There are two types of cooling towers in use today, forced draft and natural draft. The towers at TMI, pictured here, are of the natural draft type, characterized by their imposing height and distinctive hourglass shape. Natural draft cooling towers have come to be a symbol of nuclear power, and certainly they're the most memorable sight to be seen at Three Mile Island. The hourglass shape is designed to cause a "natural draft" (i.e. a natural convective flow of air upward through the tower), as the name suggests.

Photo: TMI Cooling TowersInside the towers, hot water from the condensers is pumped partway up the tower. It is then sprayed out onto a series of baffles, channels, and vanes known as the fill.  These structures slow and spread the falling water throughout the tower's inner cross-section, dividing it into fine droplets and exposing maximum surface area to the air.

As the water falls, the fine droplets give up some of their heat by evaporation. Meanwhile, the warmed air rushes upward, carrying away the water vapor and drawing more cool air into the openings at the bottom of the tower. The remaining liquid water, which has now been greatly cooled, falls to the bottom of the tower and is collected.

The evaporative nature of the cooling towers' operation explains the large clouds of water vapor (similar to the foggy breath you exhale on a cold winter day) that we see above the cooling towers. This is one of the only outward signs that a nuclear power plant is in operation. The photo at left was taken after the accident, so there are no vapor clouds.

This process, by its very nature, evaporates a lot of water. To replace that volume, fresh water is drawn in from the Susquehanna River and added to the reirculating volume of tertiary, or "circ" water. It is seldom necessary for circ water to be released back to the river, although it would be entirely safe to do so.

The cooling towers exist primarily as an environmental protection measure. Dumping hot water into the river would be substantially cheaper, but would cause irreparable damage to the river's ecology.