2 Forschungszentrum Karlsruhe GmbH, IHM, Postfach 3640, 76021 Karlsruhe, FRG

The helium purge gas loop for tritium extraction from the ceramic breeder blanket (Helium Cooled Pebble Bed, HCPB) of ITER includes a liquid nitrogen cooled cold trap for the retention of tritiated water produced in the breeder. Including 0.1% hydrogen in the He purge gas enhances the tritium extraction from the blanket as it allows an isotopic exchange process to take place between the protium in the gas phase and the tritium generated by nuclear reactions and fixed in the breeder. Purging the breeder with hydrogen inevitably produces some tritiated water which can be removed very efficiently by the mean of a cold trap working at –100°C. The final He purification requires a liquid nitrogen cooled molecular sieve adsorber bed to be included in the loop to extract the various isotopic species of hydrogen (H₂, T₂, HT) as well as N₂ and O₂.

The existing cold trap has already been operated in a preliminary test rig under once through

conditions to obtain layout data for the final design. The matrix of operation parameters and

measured humidities in the helium at the outlet of the trap allow us to define the cooled surface needed for higher flow rates. The results of these humidity measurements confirm the efficiency of the apparatus.

To demonstrate the efficiency of the hydrogen extraction from the helium at the working temperature of 78 K, the test rig will also house a liquid nitrogen cooled adsorber bed, designed and presently under construction at the Forschungszentrum Karlsruhe. To permit a selective regeneration mode between hydrogen and the co-adsorbed impurities (mainly nitrogen and oxygen) the molecular sieve’s desorption process should be achieved in two separate stages by raising the temperature first to 150 K and then to 573 K. In the first stage only hydrogen isotopes will be released and in the second the remaining impurities will be released by further heating. The first stage will be controlled by a flow of evaporated liquid nitrogen heated up to 150 K and passed through a tube spiral placed around the molecular sieve container. Complete regeneration is achieved by the mean of electrical heaters installed along the adsorber bed.

As next step, a test facility on a technical scale (one sixth of the flow rate of the ITER-HCPB)

will be built in a glovebox in the Tritium Laboratory Karlsruhe (TLK) for tritium experiments

with the two main components of the extraction process, cold trap and molecular sieve bed.