Breeder-reactors by keralaguest


									                              Breeder reactors

        I have been told that I often start my posts by remarking that a friend asked me
about such and such a topic. This is another one of those. Having read my earlier stuff on
nuclear energy, someone asked me about breeder reactors, and whether they were
perpetual energy machines. Well, they are not perpetual energy devices in any sense of
the word, but they are a type of reactor which can produce more fuel than it consumes. I
know that this sounds like perpetual motion, but it really is not, as the fuel is not
produced out of nowhere, but rather is produced by converting an unusable (i.e. non-
fissile) isotope of uranium into a useable (i.e. fissile) isotope of plutonium. Furthermore,
unlike perpetual motion machines, they cannot run forever, since the plutonium does
not in its turn produce more fuel, and so on ad infinitum.
        Anyway, let’s first look briefly at how normal reactors work, and all this will
become clear: All nuclear reactors today generate energy through fission. Many people
would love to invent fusion reactors, which are much cleaner, but for the moment no
one knows how to maintain a sustainable but controllable fusion reaction. All we can do
to date, insofar as fusion reactions is concerned, is build hydrogen bombs (for more on
all this, please refer to my earlier posts). Anyway, fission is the process by which an
atomic nucleus splits into two or more smaller (‘daughter’) nuclei, whilst simultaneously
releasing some neutrons and some energy in the process.

A diagram of a fission reaction involving uranium 235 (which is an isotope of uranium
with 92 protons, and 143 neutrons in its nucleus). If you are not sure about what all this
means, please refer to my earlier posts on atomic energy.

       The heat and electromagnetic radiation released during fission, is derived from
the conversion of a small amount of matter into energy (the amount of energy released
may be calculated using Einstein’s most famous formula: E = mc2). This energy can be
used to boil water and thusly to drive steam turbines which are connected to electricity-
producing generators.
       The method by which all of this is accomplished is by initiating a controlled but
sustained chain reaction which is carefully damped so as to prevent it from running-
away and becoming an atomic bomb, but yet is not so heavily attenuated as to cause the
reaction to cease altogether.
       The driving force behind fission reactions are neutrons, and the secret to
controlling and damping the reactions so they do not become dangerously excessive lies
in absorbing a certain percentage of the freed neutrons (usually using a combination of
both control rods made of silver-indium-cadmium alloy [80%, 15% & 5%], boron,
hafnium, hafnium diboride or dysprosium titanate, which are inserted into or
withdrawn from the rector core as needed, as well as boron compounds dissolved in the
reactor coolant).

A schematic diagram of a nuclear power plant.

As the control rods are withdrawn, the reaction proceeds and the core heats up. This hot
core is continuously flushed with a fluid coolant in order to transfer the heat to the heat-
exchanger / steam-generator, where it is then used to boil water. This steam, in its turn, is
used to drive turbines which are connected to electricity-producing alternators.

This is what would happen if the reactor was not kept in check by means of control rods.
You would have an exponentially-growing cascade of fission reactions (since every 235U
nucleus which splits, releases 3 neutrons which could potentially split 3 further nuclei).
This run-away series of chain-reactions would very quickly produce so much heat that the
reactor core would blow up. This is basically how an atom / fission bomb works.

        The neutrons produced when a uranium nucleus splits then go on and slam into
other uranium 235 nuclei and cause those to split in their turn, and so on. Now, in order
for uranium 235 to fission efficiently, it needs slow-moving neutrons, but the neutrons
which are produced in fission reactions are fast-moving. Thus, they are typically slowed
down by a moderator substance (water, or helium usually) so as to enhance the number
of fission reactions they can provoke, and thus work efficiently.
        In a breeder reactor by contrast, these fast neutrons are deliberately not slowed
down. You see, although fast-moving neutrons may not be as efficient at provoking
fission reactions in uranium 235, they can be readily absorbed by another isotope of
uranium, uranium 238 (which is far more common than 235U), which then undergoes a
series of nuclear reactions and eventually becomes plutonium 239.

This is how uranium 238 becomes plutonium. A 238U nucleus, which contains 146 neutrons
and 92 protons captures a fast neutron, and becomes uranium 239 (with 92 protons and
147 neutrons). This 239U however is unstable, and emits an electron (i.e. one of the neutrons
in the nucleus changes into a proton and an electron), and becomes neptunium 239 (with
93 protons and 146 neutrons). This isotope of neptunium too is unstable though, and it in
turn emits another electron, and becomes plutonium 239 (with 94 protons and 145
neutrons). 239Pu is also unstable, and highly fissile, but is not as unstable as 239Np, and this
can be harvested from the reactor core and used to fuel other fission reactions, be it in new
reactors, or in bombs.

        Thus, in breeder reactors, the coolant which is often used is liquid sodium, which
is an inefficient neutron moderator so the neutrons produced by the fission reaction
retain their high energies, and are thus easily able to initiate the conversion of uranium
238 (238U) into plutonium 239 (239Pu) via the intermediate reaction product neptunium
239 (239Np).
        The primary reason for using breeder reactors is that the vast bulk (~99.3%) of
naturally occurring uranium consists of the relatively stable isotope uranium 238, and
only a very little bit (~0.7%) of the unstable (and thus easily fissile) isotope uranium
235. In fact, there are also very, very small portions of other isotopes present (~0.005%
of 234U for example), but these isotopes are irrelevant to our discussion.
        In order for a fission reaction to proceed, the naturally occurring uranium 238
needs to be artificially enriched with uranium 235, which, whilst easy enough to
understand conceptually, is an extremely technologically complex and demanding
process to actually implement (for more on this see my earlier posts). For nuclear
reactors, typically, low-enriched uranium is used (containing ~3-8% 235U), whilst for
making bombs, the enrichment can be extremely high (up to ~90% 235U), though to a
large extent the requisite level of enrichment depends upon the specific design of the
bomb being manufactured.

        This is where breeder reactors come in: they can convert the unusable (non-
fissile) uranium 238 into useable (fissile) plutonium 239. This is where the breeder part
in their name comes from. There is no perpetual motion involved; in the process of
producing energy from the fission of 235U, they also, coincidentally, convert a non-fissile
element into a fissile one (the old alchemists dream realised by the way, the
transmutation of one substance into another!).
        Once a breeder reactor has exhausted its uranium 235, the core can be removed
and reprocessed to extract the plutonium 239, which can then be used to power other
reactors, or to make bombs. Depending upon their design, breeder-reactors can actually
make more fuel (up to 30% more) than they consume.
        Plutonium 239 is even more fissile than uranium 235, and in a typical (non-
breeder) nuclear reactor, some percentage of the 238U in the core does get converted
into 239Pu anyway, which then undergoes fission and contributes to the overall energy
output of the reactor. In a breeder reactor, one of the design objectives, particularly if
the aim is to produce fuel for nuclear weapons, is to prevent too much of the plutonium
239 from undergoing fission whilst it is still in the breeder reactor. Again, using fast
neutrons helps to achieve this aim, as they provoke far fewer fission reactions than do
slow neutrons.
        There are some serious issues with all of this however, as the process of
extracting plutonium, which is highly radioactive, is fraught with potential danger, and
requires careful handling, as well as proper public scrutiny and oversight.

In gun-type atomic weapons, the typical fuel is highly-enriched uranium, whereas in the
implosion-type bomb design, the preferred fuel is plutonium 239.

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