Liquid Metal Fast Breeder: Right Answer, Wrong Question
Soon after the development of the first nuclear reactors, scientists and engineers began to discuss the possibility of a nuclear fuel shortage. As far as these nuclear pioneers knew, there was a rather limited supply of uranium that was concentrated in certain areas of Africa and Eastern Europe.
The situation was considered even more critical since the first reactors depended on U-235 for fuel and that isotope only represents 0.7% of natural uranium. According to early projections, the era of nuclear fission would only last for a few decades before the fuel ran out if nothing changed.
The uranium situation provided an incentive to seek ways to increase the fuel supply by developing reactors that could make better use of the material that was available. Designers began thinking of ways to produce useful energy using U-238, which represents 99.3% of natural uranium. Using U-238 as fuel could increase the energy value of the world’s uranium stocks by a factor of 100 or more.
The sense of urgency to find a solution was enhanced by the United States Atomic Energy Commission’s stated desire for additional stocks of material that could be used for weapons production. The government made firm offers to purchase plutonium at prices that could produce a healthy profit for suppliers. As usual, monetary incentives worked their magic and a reactor concept was developed that optimized plutonium production.
As was realized quite early in the Manhattan Project, it is not difficult to design a reactor system that combines excess fission neutrons with U-238 to produce plutonium, a fissile material with physical properties similar to U-235.
Most fission reactions produce more than two times as many neutrons as are absolutely necessary to sustain a chain reaction. For each fissile atom that is consumed in fission, it is possible to produce at least one new atom of plutonium.
It is even possible to design the system so that an average of more than one neutron per fission would be absorbed by U-238. These systems became known as “breeder” reactors. The Atomic Energy Commission’s generous prices for plutonium produced an incentive to maximize its production.
Optimizing the Economics
There are two ways to maximize the rate at which U-238 is converted into plutonium. The first way is to choose a fission reaction that produces the maximum number of neutrons. If more neutrons are produced by each fission, there will be more neutrons available to convert U-238.
The second way to maximize the conversion efficiency is to ensure that as many neutrons as possible are absorbed by the U-238 and the fissile fuel elements instead of being parasitically absorbed in other core materials and in the coolant.
These two thoughts became the drivers behind the idea of a liquid metal fast breeder reactor (LMFBR).
- Plutonium-239 fission with fast neutrons produces an average of 3.04 neutrons per fission, more than any other type of fission.
- Metal coolants are poor moderators, leaving more neutrons at fast energy levels.
- Metals have high heat transfer coefficients, allowing high power density cores.
- Metals useful for cooling have low neutron cross sections.
- High power density cores could produce more plutonium with a smaller initial investment, thus allowing a higher rate of return when the plutonium was sold to the government.
There are some difficulties with the concept, however, that have prevented its widespread use.
- Sodium, the most commonly used metal, reacts explosively in air and water.
- Metals tend to solidify if not kept above a certain temperature; requiring complex maintenance procedures.
- Sodium becomes more radioactive than water when used as a coolant, again complicating routine maintenance.
- Metals cannot be used as working fluids, making a secondary cooling system a requirement. Normally, this is a steam system which causes some problems in design because of the sodium – water reaction possibility.
- There is an adequate supply of plutonium in the world for all current and projected needs; there is no market for any excess that can be produced in a reactor.
Several LMFBRs have been built, with mixed results. Some of the difficulties associated with the plants are simply a result of the natural growing pains of any new technology. Other barriers, mostly related to the high price of the proposed systems when compared to alternative power sources, are more difficult to overcome. No commercially viable LMFBRs are currently in operation, but test programs proceed.
Liquid metal is not the only possible coolant for a breeder reactor; by many measures it is not even the best coolant. LMFBRs maximize the production of plutonium, but that is no longer the desired result. Under current market conditions, a successful breeder concept will be one that maximizes the owner’s return on investment in the atomic plant as an electrical power production machine.