From: Holly Harrington [HMH1@nrc.gov]

Sent: Wednesday, March 23, 2005 7:04 AM

To: Bennettg@wfps.k12.mt.us

Subject: NRC Response

Dear Ms. Bennett:

You wrote to the U.S. Nuclear Regulatory Commission asking a question for your algebra students about the shape of nuclear power plant cooling towers. I posed the question to one of our experts here and he wrote a very lengthy response. Since you might enjoy it, I've pasted the whole answer below.

best wishes,

Holly Harrington

NRC Office of Public Affairs

Cooling towers exist largely as an environmental protection measure. The cooling towers are part of a closed cycle cooling system which minimizes discharge of waste heat to nearby rivers or lakes. Discharging plant cooling water, heated to approximately 90 to 100 degrees Fahrenheit, directly into a river or lake would be substantially cheaper, but could have and ecological impact. There are two types of cooling towers forced draft and natural draft cooling towers. Forced draft cooling towers use large motor driven fans to circulate the air through the tower. Hyperbolic cooling towers, as seen at some power plants, both nuclear and fossil fuel fired, rely on the natural circulation of air and are less expensive to operate than forced draft towers.

Large hyperbolic cooling towers that are frequently seen near power plants are designed to allow air to enter at the bottom and flow through the chimney like shape and come out at the top. The flared shape of the lower tower allows a large amount of air to enter at the bottom without causing too much wind disturbance at the base of the tower. Inside the tower, heated water from the plant condensers is pumped to the lower third 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 and pumped back to the plant much like the coolant exiting an automobile radiator.

The heated air starts to move up rapidly. The increased velocity through the constriction causes a drop in pressure which helps to draw cool dry air into the bottom of the cooling tower, enhancing the chimney effect. The flared shape at the top is to slow down the air velocity exiting the tower, thereby minimizing disturbance in air flow near the top of the tower. The flared top reduces the velocity of the exiting vapor which helps match the pressure of vapor exiting the tower to that of the ambient air. Also, the decreased the exit velocity of the air caused by the flared top helps to minimize vapor carryover. The evaporative nature of the cooling towers' operation explains the large clouds of water vapor (no, it's not steam) that we see above the cooling towers.

The shape of the tower is called "hyperbolic paraboloid." One of the main reasons for the shape is that being curved in both directions makes the tower much more rigid than a cylinder. The hyperbolic shape is also easier to construct because the curved shape can be generated by straight lines.