The Most Complex Reinforced Concrete Basemat Ever Made?

To say that the world’s biggest nuclear fusion reactor requires strong foundations is an understatement. Amongst the hills of Cadarache, near Marseille in the South of France, civil engineers have completed the monumental task of pouring 14,000 cubic metres of concrete to make a base slab for that exact purpose. The completion of the foundation which is roughly the size of a football pitch, marks the start of a new chapter for the site of ITER (the International Thermonuclear Experimental Reactor) the purpose of which is to test whether plasma fusion is a practical method of energy generation.

The nuclear reactor, called a tokamak, needs some serious concrete engineering to support it safely. The massive machine, weighing roughly the same as a fully fuelled Boeing 747, will sit on a basemat of concrete 1.5m thick. The concrete basemat will not only withstand the actual weight of the machine but also deal with the forces generated by the electromagnetic effects of the movement of plasma and its interaction with each of the reactor’s massive electromagnets.


Following the excavation of a seismic isolation pit, a 1.5m basemat and retaining walls were constructed to create a ground support structure, giving protection to the buildings and their equipment in case of earthquakes. On top of this are 493 columns, each topped with anti-seismic pads which will absorb any movement should the area suffer from any earthquake tremors.

On top of these columns sits another concrete basemat called B2, also 1.5m in depth, which is the concrete floor on which the tokomak itself will sit. The B2 basemat is made of reinforced concrete with 40mm bars laid in a specific pattern corresponding to the forces that will be acting upon the slab.

Seismic building specialist Laurent Patisson, who is responsible for the safety of the ITER site explains how this extraordinary project required special measures:

"We have up to 26 layers of rebar in some areas. The 40mm thickness of the bars is standard for infrastructure concrete – like bridges – but on top of the basemat we’ll be building some radial walls…they’ll have 50mm reinforcement. That’s not common at all."

With over 14,000 cubic metres of concrete to pour, it made sense for ITER to have its own concrete batching plant onsite. Over 95% of the material used in the project is standard concrete mixed with the ground-up bedrock that was removed during the excavation of the site. The B2 basemat was divided up into 15 sectors which were poured individually, each taking a day to pour, then a further 10-15 days to set. The pouring of this huge volume of concrete was done over a 9-month period.

event_18_Visite chantier ITER

Completion of the entire basemat marks the end of the first phase of construction which started back in August 2010 and represents an investment of €100 million. The EU is funding half of the cost with the rest being made up in equal contributions from China, Japan, India, Russia, South Korea and the USA.

ITER Director General Osamu Motojima said:

"With the completion of the B2 slab, a new chapter opens in the history of our project. We have built the floor, now come the walls, then the roof, and after that the machine itself."

Laurent Patisson is proud to have reached this significant milestone in the ITER project’s history too, adding: “I believe it’s the most complex reinforced concrete basemat ever made.”

The facility is expected to reach full operation in 2027.


Images and Video courtesy of the ITER Organisation