Recently, I have been focusing on nuclear breeder reactors which can generate more fissile material than they burn. These reactors can create weapons grade plutonium, generate electrical power and burn nuclear waste. Today I am going to talk about fast breeder reactors (FBR). These reactors utilize unmoderated neutrons which have not been slowed down by some moderator.
Ordinary water is generally not used as a coolant because water slows down neutrons. There have been some new designs which would use supercritical water held at the exact point between liquid water and steam as a coolant for a breeder reactor. An early FBR used mercury for a coolant and plutonium for fuel. It was found that mercury had some serious disadvantages as a coolant. Lead has been used as a coolant in reactors for naval vessels. Currently, big FBRs use molten sodium as a coolant. Molten salts may be used in future FBRs with the light metal fluorides being replaced with heavy metal chlorides. There are also designs that use gases such as helium for a coolant that have garnered some interest.
It requires highly enriched plutonium or uranium to fuel a FBR. When an atom of natural U-238 absorbs a neutron, it can emit two electrons and become an atom of Pu-239 which highly fissile. There are moderators in a conventional nuclear reactor for the express purpose of slowing down neutrons to make it more likely that the neutron will trigger the desired fission. Most fast neutron will pass right through atoms of plutonium and uranium without causing fission. FBRs cannot be fueled with natural uranium. Spent fuel from an FBR can be reprocessed and used again to burn up more of the fissile material. Most existing FBRs are fueled with a MOX or mixed oxide fuel with both uranium and plutonium. The Russians are building a lot of FBRs these days and using highly enriched U-235 for fuel. Regular nuclear reactors are fueled with uranium enriched to have about five percent of U-235. FBRs usually have the uranium enriched to more than twenty percent U-235. U-235 decays, creating fission products and emitting two neutron for every decay. In an FBR, these fast neutrons are absorbed by U-238 atoms which emit two electrons and become Pu-239 atoms. These Pu-239 atoms decay, generating fission products and more fast neutrons than U-235. The material that is intended to produce fissile isotopes is either mixed into the fuel or placed in a blanket around the core of the reactors. FBRs generally use neutron absorbing control rods to keep the reactor from going supercritical and melting down.
In 2008, the International Atomic Energy Agency published a report that said that there had been a falling of interest in most nations with atomic technology in the development of FBRs. Only Russia, Japan and France were still proceeding with research on FBRs and that research has been impeded by a lack of qualified personnel. The U.S. has a FBR that is having serious problems. The Japanese cannot seem to get their FBR to work. Only the Russians have embarked on a program to build FBRs for commercial purposes such as breeding nuclear fuel and burning nuclear waste.
