Episode: 4553
Title: Nuclear Reactor Technology - Ep 4 Less Common Reactor Types
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr4553/hpr4553.mp3
Transcribed: 2026-07-31 16:13:48 (official HPR transcript)

---

This is Hacker Public Radio Episode 4553, for 2026-01-14
Today's show is entitled, "Nuclear Reactor Technology - Ep 4 Less Common Reactor Types"
The host is Whiskeyjack and the duration is 00:17:34
The flag is Clean, and the license is CC-BY-SA
The summary is "Some of the less common historical reactor types."
This episode is the fourth in an eight-part series on nuclear reactor technology.
Less common reactor types. In this episode we discussed some of the less common historical
reactor types. These are a mixture of less common commercial types and some experimental
or research reactors. I will cover advanced or future designs in another episode.
Miner successes. First I'll start with some reactors that had minor successes, but for
one reason or other, didn't find widespread sales.
Magnocks. This is a British design, but saw very limited use outside of the UK.
The name comes from the type of fuel used. This used a non-oxidizing, magnesium aluminum
sheath around the fuel. The first nuclear reactors anywhere to produce power for the grid
on a large scale were the Magnocks reactors at Calderhal with 460 megawatt units starting
in 1956. Other claimants to this title were experimental reactors that produced anywhere
between less than a kilowatt to no more than 5 or 6 megawatt.
It is characterized by the following features. The moderator is graphite. The coolant
is carbon dioxide. It used natural uranium. The moderator, fuel and coolant are contained
in a steel and concrete pressure vessel. The carbon dioxide coolant is used to create steam
and a steam generator, refueling is done online. This design was succeeded by the AGR.
The Magnocks reactors are currently all shut down or dismantled due to age.
AGR advanced gas cooled reactor. This is a British design, but not used outside of the UK.
These were based on the earlier Magnocks reactors. It is characterized by the following features.
The moderator is graphite. The coolant is carbon dioxide. It uses uranium and rich to 2.5
to 3.5%. The older Magnocks reactors use natural uranium. The moderator, fuel and coolant
are contained in a steel and concrete pressure vessel.
The carbon dioxide coolant is used to create steam and a steam generator, refueling is
done online. The operating temperatures are much higher than for other reactors mentioned
so far. This allows for higher thermal efficiency and so allows more power to be generated
from a smaller reactor. This design is no longer being developed. The AGR reactors are being
faced out as a reach end of life. The design was successful from a technical perspective,
but did not sell well outside of the UK.
LWGR, light water graphite moderated reactor, also known as RBMK. This was originally
a Soviet design and never built outside of the Soviet Union. It was unusual in how it combined
features and ways not seen in other reactors. It is characterized by the following features.
The moderator is graphite. The coolant is light water. Fuel is contained in vertical
tubes running through the graphite moderator. Steam is boiled directly in the fuel channels
and there is no separate steam generator. It uses slightly enriched uranium, refueling
is conducted online with the reactor running at full power by depressurizing one tube at
a time. There is no containment building. The reactors at Chernobyl were of this design.
These are not related to the more common VVER designs which originated in the Soviet Union
at about the same time. The VVER reactors are fairly conventional PWR designs.
Historical oddities are dead ends. Next, I'll cover some historical oddities or dead
ends. Some of these designs may make a comeback, however, so don't write them off completely.
In many cases, there was nothing inherently wrong with the reactor design. The design
owners in many cases simply decided that there was a better market for some other design
they had. Organically cooled reactors. Organically cooled reactors use an organic oil compound
as either a coolant or both as a coolant and moderator instead of water. The reason
for using an organic oil was to allow higher operating temperatures without high pressures.
This allows for the reactor itself to be unpressurized and thus simpler and cheaper while
the high pressure parts are combined to the steam generator in steam circuit. Higher operating
temperatures allow for higher thermal efficiency in the steam cycle, allowing more power
to be produced from a given size of reactor. Organically cooled and moderated. This uses
the organic fluid as both coolant and moderator. The main example of this is the Pickwood
Nuclear Power Plant built as an experiment in the U.S. in the early 1960s. This generated
43.5 megawatts. The basic design was similar to the American PWR designs in that the fuel,
moderator and coolant were together in one tank and steam was generated separately. However,
the reactor vessel was an impressurized due to the high boiling point of the organic coolant.
The coolant was a standard commercially available heat transfer fluid used at industry. The
design suffered from the following of surfaces due to the breakdown of the organic fluid
and the build-up of sludge in the reactor vessel. It was shut down after less than three years
of operation. Organically cooled and heavy water moderated. This uses an organic coolant but the
moderator is heavy water and is separate from the coolant. The main example of this was the
WR-1 reactor in Canada. This was a research reactor and was not intended to produce electric power.
Nonetheless, it was large enough to produce 60 megawatts of heat. The design was similar to PHWR
or also known as Kando, reactors, and that it consisted of a number of tubes,
holding the fuel and coolant, which ran through a large, unpressurized tank of heavy water.
Unlike normal Kando designs, the tubes ran vertically rather than horizontally.
These tubes were unpressurized and the coolant operated at relatively high temperature.
Since this was a research reactor, there were also tubes which were used to hold experiments
which were to be irradiated. Fueling was online. Fuel was uranium carbide rather than the
normal uranium oxide. This was cheaper and easier to make than oxide fuel and was chemically
compatible with the organic coolant. Because the oil was non-corroso, the metals used to build
the reactor could be less complex and expensive than once it had to withstand both radiation
and high temperature water. The higher operating temperatures would have allowed for higher
efficiency if a steam cycle was added. It ran for two decades from the mid-1960s to the mid-1980s,
when it was shut down as Canada's national nuclear lab decided to start to consolidate operations
onto fewer sites. This used a different organic coolant than the Pekkor reactor in the U.S. did.
It did not suffer the same following in sludge problems. The lack of problems may have been
due to a combination of factors, including not using the coolant as a moderator,
and also better control the chemistry. The design of the Pekkor reactor in the U.S. was based on
their PWR designs and so inherently had large volumes of stagnant fluid, something that was not
possible with a two-based design such as WR1. Preliminary work was done on turning this into a
large power reactor design, but this did not proceed as Canada decided to focus on further
development of existing water-based designs instead. HTGCR, high temperature gas cooled reactor.
I have mentioned the bagnox and HDR reactors, but there were a variety of other gas cooled
reactor designs. Some of these were built but generally as one off prototypes. These other
reactors are all characterized by the following features, helium gas cooled, high temperature
operation, graphite moderator. The fuel is sometimes referred to as prismatic, as it is in
the form of blocks rather than the spheres used in pebble bed reactors. I'll mention pebble bed
reactors in more detail later. It's not clear to me how refueling is carried out. Some used
uranium thorium mixed oxide fuel. A variety of reactors are built in the 1960s, 70s and 80s,
but always as one off prototypes. No gas cooled reactor other than the U.K.'s
magnox and HDR designs have been commercial successes. However, there is renewed interest into
this type of reactor today, mainly in connection with using the heat to directly power large
process industries to replace burning fossil fuels in chemical or metallurgical applications.
HWLWR, heavy water light water reactor or, also known as SGHWR, steam generating heavy water
reactor. This was an odd style of reactor in that it combined features of several different
reactors. There were a number of different reactors designed in several countries which use this type,
although none went beyond single prototypes. Countries building them included Canada, the U.K.,
Japan and Italy. Like a can do or PHWR reactor, they use a heavy water moderator in an
unpressurized calendria. However, the coolant was light water rather than heavy water.
The pressure tubes are oriented vertically rather than horizontally as found in a can do.
There were all boiling water reactors in which the coolant boiled directly in the
coolant channels, rather than an A separate steam generator. There seemed to be nothing inherently
wrong with the concept, it seems they just did not for any advantages over other reactors already
being built in those countries. Ractors making a comeback, next I will discuss a couple of types
of reactor that have been around for a long time, but have recently started making a major
comeback in terms of interest. Pebble bed reactors, this reactor type has a long history and its
basic ideas go back to the very early days of nuclear power. Series attempts at commercializing
it however, first took place in Germany or West Germany as it was known at the time in the early
1960s. AVR. The first one was the 15 megawatt AVR reactor in the western most part of Germany.
The concept behind the Pebble bed reactor is the fuel and graphite moderator where a single unit
contained in many small spheres, each about the size of a billiard ball. The reactor was basically
a bin full of these spheres. The coolant was helium gas which would circulate between the spheres.
Fresh fuel spheres would be added at the top of the bin and spent fuel removed through the bottom.
It ran from the late 1960s to late 1980s.
THTR 300. The AVR was succeeded by the larger THTR 300 in the early 1980s.
This supplied 300 megawatt's electricity to the grid, operating until the late 1980s.
The helium coolant fed a steam generator which in turn operated a conventional steam plant.
The reactor vessel was made from pre-stress concrete, the fuel with the mixture of uranium 235
and thorium. South Africa, China and the US. Pebble bed technology was in licensed to a group in South
Africa who pursued development for many years before money ran out.
The technology was also licensed to a group in China.
Chinese development led to first the HTR 10 prototype reactor in the 1990s and early 2000s
and then the HTR PM reactors in which a pair of reactors feed a single
210 megawatt steam turbine generator. This started operation in 2021.
This Chinese plant is the one which people often refer to when they talk about small modular
reactors in China. Part of the South African design team were then hired by an American company
called X-energy who wanted to take the German and South African technology and sell it to the SMR market.
Making a comeback? At this point it's hard to say whether pebble bed technology is actually
making a comeback although there is renewed interest in it. MSR or molten salt reactors.
The distinguishing feature of molten salt reactors is that they use molten chloride or fluoride
salts as a coolant. Since the molten salt has a high boiling point, the reactor is
unpressurized and steam is created in a steam generator. Slow or fast neutron reactors,
molten salt reactors may be slow or fast neutron reactors. Slow neutron reactors would typically
have a graphite moderator. Fast neutron reactors would not have a moderator. I will discuss
fast neutron reactors in more general terms in another episode. Fuel. The fuel may be uranium,
plutonium, thorium or mixture of these. The fuel may be in separate fuel pins like
another reactor types or maybe dissolved in the salt. Some interest in molten salt reactors
seems to be focused on using them to consume stored spent fuel from other types of reactors,
such as light water reactors. The use of spent fuel is intended to provide a means of getting
rid of long-lived radioactive waste from spent fuel by burning it in nuclear reactions.
Salts. The salts are usually sodium, magnesium and calcium chloride or lithium fluoride salts.
Chloride salts are typically used with uranium fuel in fast neutron reactors.
Fluoride salts are typically used with thorium fuel in slow neutron reactors.
Chloride salts are much less expensive than lithium fluoride salts,
which require isotopically pure lithium-7. Why some variants used is all fuel?
One major reason for dissolving the fuel in the salt, coolant, is to allow the removal of
Protaktinium 233, which is produced as an intermediate product in the process of transmuting
thorium 232 to uranium 233. Protaktinium 233, or PO 233, is a major neutron observer,
and so interferes with the thorium reaction. To Protaktinium 233 decays into uranium 233 with a
half life of 27 days after which it could be fed back into the fuel stream.
Various design for thorium fuel would typically have a separate blanket of additional
thorium around the reactor core in order to produce more uranium 233 from neutrons that would
otherwise escape. Protaktinium 233 is not a significant problem in the blanket.
History experiments with molten salt reactors date back to the 1960s and 70s.
Recently interest has revived, with research being conducted in a number of countries on both
dissolved and solid fuel versions. Types of molten salt reactor, there are many different
variants of molten salt reactor, so I won't try to describe them all here. Some used solid fuel,
some used fuel dissolved in the coolant, some used spheres similar to those used in
pebble bed gas cooled reactors. Many of the reactor types described elsewhere in series have a
molten salt equivalent. Pros and cons, pros. On the positive side, molten salts remain liquid up
to a very high temperature, allowing that for the generation of high temperature steam which
can drive a turbine efficiently. The high coolant temperature also allows for creating high
temperature process heat for direct use in industry. The salts are non-corrosive to special
alloys develop to handle them. The salts are not damaged by radiation. The dissolved salt
reactors can be designed, such as an increase in temperature, cause a thermal expansion of the salt,
which in turn reduces the rate of reaction, providing a passive self-regulating safety feature.
Cons. In the dissolved fuel versions, the primary coolant is highly radioactive,
making maintenance more difficult. The chemistry of the salt must be carefully maintained to
prevent it from becoming corrosive. Lithium salts must be enriched, rather like uranium is often
enriched, to nearly pure lithium 7 since lithium 6 will produce radioactive tridium when
fissioned by neutrons. Supplies of fairly pure lithium 7 are limited as it is mainly produced
as a byproduct of producing tridium. Birlium is used in some compositions and is highly toxic.
Continuously chemically processing the coolant in the dissolved fuel variants adds complexity to the
system. Overall, there are a very large number of molten salt designs. Of the less common
reactor types, molten salt reactors are undoubtedly the ones which are seeing the most current
interest in terms of numbers of new designs and the amount of current R&D. Conclusion. In this episode,
we discussed some of the less common historical reactor types. As we have seen, there have been a
number of different reactor designs, which are less commercially successful for one reason or another.
Some of them may be making a comeback, however, particularly as the basis for a small reactor.
In the next episode, we will describe fast neutron reactors. This concludes the fourth episode
of a N8 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.
Hosting for HPR has been kindly provided by an AnHonestHost.com, the Internet Archive,
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.