Every morning I read the online editions of the two main papers in Salt Lake City, because I was born and raised in Utah and want to keep up with what’s going on in my boyhood home. This morning there was a very interesting article:
Deseret News: Utah nuclear plant construction a step closer
I had no idea anyone was really seriously thinking about putting a nuclear power plant in Utah. For many years now, the state has fought the Skull Valley Band of Goshute Native Americans from building a dry-cask storage site on their tribal lands. Personally, I can’t understand why they would be so terrified of dry-cask-stored nuclear waste and yet be fine with the fact that at Dugway Proving Grounds the US has been storing chemical weapons for many years. But that’s another story.
No, my real question about a nuclear power plant in Utah is the question any good Westerner asks about any development: where’s the water going to come from? Water is everything in the West. Without water, there’s no crops, no development, no people. We are blessed with giant natural water storage towers called mountains, and each winter we would watch the snowpack carefully, knowing that that’s all the water we would be getting for the next year.
A typical light-water reactor, like any large thermal power plant, uses a lot of water for cooling. Using cooling water is essential because of the physics of the power conversion system. Light-water reactors use the Rankine cycle with water/steam as the working fluid. Water is boiled under high pressure until it’s all steam, then expanded through a steam turbine to make work (and electricity), then condensed at low pressure back to a liquid, where it can then be pumped back up to high pressure for a tiny fraction of the work generated during expansion.
The Rankine cycle is quite elegant, taking advantage of the thermodynamic efficiency of isothermal heat addition and rejection. In normal speak, it means that the best way to make work (and electricity) is to somehow add thermal energy without making something hotter, and to take thermal energy away without making it colder. That may seem impossible, but nature has given us one simple process to do it–the changing of phases, from liquid to gas, and from gas to liquid.
When you are bringing water to a boil, stick a thermometer in the pot. You’ll see the temperature creep up closer and closer to 100 degrees Celsius until boiling starts. Once the water starts to boil, the temperature won’t change anymore. It will stay at 100 degrees C until the water has all boiled away to steam. That’s isothermal heat transfer. Your stove is moving thermal energy into the water, but the water isn’t getting hotter (isothermal) because it is undergoing a phase change (boiling). It’s really very elegant.
The concern for Utah comes from the reverse process: condensation. After the water has been turned to steam and expanded through the turbine, it has to be condensed (turned from steam to water) by cooling it. And all of that thermal energy in the steam has to go somewhere. Here in the Tennessee Valley of Alabama, we have a large river where we can conveniently dump the heat from the three reactors at Browns Ferry. But I’ve seen the Jordan River, and the Bear, and the Weber and Ogden and Logan Rivers. They’re barely ditches compared to the mighty Tennessee. Even the Colorado, for all its fame, isn’t much to compare to the Tennessee.
And very few Utahns live near the Colorado.
What about the Great Salt Lake? I grew up four miles away from that body of water. I’ve never been there. One of the many problems of the Great Salt Lake is that it is so shallow and slopes so gradually that the water intake pipes and return pipes would have to be many miles long. Also the lake is so saline that the corrosion could be horrible. The land around most of the lake is very marshy. (I did try to go to the lake one day, but after half-a-mile of wading through knee-deep mud, I gave up and went back home)
Many of you know that I am an afficiando of the Brayton cycle, which does not use the phase change effect for heat addition and rejection. The working fluid of the Brayton cycle is always a gas. Because of that, there’s no way to add heat to a gas without it getting hotter, and there’s no way to take it out without the gas getting colder. Isothermal heat transfer simply isn’t possible. So right off the bat, the Brayton cycle has a major strike against it relative to the Rankine cycle. Why consider it?
Because if you have a large difference in temperature between heat addition and heat rejection, the Brayton can actually outperform the Rankine. This is because any working fluid you might choose for the Rankine cycle, be it water, mercury, potassium, etc, has basic thermodynamic limitations in its temperature range. The gases used in the Brayton cycle (typically helium or carbon dioxide) have far fewer, if any, such temperature limitations.
So we should just retrofit light-water reactors with Brayton cycles instead of Rankine cycles, right? No. The maximum coolant temperatures of the light-water reactor are such that a Brayton cycle would perform much worse than a Rankine cycle in a light-water reactor. There’s not much you can do about it, either, because if you go much hotter, you’ll melt the centerline of the oxide fuel of the reactor.
But reactors that can achieve much higher coolant temperatures, such as the gas-cooled pebble-bed reactor or the liquid-fluoride reactor, can utilize Brayton cycles to their advantage, and they incur far less penalty when they raise their heat rejection temperatures to the point where they can use air-cooling for the reactor instead of water cooling.
What does this mean for Utah? In my opinion, it means that high-temperature reactors, of whatever sort, using gas turbines and heat rejection to air, are going to be much more successful in the arid West than the conventional reactors that use water cooling.