How will the power generated by nuclear fusion reactions be converted into electricity? That is not a question that ITER has been designed to answer explicitly, since the heat generated during ITER operation will be captured by cooling water circulating in the in-vessel components and vacuum vessel walls and dispersed through cooling towers rather than being used to generate electricity. ITER will certainly help to provide some of the potential answers, however, thanks to the testing in the ITER machine of prototype test blanket modules. In addition to demonstrating the generation of tritium within a closed fuel cycle, ITER's Test Blanket Module Program has a second purpose—experimenting with different coolants for the future power-to-electricity conversion cycle. 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...?'

As the October sun sets on the ITER worksite, the cladding of the neutral beam power buildings takes on a golden hue. One after the other, each of the scientific buildings on the platform is given the ITER signature finish (mirror-like stainless steel alternating with dark grey-lacquered metal) to compose a striking architectural imprint on the Provencal landscape. This 'golden hour' image also captures the new banner that was recently installed on the north wall of the Tokamak Building. For three and a half years, the former banner had sent a message of collective pride celebrating the delivery of the building, home of the ITER Tokamak. Now that the machine has entered the assembly phase, the message is one of commitment. 'Safely assembling ITER' is a prerequisite for 'a future with fusion energy.'

Twenty years—that is how long it took to design, manufacture and deliver the cold valve boxes that regulate the flow of cryogens to the tokamak's vacuum system. Last week, as the first of the eight first-of-a-kind components was undergoing site acceptance testing, the ITER vacuum team organized a 'pat on the back' moment to celebrate the completion of this unique conceptual and industrial adventure. Procured by Europe, manufactured by Research Instruments (Germany) and its subcontractor Cryoworld (Netherlands), the eight cold valve boxes are 4-tonne components measuring more than 3 metres in height and approximately 2 metres in diameter. Each one is equipped with 25 cryogenic valves, relief systems, and pressure and temperature sensors that are specifically designed to operate under the harsh ITER environment. Located in the lower port cells, six boxes are destined for the torus cryopumps and two for the cryostat. Preliminary design activities began in 2004 at the French Alternative Energies and Atomic Energy Commission (CEA), the procurement agreement was signed in 2017 with the European Domestic Agency Fusion for Energy, and a final design was achieved in 2020. Designing and manufacturing the complex components was particularly challenging, as they will be exposed to the intense magnetic field inside the machine and the high neutron flux from the fusion reaction. The cold valve boxes will operate within an exceptionally large temperature range: from 4K (minus 269 °C) when distributing supercritical helium to the cryopumps, to 200 °C when hot gas is blown into the boxes during the regeneration process. The functional tests presently performed in the ITER vacuum lab will be followed by cold tests at liquid nitrogen temperature (80K, minus 196 °C). An installation is being prepared in the cryoplant to test at 4K.

In October 2011, when ITER organized its first 'Open Doors Day,' there was little to show and much to leave to the public's imagination: the Poloidal Field Coils Winding Facility wasn't yet finalized, the electrical switchyard was just a forest of pylons, and at the bottom of a large 17-metre-deep excavation on the platform, installation work had just begun on the anti-seismic system of the Tokamak Complex. The visitors of twelve years ago however sensed that something unique was beginning to take shape. Last week on Saturday, as the doors of ITER were opened wide once again, the immensity and complexity of the project hit each and everyone of the approximately 800 participants. Conceived and organized for the public, Open Doors Day is also a unique opportunity for 'ITER workers," whichever organization or company they belong to, to share their passion and expertise. This year, a record 75 volunteers, including ITER Director-General Pietro Barabaschi, guided the public and provided explanations and perspective. New features are offered at every event, as progress is made on the worksite and more components are finalized and delivered. On Saturday, the public particularly appreciated the viewpoints through plexiglass panes into the Tokamak assembly pit—a bit like an aquarium view on a mysterious subterranean world. Also impressive was the recently cocooned cryostat top lid, which gave a sense of the exceptional dimensions of the ITER machine. And of course there was the magic of physics made fun—the plasma ball, the expanding marshmallows, the tricks magnetism can play... all courtesy of the usual partners Petits débrouillards, United Crocos of Marseille and, last but not least, the in-house 'plasmagicians.'      

In its final deuterium-tritium experimental campaign, Europe's JET tokamak device demonstrated plasma scenarios that are expected on ITER and future fusion power plants, offering critical insights into key aspects such as heat exhaust, managing fuel retention, and the effect of fusion neutrons on cooling systems and electronics. The experimental campaign at JET was conducted by over 300 scientists participating in EUROfusion from across Europe together with engineering and scientific technical staff at the United Kingdom Atomic Energy Authority. JET is the only existing facility of its kind that can already operate with the high-performance deuterium-tritium fuel mix that will be used in future fusion power plants. While most fusion experiments use fuels like hydrogen or deuterium alone, testing with deuterium-tritium mix brings scientists and engineers as close as possible to the conditions of a real fusion power plant. In late 2021, the team at JET introduced the high-performance deuterium/tritium fuel mix for only the second time in the device's history. This DTE2 campaign resulted in fusion energy record output of 59 megajoules during a five-second pulse in December 2021. Other highlights of the campaign were: the first direct observation of the fusion fuel keeping itself hot through alpha heating, confirmation of predictions for heat transport inside the plasma, successful tests of tritium recovery, and the demonstration of heating and control techniques relevant to future fusion reactors*. The JET team has now completed its third and final campaign, DTE3. In a press release published last week, the team reports that it was able to replicate the high-fusion-energy experiments from DTE2, highlighting the reliability and maturity of JET's operational methodologies that are essential also for ITER's success. The campaign also tested optimized scenarios and novel operational strategies, enhancing scientists' understanding of deuterium-tritium plasmas; tested heat exhaust solutions, and focused on the effect of 14.1 MeV fusion neutrons on cooling systems and and electronics (the latter in collaboration with CERN). (See a detailed list of DTE3 achievements here.) JET commenced operation in 1983 as a joint European project, undergoing several enhancements to improve its performance over the years. In 1991, JET became the world's first reactor to operate using a 50—50 mix of tritium and deuterium. The facility set numerous fusion records including a record Q-plasma (the ratio of the fusion power produced to the external power put in to heat the plasma) of 0.64 in 1997 and a fusion energy record output of 59 megajoules in a five-second pulse in December 2021. Built by Europe and used collaboratively by European researchers over its lifetime, JET became UKAEA property in October 2021, celebrated its 40th anniversary in June this year, and will cease operations at the end of 2023. *See a full report of DTE2 results in this special issue of Nuclear Fusion.

Join the ITER Organization at the United Nations Climate Change Conference, COP28, in Dubai from 30 November to 12 December. You can find the ITER pavilion in the Blue Zone, B7, Building 88, on the second floor. Follow us on social media to stay abreast of the dynamic series of talks, panel discussions and events planned around fusion energy.

Global technology solutions provider NTT Corporation (Japan) and the ITER Organization are planning joint experiments on predicting anomalies in ITER plant facilities. The early detection of anomalies and failures can reduce equipment downtime and increase the efficiency of the ITER Tokamak's operational phases. NTT's artificial-intelligence (AI) powered anomaly prediction technology 'DeAnoS' (for Deep Anomaly Surveillance) will be used to understand the normal status of selected ITER plant equipment, to detect faults, and to predict anomalies. The ITER Organization has already provided some operational data from the cooling water system to set up this ambitious collaboration, and will scale up with data from other equipment in the years ahead. These experiments result from a Cooperation Agreement signed with NTT in 2020. See the press release issued by NTT here.