7. The efficiencies of nuclear power plants are less than those of
fossil-fuel plants. Why?
Answers
Answer:
Current nuclear plants have a lower thermal efficiency than fossil fuel plants simply because they are run at a lower temperature.
Today's fossil fuel plants (thermal plants, not including gas turbines) have a furnace burning the fossil fuel which then heat up water in a heat exchanger. The furnace itself can basically be any temperature they want. You want higher temperatures, just burn fossil fuels more aggressively. The pipelines in the steam generator portion is toughened to withstand huge amounts of pressure, so many of the fossil fuel plants are actually capable of producing supercritical steam. It's at these pressures where high temperatures (500 degrees C and higher) allow for higher thermal efficiency.
On the other hand, nuclear reactors combine both the moderator and coolant into one. So the upper limit of a nuclear reactor’s temperature is the temperature at which the coolant boils. A higher pressure allows for a higher boiling point, but the pressure is limited to how tough the pressure vessel that the fuel is sitting in is. Current PWRs are capable of withstanding about 150 bar. This limits the temperature of the coolant to be no more than 300 degrees C. Any higher and you risk the coolant boiling, exceeding the pressure limit in the reactor vessel. Hence, you get a lower thermal efficiency.
The running temperature of the reactor is purely an engineering problem. Make the pressure vessel tougher and you can achieve higher temperatures (this, of course, requires even better forging capabilities). Alternatively, use something other than water and you can reduce the pressure required to maintain said temperature.
Gen IV reactors address the above engineering problem. Supercritical water reactors (SCWRs) use better manufacturing capabilities to make pressure vessels capable of withstanding water at supercritical temperatures. Alternatively, molten salt reactors, gas cooled reactors, and molten metal cooled reactors use a material either with a significantly higher boiling point so that you can maintain roughly atmospheric pressure (molten salt reactors and molten metal cooled reactors), or just design your reactor to be cooled by gas to begin with (gas cooled high temperature reactors) and bypass all the pressure requirements.
All these reactors are fully capable of thermal efficiency comparable or even exceeding that of other thermal reactors.