Cooling towers: the white plume is water vapour

A nuclear plant converts barely a third of the reactor's thermal energy into electricity; the other two thirds have to go somewhere. A 3,000 MWt reactor delivers about 1,000 MWe to the grid and rejects about 2,000 MWt as waste heat. The cooling tower is the equipment that carries that heat into the air; it is the last stage of the turbine's steam cycle, not a safety system for the reactor.

The water rising through the tower is not the water that flows through the reactor. Steam leaving the turbine condenses against a third cooling-water circuit, separate from the primary circuit, that never touches the fuel or the water surrounding the core. That condenser cooling circuit is what gets cooled in the tower and returns to repeat the cycle; at plants that cool with a river, a lake or the sea instead, the same circuit returns the water to its source a few degrees warmer, with no tower involved.

The hyperbolic natural-draft tower, the concrete-shell shape associated with nuclear power, has no fans: hot water from the condenser is sprayed over a fill near the base, cold air enters at the bottom and rises by convection because warm, humid air is lighter, and that draft carries the heat up and out through the top opening, which at large plants exceeds 200 metres. A fraction of the water evaporates on contact with the air and carries away the latent heat of vaporization; the rest, now cooler, falls to the basin at the base and returns to the condenser. That vapour is pure water, with nothing dissolved in it, because vapour cannot carry what was in the liquid water; what you see above the tower is that vapour condensing back into microscopic droplets as it mixes with cooler air, the same thing that makes your breath visible on a cold day.

That is why the plume depends on the weather: on a warm, dry day the surrounding air absorbs the vapour without it condensing, and the tower keeps working the same even though nothing is visible leaving it. Towers are not a nuclear technology either: any steam-cycle thermal plant, coal, gas or a refinery, has to reject the same waste heat, and many use identical towers. And a large share of the world's nuclear plants have no towers at all because they cool with a river, a lake or the sea: Atucha I and II, on the Paraná de las Palmas river, and Embalse, on the Río Tercero reservoir, both in Argentina, have no hyperbolic towers; Gravelines, in France, cools with water from the North Sea. Cattenom and Chooz, also in France, and Three Mile Island, in the United States, do have them.

Where towers exist, the water use is real: a 1,000 MWe plant with cooling towers evaporates on the order of 75 megalitres a day, about 25 Olympic pools, according to the World Nuclear Association. What actually leaves a nuclear plant through a stack are the process's gaseous effluents, tiny quantities of noble gases, iodine and tritium that are filtered, held and monitored continuously before release; the additional dose to a person who lived all year at the site boundary is, according to the NRC, a small fraction of what they already get from natural background radiation.