Breeding

It is also possible to create more fuel than a reactor consumes by converting uranium 238 to plutonium 239 in a process called breeding. As mentioned, uranium 238 is converted into plutonium 239 by the absorption of a neutron and subsequent radioactive decay. In a conventional reactor, the efficiency of the breeding process is not sufficient to make up for the uranium 235 consumed. In certain types of reactors, called breeders, the conversion of uranium 238 to plutonium 239 is enhanced. These reactors typically use fuel containing about 20% plutonium in the fuel and a liquid metal such as sodium as a coolant. Such reactors produce more plutonium than they consume and are, thus, capable of extracting 20 times as much energy from the original uranium fuel as a conventional reactor.

An unusual type of breeder reactor that has recently gained interest is the traveling wave reactor. Initially proposed and studied in 1958 by Saveli Feinberg, who called it a "breed-and-burn" reactor [4], the design generates its own fuel, plutonium 239, by breeding it from uranium 238. Unlike conventional breeders that breed plutonium to be extracted later for fabrication into fuel, the design of the reactor is such that the plutonium that is produced continuously replenishes that which is burned and is sufficient to keep the reactor operating and producing power without the addition of fuel for many years. The traveling wave reactor is so named because the region where the fissions occur and the power is generated moves through the reactor much like a wave moves across the surface of the water. As the fuel in the reactor is used up by generating power, the power producing region moves down the fuel.

The original concept in a modified form is under development by TerraPower. The TerraPower traveling wave reactor [5] is a pool-type breeder reactor cooled by liquid sodium and fueled mostly by depleted uranium. It requires only a small amount of enriched uranium or other fissile fuel such as plutonium to initiate the fission and breeding process. Unlike the original concept, the power producing region doesn't move in the TerraPower design. Instead the fuel is periodically moved in the core to ensure breeding and continued operation. According to analysis performed by TerraPower, the core could operate for 40 years without refueling and could be fueled with depleted uranium or spent fuel from existing light water reactors [6].

The concept has many advantages. It provides a way to dispose of depleted uranium by extracting the large amount of energy still remaining in the uranium. From a proliferation standpoint, there is no need to separate the plutonium from the uranium drastically reducing the likelihood of diversion to weapons production. Still, it remains to be seen if the engineering issues that have limited the deployment of sodium cooled fast breeder reactors can be overcome.

Currently, it is estimated that there are sufficient proven reserves of uranium to fuel the existing reactors for about 80 years [7]. The Energy Information Administration (EIA) estimates that current proven reserves could support the existing reactors as well as those expected to be built between now and 2030. The EIA acknowledges that there is currently insufficient production capacity and that new mines and processing facilities are needed to support this expansion [8]. Long term, the use of recycling and deployment of breeder reactors would extend the fuel supply to 1,000 years at current rates of consumption.

Thorium, a widely abundant element, may also be used as a material for breeding. Through the absorption of a neutron, the naturally occurring isotope of thorium, thorium 232, may be converted to uranium 233, which like uranium 235 is relatively easy to fission. India is embarking on a program to use a thorium-uranium fuel cycle since it lacks sufficient reserves of uranium but has abundant domestic reserves of thorium. Thorium reserves are sufficient to power a breeder-based economy for even longer than a uranium based breeder economy.

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