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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.