For natural uranium reactors, primary selection criteria is a low neutron cross section. A material that absorbs more than its share of neutrons would prevent the reactor from being able to produce any power at all.
After making the coolant and moderator choices, certain other details moved higher on the priority list. Core engineers needed to choose a cladding material, fuel material composition, and fuel element configurations. The choices designers made for the first reactors played an important role in the long term competitiveness of the early gas cooled reactor designs.
Nuclear engineers traditionally use a protective covering – known as cladding – to seal the heavy metal fuel so that the radioactive byproducts of nuclear fission are retained in a controlled location. Cladding’s main function is to keep the plant systems as free from contamination as possible. Cladding material selection is based on required operating temperature, neutron economy, and compatability with the coolant and other core materials.
For natural uranium reactors, primary selection criteria is a low neutron absorption cross section. A material that absorbs more than its share of neutrons would prevent the reactor from being able to produce any power at all.
The earliest natural uranium reactors – designed for solely for plutonium production – used aluminum cladding. Aluminum is a corrosion resistent metal with a very low neutron affinity, but its melting point is too low for use in a power reactor.
The European reactor designers had a choice of Magnox, an alloy of magnesium oxide and aluminum or a magnesium zirconium alloy. There are few differences between the two materials. The British chose Magnox while the French chose MgZr. Both materials’ melting point is high enough to enable useful steam production, but their limiting temperature is much lower than materials used in other types of power systems. This leads to low thermal efficiency and higher power costs. The name “Magnox” is widely used to refer to the early gas cooled reactors because it provides a simple way to uniquely identify the group.
While the American submarine reactor designers had to be concerned about interactions between the fuel and hot water in case of a cladding failure, this was less of a concern for the French and British. As long as its temperature is kept below 1000 F, CO2 does not interact chemically with uranium metal.
Efficient heat transfer was the primary consideration for the Europeans; because they were using a gaseous coolant and a cladding that was only capable of limited temperatures, they needed to choose a fuel material with high thermal conductivity. Uranium metal has a much higher thermal conductivity than uranium dioxide does. The use of metal, rather than metallic oxide, also improved the neutron economy and eased the task of recycling the fuel material to extract useful isotopes.
Choosing metallic uranium had its price, however. Uranium metal is more susceptible to irradiation damage, and it undergoes significant dimensional changes at relatively modest temperatures.
These characteristics put a low limit on the potential fuel burn-up and the potential efficiency of the system. The early Magnox fuel required replacement after only producing about 3500 MWdays/tonne of heavy metal. For comparison, modern light water reactor fuel is replaced after producing approximately 40,000 MWdays/tonne of heavy metal. The theoretical limit is about 1,000,000 MWdays/tonne of heavy metal.
Magnox designers partially overcame this low fuel burn-up by designing the reactors to be refueled while producing electrical power. This capability improves operating economy by allowing the production facility to generate revenue during a larger portion of the year.
The low fuel burn up, though a significant disadvantage for single purpose reactors producing only electrical power, actually improved the weapons material production ability of the Magnox reactors. Fuel elements with short active lives are better sources of bomb grade plutonium than those that have remained in a reactor long enough to produce significant quantities of Pu-240, an isotope that increases the difficulty of bomb construction.
The ability to produce good bomb material was a strong selling point to the governments that provided the initial funding needed to build the Magnox reactors. Later, however, this capability, rather than being valued by the market, became a significant inhibitor to their commercial viability.
Finned Fuel Elements
The low heat capacity of gas coolants compared to liquid coolant like water or sodium encouraged heat transfer engineers to design finned fuel elements to increase the surface area and to enhance gas mixing. One can get a good idea of what finned heat exchangers look like by closely observing the design of car radiators or air conditioning coolers.
While the fins improved the reactor performance, they increased the cost and complexity of fuel element manufacture compared to the simple tubes found in pressurized water reactors. Magnox fuel designers also kept trying out new fin patterns to enhance fuel performance, thus preventing standardization and mass production economies. The high cost of finned fuel elements became a factor in the market battle between American light water reactors and the British/French type of gas cooled reactors.