From river to droplets and mist

 

Evaporation cools—we experience it on a daily basis when we step out of the shower. As water molecules transition from liquid to gaseous (vapour), they draw heat from the skin and generate a cold sensation throughout the body. This is one of the principles at work in the ten cooling cells of the heat rejection zone, a 6,000-square-metre facility located at the north end of the ITER platform.

 

A squat black building accommodates the main feature of the ITER heat rejection system: ten cooling cells that act like the familiar concrete cooling towers of an industrial power plant, whether thermal or nuclear. But whereas the upward airstream that accelerates the evaporation process occurs naturally in a conventional cooling tower, it is induced by large fans in the ITER cooling cells.

 

The advantages of induced draft cooling towers are size, cost and flexibility: their footprint is much smaller than that of a concrete cooling tower, their construction cost is significantly lower, and their flexibility is well adapted to the intermittent nature of ITER operation.

 

The efficiency of the cooling process depends of the surface through which the heat is exchanged. Similar to cooling fluid in a car engine passing through the honeycomb structure of the radiator, hot water entering the cooling cells¹ is sprinkled by a set of 4,540 spray nozzles into a stack of corrugated plastic sheets called a "fill pack." If unfolded, the total exchange surface provided by the fill pack would be on the order of 704,000 square metres, the equivalent of some 70 football fields.

 

Efficiency also depends on how long the droplets remain in contact with the fill-pack exchange surface. Left to the natural effect of gravity, the duration of contact would not be sufficient and droplets falling from the spray nozzles would not have enough time to cool to the required temperature. To remedy this, the droplets' fall is slowed by the upward draft induced by the large fan on top of every cell.

 

This upward draft creates a drag force, however, that could cause the droplets to scatter and be lost. And water—in the ITER heat rejection system like everywhere else—should not be wasted.

 

 

At the end of the process the water, now considerably cooler than when it entered the cells (27 °C), falls into the cold basin under the towers. From there, it is transported by huge vertical pumps to the different heat exchangers of the system, where it goes back to cooling the plant systems and the Tokamak secondary loop.

 

And the cycle repeats itself endlessly, extracting and rejecting the massive quantity of heat generated by the tokamak, its auxiliaries and the installation's plant systems. At full power the combined load that needs to be evacuated is of the order of one gigawatt, approximately half of it generated by the heat from the fusion reaction collected by the plasma-facing wall inside the vacuum vessel.

 

If ITER were an industrial fusion plant, the heat in the plasma-facing wall would be converted into electricity by way of steam generators, turbines and alternators. But as a research installation—with a non-steady-state experimental device—installing electricity-generating equipment would not have made sense.

 

On the roof of the squat black building that accommodates the cooling cells, seven out of ten funnel-like fan cylinders are in place. The remaining three are being assembled and will be lifted in the coming weeks. Inside the cells, 80 percent of the fill pack units are now stacked together under the spray nozzles, and about half of the 4,000 drift eliminator sheets are glued together and ready to be positioned above the fill packs, under the large fans.