Hat is a Fast Breeder Reactor
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Transcript of Hat is a Fast Breeder Reactor
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hat is a Fast Breeder Reactor?
Natural uranium consists of 99.3% Uranium-238 and 0.7% Uranium-235. Of these two forms only
Uranium-235 can be used as a nuclear fuel. In a conventional thermal reactor, during operation someUranium-238 is transformed into Plutonium-239 which can also be used as a nuclear fuel. By recycling
this plutonium to make new fuel it may be possible to exploit at most about 2% of the potential fuel
value of the world's natural uranium resources.
However, a fast breeder reactor can convert Uranium-238 into Plutonium-239 at a rate faster than it
consumes its fuel. By repeated recycling of the fuel it should be realistically possible to exploit 50% ofthe fuel value of the uranium feed. This means that fast reactors could extend the energy output from
the world's uranium fuel reserves 25 fold.
About the term "Fast"
A neutron released from the fission of Uranium-235 (or Plutonium-239) has a high energy. To increase
the probability of the neutron causing the fission of another nucleus of the fuel material - and therebycontinuing the chain reaction - either its energy must be reduced, or the concentration of fissionable
target nuclei must be increased.
The approach of reducing the neutron energy led to the development of the nuclear reactors which are
now widely used for power production. In these reactors the energy of the neutron is reduced by
"bouncing" it off the atoms in a so-called moderator material until the neutron is in thermal equilibriumwith the atoms with which it is interacting. The neutron is then termed a "thermal neutron" and reactors
using this principle are called "thermal reactors".
The alternative approach of increasing the concentration of fissionable material and using fissioncaused by high energy or "fast" neutrons led to the development of "fast" reactors.
About the term "Breeder"
If a neutron is captured by a Uranium-238 nucleus the following reaction takes place:
The result is that Uranium-238, which is very difficult to fission, is transformed into Plutonium-239which can be fissioned much more easily. This means that a useful reactor fuel can be made from an
otherwise useless natural resource. The symbol +n represents the gain of a neutron by capture and b -
represents radioactive decay by beta emission with the half-life shown below the arrow.
As mentioned above, for the chain reaction to continue, on average one neutron released during fission
must go on to cause the fission of another nucleus. The average number of neutrons released by fission
depends on several factors but is usually around 2.5. Some of these neutrons are inevitably lost as theyare captured by the reactor structure or coolant but if, on average, more than one of the available 1.5
neutrons is captured by Uranium-238 then the total number of atoms in the reactor that can be fissioned
will increase as the reactor operates. This is the principle of the "breeder" reactor.
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All reactors contain Uranium-238 in their core and so the reaction which produces Plutonium-239
occurs to some extent in all reactors. However, only in a reactor using fast neutrons for fission is it
possible to "breed" more new fuel than the reactor consumes; this is what a Fast Breeder Reactor does.
The amount of breeding that takes place in a fast reactor depends on the size and design of the reactor
core and the concentration of the fissile material that is in it. Some fast reactors may be deliberately
designed not to breed at all. It is important to realise that not all fast reactors are breeder reactors.
Breeding and burning
In the early days of research in fast reactor technology it was considered very important to maximisethe amount of plutonium that was bred in order to fuel an increasing number of reactors that would
follow. Two important changes have taken place since then which have changed this ...
Around 1960 it became clear that the economics of operating a fast reactor depend more on minimising
the number of times that its fuel must be recycled than on simply maximising the breeding. Fuel
assemblies cannot stay in the fast reactor core indefinitely for a number of reasons, principally because
the fissile material is gradually consumed. Breeding does take place in the fuel but because it also takesplace in the blanket there is a net loss of fuel in the core centre and a net gain around the edge where it
cannot contribute to powering the reactor. In addition the steel fuel pins which hold the fuel are
weakened by the gradual swelling of the fuel pellets, chemical attack by the fuel and damage by fastneutrons. So from the early 1960's until the end of the 1980's the emphasis was on maximising the
amount of energy which could be extracted from a fuel assembly before it had to be removed for
reprocessing - the energy extracted is usually referred to as the "burn-up" of the fuel. The results of theyears of development carried out in this field were impressive; a twenty-fold increase in the burn-up
was achieved by using oxide fuel in specially developed alloy fuel pins.
Since the early 1990's the picture has again changed. The widespread use of thermal reactors meansthat there is now an ample stockpile of plutonium. Plutonium from dismantled nuclear weapons may
also become available with the end of the Cold War. A fast reactor which is configured to consume
plutonium is called a "burner" reactor. The fast neutrons in the core of a fast reactor have anotherpotential use: for a long time the disposal of a certain type of nuclear waste produced in thermal
reactors which belong to the category of elements called actinides has been a problem. By putting this
nuclear waste into the fast reactor it can be broken down into materials which are more easily disposedof. The plutonium burning and actinide disposal project is led by France with the participation of the
United Kingdom and Japan.
The fast reactor is very versatile, a reactor can be designed so that by changing the core it can be used
to create more plutonium than it destroys or destroy more plutonium than it creates according to what is
required.
The Fuel Cycle
As explained above, fuel cannot stay in the reactor indefinitely. To recover the useful materials in the
fuel it must be sent to a reprocessing plant for chemical separation. The recovered unburnt and bredplutonium is then returned to the fuel fabrication plant to be put into new fuel rods.
Fast Reactor DesignThe coolant
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The coolant which passes through the core of the fast reactor must not moderate (slow down) the
neutrons emitted from the fission reaction. This limits the choice of coolant slightly, in particular water
is unsuitable as a coolant because it is a very effective moderator. To avoid moderating the neutrons
suitable coolants are liquid metals and inert gases (e.g. helium). Little research has been done on inertgases in fast reactor applications, liquid metals are the preferred option due to their excellent heat
transfer properties.
The liquid metal coolant would preferably remain in the liquid state below or as close as possible to
ambient temperatures (to avoid expensive pre-heating of coolant filled vessels and pipes), have a
boiling point which gives a margin of safety above the proposed reactor operating temperatures and beobtainable in sufficiently large quantities at a reasonable cost. Possible choices of liquid metal coolant
would include mercury, lead, sodium and a sodium-potassium mixture; except for lead, all these have
indeed been used as fast reactor coolants and designs with lead coolant have been made. However,mercury and lead have problems of chemical toxicity which make them unattractive and mercury is
also very expensive. The sodium-potassium mixture - often referred to as NaK - has the advantage of
remaining liquid at room temperature, obviating the need for pre-heating, but in other respects is more
difficult to handle than sodium. Fast reactor engineers around the world have therefore arrived upon thesame conclusion that the best coolant is sodium.
The reactor layout
One of the most important choices facing the designer of a fast reactor is the layout to be adopted for
the primary circuit which includes the reactor. There are two main concepts, the "pool" and the "loop".In a pool-type reactor one large vessel holds the core, the Intermediate Heat Exchanger (IHX) which
passes heat to the secondary loops, and the pump which circulates the primary sodium. In a loop-type
reactor, the core, IHX and pump are each in their own smaller vessels linked by pipes. The layouts are
illustrated below, sodium flows upwards through the core (shown yellow) in both designs:
The choice is not simple since each has its own advantages and disadvantages. The vessel of the pool is
a very simple design with no branches to cause stress concentrations. It can be arranged so that hot
coolant never comes into contact with the vessel wall. The disadvantages of the pool are that the vesselis so large that it must be fabricated on-site where quality assurance is more difficult. Once in operation
its internal structures are difficult to inspect. The reactor vessel of the loop-type, being much smaller,
can be built in a factory and transported to the site. The pipework of the loop reactor may be longer andmore complicated but it is easier to inspect.
Both types of reactor have been built but, to date, there is less experience with large scale loop-type
reactors. Monju which is a loop-type reactor will provide an interesting comparison with the Europeanpool-type prototypes.