Breeder Reactor

A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile from fertile material like uranium-238 or thorium-232. Breeders were at first considered superior because of their superior fuel economy compared to light water reactors. Interest in breeders reduced after the 1960s as more uranium reserves were found, and new methods of uranium enrichment reduced fuel costs.

Breeder reactors could in principle extract almost all of the energy contained in uranium or thorium, decreasing fuel requirements by nearly two orders of magnitude compared to traditional once-through light water reactors, which extract less than 1% of the energy. This could greatly dampen concern about fuel supply or energy used in mining. In fact, with seawater uranium extraction, there would be enough fuel for breeder reactors to satisfy our energy needs for as long as the current relationship between the sun and Earth persists, about 5 billion years at the current energy consumption rate (thus making nuclear energy as sustainable in fuel availability terms as solar or wind renewable energy).

Nuclear waste became a greater concern by the 1990s. Breeding fuel cycles became interesting again because they can reduce actinide wastes, particularly plutonium and minor actinides. After the spent nuclear fuel is removed from a light water reactor, after 1000 to 100,000 years, these transuranics would make most of the radioactivity. Eliminating them eliminates much of the long-term radioactivity of spent nuclear fuel.

In principle, breeder fuel cycles can recycle and consume all actinides, leaving only fission products. So, after several hundred years, the waste's radioactivity drops to the low level of the long-lived fission products. If the fuel reprocessing process used for the fuel cycle leaves actinides in its final waste stream, this advantage is reduced.

There are two main types of breeding cycles, and they can both reduce actinide wastes:

• The fast breeder reactor's fast neutrons can fission actinide nucleii with even numbers of both protons and neutrons. Such nucleii usually lack the low-speed "thermal neutron" resonances of fissile fuels used in LWRs.
• The thorium fuel cycle simply produces lower levels of heavy actinides. The fuel starts with few isotopic impurities (i.e. there's nothing like U238 in the reactor), and the reactor gets two chances to fission the fuel: First as U233, and as it absorbs neutrons, again as U235.

A reactor whose main purpose is to destroy actinides, rather than increasing fissile fuel stocks, is sometimes known as a burner reactor. Both breeding and burning depend on good neutron economy, and many designs can do either. Breeding designs surround the core by a breeding blanket of fertile material. Waste burners surround the core with non-fertile wastes to be destroyed. Some designs add neutron reflectors or absorbers.

Today's LWRs do breed some plutonium. They do not make enough to replace the uranium-235 consumed. Only about 1/3 of fissions over a fuel element's life cycle are from bred plutonium. However, LWRs are not able to consume all the plutonium and minor actinides they produce. Nonfissile isotopes of plutonium build up. Even with reprocessing, reactor-grade plutonium can be recycled only once in LWRs as mixed oxide fuel. This reduces long term waste radioactivity somewhat, but not as much as purpose-designed breeding cycles.