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A breeder reactor is a nuclear reactor that creates more fissile material than it consumes. It requires an initial charge of fissile material, such as highly enriched uranium or plutonium, and a supply of fertile material, such as natural uranium, depleted uranium or thorium. Excess neutrons generated in the fission reaction are absorbed by the fertile isotope, which is transmuted to a fissile isotope.
Breeder reactors can utilize nearly 100% of the energy contained in uranium and thorium ores, while the reactors currently used for nuclear power generation can use at most 1%. They can also reuse waste from existing reactors as fuel, until nothing but fission products is left. Since the supply of fertile materials on Earth is extremely large, the introduction of breeder reactors would turn fission-based nuclear power into a technology that is sustainable for nearly as long as solar power, e.g. until the Sun burns out.
 Breeder reactor fuel cycles
There are two fuel cycles investigated for use with breeder reactors:
- Uranium-plutonium fuel cycle. The fertile material is uranium-238, the major isotope in natural uranium. The fissile material is plutonium.
- Thorium-uranium fuel cycle. The fertile material is thorium-232, the only naturally occurring isotope of thorium. The fissile material is uranium-233.
Both cycles have advantages and disadvantages, especially depending on the kind of reactor used.
 Fast vs slow breeder reactors
When a fissile atom fissions from impact by a neutron, it can release multiple neutrons. Some of those neutrons can cause further fission events. Some of those neutrons can hit the breeder fuel (ex: U238 and thorium) and transmute it into the fissile fuel (ex: P239 and U233). The released neutrons are usually fast neutrons.
A fast reactor is a nuclear reactor which does not use a moderator - a substance that slows down neutrons. A slow reactor uses moderator to slow down neutrons substantially. This is called a "thermal" spectrum, and its neutrons are called "thermal" neutrons. The most common moderator is water, and graphite is also common.
Fast vs thermal neutrons have different probabilities of impacting atoms, and have different probabilities on impact of whether causing a fission event or some other event. Uranium-235 fissions easily with slow (thermal) neutrons, but not very well with fast neutrons. Fast neutrons are more efficient at converting uranium-238 into plutonium. The physics works out so that uranium-plutonium must be fast reactors in order to break even on transmuting enough U238 to P239 to continue the reaction and not require additional fissile fuel. However, for the thorium-uranium fuel cycle, the physics allows both fast and thermal spectrum reactors to achieve a self sustaining breeding of fissile fuel (U233) from the input breeder fuel (thorium).
Due to the physics and engineering of the reactors, coolant must pass through the core, and thus water cannot be used as a coolant for a fast reactor because water is a moderator. Thus, they they are somewhat more complicated. Usually they use liquid metals or inert gases as the coolant - mostly sodium or helium, but some Russian navy reactors also used lead or a lead-bismuth alloy. Research reactors sometimes use a sodium-potassium alloy, which is liquid at room temperature and avoids the problem of coolant solidification when the reactor is turned off, but is also more expensive. The coolants of choice for a fast breeder reactor also are alkali metals or alkali metal alloys which tend towards having somewhat violent chemical reactions with a wide variety of substances, including water (in even the slightest moisture content levels) and to a lesser extent oxygen.
Fast breeder reactors got the majority of funding during the Cold War because they allowed for production of plutonium to go in nuclear weapons. They are also less attractive for producing energy because fast-reactors tend towards being unstable or "twitchy" in their response to control input adjustments.
 Liquid Fluoride Thorium Reactor
The Liquid Fluoride Thorium Reactor (LFTR) is a radical proposed new breeder reactor design. It uses the thorium-uranium cycle, it has a thermal spectrum, and it uses a liquid fuel unlike all other modern reactor designs. The fuel consists of thorium and uranium in solution in a fluoride salt, usually with beryllium and lithium. The idea has been around since the birth of the nuclear age, and has been researched and pursued to some degree, but far less than other technologies. LFTR remains experimental and unproven.
Oak Ridge National Laboratory ran a mock of the core of a LFTR for 5 years in the Molten Salt Reactor Experiment (1965-1969) which demonstrated many of the fundamentals of LFTR. The light water reactor at Shippingport, Pennsylvania in 1982 showed that there was 1.39% more fissile material was present in the core than was put in after a 5-year run.
The purported benefits include cheap electricity, better safety, good or better proliferation characteristics, incredibly large availability of fuel, and less nuclear waste by-volume and by-lifetime. The purported negatives, like that of any new inovative experimental technology, are slim to none.Do You Believe That?
Contrary to the myths perpetuated by some LFTR proponents, it is likely not proliferation proof. However, it is likely more proliferation proof than conventional nuclear reactors.
 Traveling wave reactor
The traveling wave reactor (TWR) is a vaporware breeder reactor which purports to be an improvement over existing breeder reactor designs. It's gone through several incarnations over the years. The basic idea is that you can build a box of stuff that will generate heat (and thus electricity) for 60 years without maintainance or operator intervention, aka walk-away safe. The proponents claim that there will be areas of high breeding, and the areas will slowly move across the box, aka a "wave" of high breeding over time and space (hence the name). In practice, it violates simple physics and engineering, and will likely remain vaporware for the foreseeable future.
One of the big benefits pushed by supporters is that it's immune to operator error because there is no operator input. It's just a box that - once constructed - you never have to fiddle with, with suggestions that the waste be permanently stored in that box.
The design caught the attention of Bill Gates, which means that either (A) they're something really really interesting or (B) a rich retired software mogul likes them.
 Existing breeder reactors
Currently there are three breeder reactors used for power production operating in the world:
- BN-600 reactor in Beloyarsk, Russia.
- Monju reactor in Tsuruga, Japan.
- China Experimental Fast Reactor near Beijing, China.
A prototype of a 1200 MW commercial breeder reactor, Superphénix, was operating in France between 1986 and 1997. It was closed following an executive decision of a newly elected prime minister, Lionel Jospin, allied with the French Green Party.
The anti-nuclear movement opposes the development of fast reactors. Their main concern is that breeder reactors are expensive, and the prototypes had engineering problems. That's not really surprising, considering some of these organizations also oppose research into fusion. Nuclear reprocessing, another technology which the movement is allergic to, is a mandatory part of the fuel cycle. Additional criticism comes from proliferation experts, which worry whether the proposed anti-proliferation features will be sufficient.
- ↑ Bernard L. Cohen. "Breeder reactors: A renewable energy source". American Journal of Physics, 51(1), January 1983, p. 75–76. doi:10.1119/1.13440.
- ↑ http://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment
- ↑ WNA: Thorium
- ↑ http://www.energyfromthorium.com/forum/viewtopic.php?f=51&t=3746#p47103 From one of the leading modern advocates of the thorium fuel cycle, Kirk Sorenson, downplaying the myth at large that LFTR is proliferation proof.
- ↑ World Nuclear News: Chinese fast reactor starts supplying electricity
- ↑ http://weblog.greenpeace.org/nuclear-reaction/2010/02/fast_breeder_reactors_60_years.html
- ↑ http://www.greenpeace.org/international/en/press/releases/ITERprojectFrance/
- ↑ http://www.greenpeace.org/international/en/campaigns/nuclear/waste/reprocessing/