Breeding Pu-239 from U-238. Source: Fisslematerials.orgA breeder reactor is a type of fission nuclear reactor secifically designed to create more fissile material (nuclear fuel) than it consumes. It does so by using neutrons produced in the fission process to make new fuel--for example, to convert the stable isotope ²³⁸U into the radioactive isotope ²³⁹Pu.

Two types of breeder reactors have been proposed:

• Fast breeder reactor or FBR. The superior neutron economy of a fast neutron reactor makes it possible to build a reactor that, after its initial fuel charge of plutonium, requires only natural (or even depleted) uranium feedstock as input to its fuel cycle.  The term "fast breeder" refers to reactors can actually produce more fissionable fuel than they use.  This  is possible because the non-fissionable ²³⁸U is 140 times more abundant than the fissionable ²³⁵U and can be efficiently converted into ²³⁹Pu by the neutrons from a fission chain reaction.
• Thermal breeder reactor. The excellent neutron capture characteristics of fissile ²³³U make it possible to build a moderated reactor that, after its initial fuel charge of enriched uranium, plutonium or MOX, requires only thorium as input to its fuel cycle. Thorium-232 produces ²³³U after neutron capture and beta decay.

Breeding ratio

One measure of a reactor's performance is the breeding ratio: the ratio of the number of fissionable atoms produced in a breeder reactor to the number of fissionable atoms consumed in the reactor.  In normal operation, most large commercial reactors experience some degree of fuel breeding. It is customary to refer only to machines optimized for this trait as true breeders, but industry trends are pushing breeding ratios steadily higher, thus blurring the distinction.

In the liquid-metal, fast-breeder reactor (LMFBR), the target breeding ratio is 1.4 but the results achieved have been about 1.2 . This is based on 2.4 neutrons produced per ²³⁵U fission, with one neutron used to sustain the reaction.

The time required for a breeder reactor to produce enough material to fuel a second reactor is called its doubling time, and present design plans target about ten years as a doubling time. A reactor could use the heat of the reaction to produce energy for 10 years, and at the end of that time have enough fuel to fuel another reactor for 10 years.

Basic operating principles

Neutrons produced by fission have high energies and move extremely quickly. These so-called fast neutrons do not cause fission as efficiently as slower-moving ones so they are slowed down in most reactors by the process of moderation. A liquid or gas moderator, commonly water or helium, cools the neutrons to optimum energies for causing fission. These slower neutrons are also called thermal neutrons because they are brought to the same temperature as the surrounding coolant.

In contrast to most normal nuclear reactors, however, a fast reactor uses a coolant that is not an efficient moderator, such as liquid sodium, so its neutrons remain high-energy. Although these fast neutrons are not as good at causing fission, they are readily captured by an isotope of uranium (²³⁸U), which then becomes plutonium (²³⁹Pu). This plutonium isotope can be reprocessed and used as more reactor fuel or in the production of nuclear weapons. Reactors can be designed to maximize plutonium production, and in some cases they actually produce, i.e., "breed" more fuel than they consume.

Breeder reactors are possible because of the proportion of uranium isotopes that exist in nature. Natural uranium consists primarily of ²³⁸U, which does not fission readily, and ²³⁵U, which does. Natural uranium is unsuitable for use in a nuclear reactor, however, because it is only 0.72 percent ²³⁵U, which is not enough to sustain a chain reaction. Commercial nuclear reactors normally use uranium fuel that has had its ²³⁵U content enriched to somewhere between 3 and 8 percent by weight. Although the ²³⁵U does most of the fissioning, more than 90 percent of the atoms in the fuel are ²³⁸U--potential neutron capture targets and future plutonium atoms.

²³⁹Pu, which is created when ²³⁸U captures a neutron, forms ²³⁹U and then undergoes two beta decays, happens to be even better at fissioning than ²³⁵U. ²³⁹Pu is formed in every reactor and also fissions as the reactor operates. In fact, a nuclear reactor can derive a significant amount of energy from such plutonium fission. But because this plutonium fissions, it reduces the amount that is left in the fuel. To maximize plutonium production, therefore, a reactor must create as much plutonium as possible while minimizing the amount that splits. This is why many breeder reactors are also fast reactors. Fast neutrons are ideal for plutonium production because they are easily absorbed by ²³⁸U to create ²³⁹Pu, and they cause less fission than thermal neutrons. Some fast breeder reactors can generate up to 30 percent more fuel than they use.

The U.S. constructed two experimental breeder reactors, neither of which produced power commercially. The Enrico Fermi Nuclear Generating Station in Michigan was the first American fast breeder reactor but operated only from 1963 until 1972 before engineering problems led to a failed license renewal and subsequent decommissioning. Construction of the only other commercial fast breeder reactor in the U.S., the Clinch River plant in Tennessee, was halted in 1983 when Congress cut funding. Elsewhere in the world, only India, Russia, Japan and China currently have operational fast breeder reactor programs; the U.K., France and Germany have effectively shut down theirs.

There are a number of reasons why breeder reactors have not yet realized their potential.  One serious involves the use of liquid sodium as coolant. Sodium is a highly corrosive metal and in an LMFBR it is converted into a radioactive form, sodium-24. Accidental release of the coolant from such a plant could, therefore, constitute a serious environmental hazard.

Safety concerns

Plutonium is a very toxic substances and has a half-life of 24,000 years, means that it poses significant long-term environmental and human health problems. The plutonium produced in a breeder reactor can be removed and, with additional processing, be used in nuclear weapons.

Another problem is that, to extract the plutonium, the fuel must be reprocessed, creating radioactive waste and potentially high radiation exposures in the event of an accident. Indeed, the use of a breeder reactor implicitly assumes nuclear reprocessing of the breeder blanket at least. In practice, all proposed breeder reactor programs involve reprocessing of the fuel elements as well. This is important due to nuclear weapons proliferation concerns, as any nation conducting reprocessing using the traditional aqueous-based PUREX family of reprocessing techniques could potentially divert plutonium towards weapons building. In practice, commercial plutonium from reactors with significant burnup would require sophisticated weapon designs, but the possibility must be considered.

For these reasons, in the U.S., President Carter halted such spent fuel reprocessing, making the use of breeder reactors problematic. France, Israel, the United Kingdom, Pakistan, Russia, and India reprocess nuclear fuel as of 2008.

Sources

Karam, P. Andrew, How do fast breeder reactors differ from regular nuclear power plants?, July 17, 2006, Scientific American.

Nave, Carl R., Plutonium Breeding Ratio, Hyperphysics, Department of Physics and Astronomy, Georgia State University, Accessed 1 November 2008.

Ren Ruan Shi, Liquid Metal Fast Breeder Reactor, Department of Nuclear Engineering, University of California, Berkeley, Accessed 1 November 2008.

Wikipedia contributors, Breeder reactor, Wikipedia The Free Encyclopedia, Accessed 1 November 2008.