Episode: 4573
Title: Nuclear Reactor Technology - Ep 6 Thorium Reactors
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr4573/hpr4573.mp3
Transcribed: 2026-07-31 16:14:14 (official HPR transcript)

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This is Hacker Public Radio Episode 4573, for 2026-02-11
Today's show is entitled, "Nuclear Reactor Technology - Ep 6 Thorium Reactors"
The host is Whiskeyjack and the duration is 00:16:57
The flag is Clean, and the license is CC-BY-SA
The summary is "The use of thorium in nuclear power, what thorium is, what sort of reactors can use it."
This episode is the sixth in an eight-part series of nuclear reactor technology.
In this episode, we will describe the use of thorium and nuclear power, including what thorium
is, how it differs from uranium, and what sort of reactors can use it.
What is thorium?
Thorium is a slightly radioactive metal, which can be used for nuclear fuel, and which
is roughly three times as abundant as uranium.
Really all naturally occurring thorium is a single isotope, TH-232.
Other isotopes such as TH-228, TH-230, and TH-234 exist only in trace amounts, and so
can be ignored for our purposes.
Thorium oxide has long been used in applications such as gas, metals, and high-temperature
ceramics, but has seen little use in nuclear power beyond some experimental reactors.
While thorium differs from uranium, while thorium can be used in nuclear fuel, thorium 232
is fertile rather than fizzile.
That means that pure thorium is not itself directly capable of sustaining a nuclear
reaction.
Instead, it must be turned into a fizzile isotope by bombarding it with neutrons.
In this respect, it is similar to uranium 238, which is the main isotope of uranium, which
can be turned into fizzile and non-fizzile plutonium.
In the case of thorium 232, it can be turned into uranium 233, which is a fizzile isotope
much like uranium 235.
However, unlike uranium 235, uranium 233 is not considered to be useful from making
nuclear weapons.
Sources of thorium Currently, the largest non-reserves of thorium are in India, followed
by Brazil and Australia.
However, since there is currently a very limited market for thorium, there has not been
a lot of exploration done, and it is possible that very large reserves exist elsewhere.
As a result of this, take attempts to list thorium reserves by country with a very large
grain of salt, except for a few very large cases.
India's large reserves of thorium, however, explains their interest in using it as fuel.
China also have an interest in thorium, such that it is often produced as a byproduct of
rare earthbinding and refining.
As a result of this, there are large reserves of thorium already mine there, which could
be used as fuel.
Why there is interest in using thorium as a fuel?
There are several reasons why people are interested in using thorium as a nuclear fuel.
Abundance of thorium Thorium is roughly three times as abundant as uranium in the earth's
crust.
This is based on statistical sampling of rocks, not in terms of finding commercial order
deposits.
However, the world is not currently short of uranium, so the relative abundance of
thorium is not a significant advantage at this time.
Some countries have a lot of it.
Some countries in India in particular have far more thorium than they do uranium.
As a result of this, they see thorium as a strategic and security advantage in giving them
a sovereign independent supply of energy.
Thorium breeder reactors are simpler than uranium breeder reactors.
Thorium 232 to uranium 233, the a neutron bombardment, is analogous to converting uranium 238
to various plutonium isotopes in terms of turning fertile isotopes into fizzile ones.
Listen again to the previous episode on fast neutron reactors if you need to refresh
your on those, but fast neutron breeder reactors are more expensive and difficult to design
and operate than thorium reactors.
Thorium can be used in reactors similar to certain types which have been in large-scale
commercial use for decades, and so are less complex and expensive than, for example, a
liquid sodium-fast neutron reactor.
No new technologies required to use thorium in commercial power reactors.
Supposed lower nuclear weapons potential.
Some people are of the view that there is less potential for creating nuclear weapons if
thorium is used as fuel than is the case for uranium.
However, I am not convinced of this as an argument.
Commercial uranium cycle power reactors are not suitable for creating weapons-grade plutonium.
Plutonium or enriched uranium are required for thorium cycle reactors.
A country which has a large number of thorium cycle reactors will need either uranium
enrichment or plutonium processing facilities while natural uranium reactors require neither.
On this basis, the least problematic fuel would appear to be natural uranium.
What is thorium breeding?
As mentioned previously, thorium by itself cannot be used as a nuclear fuel.
By using plutonium or enriched uranium as driver elements, however, thorium can be converted
to fizzile uranium 233 which can be used to sustain a reaction.
There are different configurations for the fuel elements which I won't go into here as
this would require too much detail.
Essentially, though, a mix of thorium 232 and either plutonium or enriched uranium 235
would be loaded into the reactor.
The reaction would be started off by the plutonium or uranium 235.
Bombardment of the thorium 232 by neutrons would gradually convert it to uranium 233.
As uranium 233 accumulated, it would also efficient and contribute to the power output,
making up for the using up of the plutonium and uranium 235.
By the end of the life of the fuel, all or nearly all of the energy would be coming from the
created uranium 233.
The fuel is then removed from the reactor and reprocess to recover the remaining uranium 233.
This uranium 233 may then be used as a driver fuel for a fresh fuel load.
For reactors that are refueled continuously while online, that is while operating,
fresh fuel is continuously fed in and used fuel extracted rather than being distinct
loading and unloading phases of operation.
Breeding ratio.
The amount of uranium 233 created in this process will depend upon the reactor and fuel design.
This may vary from not much uranium 233 generated all the way up to theoretically at least,
more uranium 233 being extracted from the spent fuel than the amount of driver fuel used to
start the reaction.
That is, the breeding ratio would be greater than one.
However, this has not been demonstrated and most fuel cycles assume that a continual source of
driver fuel will be required, generally from some sort of uranium cycle reactors.
What sorts of reactors can use thorium?
Most of the types of nuclear reactors we have discussed so far in the series can use thorium fuel to some extent.
However, some are more suitable for it than others.
pH WRs, that is heavy water reactors, including can do.
Large-scale commercial pressurized heavy water reactors have been used for decades around the world.
These have run on natural uranium but are also well suited to using thorium fuel due to their
efficient use of neutrons.
India is one of the countries using this type of reactor and they have done a great deal of R&D
on using them with thorium.
A typical thorium fuel for them would consist of 5% reactor-grade plutonium,
plus thorium, about 80% of the energy generated would come from the thorium portion of the fuel.
An alternative to plutonium would be low enriched uranium.
None of the commercial reactors of this type are currently running on thorium,
but this is an area of ongoing research, particularly in India.
HTR, high temperature gas-cooled reactors, as mentioned in a previous episode,
some of the experimental gas-cooled reactors have used thorium fuel.
Examples of these are the AVR and THTR 300 reactors in Germany in the 1960s to the 1980s.
These were pebble bed reactors in which the fuel was in the form of spheres of fuel,
enriched uranium and thorium, plus a graphite moderator, and formed into billiard ball size spheres.
The reactors were cooled by helium gas.
Pebble bed reactor technology is being revived for the small modular reactor market,
but these doer ones don't involve thorium.
Moulton salt reactors. Moulton salt reactors use molten salt as a coolant.
When used with dissolved thorium fuel, the salt would be a fluoride salt in order to be chemically
compatible with thorium. To be used in a thorium fuel cycle, they would normally have a graphite
moderator. From the perspective of thorium, there are two variants of molten salt reactor
that are of interest. One type contains a fuel inside separate fuel pins, like in most other
reactor types. The other dissolves a fuel in the molten salt coolant.
In the latter case, there would be a chemical processing loop that extracts unwanted
fission products that is elements that were created in the reaction that we don't want present,
and uranium 233 from the salt on a continuous basis. Moulton salt reactors were discussed in
another episode as they can use many types of fuel besides thorium. Not all developers of molten
salt reactors are convinced they are well suited to using thorium. With the head of one company
citing the difficult chemistry problems that thorium presents, and the effects this has on the
types of molten salt used as compared to uranium. Lightwater reactors. Lightwater reactors,
both the common pressurized water and boiling water reactor types, could use thorium fuel,
but are not well suited to it. They would not achieve very good breeding ratios in terms of
converting thorium 232 to uranium 233. A major drawback of these reactors is their lower
neutron economy due to the use of a less effective moderator. However, there is some attraction
in terms of using them to get rid of surplus plutonium if that is what is desired.
One of the features of thorium mixed oxide fuel is that unlike uranium-based mixed oxide fuels,
it can burn plutonium without creating more plutonium for uranium 238.
I'm not sure what the attraction of this is, although there are apparently
is at least some minor interest in this from some quarters. Fast neutron reactors, thorium
could be used in place if uranium 238 in fast neutron reactors. However, there is no real reason
to do this when surplus uranium 238 is available, and the latter provides a better breeding ratio
that is, produces more physon material from a given amount of fuel. The challenge is facing
thorium fuel reactors. The main challenge facing the use of thorium is that it does not
offer a lot of practical advantages over uranium. Like fast neutron reactors, using thorium
was seen as a solution to a perceived future shortage of uranium. However, that shortage didn't
materialize instead large new reserves of uranium were found. The cost of uranium is only a small
part of the total cost of electric power from nuclear power plant, so uranium prices could
rise significantly without affecting overall cost much. With enough of a rise in prices,
uranium could be economically extracted from sea water, opening up a very large potential
resource as there are very large amounts of it dissolved in the sea. Uranium fuel is already
tested, qualified, and available in quantity on the commercial market. Thorium is not,
and overcoming that will require a large investment. Thorium is more expensive the process of
defuel due to its chemical nature and the exceptionally high melting point of its oxide.
A practical thorium fuel cycle requires reprocessing of spent fuel, something that is not required
for a once-through fuel cycle with uranium, and is done with uranium in only a few countries today.
This reprocessing is another additional cost. The uranium 233 and leftover thorium in spent fuel
contain highly radioactive isotopes such as uranium 232 and thorium 228, which make reprocessing
difficult and expensive. This is not to say that thorium does not have some potential in the
long run. Rather, it means that the advantages of thorium are offset by problems which come along
with it. Overall, the balance of present in most countries favors using uranium in a once-through
fuel cycle. Thorium in India, an example use case, India is probably the prime example of a
country that has had a long-term interest in thorium-based nuclear power. This is an area they
have been researching and developing for decades. The main reactor types using India are also
well suited to being adopted to thorium fuel. These are pressurized heavy water reactors
originally derived from Canadian designs. Indian scientists and engineers continue to develop
these reactor designs into once which maintain the original principles, but are currently independent
Indian designs. Because of this reactor types of very efficient use of neutrons, it is well suited
to using thorium fuel. Why is India pursuing using thorium? The reason that India are pursuing
using thorium is that they have abundant supplies of thorium, but very limited supplies of uranium.
While Canada uses a similar type of reactor, it has abundant supplies of uranium and so has
had much less incentive to put much effort into thorium fuel. How would a thorium fuel cycle work in
India? The basic fuel cycle would work as follows. Uranium cycle fast-breeder reactors would produce
plutonium from uranium and uranium 233 from a blanket of thorium around the uranium core.
The plutonium in uranium 233 would be incorporated into mixed oxide fuel as plutonium
thorium and uranium 233 thorium fuel. This mocks fuel would be used in pressurized heavy water
reactors. Each fast-breeder reactor could supply fuel for multiple pressurized heavy water reactors.
The spent fuel in additional blanket material from the pressurized heavy water reactors
would be reprocessed to extract the leftover uranium 233. This recovered uranium 233 would then
be fed back into the fuel cycle to make more fuel with top-ups from fast-breeder reactors added
as required. This fuel cycle would allow India to use its thorium reserves with a relatively
small amount of uranium input. Current status India are working on the fast-breeder reactors
which would be used to provide the seed material for thorium fuel in their pressurized heavy water
reactors. India continue to pursue using thorium but they need to build up the rest of the fuel
cycle which enables it before they can switch to thorium on a large scale.
Reportedly they already use some thorium fuel in some of the fuel channels of their heavy water
reactors in order to smooth out the power profile but details of this are hard to come by.
India are expected to undergo rapid economic growth, industrial development,
and population movement into very large urban centers over the course of this century.
Meanwhile coal resources are becoming depleted as well as being a source of serious air pollution.
India hopes to use thorium to provide a secure domestic energy resource which can replace coal
and power their future development. Conclusion thorium is an abundant material that is seen as an
alternative to uranium and nuclear power. Experimental thorium power reactors take back to at least
the 1960s. No new reactor technology is required to use thorium. Existing well proven reactor
designs which have been in use for decades can use thorium as fuel. The common light water
reactor designs that are popular in some countries however are not well suited to using thorium.
Initial interest in thorium was mainly driven by a perception that uranium would be in short
supply in future and slow neutron thorium reactors were cheaper and simpler than fast neutron
uranium reactors. However huge new high-grade supplies uranium were founded in a number of countries
causing uranium prices to fall and reducing interest in finding alternatives. While some
R&D continues on thorium fuel in a number of countries, the mainstream of development continues to
be on uranium-based fuel. Some countries with abundant thorium reserves so maintain a major
interest in thorium with India being the prime example. In the next episode, we will describe
small modular reactors. This concludes the sixth episode of an eight-part series on nuclear
reactor technology.
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