ENEA and CEA have developed a membrane process and a palladium-based reactor to extract tritium from JET’s housekeeping waste. Compared to alternative systems, the membranes in the new device suffer less mechanical stress and the overall process is more energy efficient. Both Associates have jointly filed patents for their inventions, as the technology may also be employed in other nuclear facilities or used to produce pure hydrogen for clean energy applications such as polymeric fuel cells. The work was carried out within the JET Fusion Technology Task Force.

Housekeeping waste comprises gloves, masks, personal gas filters or over clothes that have been used inside JET’s reactor chamber. A nuclear facility produces around 0.2 kg housekeeping waste per hour and worker. Reducing the tritium content of this waste is important, because the disposal costs depend on the level of contamination and valuable tritium can be reused. The usual detritiation techniques, however, produce tritiated water, which must undergo a second process to recover the tritium.
ENEA and CEA have adapted the concept of palladium silver alloy membrane reactors – a technique which is also used to detritiate JET exhaust gas. These membranes are permeable for hydrogen isotopes, but not for molecules and thus enable the separation of tritium. The waste is ground up and heated to about 120 °C and a carrier gas stream takes up the tritium in form of tritiated water vapour. Inside the membrane reactor, the tritiated gas flows through a 50 cm long membrane tube with a diameter of 10 mm and a wall thickness of 0.150 mm. Outside the tube, pure hydrogen gas flows in the opposite direction. These hydrogen atoms enter the tube and replace the tritium atoms in the waste gas. A catalyst in the tube promotes this isotopic exchange reaction. The tritium atoms exit the tube through the membrane and leave the reactor with the outside gas stream.

Fusion power burns the nuclei of hydrogen isotopes tritium and deuterium. Tritium is a radioactive element and contaminates the reactor chamber. It also is practically non-existent in a natural state and is therefore very expensive. Thus a fusion power plant must be able to recover and reuse all unused tritium. While scientific fusion experiments usually use with deuterium plasmas, JET is the only currently operating device that has used tritium in the past and which has another deuterium-tritium experimental campaign planned for 2015.

CEA developed the detritiation process and ENEA built the reactor. Its design overcomes major two complications that were evident in previous versions. Firstly, during operation, the long and thin-walled membrane tube elongates up to about two percent because it takes up hydrogen. If it is mounted in a fixed manner, it compresses cyclically. The mechanical stress may cause defects in the crystal structure of the membrane, and thus cause it to lose its property of being selectively permeable for hydrogen isotopes only. The tube fixtures in the new reactor eliminate any harmful compressive mechanical stress. Secondly, the tritium exchange reaction is most efficient at temperatures between 300 and 400 °C. The new reactor heats only the membranes via an electric current running through the tube (direct ohmic heating). It requires approximately half the heating power of traditional systems which heat the gas and the reactor shell as well. This high level of efficiency makes the reactor more attractive for producing pure hydrogen in clean energy applications.

Tests performed at CEA show that the Pd-membrane reactor is able to achieve a tritium decontamination factor of 10 for the gas. This thus demonstrates that the method can be applied to the treatment of housekeeping waste. The capacity of this laboratory device is sufficient to detritiate incoming waste from JET. However, if all of JET’s housekeeping waste inventory was to be treated, this prototype would need to be scaled up to a multi-tube version.

X. Lefebvre, et al., Preliminary results from a detritiation facility dedicated to soft housekeeping waste, Fusion Eng. Des. (2012) (Link dahinter: http://www.sciencedirect.com/science/article/pii/S0920379612001366 )

S.Tosti,et al., Design of Pd-based membrane reactor for gas detritiation, Fusion Eng. Des.(2011) (Link dahinter: http://www.sciencedirect.com/science/article/pii/S0920379610004928 )