CO2: First Choice for Power Reactors

During the period from 1946 until 1954, the single most important constraint governing the development of peaceful uses of atomic power was the Atomic Energy Act of 1946. This American law – passed after a failed attempt to establish an international control regime for nuclear materials – made it illegal to trade in nuclear knowledge or nuclear materials. Congress, seeing the lead that the U.S. had developed by its Manhattan Project, decided that they would declare an American monopoly over the atom.

Logically enough, support for this idea was not universally shared. Great Britain and France, two nations with long histories of nuclear science and technology were determined to make progress in this new field of endeavor. Though the Americans had some valuable assets, the Europeans recognized ways to work around their lack of similar assets.

Limited Choices

The coolant/moderator choices for the European programs were far more limited than those available to Admiral Rickover. The limiting factor was the fact that there was no enriched uranium available. The American government owned the only facilities in the world capable of large scale separation of the two isotopes of uranium and it was not selling either the material or the technical know-how needed to build a plant. The French and British reactors had to be able to operate on natural uranium.

In order to use natural uranium to fuel a reactor, it has to be extremely stingy with neutrons. The coolant moderator combination had to be one that did not absorb neutrons to any great extent. To make natural uranium “burn” every available neutron has to find its way into fuel material. Using a neutron absorbing coolant/moderator – like the ordinary water that formed the basis for the U. S. submarine reactors – would be like trying to burn wood while spraying it with a garden hose.

There are only two known coolants compatible with natural uranium. Heavy water – because it contains an isotope of hydrogen that has already absorbed an extra neutron and has no real affinity for absorbing another one – and one of several different gases with a low neutron affinity and a low molecular density.

Heavy water is a costly, specialized material that was available only in small quantities from a limited set of suppliers. Since one goal of their nuclear programs was to achieve nuclear independence, neither France nor Great Britain selected heavy water. (As an aside, Canadian designers were able to design their reactors to take advantage of an ample supply of heavy water.) The elimination of the heavy water option limited the coolant choices to gases like air, carbon dioxide, nitrogen, and helium.

The selection of gas cooling required the designers to also choose a separate material to moderate (slow down) the neutrons. In a PWR, the coolant – water – happens to also be a good moderator, but gases are too low in density to have much effect on neutron speeds within a reactor.

The two moderators available for use in gas cooled reactors are graphite, a common, low cost material that has excellent heat resistent properties, and a long history of industrial use, and beryllium, a toxic, expensive, rare metal. Essentially all gas cooled reactors use graphite as the neutron moderator.

Which Gas?

The earliest British reactors – constructed at Windscale – used air for cooling. Air has the advantage of being widely available and very inexpensive. However, the Windscale reactors were specially designed for weapons material production and were not suitable for producing electricity. Like the American production reactors at Hanford, they were low temperature, open cycle systems where the coolant passed one time through the reactor and was released to the environment.

For elevated temperature operations, as would be needed to produce useful power, the combination of air and graphite was considered less than ideal. Graphite, a form of pure carbon, has a very high melting point, but it will burn if there is an oxygen supply. In addition to avoiding air for the dual purpose reactors, the designers decided to reduce the size of the needed heat transfer surfaces by operating the system at elevated pressure.

The remaining gaseous coolant choices included nitrogen, helium and CO2. Nitrogen had the advantage of being cheap and readily available, but its slight affinity for neutrons limits its usefulness in a pressurized, natural uranium reactor. (Modern British gas cooled reactors have systems that inject nitrogen into the coolant system as a means of reducing core activity in case the normal shutdown mechanisms do not function.)

Helium would have been an excellent coolant choice, but it was not available. As a policy artifact from World War I, the U. S. – which has high quality, low cost sources of helium from natural gas wells in Kansas, Oklahoma, and Texas – still maintained national control over the material in case it was needed for lighter than air ships in a time of war. In Europe, helium was a laboratory quantity material, painstakingly distilled from the atmosphere.

Thus, by a process of elimination, both Great Britain and France – acting independently – arrived at the same basic coolant/moderator selection of graphite moderated reactors cooled by pressurized carbon dioxide.

This choice was not perfect; temperatures above a few hundred degrees Celsius, CO2 reacts in several different ways with graphite, an effect that is enhanced under pressure. Until their owners learned more about the chemical behavior of CO2 at high temperature, CO2 cooled reactors were forced to operate at gas temperatures of less than 350 C. This limited total thermal efficiency of only 20 percent, which did not compare favorably with the 30 – 40 percent efficiency available from conventional power plants. A lot of room remained for improvements.

Like the American pressurized water reactors, the reactor coolant was only used as a means of transferring fission heat from one place to another. It was piped from the reactor through a heat exchanger resembling a conventional steam boiler. From that point on, the machinery used in the gas cooled nuclear stations was standard steam turbine equipment.

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