The Tertiary (Circ) System

The tertiary loop picks up cool water, generally from a nearby river or lake, and carries it through the condensers to cool and condense the turbine outlet steam. The water is then cooled evaporatively by circulating it through cooling towers.

Report of the Presidential Commission

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.

Inside the towers, hot water from the condensers is pumped to the top of the tower. It is then sprayed out onto a series of baffles, channels, and vanes known as the fill, which slows and spreads the falling water throughout the tower's inner cross-section.

As the water falls, the fine droplets are exposed to air, and give up some of their heat by evaporation. 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 water, thus 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 (or more precisely, tiny droplets of condensed water vapor similar to the foggy breath you exhale on a cold winter day) that we see above the cooling towers, one of the only outward signs that a nuclear power plant is in operation. The photo at left was taken after the accident, hence no vapor clouds.

After being cooled to nearly ambient temperatures, the water is released back into the environment. In the case of TMI, this means that the water is dumped right back into the Susquehanna river from which it came (less, of course, that which is lost to evaporation.)

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.