Fast Breeder Reactors

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.

Breeding Plutonium-239

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.

Plutonium 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.

Liquid-Metal, Fast-Breeder Reactor

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.

Illustration

Liquid Sodium Coolant

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 liquifies 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

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