French nuclear regulator halts assembly of huge fusion reactor | Science

France’s nuclear regulator has ordered ITER, an international fusion energy project, to hold off on assembling its gigantic reactor until officials address safety concerns. This month, the ITER Organization was expecting to get the green light to begin to weld together the 11-meter-tall steel sections that make up the doughnut-shaped reactor, called a tokamak. But on 25 January, France’s Nuclear Safety Authority (ASN) sent a letter ordering a stoppage until ITER can address concerns about neutron radiation, slight distortions in the steel sections, and loads on the concrete slab holding up the reactor. ITER staff say they intend to satisfy ASN by April so they can begin to weld the reactor vessel by July. “We’re working very hard for that,” says ITER Director-General Bernard Bigot.

Fusion is often promoted as the green energy source of the future, generating carbon-free power by fusing together hydrogen isotopes in the same way as the Sun. Getting the isotopes to meld requires extreme temperatures and every fusion reactor built so far has consumed more heat than it produces. ITER is designed to show net energy output can be achieved, but it comes at a high cost—estimates start at about $25 billion for its construction—because of the complexity of the reactor and the huge superconducting magnets required to keep the hot gases in place. A partnership between the United States, Europe, Russia, India, Japan, China, and South Korea, ITER is scheduled to start operations in 2025, although it won’t be fueled with the power-producing isotope tritium until 2035.

In 2012, ASN validated ITER’s overall design and authorized construction to start. But it imposed several “hold points” in the construction process when ITER must demonstrate that the reactor meets safety requirements. One of those points comes when workers are set to lower and weld together the first two of nine reactor sections, each weighing 1200 tons, because the process is irreversible: The welded sections are too heavy to remove from the pit if any later changes or inspections are required.

The ASN letter highlights three problem areas. The first concerns loads on the structure holding up the tokamak. Known as the B2 slab, it is a 1.5-meter-thick block of reinforced concrete the size of two U.S. football fields. It rests on 493 seismic dampers to isolate the reactor from earthquakes. It’s designed to support 400,000 tons, but ASN wants reassurance that, following some design changes during construction, the loads on the slab are still within safety limits. “We have to complete a modeling of the mass as built,” Bigot says.

A second concern is over radiation protection for staff working near the reactor once it begins operations. The main radiation coming out of the reactor will be high-energy neutrons, which are stopped by the thick concrete walls in the building that will surround the reactor. No one will be in the reactor building when it is operating, Bigot says. But over its lifetime, the reactor itself becomes radioactive from the neutron bombardment, creating a complicated radiological environment for workers who enter the building when the tokamak is not in operation. Existing “radiological maps do not make it possible to demonstrate control of limiting exposure to ionizing radiation,” ASN says, according to a translation of its letter.

Bigot says ASN usually only requires nuclear facilities to produce a 2D model of potential radiation exposures. But ITER built a 3D simulation to predict neutron fluxes more precisely. ASN wants more evidence that this model is as robust as the simpler one, Bigot says. “We have to demonstrate that our choice is the best option.”

A third concern is over welding the first two tokamak sections. Following their construction in South Korea, managers discovered slight deformities in the surfaces that must be welded together. ITER staff developed a fix that would involve both robotic and human welders, but ASN is not convinced. Bigot says he now has a report from the Spanish company that developed the robotic welding system. The company tested the process on a full-scale mockup and showed it will be possible for workers to get into the confined spaces needed to make the welds. That report will form part of ITER’s April response to ASN.

Delays from COVID-19 had already pushed back the planned start of welding to July. If ASN accepts ITER’s explanations in April, that timetable is still achievable, Bigot says. Although some at ITER are concerned about the threat the stoppage poses to the schedule, Bigot says it should be possible to make up time. He says he also understands ASN’s desire to be careful with a machine that will be the first of its kind. “They want to understand very precisely the safety risks,” Bigot says. “So it’s not surprising they’re taking a little more time.”