerated in order to sustain high decontamination factors. In a series of tritium experiments with the CAPER facility at TLK a method has been developed to determine the actual performance of the 2nd process step in both possible operation modes. During this experimental campaign the permeator has been operated with DT mixed with tritiated methane under conditions that promote coking by hydrocarbon cracking. The reduction of both, the permeator performance and the decontamination factor, has then been determined by measurements of process time constants and gas compositions. The experimental results will be presented and the feasibility of performance monitoring of the 2nd step of the TEP system of ITER will be described. Details of the regeneration process will be reported and possible consequences for the design of the TEP system will be discussed. (author)