Fast Reactors

03 Fast versus Slow Neutrons

"Fast neutron" reactors are ones which use the "fast neutron" reaction.

This is as opposed to "slow" or "thermal" neutron reactors which use a slow neutron reaction.

Nearly all reactors in use today use a slow neutron reaction.

04 Moderators

06 No Moderator in Fast Neutron Reactors

07 Burners versus Breeders

08 Fast Fission Fuel Cycle

08 "Typical" Fuel

09 Other Methods

10 Reprocessing

11 Fuel Types

11 Oxide

12 Metal

13 Nitride

14 Carbide

15 Coolant

16 Liquid Sodium

18 Liquid Lead or Lead-Bismuth

19 Helium Gas

20 Molten Salt

21 History of Fast Neutron Reactors

21 Origins

22 Reasons for Developing Them

23 Reasons They are Still Being Developed

24 This is a Proven Technology

25 Plutonium Stockpiles

26 Pros and Cons of Fast Reactors

If fast reactors are more expensive and difficult to operate than slow reactors, why is there any interest in them?

27 Pros

Fast neutron reactors can use all of the uranium supply by converting the U-238 to plutonium as well as using the U-235.

Slow neutron reactors can only use the U-235 plus converting a very small proportion of the U-238 to plutonium.

This means that a given amount of fuel will go much further when used with a fast neutron reactor than a slow one.

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Some (but not all) fast neutron reactors can produce more plutonium than they use.

This extra plutonium can be used to make uranium-plutonium mixed oxide (or MOX) fuel to be used in slow reactors, or it can be used to power a thorium fuel cycle. 

So the higher cost of the fast neutron reactors can be offset by having it produce fuel for several slow neutron or thorium reactors.

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They can also use up or "burn" radioactive waste. That is, highly radioactive elements which are a byproduct of fuel use but not usable as fuel by themselves can be separated from the spent fuel and fed back into the reactor where the additional radiation will convert them into elements or isotopes which are either not radioactive or which are otherwise easier to dispose of.

30 Cons

There are a number of cons however, as otherwise there would be a lot more fast neutron reactors in the world.

Since water, even "light" water, is a moderator, fast neutron reactors cannot use water as a coolant.

Other alternative coolants must be used, and these complicate the design of the reactor and make it more difficult to operate.

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Alternative compatible coolants may be corrosive, and so new materials may need to be developed for both the reactor vessel and the fuel cladding.

Alternative coolants are often opaque, making it difficult to inspect the reactor.

The fuel cycle requires reprocessing spent fuel, which means that reprocessing facilities have to be set up, which is an additional expense.

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Fast neutron reactors were primarily developed on the premise that uranium supplies were limited and would soon become very expensive. However new very large and very high grade uranium deposits were discovered in Canada, Australia, and Kazakhstan, causing uranium prices to fall rather than rise. As a result it is much cheaper to operate a once-through fuel cycle than to build fast neutron reactors.

33 Future Prospects

Currently fast neutron reactors are not economically competitive with slow neutron reactors for electric power generation so there isn't a lot of interest from prospective customers.

Originally interest in them was driven by a belief that the world would run short of uranium.

However, higher uranium prices sparked increased mineral exploration which resulted in finding large high grade reserves of low cost uranium, undercutting the need for economizing on its use.

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There is still ongoing R&D though as they offer several other use cases.

One is to get rid of radioactive waste elements by turning them into non-radioactive or less radioactive isotopes or elements.

The other is to provide a supply of plutonium for fuelling thorium reactors.

35 Conclusion

This has been a short overview of fast neutron reactors, including their history, uses, and underlying design features.

In the next episode we will describe the use of thorium in nuclear power, including what thorium is, how it differs from uranium, and what sort of reactors can use it.