Breeder reactor

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A breeder reactor is a nuclear reactor that breeds fuel. A Breeder consumes fissile and fertile material at the same time as it creates new fissile material. Production of fissile material in a reactor occurs by neutron irradiation of fertile material, particularly Uranium-238 and Thorium-232. In a breeder reactor, these materials are deliberately provided, either in the fuel or in a breeder blanket surrounding the core, or most commonly in both. Production of fissile material takes place to some extent in the fuel of all current commercial nuclear power reactors. Towards the end of its life, a uranium (not MOX, just uranium) PWR fuel element is producing more power from the fissioning of plutonium than from the remaining uranium-235. Historically, in order to be called a breeder, a reactor must be specifically designed to create more fissile material than it consumes.

One measure of a reactor's performance is the "Breeding Ratio." Historically, attention has focused upon reactors with high breeding ratios, so that they produce more fissile material than they consume. Such designs range from a breeding ratio of 1.01 for the Shippingport Reactor [link] running on thorium fuel and cooled by conventional light water to the Russian BN350 liquid-metal-cooled reactor with a breeding ratio of over 1.2. [link] Theoretical models of gas-cooled breeders show breeding ratios of up to 1.8 are possible as an upper limit. [link]

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. [link]

All commercial Light Water Reactors breed fuel, they just have breeding ratios that are very low compared to machines traditionally considered "breeders." In recent years, the commercial power industry has been emphasizing high-burnup fuels, which are typically enriched to higher percentages of U235 than standard reactor fuels so that they last longer in the reactor core. As burnup increases, a higher percentage of the total power produced in a reactor is due to the fuel bred inside the reactor.

At a burnup of 30,000 Gigawatt days/ton heavy metal, about thirty percent of the total energy released comes from bred plutonium. At 40,000 Gigawatt days/ton heavy metal, that percentage increases to about 40 percent. This corresponds to a breeding ratio for these reactors of about 0.4 to 0.5. Namely, about half of the fissile fuel in these reactors is bred there. [link]

This is of interest largely due to the fact that next-generation reactors such as the European Pressurized Reactor and AP-1000 are designed to achieve very high burnup.[link] This directly translates to higher breeding ratios. Current commercial power reactors have achieved breeding ratios of roughly 0.55, and next-generation designs like the AP-1000 and EPR should have breeding ratios of 0.7 to 0.8, meaning that they produce 70 to 80 percent as much fuel as they consume, improving their fuel economy by roughly 15 percent compared to current high-burnup reactors.

In addition to this, there is some interest in so-called "reduced moderation reactors"[link] which are derived from conventional reactors and use conventional fuels and coolants, but are designed to be reasonably efficient as breeders. Such designs typically achieve breeding ratios of 0.7 to 1.01 or even higher.

Breeding of fissile fuel is a common feature in reactors, but in commercial reactors not optimized for this feature it is referred to as "enhanced burnup". Up to a third of all electricity produced in our current reactor fleet comes from bred fuel, and the industry is working steadily to increase that percentage as time goes on.

Two types of traditional breeder reactor have been proposed:

The 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. This fuel cycle has been termed the plutonium economy.

The thermal breeder reactor. The excellent neutron capture characteristics of fissile Uranium-233 make it possible to build a heavy water 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 Uranium-233 after neutron capture and beta decay.

Use of a breeder reactor assumes nuclear reprocessing of the breeder blanket at least, without which the concept is meaningless. 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 could potentially be diverting plutonium towards weapons building.

The fast breeder reactor

Several prototype FBRs have been built, ranging in electrical output from a few light bulbs (EBR-I, 1951) to over 1000MWe. As of 2006, the technology is not economically competitive to thermal reactor technology; but Japan, China, Korea and Russia are all committing substantial research funds to further development based on existing LMFBR designs, anticipating that rising uranium prices will change this in the long term. Looking further ahead, three of the proposed generation IV reactor types are FBRs:

The Gas-Cooled Fast Reactor (GFR) cooled by helium.
The Sodium-Cooled Fast Reactor (SFR) based on the existing LMFBR and Integral Fast Reactor designs.
The Lead-Cooled Fast Reactor (LFR) based on Soviet naval propulsion units.

As well as their thermal breeder program, India is also developing FBR technology, using both uranium and thorium feedstocks.

The thermal breeder reactor

The Advanced Heavy Water Reactor is one of the few proposed large-scale uses of thorium. As of 2006 only India is developing this technology. Indian interest is motivated by their substantial thorium reserves; almost a third of the world's thorium reserves are in India, which in contrast has less than 1% of the world's uranium. Their stated intention is to use both fast and thermal breeder reactors to supply both their own fuel and a surplus for non-breeding thermal power reactors. Total worldwide resources of thorium are roughly three times those of uranium, so in the extreme long term this technology may become of more general interest.

The Liquid Fluoride Reactor was also developed as a thermal breeder. Liquid-fluoride reactors have many attractive features, such as deep inherent safety (due to their strong negative temperature coefficient of reactivity and their ability to drain their liquid fuel into a passively-cooled and non-critical configuration) and ease of operation. They are particularly attractive as thermal breeders because they can isolate protactinium-233 (the intermediate breeding product of thorium) from neutron flux and allow it to decay to uranium-233, which can then be returned to the reactor. Typical solid-fueled reactors are not capable of accomplishing this critical step in thorium conversion to energy.

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