Episode: 4543
Title: Nuclear Reactor Technology - Ep 3 Reactor Basics
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr4543/hpr4543.mp3
Transcribed: 2026-07-31 16:13:36 (official HPR transcript)

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This is Hacker Public Radio Episode 4543, for 2025-12-31
Today's show is entitled, "Nuclear Reactor Technology - Ep 3 Reactor Basics"
The host is Whiskeyjack and the duration is 00:11:22
The flag is Clean, and the license is CC-BY-SA
The summary is "Basic features of reactors and descriptions of the most widely used commercial reactor types."
This episode is a third in an eight-part series on nuclear reactor technology.
In the previous episode, we covered types of nuclear fuel, recycling of nuclear fuel, uranium
and thorium resources, and medical isotopes.
In this episode, we will describe the basic features and characteristics of reactors
together with descriptions of the most widely used commercial reactor types.
All commercial power-efficient reactors use similar principles, a nuclear reaction
in the fuel generates heat.
The heat is carried away by a coolant to a turbine which spends a generator to create electricity.
Fast versus slow reactors, reactors can be placed into more or less two categories,
fast and slow or thermal reactors, the latitude terms are equivalent.
The term refers to the type of nuclear-efficient reaction which predominates in the reactor.
Slow neutron reactors, slow or thermal reactors use slow or thermal neutrons.
Slow neutron reactors are by far the most common type in commercial service.
They are cheaper and easier to build and operate.
However, they can only use a small part of the nuclear fuel.
With a straightforward uranium fuel, they can only use the U-235 isotope, plus part of any
plutonium which is created during the reaction.
With uranium plutonium mox fuel, they can use the physal plutonium isotopes, but not the others.
Some U-238 will be converted to physal plutonium, and then consumed in the reaction, but for
most reactor types, the majority of the energy comes from the U-235.
Thorium plutonium, or Thorium uranium mox fuel, can be used as slow reactors, but the fuel
cycle requires a continual input of fresh physal fuel, plutonium in most proposals, from
uranium cycle reactors.
Nearly all commercial power reactors have been some form of slow reactor.
Fast neutron reactors, fast reactors use fast neutrons.
Fast reactors can use fuel more efficiently, but a more complex and expensive to build
and operate.
At least, some designs of fast reactors can produce more plutonium from U-238 than they
consume.
This allows a fast reactor to produce plutonium for several slow reactors using uranium
or Thorium-based mox fuel.
Only a small number of fast reactors have been built.
The commercialization of fast reactors will probably depend upon uranium becoming much
more expensive and so making recycling a few more attractive.
At present, uranium is so abundant and cheap that there isn't a lot of incentive to
economize on it.
Some most fuel is used in a one-through fuel cycle.
A reactor moderators, a moderator is a material that is used to slow down neutrons so
they are more likely to react with the nuclear items in the fuel.
Some sort of moderator is normally required to have a self-sustaining reaction for slow
neutron reactors.
The main moderator materials are water, heavy water, and graphite.
Fast reactors do not use a moderator, instead they use the unmotorated fast neutrons directly.
However, this means that fast reactors must be built from materials which won't unintentionally
moderate the reaction.
Light water.
Ordinary water is commonly referred to as light water to distinguish it from heavy water.
Light water is a moderately good moderator, although not as good as some of the common alternatives.
Reactors using light water as a moderator must use enriched uranium as fuel.
Heavy water.
Heavy water differs from ordinary water, and that is molecules contain heavy hydrogen,
more commonly known as deuterium.
While regular hydrogen has a nucleus containing only a proton, deuterium has a proton
and a neutron.
Deuterium is often used in the industry to refer to heavy water, although technically
it's only the hydrogen atoms which are deuterium.
Deuterium is extracted from normal water, but any of several different processes which
I won't go into here.
Heavy water is a very efficient moderator.
Due to this, reactors using heavy water moderators can be fueled on natural unenriched uranium.
Some proposed designs have used slightly enriched uranium, but this is just to reduce the
size of the reactor.
Graphite.
Graphite is a form of carbon.
Graphite is also a good moderator.
Reactors using a graphite moderator can also be fueled on natural uranium.
However, some graphite moderated reactors have used enriched uranium.
Unmoderated.
As mentioned previously, fast reactors not use a moderator as they use fast neutrons directly
in the reaction.
Coolants.
In commercial-fission power reactors, the nuclear reactions used to produce heat.
This heat is then usually used to boil water to produce steam, which a steam being used
to drive a steam turbine, which spins a generator to produce electricity.
Common coolants.
The common coolants in commercial-fission power reactors are light water, heavy water, or
a gas such as helium or carbon dioxide.
Alternative coolants.
Some alternative coolants which have been tried or proposed are a liquid metal of some
sort, an organic oil-coolant, molten salt.
Depending on the design, a reactor may use the same material for the coolant as it used
for the moderator, or they may be different.
Primary and secondary coolant loops.
The primary coolant is the coolant that comes in direct contact with the nuclear fuel
assemblies.
In many reactor types, this coolant is then passed through a heat exchanger to heat a secondary
coolant.
The secondary coolant will then typically be turned into steam to drive a turbine.
The primary and secondary coolant may be the same material or different materials.
The secondary coolant is usually light water.
Some reactor types do not have a secondary coolant loop.
Instead, they boil water directly in the reactor, and this steam is then passed directly
to the steam turbines.
Steam generation.
Most commercial power reactors are used to generate steam, which is used to drive a turbine,
which in terms spins a generator to make electricity.
The steam is normally created in one of two ways.
By far, the most common way is to run the hot coolant through a heat exchanger, known
as a steam generator.
The coolant boils the water in the steam generator.
The steam is then used to drive a turbine.
The steam that runs through the turbine is kept separate from the coolant material, and
the water that goes through the turbine never passes through the reactor itself.
This is the most common design.
The other common method is to allow the coolant water in the reactor to boil, and
then using that steam to directly to drive a turbine.
Braton cycle gas turbines.
An alternative to steam turbines is to use a hot gas such as helium to drive a gas turbine
directly.
This is called a Braton cycle.
That's BR-A-Y-T-O-N.
This removes the need to have steam generation equipment, but the very hot gas presents
other difficulties in terms of materials and design.
Braton cycle turbines have seen very little use in nuclear power.
Refueling method.
The uranium fuel must be replaced on a regular basis, with the fuel lasting no more than
a couple of years or so.
Offline refueling refers to shutting down the reactor to replace the fuel.
Online refueling refers to refueling a reactor continuously while it is in operation.
Refueling method used is determined by the reactor's design.
Main commercial reactor types.
The following gives a brief overview of the most common commercial reactor types.
I will cover the less common models, including some more unusual ones, later on in other
episodes.
PWR.
Percerized water reactor.
This is the most widely used style of commercial power reactor.
The name is usually abbreviated as PWR.
These are made by a number of companies in a wide variety of countries around the world.
It is characterized by the following features.
The moderator is light water.
The coolant is also light water.
The moderator and coolant are mixed together.
There is no distinguishing between them.
Steam is generated in a separate steam generator and does not boil in the reactor.
It uses enriched fuel, usually 3 to 5% uranium 235.
Refueling is conducted offline with the reactor shut down.
The reactor itself is a large metal pot called the reactor vessel, containing the fuel,
moderator, and coolant altogether.
PWR.
Boiling water reactor.
This is less common than the PWR, but there are a large number of them.
The name is generally abbreviated as PWR.
There are most common in the USA.
A PWR is very similar to a PWR, with the exception that the primary coolant is
allowed to boil in the reactor vessel and then drives a steam turbine directly.
There is no steam generator or secondary coolant.
Other than this, the PWR and PWR are very similar.
PHWR, pressurized heavy water reactor.
This is also known as a can-do reactor named after the most common model.
It was developed in Canada and also used by a number of other countries in Europe, Asia,
and South America.
India developed their own version based on Canadian technology and these are used in India.
It is characterized by the following features.
The moderator is heavy water, the coolant is also heavy water.
The moderator and coolant are kept separate and do not mix.
Steam is created in a separate steam generator and does not boil in the reactor.
It uses natural uranium fuel, no enrichment is required.
The reactor itself is a set of hundreds of separate pressure tubes,
running through large, un-pressurized tank of heavy water, known as the Kalandria.
Re-fielding is conducted online with the reactor running at full power by depressurizing
one-two at a time.
The Kalandria contains the moderator.
The heavy water in the Kalandria remains that ambient temperature and is not heated.
The fuel is in the pressure tubes in fuel bundles.
These fuel bundles are much shorter than the fuel rods used in PWRs or BWRs.
The coolant runs through the pressure tubes around the fuel bundles.
The reactor design is very flexible in terms of the sort of fuel it could run on due
to its excellent neutron economy.
Conclusion We have covered the main reactor characteristics.
These characteristics can be mixed in various ways to give different reactor types.
The characteristics also affect the type of fuel that can be used.
We also covered the three main commercial power generation reactor types.
In the next episode, we will describe some of the less common historical reactor types.
This concludes the third episode of an eight-part series on nuclear reactor technology.
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