Nuclear Fission Vs. Combustion: Inexpensive Machines and Cheap Fuel
Nuclear fission is still in its adolescence, especially when compared to combustion, its major competitor. There is room for process innovations that will improve efficiency, increase flexibility and reduce machinery complexity. Changes in each of these areas offers the opportunity for major cost reductions.
The next time you travel on a large jet, look out the window. The engines hanging under the wing offer a clue leading to one path for reducing the cost of uranium based electricity.
Jet engines are impressive machines. They are compact, mechanically simple and lightweight. The engines on a 747 can produce approximately 40 megawatts (MW) of useful power. In comparison, a steam plant with the same output would fill up a large portion of the interior of the aircraft and its weight would prevent take off.
All operating nuclear stations use steam engines to convert heat into useful work. Engines similar to jet engines might be a better choice for advanced systems.
A jet engine heat cycle is rather simple. Air is compressed and fed into a combustion chamber where fuel is injected. The fuel/air mixture burns, adding heat to a mixture of combustion products. The combustion gases are expanded through a turbine that drives the compressor. The gases are then accelerated through a nozzle to produce thrust. There is one major moving part, the rotor on which the compressor and turbine turn.
It is becoming a common practice to use the same technology to produce electricity by adding a power turbine to the machine. Instead of producing thrust, these machines can turn a generator to produce electricity.
These aeroderivative gas turbines have had a niche in the electrical generation industry for several decades. They are much cheaper than large steam plants, they can be purchased and installed in short period of time, they can be started and stopped as needed, and they do not require extensive maintenance.
One Fatal Flaw
Gas turbine engines could very well provide the backbone of the electrical power system except for a fatal flaw. The fuel cost for gas turbines is higher than it is for steam plants.
The reasons for the high cost are two fold. Because the combustion products flow directly through a rapidly spinning turbine, the fuel cannot contain contaminants that can damage the turbine blades. Sulfur would cause major corrosion problems while particulates like ash would physically wear out the blades. Logically enough, clean fuel costs more than dirty fuel.
The second reason is being overcome by technology for the newest gas turbines. Until recently, material limitations in the turbines kept the efficiency of the machines lower than top-of-the-line steam plants.
Massive research and development efforts since the fuel price shocks of the 1970s have improved gas turbine efficiency to the point where the machines rival and often exceed the best that can be offered in a steam plant.
Even with the high cost of clean fuel, the efficiency improvements combined with low initial costs have put gas turbines in contention for most utility power projects. The machines come in a variety of sizes to match the utility’s need, whether the need is for 10 MW of emergency peaking power or 1000 MW of baseload power.
Nuclear Gas Turbines
A machine that combines the low cost and ease of use of gas turbines with the low fuel cost and zero emissions of a nuclear plant would be a huge improvement over current systems. Paraphrasing a famous commercial, such machines would “Taste great and be less filling.”
It surprises most people involved in the power industry to find out that nuclear gas turbines are indeed quite possible. The idea is not new. The first proposal for a nuclear heated gas turbine was published in 1945, fifty years ago.
In 1960, Holmes Crouch had the following to say about nuclear gas turbines, “The ‘ultimate’ nuclear plant for merchant ship propulsion appears to be some form of direct cycle reactor-turbine, eliminating steam and other forms of intermediate heat exchange.”
Time For a Change
In the past, however, reactor material temperature limits and the widespread use of steam plants prevented the commercial introduction of nuclear gas turbines. Times and technology have changed. Gas reactors with adequate temperature capabilities have been extensively tested and operated over long periods of time.
It seems imminently logical to match those reactors with the best heat engine available to produce flexible, cost competitive power. If the price of the nuclear machines is similar to that of the combustion machines, and the fuel can be manufactured for the same price as current commercial nuclear fuel, the total generating cost could approach one or two cents per kilowatt hour.