Turning neutrons into electricity

In the current ITER program, the power produced will be mainly captured by cooling water circulating in the ITER shielding blanket under pressure at low temperature (4.0 MPa and 70°C at the inlet), which gets sent to ITER's heat rejection system. In next-phase DEMO machines and beyond, the power carried by the coolant will be transferred to a power cycle fluid through a heat exchanger. For this, a high-temperature coolant will be required for economically acceptable power cycle efficiency. With this in mind, ITER's test blanket modules will be tested with their own independent cooling systems.

The test blanket modules are structures situated in dedicated vacuum vessel port plugs. The four modules planned for initial testing on ITER are each about the size of a person (1.7 x 0.5 x 0.6 metres). They will be mounted on two of the equatorial ports of the tokamak—Port 16 and Port 18—especially reserved for that purpose.

Directly facing the plasma, and supported by huge ancillary systems, the test blanket modules will be used to demonstrate the removal of the enormous heat generated by the neutrons escaping the plasma. This cooling process will be useful in demonstrating the feasibility of converting that heat into useable electrical power—which would be the basis for a future fusion power plant.

The process is being overseen by ITER's Nuclear Technologies Program Manager, Mario Merola. After working for the European Fusion Development Agreement (EFDA) in Garching, Germany, Merola joined ITER back in 2006. He now leads a team of 80 staff who manage all the components that will face the plasma—including the blanket, divertor, tritium breeding, hot cell, and remote handling projects.

"Like so many things in fusion science, the principles are relatively simple," says Merola, "but in practice they are both difficult and complicated to implement. In terms of both the scale of the project and the materials being used, we are dealing with challenges of a scale that have never been faced by scientists or engineers ever before—and of course that's what makes it such interesting work!"

There are two standard ways to produce electricity from the cooling of the heat generated by the nuclear fusion reaction: water cooling, and helium cooling. Both will be trialled at ITER via the Test Blanket Module (TBM) Program, comprising a total of four projects already underway by ITER Members: water-coolant projects are being developed by Japan and by Europe; while helium-coolant projects are under development in China, and also in a joint project between Korea and Europe. The four projects will be trialled simultaneously at ITER.

Water coolant

The first method—the water-coolant test blanket system—employs the same thermodynamic cycle used in a pressurized water reactor (PWR) in nuclear fission. Water is pumped under high pressure (15.5 MPa) to the test blanket module, where it is heated by the neutrons emitted from the plasma, reaching a temperature of 325 °C. The water remains liquid due to the high pressure, and is used to transfer heat away from the tokamak to lower-pressure water in a secondary coolant circuit where steam is generated. In a power station, this vapour would then be routed through turbines to generate electricity.

"Water cooling has clear benefits," says Merola, "in that it's based on technology that's already widely in use in conventional nuclear power plants all around the world. The drawback, however, is that it's not so very efficient—we only get around a third of the power back from the conversion from thermal to electrical power."

Helium coolant

The second method—the helium-coolant test blanket system—works on essentially the same principles, but at lower pressure (8 MPa) and higher temperatures (500 °C). Among the possible gases to be used as a reactor coolant, helium is the preferred choice as it has chemical inertness, combined with high thermal conductivity and specific heat.

"Standard thermodynamic rules mean we get considerably higher efficiency with the higher temperature, with conversion rates in excess of 40%," says Merola. "But helium also has its own drawbacks, of course. It provides about as much shielding from neutrons as a vacuum—so we need to compensate with extra shielding, usually in the shape of boronized steel. We also need a higher flow velocity for helium, which means higher pumping power."

So which coolant will be most likely to prevail?

"It depends on who you ask," laughs Merola. "But my own feeling is that we are likely to see the same pattern as in nuclear fission, with both coolants being used in power plants—but probably more water than helium."

Thinking beyond current constraints, are there any better ways of converting the heat from nuclear fusion reactions into electrical power?

"Well, that means going into fantasy science!" says Merola. "If you could have deuterium/helium-3 nuclear fusion, for example, then you would be able to envisage direct conversion of the fusion power into electrical power. Helium-3 only has one neutron, so the reaction wouldn't produce neutrons as an outcome, only charged particles. And these could convert most of their kinetic energy directly into electricity."

It sounds ideal. But deuterium/helium-3 nuclear fusion can only take place at temperatures exceeding one billion degrees Celsius—currently well beyond technical capacity. There is also the not-insignificant matter of finding the helium-3, one of the rarest materials on Earth. It does exist in greater quantities on the moon, but moon mining at any industrial level is still some way off into the future.

Another possibility would be hydrogen/boron-11 fusion, which also produces only charged particles—and the fuel is available on Earth. However, the required temperature for hydrogen/boron-11 fusion is billions of degrees Celsius, making this option also too futuristic today.

"It may be fantasy science," says Merola, in closing, "but deuterium/helium-3 or hydrogen/boron-11 nuclear fusion is really scientifically beautiful. Perhaps in a few generations...?"