hpr-knowledge-base/hpr_transcripts/hpr4593.txt

Episode: 4593
Title: Nuclear Reactor Technology - Ep 8 Generation Four Reactors
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr4593/hpr4593.mp3
Transcribed: 2026-07-31 16:14:41 (official HPR transcript)

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This is Hacker Public Radio Episode 4593, for 2026-03-11
Today's show is entitled, "Nuclear Reactor Technology - Ep 8 Generation Four Reactors"
The host is Whiskeyjack and the duration is 00:11:29
The flag is Clean, and the license is CC-BY-SA
The summary is "Future reactor technologies, particularly what are referred to as Generation Four reactors."
This episode is the eighth and final one in an eight-part series on nuclear reactor technology.
In this episode, I will talk about future reactor technologies, particularly what are referred to as generation four reactors.
Some of these will simply be additional developments of reactors that have already been discussed in this series,
but this will show what technologies are seen as being most promising today.
What is generation four?
Generation four international forum is an international organization whose membership is composed of many of the countries that are researching advanced vision reactors.
Their goal is to conduct a number of joint research projects to advance the state of the art.
The members agreed to participate in and share a research on advanced technologies.
Research subjects.
Here are the subjects undergoing research at this time.
Half of them are some form of fast neutron reactor.
The names they use here are for the specific projects and not intended to encompass all possible reactor types.
Lead fast reactors.
Lead cooled fast reactors use a liquid lead or lead bismuth cooling.
As the name implies, they use the fast neutron reaction.
Lead has a high boiling point up to 1,743C, favorable neutronic properties, and does not react strongly with air or water.
The reactor would operate at atmospheric pressure.
Fuel is your radium, Epletonium mixed oxide, or nitrite.
The main advantages of this reactor are lower capital and construction costs because of operating at atmospheric pressure,
and design simplifications which lead permits.
The problems we worked on are related to materials and chemistry.
Sodium fast reactor.
Sodium cooled fast reactors use a liquid sodium coolant.
As the name implies, they use the fast neutron reaction.
The reactor would operate at atmospheric pressure.
Fuel is your radium and plutonium mixed oxide or metal.
They have a great deal of flexibility in terms of how much plutonium they use or create.
However, the coolant is highly reactive with air and water, and requires an oxygen-free environment.
There is a long history of experience with sodium fast reactors to build on.
They are very well suited to producing plutonium with which to fuel other reactors,
including providing the plutonium driver for thorium fuel,
eliminating the need for uranium enrichment.
Gas cooled fast reactor.
Gas cooled fast reactors use helium gas coolant.
As the name implies, they use the fast neutron reaction.
The reactor would operate at high pressure.
Fuel is uranium and plutonium carbide.
Helium is non-corrosive, and unlike lead or sodium, it is transparent,
easing inspection of the reactor.
The high temperature helium can be used directly to provide process heat to industry,
as well as for producing steam for electricity generation.
However, without a moderator, the system has very low thermal mass,
meaning that temperatures can rise more quickly in the event of a loss of coolant.
It requires very pure helium coolant as hydrogen and cause
embrittlement of metals.
Very high temperature reactor.
Very high temperature reactors use helium gas coolant.
Unlike gas cooled fast reactors, they use a slow neutron reaction.
The reactor would operate at high pressure.
Fuel is uranium and plutonium oxide, or uranium and thorium oxide,
mixed with graphite in billiard balls, size spheres, or in blocks.
The spherical fuel variants are essentially pebble bed reactors.
Operating temperatures are 700 to 850C, and possibly more than a thousandC later.
As well as producing electricity, the heat could be used directly for process heat for industry,
including producing hydrogen.
There is a great deal of experience with gas cooled reactors of various types of drop-on.
Molten salt reactors.
Molten salt reactors use various molten salts as coolant.
Different variants use slow or fast neutron reactions.
Some will use conventional solid fuel, while others use fuel that is dissolved in the salt coolant.
Where the salt is dissolved in the coolant, there will be online chemical processing
to remove waste products and keep the chemistry and balance.
Advantage of molten salt over water coolant are operating at ambient pressure and higher temperatures.
Heat can be used directly in some industrial processes.
Problems to be solved include corrosion caused by molten salt and maintaining the correct chemistry
to salt when using dissolved fuel.
Supercritical water reactors.
Supercritical water reactors are water-cooled reactors that operate at temperatures and pressures
above the critical point of water, which is 374C and 22.1 megapascals.
Supercritical water has a behavior somewhere between that of liquid water and gaseous steam.
At the critical point, the supercritical fluid has a density of about one-third that of liquid water.
These can use slow or fast neutron reactions.
Fuel is uranium oxide, plutonium uranium mixed oxide, or plutonium thorium mixed oxide.
The moderator is light water, heavy water, or no moderator in the case of fast reactors.
They can be pressure vessel reactors, similar to boiling or pressurized water reactors,
or Kalandria and pressure tube reactors, similar to can-do reactors.
Supercritical water has been well-established in industry, but the first supercritical boiler being built in 1960.
Modern fossil fuel-fired steam generating plants use supercritical water boilers and turbines.
Supercritical water reactors can expect to have thermally efficiencies of 44 to 48%.
Current water-cooled reactors have thermally efficiencies of 34 to 36%.
The benefits of higher efficiencies are to get more power from a similar size of plant reducing capital costs.
Additionally, the characteristics of supercritical water allow reactors to be simplified further reducing costs.
Using supercritical water would allow nuclear power plants to use the same sorts of steam turbines as are used in fossil fuel plants,
reducing costs and increasing the supply chain availability.
With supercritical water, there is only a single phase rather than separate liquid and gas phases.
This simplifies the design and reduces the number of components, as no steam generators are needed.
Supercritical water reactors combine decades of experience with water-cooled reactors, with decades of experience of the supercritical water in multiple industries.
This is seen as the natural evolution of water-cooled reactors.
Cost estimates from studies done in Europe in Japan suggest cost reductions of 20 to 30%.
The main challenges revolve around developing materials that are compatible with both the very high temperature water and with radiation, particularly for the fuel cladding.
Concept designs have been made for both large and SMR size supercritical water reactors.
Episode conclusion. In this episode, we looked at the reactor types being studied under an international organization called the Generation 4 International Forum.
All of these reactor types, except for supercritical water reactors, are not new, and we have looked at them previously.
Supercritical water reactors themselves represent the natural evolution of water-cooled reactors.
I suspect that many of these research projects will not result in commercially successful results, such as the nature of R&D.
The supercritical water reactors would, on the surface, seem to have the most promise in terms of commercial use.
As they focus on bringing two very well-established technologies together, water-cooled reactors and supercritical water.
However, I'm not an expert in this field, so I'm just making an educated guess on that.
Series conclusion. This is the end of the series on nuclear reactor technology.
Episode 1 covered nuclear basics, including basic terminology and several verses military nuclear material.
Episode 2 covered nuclear fuel, including the different types, recycling of spent fuel, uranium, and thorium resources, and medical isotopes.
Episode 3 covered reactor basics, including slow versus fast reactors, moderators, coolants, steam generation, refueling methods, and the three main commercial reactor types.
Episode 4 covered the less common reactor types, including types which are no longer used, some historical developmental dead ends, and some types which may possibly be making a comeback.
Episode 5 covered fast reactors, including the different types, some of their history, why they were developed, and why they have so far only seen limited use.
Episode 6 covered thorium reactors, including what is thorium and how it differs from uranium, why there is interest in thorium, what sorts of reactors can use thorium, and why thorium has not yet seen widespread use.
Episode 7 covered small modular reactors, or SMRs, what is the reason for developing them, what are the different ways they may be used, and where they are currently being built.
Episode 8 covered generation 4 reactors, which is a collection of future technologies.
I hope that this series has been useful and informative on how nuclear reactors work, and what the different types of reactors and different types of fuel are.
I have focused on the past and present, without looking very much beyond what is already developed except in this final episode.
I have focused on the reactors, fuel and medical isotopes, without much discussion of mining, refining, converting, enrichment, fuel fabrication, or disposal.
I also haven't talked much about the rest of a functioning power plant, which includes cooling, steam turbines, generators, transformers, control systems, refueling systems, switch gear, transmission grid connections, grid coordination, and many many other things.
And of course, those entire grid itself, a very complex thing when operated at scale.
Nonetheless, we count on the lights going on when we turn on the light switch, while seldom thinking about all the things that go on behind the scenes to make that happen.
As the recent blackout in Spain shows, that is something that we can't take for granted.
With plans for net zero amounting to essentially the further electrification of everything, we need reliable sources of electrical energy to make that happen.
Without reliable electrical energy, available at the touch of a switch, we don't even have a Stone Age civilization, let alone a modern one.
So think about that the next time you turn on a light or listen to a podcast or do nearly everything else in your daily life.
This concludes the 8th and final episode of an 8-part series on nuclear reactor technology.
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