France's nuclear safety regulator, the Autorité de Sûreté Nucléaire (ASN), has ordered the start of the assembly of the tokamak of the International Thermonuclear Experimental Reactor (ITER) fusion project to be postponed until a number of safety issues have been addressed.

ASN inspectors at the ITER construction site in Cadarache (Image: ITER)

In 2012, ASN approved the proposed ITER design and authorised the ITER Organisation and its domestic agencies to proceed with construction, manufacturing and installation of the ITER facility at Cadarache in southern France. However, as with any nuclear installation, ITER is required at each stage to demonstrate that its safety-relevant buildings, civil structures, systems and components conform to the approved design and meet the safety case in the "as-built" or "as-installed" condition.

On 25 January this year, ASN issued a letter in which it stated that it could not release the ITER tokamak assembly "hold point" as expected on 1 February and requested some further clarifications.

The specific hold point now under review was established by ASN in November 2013. Under this hold point ITER cannot begin to weld the first two sections of the vacuum vessel together in the tokamak pit - an assembly step considered irreversible - until certain safety aspects related to the B2 slab (a 120-metre-long, 80-metre-wide and more than 1.5-metre-thick mass of reinforced concrete supporting the Tokamak Complex Building) have been validated.

First, ITER must demonstrate the as-built safety performance of this B2 slab. Second, ITER's radiological maps, which calculate the shielding effects of concrete and steel barriers that contain the radiation from the machine, must demonstrate that radiation levels will be safe wherever humans are present on the ITER site without requiring extra shielding material. ASN has also asked for more analysis to validate the vacuum vessel welding process, to account for some limited dimensional non-conformities the ITER Organisation has identified at the interface between the two vacuum vessel sectors now on the ITER site, which require adjustments to the robotic welding procedure.

"Technical discussions and requests for information are a normal part of the process, especially in a 'first-of-a-kind' machine such as ITER, which is also of course a 'first-of-a-kind' facility for ASN," the ITER Organisation noted. "This ASN action has not impacted the ITER assembly schedule to date."

The first full sector subassembly - consisting of the double-walled vacuum vessel sector, a tightly fitted thermal shield and two toroidal field coils - has been completed and will be transferred into the tokamak pit in the coming weeks, it added. The fabrication of the second subassembly has begun, but it will take several months to complete, and still more time before it is inserted into the tokamak pit and preparations can be made for welding.

"Experts are vigorously preparing the additional information and analysis required, with the goal of providing all answers to the ASN by the end of April," the ITER Organisation said. It expects this will allow ASN sufficient time to consider the additional information and release the hold point without impacting the ITER assembly schedule.

ITER is a major international project to build a tokamak fusion device designed to prove the feasibility of fusion as a large-scale and carbon-free source of energy. The goal of ITER is to operate at 500 MW (for at least 400 seconds continuously) with 50 MW of plasma heating power input. It appears that an additional 300 MWe of electricity input may be required in operation. No electricity will be generated at ITER. First plasma is planned for 2025, with deuterium-tritium fusion experiments commencing in 2035.

The European Union is contributing almost half of the cost of its construction, while the other six members (China, India, Japan, South Korea, Russia and the USA) are contributing equally to the rest.

Researched and written by World Nuclear News

Date: Wednesday, 02 March 2022
Original article: world-nuclear-news.org/Articles/ITER-tokamak-assembly-put-on-hold-by-regulator