UK scientists develop disruptive membrane

RESEARCHERS have pilot-tested a graphene-based membrane system that promises to radically reduce the energy costs and size of plant needed to produce heavy water for the nuclear power industry.

The nuclear industry produces more than 1000 t/y of heavy water for use as a moderator in nuclear reactors to slow down neutrons so they react specifically with U-235. The ‘heavy’ in heavy water refers to the higher than natural presence of the hydrogen isotope deuterium. This consists of one proton and one neutron, compared to the far more common hydrogen isotope, protium, which has no neutron.

The key challenge with producing heavy water is that deuterium occurs in a low natural abundance of 0.015% so the industry must process large volumes of water to concentrate it to the 20% needed for initial enrichment by industry. Current methods require hundreds of separations stages and as a result today’s heavy water plants are costly to build and use lots of energy in their operations.

Now a team of scientists at the University of Manchester, UK, say they have developed a new separation technique that reduces current costs and energy uses to such a degree that the nuclear industry should consider the rapid introduction of this disruptive technology.

“The potential gains are high enough to justify its introduction even in the highly conservative nuclear industry,” said researcher Marcelo Lozada-Hidalgo.

The team has created a separation technique that uses a graphene-based electrochemical pump to separate the two hydrogen isotopes. The membrane was created by taking graphene laid down through chemical vapour deposition and hot pressing it to a polymer film. Palladium nanoparticles were then added to increase its catalytic properties and carbon cloths were used to electrically contact the membrane. Applying a voltage pumps a greater proportion of the light hydrogen isotope through the membrane, leaving behind deuterium.

“So we started with a mixture of deuterium and hydrogen on one side of the membrane…and proved that the gases that permeated through the membrane have a much lower concentration of deuterium,” said Lozada-Hidalgo. “Once you get the gases separated – which is the difficult bit – combining them with oxygen to form water is a much simpler process. That process is done all the time in the chemical industry.”

The team says the technique would use 100 times less energy than conventional heavy water plants – and that its estimate is based on a single stage separation. This promises to reduce the footprint of today’s heavy water production processes. Furthermore, the method will readily scale as the membrane could be produced at a size large enough for industrial use using conventional roll-to-roll fabrication techniques.

It also avoids the corrosive and toxic substances involved in the existing the Girdler sulphide and monothermal-NH₃/H₂ heavy water production processes used currently.

“Although the existing isotope separation technologies are well established, the described advantages of the graphene-based separation seem significant enough to justify rapid introduction of this disruptive technology even within the highly conservative nuclear industry,” the team concludes in its research paper.

The team believes its process could be further improved by increasing the operating temperature; adding separation stages; and using hydrogen produced in the process as an energy source.

Nature Communications: doi.org/b66m