Pebble bed nuclear reactors are still in the development phase and have yet to be placed into operation in any nation, but these reactors promise to eliminate many of the roadblocks that have prevented emissions-free nuclear power from generating a greater share of our electric power.
In the 1970s Dr. Rudolf Schulten conceptualized a way to produce nuclear power through the use of a large number of small, self-contained spheres that individually combine fuel, structure, and containment. Tennis ball-sized ceramic spheres, each containing a pebble-sized piece of uranium, would be tightly packed into a container in which heat-transfer fluid could be conducted at an extremely high temperature.
As revised and currently envisioned, a pebble bed reactor (PBR) producing 120 megawatts of electricity might contain as many as 360,000 of these pebbles in a reactor core cooled by a semi-inert gas such as helium or perhaps carbon dioxide or nitrogen.
Its major attraction would be a dramatically reduced need for safety features to preclude or combat a possible meltdown. The technology’s own safety features, known as Doppler broadening, work as follows:
As the small amount of uranium inside each self-contained sphere increases in temperature, it absorbs more and more neutrons in its individual sphere, which reduces the number of neutrons still available for subsequent fission. This caps the power of the reactor, in what is called a negative feedback.
This Doppler broadening occurs at around 900 degrees centigrade, which effectively caps the potential temperature in each sphere at that 900 degree level. The ceramic-coated spheres, meanwhile, will not melt at temperatures below 2000 degrees centigrade, thus precluding the chance of a meltdown.
When helium is used as the coolant, it will directly turn low-pressure turbines without intervening losses from heat exchangers. Helium is chemically inert, with the additional benefit that it cannot be transformed into a radioactive element.
Safer More Efficient
The inherent safety features of pebble bed reactors preclude the necessity for additional, redundant, costly safety features currently required for other nuclear power plant designs. That reduces costs and increases safety.
The use of gas as a coolant also significantly reduces the problem of a cooling liquid absorbing radioactive material in some type of accident. There will also be no need for piping to carry the cooling fluid, which can become brittle over time and result in ruptures. In the pebble bed the helium-filled space between the so called pebbles acts as the piping.
Also, because of its ability to operate at higher temperatures, a pebble bed reactor can be as much as 50 percent more efficient than conventional nuclear power plants. Thus pebble bed reactors deliver a great deal more electrical power from each pound of fuel.
Precision, Ease of Use
The pebble bed systems can also be temperature-controlled in a very precise manner by altering the flow of gas coolant through the system. This allows the system to operate in a narrow range of radioactive output.
Conventional light-water reactor nuclear plants control the system by inserting nonradioactive rods around the nests of radioactive rods, which alter the density of radioactivity and thus its output. These systems are more complicated and less efficient than pebble bed reactors.
Another advantage of the pebble bed design is that the reactors do not have to be shut down periodically to refuel by replacing spent fuel rods with new fuel rods. In the pebble bed design there is always an opening at the bottom of the container through which spheres can be removed while new ones are added at the top. The spheres that are removed are tested for their radioactivity. If the spheres are still active enough, they are transferred to the loader at the top.
As with conventional nuclear power plants, the pebble bed reactor is in a container with walls two meters thick. The reactor is in turn enclosed in a containment structure built to withstand the crash of any large airplane.
Perhaps the greatest benefit of the pebble bed reactor is its modular capability. Additional pebble bed units can be added as needed. Economies of scale can be realized, and several reactors can share control equipment. Small reactors can be mass-produced, which will facilitate rapid safety certification and design acceptance.
Jay Lehr, Ph.D. ([email protected]) is science director of The Heartland Institute.