Liquid Fluoride Thorium Reactor - Breeding Basics

Breeding Basics

In a nuclear power reactor, there are two types of fuel. The first is a small amount of fissile, which splits ("burns") when hit by neutrons, releasing a large amount of energy and also releasing two or three new neutrons. These can split more fissile material, resulting in a continued chain reaction. Examples of fissile fuels are U-233, U-235 and Pu-239. The second type of fuel is called fertile. Examples of fertile fuel are Th-232 (mined thorium) and U-238 (mined uranium). The amount of fertile fuel in the reactor is far bigger than the amount of fissile, but it cannot be fissioned directly. It must first absorb one of the 2 or 3 three neutrons produced in the fission process, which is called neutron capture, then it becomes a fissile isotope by radioactive decay. This process is called breeding. All reactors breed some fuel this way, but today's solid fuelled thermal reactors don't breed enough new fuel from the fertile to make up for the amount of fissile they consume. This is because today's reactors use the mined uranium-plutonium cycle in a moderated neutron spectrum. Such a fuel cycle, using slowed down neutrons, gives back less than 2 new neutrons from fissioning the bred plutonium. Since 1 neutron is required to sustain the fission reaction, this leaves a budget of less than 1 neutron per fission to breed new fuel. In addition, the materials in the core such as metals, moderators, and the ashes from the fission reaction, the fission products, absorb ("steal") some neutrons, leaving too few neutrons to breed enough fuel to continue operating the reactor. As a consequence they must add new fissile fuel periodically and swap out some of the old fuel to make room for the new fuel.

In a reactor that breeds at least as much new fuel as it consumes, it is not necessary to add new fissile fuel. Only new fertile fuel is added, which breeds to fissile inside the reactor. This type of reactor is called a breeder reactor. A LFTR is usually designed as a breeder reactor. If it breeds just as much new fissile from fertile to keep operating indefinitely, it is called a break-even breeder or isobreeder. Thorium goes in, fission products come out.

Reactors that use the uranium-plutonium fuel cycle require fast reactors to sustain breeding, because only with fast moving neutrons, the fission process provides more than 2 neutrons per fission. With thorium, it is possible to breed using a thermal reactor. This was proven to work in the Shippingport Atomic Power Station, whose final fuel load bred slightly more fissile from thorium than it consumed, despite being a very neutron inefficient reactor type (Shippingport was a fairly standard light water reactor). Such thermal reactors require much less of the expensive fissile fuel to start and have a slow, gentle response in power changes.

There are two ways to configure a breeder reactor to do the required breeding. One can place the fertile and fissile fuel together, so breeding and splitting occurs in the same place. Alternatively, fissile and fertile can be separated. The latter is known as core-and-blanket, because a fissile core produces the heat and neutrons while a separate blanket does all the breeding.

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Famous quotes containing the word breeding:

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    Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)