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Episode: 4563
Title: Nuclear Reactor Technology - Ep 5 Fast Reactors
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr4563/hpr4563.mp3
Transcribed: 2026-07-31 16:13:57 (official HPR transcript)
---
This is Hacker Public Radio Episode 4563, for 2026-01-28
Today's show is entitled, "Nuclear Reactor Technology - Ep 5 Fast Reactors"
The host is Whiskeyjack and the duration is 00:12:07
The flag is Clean, and the license is CC-BY-SA
The summary is "Fast neutron reactors"
This episode is the fifth in an eight-part series, a nuclear reactor technology.
In the previous episode, we discussed some of the less common historical reactor types.
In this episode, we will describe fast neutron reactors.
Definition, 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.
Moderators. The normal nuclear reaction is the fast one.
However, reactors which use a slow reaction are simpler and cheaper to build.
Therefore, most reactors use a moderator to slow down the fast neutrons to bake them slow neutrons.
The most common moderators are ordinary light water, heavy water, and graphite.
Light water is not the best of moderators, so reactors which use it need to enrich the uranium fuel
to a higher percentage of U235 to compensate for this.
By their nature, fast neutron reactors do not use a moderator.
Indeed, a moderator is detrimental to the operation.
However, since even the coolant could moderate the reaction, fast neutron reactors generally use something other than water
to act as a coolant and carry the heat from the fuel to the steam generator.
Burners versus breeders.
While fast neutron reactors can produce more fizzile material than they consume, not all designs do.
Once which produce more fizzile material than they consume are called breeders.
Once which produces less fizzile material than they consume are called burners.
This described by the breeding ratio.
A reactor with a breeding ratio of less than one consumes more fizzile material than it produces.
A reactor with a breeding ratio of greater than one produces more fizzile material than it consumes.
Fast-fission fuel cycle, typical fuel.
Uranium visions more efficiently with slow neutrons than fast neutrons.
However, plutonium visions better than uranium does with fast neutrons.
Also, plutonium 239 produces 25% more neutrons than uranium.
Fast neutrons are more efficient at converting U238 to plutonium than slow neutrons.
There are multiple ways the fuel can be arranged in the reactor with different arrangements
giving different characteristics.
Other methods.
India are apparently working on fast neutron reactors that produce plutonium from uranium
and U233 from thorium at the same time.
The plutonium and U233 are then used with thorium in slow neutron reactors.
Reprocessing.
Reprocessing with spent fuel is fundamental to the fast-fission fuel cycle
in most fast reactor designs, as that is how excess plutonium is harvested and used in other reactors.
Reprocessing involves chemically separating different elements from one another
in order to keep the desirable ones for recycling and getting rid of the undesirable ones.
Fuel types.
Here are the common fuel types for fast reactors.
Oxide.
This is similar to how fuel is made for most reactors.
It has low thermal conductivity.
It does not react with sodium or lead.
The characteristics are well known.
Metal.
It has a high thermal conductivity.
It is easier to reprocess.
It has a tendency to self-regulate.
As temperature increases, the metal expands.
This causes the reaction to slow down as the atoms get further apart.
If the coolant stops flowing, the reaction tends to shut itself down.
This acts as an inherent safety feature.
Nitride.
Nitride fuel, that is fuel incorporating nitrogen, has a much higher thermal conductivity
than oxygen fuel.
However, this fuel is more difficult to make, and is still subject to further R&D.
Carbide.
Carbide fuel incorporates carbon.
The pros and cons are similar to those of nitride fuel.
Coolant.
Fast neutron reactors are generally categorized based on the type of coolant they use.
Coolant are generally either liquid metal, gas, or molten salt.
The coolant will transfer the heat from the reactor to a steam generator to produce steam
for the turbine.
Liquid sodium.
Liquid sodium is a common choice for fast neutron reactors.
Sometimes a sodium potassium alloy is used instead.
It does not interfere much with neutrons.
It is a good conductor of heat.
It is non-corrosive.
Premium cooled reactors are good for producing plutonium for further use in fuel.
It has a high boiling point, so the reactor can operate at ambient temperature.
On the negative side, sodium reacts chemically to air and water, meaning that a coolant
leak can result in a chemical fire.
Liquid lead or lead bismoth.
Lead or lead bismoth fast neutron reactors have advantages similar to those of sodium cooled
ones.
However, they are not as good at producing plutonium for fuel.
Helium gas.
This uses helium gas as a coolant.
This is subject to ongoing research and there isn't a lot of information about this
style.
The main advantage of gas cooling is to avoid the potential chemical reaction problems
of sodium coolant in the advent of a leak in the steam generator.
Maltons salt.
This uses a molten salt as a coolant.
I will discuss Maltons salt reactors separately, so I won't go into a lot of detail here.
History of fast neutron reactors.
The origins.
More than 2 dozen fast neutron reactors have been built.
The earliest ones date from the 1950s.
Most have been small to medium sized research or prototype reactors.
Several, though, were large power reactors.
They have been built in China, France, Germany, India, Japan, Kazakhstan, Russia, the UK and the US.
Reasons for developing them.
Interested them originated from a belief that uranium supplies were limited, so there
was a desire to design reactors that could use fuel more efficiently.
However, after the discovery of very large, high-grade uranium deposits in a number of parts
of the world, such as Australia, Canada, Kazakhstan, and other places, interesting fast neutron
reactors waned as it became evident that fresh uranium fuel would be abundant and inexpensive.
Reasons there are still being developed.
Interest remained in some quarters for two reasons.
One is that some countries, such as India, have much more thorium than uranium, and they
wish to be able to produce plutonium in order to enable a thorium fuel cycle.
The other area of interest isn't using them to burn high-level waste in order to get rid of
it.
A fast neutron reaction can consume radioactive isotopes that cannot be consumed in slow neutron
reactors.
Fast neutron reactors are proven technology.
However, they are not currently an economic technology compared to slow neutron reactors
in a era of abundant fresh uranium.
They were developed to solve a problem that hasn't arisen, which is assured as your fresh
uranium.
plutonium stockpiles, several countries built up stockpiles of civil plutonium, with the intention
of using it to fuel the start-up of fast neutron reactors.
However, the canceling of plans for fast neutron reactors left these stockpiles with no immediate
purpose.
This led to the pursuit of plans to use these stockpiles in Mox fuel for slow neutron reactors,
or finding other ways of getting rid of it.
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?
Pros Fast neutron reactors can use all of the uranium supply by converting the U238 to plutonium
as well as using the U235.
The new neutron reactors can only use the U235, plus converting a very small proportion
of the U238 to plutonium.
This means that a given amount of fuel will go much further when used with a fast reactor
than a slow one.
Some, but not all, fast neutron reactors can produce more plutonium than they use.
This extra plutonium can be used to bake uranium plutonium mixed oxide or mox fuel to be used
in slow reactors, or can be used to power a thorium fuel cycle.
So the higher cost of the fast neutron reactors can be offset by having to produce fuel
for several slow neutron or thorium reactors.
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.
Cons.
There are a number of cons, however, as otherwise there would be a lot more fast reactors
in the world.
Since water, evens, light water, is a moderator, fast neutron reactors cannot use water as
a coolant.
Other alternatives, coolants, must be used, and these complicate the design of the reactor
and make it more difficult to operate.
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.
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.
After prospects, currently, fast neutron reactors are not economically competitive with
slow neutron reactors for electric power generation, so there is a lot of interest from
prospective customers.
Originally, interest in them was driven by a belief that the world would prud 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 economic use.
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 fueling thorium reactors.
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.
This concludes the fifth episode of an eight-part series on nuclear reactor technology.
You have been listening to Hacker Public Radio at Hacker Public Radio.org.
Today's show was contributed by a HPR listener like yourself.
If you ever thought of recording podcast, click on our upload link
to find out how easy it is.
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rsync.net, and our mirror network.
Unless otherwise stated, today's show is released under a Creative Commons
Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license.