Conventional nuclear reactors use uranium-235 as their fuel. However, uranium-235 makes up less than 1% of naturally occurring uranium. Most uranium occurs as the isotope uranium-238. The only problem is that uranium-238 can't be used in conventional nuclear reactors. It doesn't undergo fission like uranium-235. However, if uranium-238 could be used as a nuclear fuel, there would be sufficient uranium to run nuclear reactors for hundreds of years.
The Breeder Reactor was developed to use uranium-238. Here's how it works. A reactor is built with a core of fissionable plutonium, Pu-239. The plutonium-239 core is surrounded by a layer of uranium-238. As the plutonium-239 undergoes spontaneous fission, it releases neutrons. These neutrons convert uranium-238 to plutonium-239. In other words, this reactor breeds fuel (Pu-239) as it operates. After all the uranium-238 has been changed to plutonium-239, the reactor is refueled.
However, there are some major problems with the breeder reactor. To begin with, plutonium-239 is extremely toxic. If an individual inhales a small amount, he or she will contract lung cancer. Also, the half-life of the material is extremely long, about 24,000 years. This could create an almost impossible disposal problem if large amounts of this material are generated.
Also, because of the nature of the reactor core, water can't be used as a coolant. Instead, liquid sodium must be used. In the event of an accident a catastrophe could develop because sodium reacts violently with water and air.
Although the breeder reactor could solve the uranium fuel problem, there are still a number of other problems that will have to be worked out.
Two Types of Traditional Breeder Reactors
In addition to this, there is some interest in so-called "reduced moderation reactors" 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.
Fast Breeder Reactors
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 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.
Under appropriate operating conditions, the neutrons given off by fission reactions can "breed" more fuel from otherwise non-fissionable isotopes. The most common breeding reaction is that of plutonium-239 from non-fissionable uranium-238. The term "fast breeder" refers to the types of configurations which can actually produce more fissionable fuel than they use, such as the LMFBR. This scenario is possible because the non-fissionable uranium-238 is 140 times more abundant than the fissionable U-235 and can be efficiently converted into Pu-239 by the neutrons from a fission chain reaction.
France has made the largest implementation of breeder reactors with its large Super-Phenix reactor and an intermediate scale reactor (BN-600) on the Caspian Sea for electric power and desalinization.
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:
As well as their thermal breeder program, India is also developing FBR technology, using both uranium and thorium feedstocks.
Fissionable plutonium-239 can be produced from non-fissionable uranium-238 by the reaction illustrated.
The bombardment of uranium-238 with neutrons triggers two successive beta decays with the production of plutonium. The amount of plutonium produced depends on the breeding ratio.
In the breeding of plutonium fuel in breeder reactors, an important concept is the "Breeding Ratio", the amount of fissile plutonium-239 produced compared to the amount of fissionable fuel (like U-235) used to produced it. 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 U-235 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.
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 Shipping port Reactor 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. Theoretical models of gas-cooled breeders show breeding ratios of up to 1.8 are possible as an upper limit.
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.
The plutonium-239 breeder reactor is commonly called a fast breeder reactor, and the cooling and heat transfer is done by a liquid metal. The metals which can accomplish this are sodium and lithium, with sodium being the most abundant and most commonly used. The construction of the fast breeder requires a higher enrichment of U-235 than a light-water reactor, typically 15 to 30%. The reactor fuel is surrounded by a "blanket" of non-fissionable U-238. No moderator is used in the breeder reactor since fast neutrons are more efficient in transmuting U-238 to Pu-239. At this concentration of U-235, the cross-section for fission with fast neutrons is sufficient to sustain the chain-reaction. Using water as coolant would slow down the neutrons, but the use of liquid sodium avoids that moderation and provides a very efficient heat transfer medium.
Liquid sodium is used as the coolant and heat-transfer medium in the LMFBR reactor. That immediately raised the question of safety since sodium metal is an extremely reactive chemical and burns on contact with air or water (sometimes explosively on contact with water). It is true that the liquid sodium must be protected from contact with air or water at all times, kept in a sealed system. However, it has been found that the safety issues are not significantly greater than those with high-pressure water and steam in the light-water reactors.
Sodium is a solid at room temperature but liquefies at 98°C. It has a wide working temperature since it does not boil until 892°C. That brackets the range of operating temperatures for the reactor so that it does not need to be pressurized as does a water-steam coolant system. It has a large specific heat so that it is an efficient heat-transfer fluid.
The Super-Phenix was the first large-scale breeder reactor. It was put into service in France in 1984.
The reactor core consists of thousands of stainless steel tubes containing a mixture of uranium and plutonium oxides, about 15-20% fissionable plutonium-239. Surrounding the core is a region called the breeder blanket consisting of tubes filled only with uranium oxide. The entire assembly is about 3x5 meters and is supported in a reactor vessel in molten sodium. The energy from the nuclear fission heats the sodium to about 500°C and it transfers that energy to a second sodium loop which in turn heats water to produce steam for electricity production.
Such a reactor can produce about 20% more fuel than it consumes by the breeding reaction. Enough excess fuel is produced over about 20 years to fuel another such reactor. Optimum breeding allows about 75% of the energy of the natural uranium to be used compared to 1% in the standard light water reactor.
Breeding Versus Burnup
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 Gigawatt days/ton heavy metal, about thirty percent of the total energy released comes from bred plutonium. At 40 Gigawatt days/ton heavy metal, that percentage increases to about forty 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.
This is of interest largely due to the fact that next-generation reactors such as the European Pressurized Reactor, AP-1000 and Pebble Bed Reactor are designed to achieve very high burnup. 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.
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.
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 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. To address this concern, modified aqueous reprocessing systems are proposed which add extra reagents which force minor actinide "impurities" such as curium and neptunium to commingle with the plutonium. Such impurities matter little in a fast spectrum reactor, but make weaponizing the plutonium extraordinarily difficult, such that even very sophisticated weapon designs are likely to fail to fire properly. Such systems as the TRUEX and SANEX are meant to address this.
Even more comprehensive are such systems as the IFR pyroprocessing system, which uses pools of molten cadmium and electro-refiners to reprocess metallic fuel directly on-site at the reactor. Such systems not only commingle all the minor actinides with both uranium and plutonium, they are compact and self-contained, so that no plutonium-containing material ever needs to be transported away from the site of the breeder reactor. Breeder reactors incorporating such technology would most likely be designed with breeding ratios very close to 1.00, so that after an initial loading of enriched uranium and/or plutonium fuel, the reactor would then be refueled only with small deliveries of natural uranium metal. A block of natural uranium metal about the size of a milk crate delivered once per month would be all the fuel such a 1 gigawatt reactor would need. Such self-contained breeders are currently envisioned as the final self-contained and self-supporting ultimate goal of nuclear reactor designers.
Thermal Breeder Reactor
The Advanced Heavy Water Reactor is one of the few proposed large-scale uses of thorium. As of 3006 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 step and thus U-234 is formed upon further neutron irradiation.