Working Principle of a Breeder Reactor

The name 'breeder reactor' comes from the fact that, in these reactors, fissionable material is bred by changing the properties of non fissionable ones. Thus, effectively, these reactors are capable of producing more fissionable material than they consume.

I really wonder, did the great Albert Einstein truly know how powerful e=mc2 was when he first came up with it? This deceptively simple looking equation governs one of the most complex processes in the history of mankind - the generation of atomic energy. Now, decades later, it has become the best possible solution for our present-day energy crisis.

However, though atomic fission in nuclear reactors is capable of generating vast amounts of energy, it isn't a perennial source. The never-ending demands of modern-times is forcing nuclear physicists to think of alternate ways to maximize the efficiency of nuclear energy production. One such method is the use of breeder reactors.

In the following sections, we shall explore the working principle of this intuitive process, but before that, we need to understand the reasons behind the development of this technology.

The main fuel that is used in almost all nuclear reactors is uranium. It has a total of 6 isotopes from uranium-233 (U-233) to uranium-238 (U-238). All these isotopes are unstable, meaning, they will undergo radioactive decay over time and change their form. Typically, their decay-rate ranges from 70 years to 4.5 billion years.

Because of such long decay times, uranium is considered to be mildly radioactive. Of the 6 isotopes of uranium, two are of importance in nuclear energy generation - U-235 and U-238. U-235 has been traditionally used in nuclear reactors, because unlike U-238, it is fissile in nature, and is therefore capable of sustaining a fission chain reaction.

For years it has powered many nuclear reactors across the globe. However, of the total naturally occurring deposits of uranium in the world, U-235 constitutes only about 0.72%, and because of its increased usage in recent years, it has begun depleting fast.

On the other hand, U-238 constitutes almost 99.28% of the total uranium deposits. But the problem in using it is that it is non-fissile. Nuclear scientists realized that, if somehow U-238 could be used, it would be able to power reactors for hundreds of years.

So they started looking for the means of making its use possible, until finally they found an answer in the form of breeder reactors. What follows here is the principle and working of breeder reactors.

Nuclear scientists, upon experimentation, discovered that though U-238 isn't fissile, it is fertile. In atomic science, a fertile material is one which, though isn't fissionable by thermal neutrons, can be converted into one by being bombarded by neutrons, which subsequently leads to the transformation of its nucleus.

This fact forms the basis of the working of a breeder reactor. When a neutron strikes a U-238 atom, it gets captured by its nucleus. This additional neutron, increases the atomic mass by a factor of one, and thus, U-238 changes to the isotope U-239.

The half-life period of U-239, that is the time taken by half the radioactive atoms in a sample to undergo decay, is about 23 minutes, after which it decays and changes form to neptunium-239, while releasing energy in the order of 1.29MeV. Nu-239, after another 2 - 3 days, further undergoes beta decay, finally forming into plutonium-239.

In breeder reactors, the core is made up of plutonium Pu-239. It is encased by a layer of non fissionable uranium-238. Plutonium being fissile, undergoes spontaneous fission and releases neutrons. These neutrons are projected towards the surrounding layer of U-238.

The uranium-238 atoms in the layer, capture these neutrons and undergo two beta decays, which change the structure of their nuclei, converting them to fissile plutonium-239. The newly formed Pu-239 atoms, again ejects more neutrons via fission.

This process continues on until all the U-238 is converted to Pu-239. Once that is done, the reactor is refueled, and it can carry on working by producing more nuclear reactions.

It is interesting to note that though originally only an x amount of fissile Pu-239 was added to the reactor, in the end, via the phenomenon of nuclei transformation, the reactor was able to 'breed' Pu-235 in multiples of that amount.

This process of Pu-239 generation produces a tremendous amount of heat. This heat is absorbed by different coolants running through the reactors, and is transported to heat exchangers. It is this heat which is collected by the heat exchangers, that is used to convert water to steam and drive the large turbines of electricity generators.

A breeder reactor is simply one which can use existent fissile material to convert non-fissionable matter into fissionable matter. As such, many different types of breeder reactors have surfaced over the years. However, the following are two of the most significant ones from among them.

It is considered to be one of the most promising types of breeder reactors. In it, U-238 is converted to PU-239 through bombardment of fast neutrons, as described in the section above. The newly formed PU-239 atoms again eject neutrons, converting more U-238 atoms to P-239.

This leads to a self-sustaining chain reaction. The heat that is released continuously during this process is absorbed by a liquid metal (sodium) coolant and transported further to be used in electricity generation.

A thermal breeder reactors use thorium instead of uranium as its main fuel. In it, thorium is converted to uranium-233, which is fissionable. For this conversion to take place, thorium atoms have to be bombarded with neutrons that have been slowed down or thermalized using neutron moderators. Hence, the reactor is named thermal breeder reactor.

The U-233 that is produced undergoes spontaneous fission, which starts a chain reaction producing a lot of energy in the form of heat. This energy is collected by water that gets turned into steam, which is used for the generation of electricity.

It is estimated that the thorium deposit is three times more abundant than uranium deposit. Hence, it may one day serve as an alternative to uranium.

1) It is estimated that the cost of construction of a breeder reactor is twice that of conventional nuclear reactors. This was one of the reasons cited for the cancellation of the Clinch river breeder reactor project.

2) Liquid sodium, which is used as a coolant in LMFBR, is very volatile when exposed to air or water. It reacts violently with both of these and produces hydrogen gas which is highly flammable. This can lead to a large-scale catastrophe in case of accidents.

3) Plutonium, which is generated in breeder reactors, is highly toxic and known to cause lung cancer in human beings. Also, its half life period is very long (24,100 years). Thus, its disposal is a serious problem.

4) Plutonium can also be easily used to make nuclear bombs. Hence, it poses a threat if it were to fall in the wrong hands.

Breeder reactors are able to convert unusable nuclear fuels into usable ones, and thus, generate a lot of energy which will be useful to us for years to come. They have a few drawbacks associated with them, but continuous research is being made into finding feasible solutions, which will allow this beneficial technology to come to the forefront of nuclear energy generation.