Breeder | Technical aspects of breeder reactors

A breeder reactor is a type of nuclear fission – nuclear reactor, which is optimized so that as much new fissile material produced by another substance is irradiated with neutrons. For example, the hardly fissile uranium 238 can be converted into the easily fissile plutonium 239 by capturing neutrons. It may even be possible to breed more fissile material than is needed for operation. However, this does not apply to a number of previously built breeder reactors; the neutron losses are too high for this.

In principle, breeder reactors could become the central component of a plutonium economy. While only a small part of the uranium 238 is converted into plutonium with the previous use of nuclear energy mainly with light water reactors and therefore the largest part of the uranium 238 remains unused, a much larger part of the uranium could be used with breeder reactors. This would increase the range of uranium reserves from a few decades to several millennia. At the same time, the problem of radioactive waste could be significantly reduced compared to the use of conventional nuclear reactors. However, this development has hardly been followed seriously for some time due to various problems that are discussed below.

Technical aspects of breeder reactors

The most commonly used technical approach

The efficient incubation of plutonium 239 from uranium 238 requires strong radiation from fast neutrons, while the neutrons in a conventional light water reactor are strongly slowed down by the water as a moderator. That is why breeder reactors are mostly built as so-called fast breeders, i. H. they work with neutrons of much higher energy without a moderator. This has significant consequences for the design and functioning of the reactor – discussed here for a typical design:

• In order not to obtain too high a critical mass for nuclear fission even without a moderator, the concentration of fissile material in the inner reactor core (the fission zone) – e.g., B. 239 Pu – be relatively high. Uranium with a degree of enrichment such as that used for light water reactors is normally not sufficient for such a breeder reactor.
• In a surrounding breeding zone (breeding mantle) there is 238 U (i.e., depleted natural uranium) that is exposed to the neutron radiation. As is usually the case with reprocessing, the hatched plutonium can be separated using chemical processes.
• Since water, with its moderating effect (slowing down the neutrons), cannot be used as a coolant, another substance has to be used for this – for example a liquid metal (mostly sodium), a liquid salt or a gas such as helium. In a heat exchanger or steam generator, the heat is then used to generate water vapor, which is used to drive a steam turbine. A second sodium circuit can preferably also be connected in between so that if the steam generator fails, the heavily contaminated sodium from the reactor core does not react with the water. In principle, there would of course be other types of use than process heat conceivable, especially when generating high-temperature heat.
• As in other nuclear fission reactors, the chain reaction is controlled with the help of neutron-absorbing control rods in order to maintain the desired output and to prevent the reactor from “running away”. This regulation is much more critical with fast breeders, however, since there are fewer delayed neutrons and an increase in output cannot be stopped automatically by the formation of vapor bubbles.

Aspects of security

Reactor safety

With regard to reactor safety, fast breeders have considerable advantages and disadvantages compared to conventional light water reactors:

• The difficult control of the reactor output has already been mentioned.
• Another field of problems arises from the use of sodium as a coolant in most cases. Sodium is very reactive chemically; it burns on contact with air with intensive smoke formation, and on contact with water it forms hydrogen, which can then easily lead to strong hydrogen explosions. Incidentally, the sodium has to be kept hot even when the reactor is at a standstill so that it does not solidify (which would probably destroy the reactor).
• On the other hand, there are also significant safety advantages of breeder reactors; Certain fatal developments in conventional reactors are not possible there, and a kind of “passive safety” (e.g., a relatively good-natured behavior in the event of loss of external cooling) can be achieved, especially with smaller sizes.

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