Racing against concrete
For eight years, between 2012 and 2020, a large concrete batching plant operated on the ITER platform. Mixing gravel and sand, cement, water and additives, it produced a wide range of concrete formulas, each adapted to the specific requirements of the ITER buildings and structures. Delivered directly to the necessary areas by way of powerful pumps and extended booms, or snaking its way through piping laid inside the buildings, approximately 120,000 cubic metres of concrete were used during the civil works phase of Tokamak Complex construction.
The age of concrete has waned on site and for the relatively small quantities still needed, ITER now depends on a batching plant located six kilometres away in the village of Vinon-sur-Verdon. Distance and delivery time are not an issue, provided concrete is transported in a mixer truck directly from the plant to the pouring site. In some instances, however, no truck, pump or piping can access the pour location and a quick solution must be found to deliver the required volumes of concrete where they are needed.
Concrete is like cake mix : depending on its formulation and viscosity, it only remains usable for a defined amount of time. “In order to remain within the workability window, certain operations in certain areas of the buildings require considerable organizational and human effort,” says Armand Gjoklaj, a civil nuclear engineer in ITER’s Civil Engineering and Interface Project. Initiated in September, the backfilling of openings created for busbars in the concrete slab between levels 3 and 4 of the Tokamak Building perfectly illustrates the challenges the teams are facing.
The openings are located just above the upper pipe chase mezzanine, which accommodates the primary loop of the machine’s cooling water system. The 1.45-metre-thick slab is made of high-density borated concrete and will act as a nuclear shielding and confinement barrier when ITER enters operation.
The area, on the rim of the tokamak pit, is restricted and subject to strict cleanliness protocols: workers must wear white lab coats and special shoes, no dust must be generated, not a single drop of concrete must be projected, and the pouring must be done under sealed plastic tents. Concrete pouring in these conditions looks almost like a clean room operation.
Although the quantities needed to fill the busbar penetrations are small (one cubic metre per opening on average) the concrete formulation is the “heaviest” of all those implemented in ITER construction. Whereas one cubic metre of standard raw concrete weighs approximately 2.2 tonnes, the formulation implemented here is more than 60 percent heavier (3.6 tonnes).
The clock starts ticking as soon as the concrete batch has been mixed at the plant. From that moment on the teams have no more than 150 minutes to deliver, via the temporary cargo lift¹, the required volume of concrete to the work area. Two-and-a half hours might seem like a comfortable amount of time, but the sequence of operations and the successive transfers from one container to another makes for a very tight schedule.
Two main constraints need to be taken into account. One is the “workability” of the concrete batch (how long it remains usable); the other is the handling capacity of the cargo lift, which is limited to 8 tonnes.
The mixing truck contains two cubic metres of high-density concrete, amounting to approximately to 7.2 tonnes. With the added weight of the skip, the load would exceed the cargo lift capacity.
Once the mixing truck has parked at the foot of the Tokamak Building in the excavated zone reserved for the Hot Cell, half of the concrete is emptied into a one-cubic-metre skip. A telescopic crane lifts the skip and empties it into two or three much smaller containers that are swiftly moved by forklift to the cargo lift platform. “By using smaller skips, one of 500 litres and two of 250 litres, we remain within the handling capacity of the forklift and we can spread the load evenly inside the cargo lift,” explains Bruno Pinard-Legry, the site engineer for the contracting companies Nuvia-GTM Sud, both subsidiaries of Vinci Construction.
Like an oversize elevator, the cargo lift moves the skips up five levels and delivers them to a waiting forklift at Level 4. The forklift (or a pallet jack for the smaller skips) transports them over a short distance to the work area, where they are emptied into a skip whose capacity matches the quantity of concrete needed to fill a busbar opening—generally in the range of one cubic metre.
Attached to the overhead crane, the skip is lifted and positioned above the opening to be filled. The skip's chute is manually guided towards the opening, a valve is opened, and concrete starts rushing out. Eighty-five minutes have elapsed since the mixing truck left the batching plant.
In the meantime, five levels below, the same sequence of filling, lifting, moving and emptying has started in order to deliver the second half of the mixing truck’s load. “The timing of the two operations is crucial. Within the 150-minute envelope, we allow no more than 15 minutes for contingencies,” says Pinard-Legry. “If we’re late and exceed the workability duration of the concrete, we’ll either have to throw it away if it is still inside the mixing truck, or evacuate it from Level 4, which would be exceedingly complicated…”
Lessons learned from previous concrete pouring operations and precise “chrono analysis” of the filling of the first busbar opening in September have resulted in a smooth process that runs like clockwork and reconciles the rough reality of concrete pouring and the demands of a clean room operation.
The last of the 15 openings should be filled this week.
¹The cargo lift presently in operation is a temporary device that will be replaced with a considerably more powerful system, designed to transport activated components (up to 120 tonnes) from the Tokamak Building to the Hot Cell during ITER operation.