Russell's Blog

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Boiling Nukes

Posted by Russell on August 17, 2007 at 8:35 p.m.
Like coal and gas power stations, nuclear power plants are heat engines. They produce power by exploiting the difference in temperature between two heat reservoirs. When most people think of a nuclear power plant (or any heat engine, really) the component that naturally dominates one's attention is the hot reservoir -- the reactor. The reactor contains most of the clever science and engineering, so it commands attention. But it takes two reservoirs of comparable heat capacity to make a heat engine.

Surprisingly, though, the electricity production of a given power plant is usually not limited by the reactor. We know how to build staggeringly enormous reactors, and even small reactors can be designed to run extremely hot. Rather, the generating capacity is limited by the heat capacity of the cold reservoir, which is a function of the natural environment in which the power station is situated. Nuclear reactors are cooled by water, so the generating capacity of a nuclear power plant is directly proportional to the quantity and temperature of water available from the environment.

So, what happens when there is a drought? Or a heat wave? Or both? The heat capacity of the cold reservoir shrinks, and the generating capacity of the power plant shrinks with it. It doesn't matter how big and fancy the reactor is if there isn't enough cooling water.

The water in the Tennessee River has gotten so hot this summer -- more than 90 degrees Fahrenheit averaged over a day -- that the TVA was forced to shut down one of the reactors at Browns Ferry. The heat capacity of their cold reservoir has shrunk so much that they can only operate two of their three reactors. The TVA is already suffering from reduced production at their hydroelectric stations due to drought conditions.

The lesson here is that nuclear power isn't simply a solution to global warming. It a technology that is threatened by global warming.

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