Thorium myths

By Nick Touran, Ph.D., P.E., 2014-03-13 , Updated 2020-09-08 , Reading time: 19 minutes

Thor by Marten Eskil Winge Dear Internet, we need to have a talk about Thorium. Almost all the borderline-miraculous things people say it does online are associated with advanced nuclear technology (e.g. breeding and low-pressure coolant systems) and apply to both thorium and uranium fuels. Generally, you can swap the word “thorium” out for “advanced nuclear”.


The things being said on the internet have become largely misleading, if not all-out inaccurate. Every internet person I meet in real life who finds out that I am a nuclear engineer asks me why we aren’t using the end-all, be-all that is thorium. Every post regarding nuclear energy is packed full of comments claiming that Thorium will end all concerns about nuclear energy and that Uranium is only in use due to some dark dark conspiracy.

Some places on the internet have become echo-chambers for this kind of thing, and while it’s great to spread awareness of thorium, blatant disregard of the associated challenges is a detriment to civilization’s energy debate. Besides, taking a moderate viewpoint lends credibility to any cause. This page will try to point people in the right direction if they get lost.

Thorium has one primary advantage, which is that it uniquely allows breeding with slow neutrons. Breeding is possible with fast neutrons in any fuel, but doing it with slow neutrons is kind of neat. Everything else you may have heard is suspect.

On this page:

Myth #1: Development of Thorium reactors was stopped because they couldn't make bombs!

Nope. It was economics. When you add a neutron poison like thorium into your core, you need to add extra enriched uranium, which is extra expensive. Simple as that.

Thorium fuel was included alongside uranium in the first core of the Indian Point 1 reactor in New York in the early 1960s. The second core had no thorium. Here’s an exchange from 1963:

G.B. SCURICINI: Can you tell me why you changed your plant over from a thorium to a uranium cycle? Is the reason purely economical or did you expect trouble from the use of thorium?

W. BEATTIE: The reason is purely economical and we did not expect to have any trouble with the thorium fuel cycle.

IAEA Conference on Operating Experience with Power Reactors. Vol. I (1963)

For the molten-salt variety of thorium reactors, a lucid description of what happened can be found on page 49 of WASH-1222 [1]. There, they describe a few privately-funded working group studies of the MSBR, including the Molten Salt Breeder Reactor Associates (consisting of the engineering firm Black & Veatch and five midwestern utilities) and the Molten Salt Group, headed by Ebasco Services, Inc. (with 5 other industrial firms and fifteen utilities involved). These groups concluded that the MSBR (basically the LFTR) is attractive and potentially cheaper than LWRs. They said that a demonstration plant is warranted, but the performance cannot be predicted with confidence. Then, a list of factors that limit industrial involvement is given. They include (verbatim):

  • The existing major industrial and utility commitments to the LWR, HTGR, and LMFBR.
  • The lack of incentive for industrial investment in supplying fuel cycle services, such as those required for solid fuel reactors.
  • The overwhelming manufacturing and operating experience with solid fuel reactors in contrast with the very limited involvement with fluid fueled reactors.
  • The less advanced state of MSBR technology and the lack of demonstrated solutions to the major technical problems associated with the MSBR concept.

Weapons were produced with graphite or heavy-water moderated production reactors and with gas centrifuge enrichment. Also, thermonuclear weapons require tritium, which is something that many Thorium MSR designs excel in producing because they often contain Lithium. The commercial LWRs had nothing to do with making bomb material. Stop the nonsense.

To be fair, you can rightly argue that U-Pu-fueled reactors got developed in the first place (in the Manhattan project of the 1940s) for weapons reasons. Back then (before enrichment), Th-fueled reactors couldn’t even go critical, much less make bombs. Natural uranium reactors were the only way to go. This gave them the technical head start that has arguably led to their dominance. However, when MSRs were finally given their chance in the 1950s and 60s, their (non-existant) inability to make bombs was not to blame for the cancellation.

Myth #2: Thorium reactors never need enrichment!

Like all reactors, Thorium reactors need some fissile material to start up. Thorium itself is a neutron poison and needs to be irradiated by neutrons (coming from somewhere) in order to start making good nuclear fuel.

Once started up, any breeder reactor (using uranium and/or thorium) can be fissile self-sufficient, meaning they breed more (or equal) fissile material than they consume.

It should be noted, however, that the key advantage of Th fuel is that it allows breeding with slow neutrons. This means that you can start up a Th-based breeder with substantially (between 3 and 10 times) less fissile material than you need to start an equivalent-powered fast breeder reactor. Once started, the fast breeder will make far more fissile material (because they make have a better breeding neutron economy), but the amount of fissile in fast spectrum reactors is always more than in slow-neutron reactors.

TL;DR: They do to start up, and U-Pu breeders like the LMFBR can do the same so it’s not Thorium specific.

Myth #3: Thorium reactors cannot make bombs!

Nuclear reactors, by their nature, split atoms in a chain reaction to release energy slowly, safely, and under control. If a nefarious operator controls such a system, they can use various means to extract the fissile nuclear fuel atoms and concentrate them into a weapon. Thus, all reactors require safeguards and inspections from the UN nuclear watchdog, the IAEA. This includes Thorium reactors.

Thorium reactors work by breeding Th-232 through Protactinium-233 (27.4 day half life) and into Uranium-233, which is fissile. Pa-233 is a pretty strong neutron absorber, so the MSBR (basically the LFTR) has to extract it from the core once it is produced and let it decay to U-233 away from the neutrons. Once the U-233 is created, it gets fed back into the reactor. Well, if you went rogue, you could build up a little excess reactivity (maybe add some low-enriched U-235?) and then divert the freshly-bred U-233 into a weapons stream to make U-233 nuclear bombs. It may be difficult to do this several times without going subcritical, but it certainly could be done. A U-233-filled bomb has been tested before, and it worked just fine.

But don’t take my word for it. Here is a declassified letter entitled LRL interest in U-233 from the USA’s top nuclear weapons design team from 1966. It says:

Letter from Livermore lab about how good U-233 is as a weapons

Figure 1. The text of the Livermore weapons scientists saying U-233 is only better than plutonium in some rare scenarios, so they don't need to ditch plutonium and switch everything over to U-233 (emphasis mine)

More information about the specifics of making clean U-233 can be found in this 1965 article by Woods entitled Clean Uranium-233, and in this (more readily available) article: Boswell, Production of U-233 with Low U-232 Content (1968).

Here’s a quote from a Frank von Hippel paper on the subject [2]:

"On the one hand, gamma radiation from U-232 makes the U-233 from high- burnup U-233-thorium fuel cycles more of a radiation hazard than plutonium. On the other hand, because of its low rate of spontaneous-neutron emission, U-233 can, unlike plutonium, be used in simple gun-type fission-weapon designs without significant danger of the yield being reduced by premature initiation of the fission chain reaction"

And another (also [2]):

"In the case of the molten-salt U-233 breeder reactor, it was proposed to have continual chemical processing of a stream of liquid fuel. Such an arrangement also offers a way to completely bypass the U-232 contamination problem because 27-day half-life Pa- 233 could be separated out before it decays into U-233."

Options to make bomb-making less favorable include fostering substantial U-232 contamination in the reactor and denaturing the U-233 with U-238 that keeps the in-reactor inventory safe. Both of these options can conceptually be bypassed in the Pa separation route though. Besides, U-232 isn’t releasing the gammas, its decay products are, and it has a 70 year half-life. So you can just chemically purify your stolen goods and then make the bomb anytime within the next few months or so.

Figure showing a
reduction in gamma dose to 3% as you chemically purify a U-232 sample.

Figure 2. A plot showing a reduction in gamma dose from U-232 daughters to 3% after chemical purification, from WAPD-TM-1422.

There are about a dozen other ways people try to amp up the proliferation resistance of various fuel cycles. But they always forget that the owner of such a plant can secretly install a chemical cell that does Pa separation.

Really, most civilian power to bombs proliferation paths are mythical, in any reactor, because they’re so difficult! But since the consequences of proliferation are so dire, nuclear power plants need to have baseline proliferation safeguards in place. Thorium-powered reactors, whether fluid fueled or not, are no exception.

Myth #4: There's more Thorium than Uranium, and that is really important!

This one is mostly true, but also partially false. The average crustal concentration of Thorium is 0.00060%, compared with 0.00018% for Uranium [3]. But, the oceanic abundance of Th is 4x10-12%, compared with 3.3x10-7% (mass percent). Considering that the oceans contain 1.4x1021 kg of water, that amounts to 56,000 tonnes of Th and 4.62 billion tonnes of Uranium. Moreover, mining the entire crust is difficult, whereas the ocean delivers to you. While seawater extraction of uranium is not yet competitive with traditional mining (it’s hovering around 4x more expensive), it is possible and may become economical in the near future. So while Myth 4 is correct with respect to the crust, it’s not necessarily relevant from a global resource perspective, and there may very well be more accessible Uranium available to us. The crust is estimated to weigh around 1.0x1022 kg, so overall, there is actually more Th. If you want to get very technical and start including asteroid and star mining, the abundance of Th in the universe is estimated at about 2x that of Uranium.

If you’re the Indian government, however, you’ll note that you have hundreds of thousands of tonnes of Th but basically zero U. So you guys might want some Th-power to secure a domestic supply! China has about an estimated 50% more known U than Th [4,5].

Another point, if you look at the known reserves of economically extractable Thorium vs. Uranium [4,5], you’ll find that they are both nearly identical (though many people argue that we can economically extract Th from lots of common sands). And remember, if we close the fuel cycle (whether using Th-U or U-Pu), the fuel resources are a non-issue for millenia.

Myth #5: Thorium reactors uniquely make waste that is safe in hundreds of years!

Undenatured Thorium cycles certainly produce fewer transuranic elements (Np, Pu, Am, Cm,+), which are the major dangerous nuclides in nuclear waste in the 10,000+ year timeframe. In fact, the long-term decay heat from Thorium-MSRs can be orders of magnitude lower than that from traditional reactors. However, this same capability exists in many other reactor concepts, including U-Pu fueled fast reactors with reprocessing. So, if someone says that MSR/LFTR waste is better than traditional LWR waste, they are correct. If they say Thorium is the only game in town that can reduce waste like this, then they are not correct.

Myth #6: Thorium reactors and Molten Salt Reactors are the same thing!

On one hand you can choose between a Th-U fuel cycle and a U-Pu fuel cycle. On the other hand, you can choose between a fluid fueled reactor (like a MSR) or a solid fueled reactor (like a LWR or a sodium-cooled fast reactor). Now, the Th-U cycle works really really well with MSRs, and that’s why they are often discussed together. There’s nothing wrong with this, but it’s nice to know what benefits come from which choice. The Th-U fuel cycle can be (and has been) used in solid fueled reactors and the U-Pu fuel cycle can be (and has been) used in MSRs.

The attributes of a system that come from choosing a fluid fueled reactor include: the ability to have passive safety by draining the fuel into cooled storage tanks, online fission product removal, low/zero fabrication cost, low fissile requirement, low excess reactivity (since you refuel online) [6]

The attributes that come from choosing the Th-U fuel cycle over the U-Pu cycle include: the possibility of slow-neutron (thermal) breeding (as demonstrated in the Shippingport LWR), the reduced production of minor actinides (see Myth #5), allowing nuclear waste to be safer without aggressive reprocessing, and the ability to use the Thorium mineral base instead of the Uranium minerals (useful if your country has Th but no U. See Myth #4).


The Wall of Shame

This section shows a few examples of articles that propagate the misinformation represented by the myths. Oftentimes, people ask us how on Earth they were able to read so much that was so wrong. The simple reason is that there is a lot of stuff out there that repeats the myths. We hope journalists will go the extra mile before publishing more of this.

  • Sam O’Nella academy – Why Thorium Rocks — A very popular Youtube video that gets it so wrong that I just can’t even. It’s like no one ever told them that they’re describing breeding and thinking that thorium is somehow the only way to breed. Of course this is not true. Uranium can breed too. The entire video is misleading.

  • Richard Martin, “Uranium Is So Last Century — Enter Thorium, the New Green Nuke” An article about the primary Thorium proponent, Kirk Sorensen. Hits every single myth, and is accidentally misleading on many accounts.

  • Pascal-Emmanuel Gobry,”This energy source could solve all of our problems — so why is no one talking about it?” Woah this is a really good yarn. Again, hits most myths but even makes up some new ones. Best quote:

    If these systems fail, the reactor starts going into meltdown. Thorium, being a lighter element, doesn’t have that problem.

    Being a lighter element has nothing to do with it. Having low-pressure coolant is what gives the MSR its safety advantage, dude. SFR, LFR, FHR, etc. are other reactors that can do this with or without Thorium. Crikey!

  • Asgard’s fire (The Economist) This article is pretty good in general. But, it has to be on the wall of shame due to this quote:

    Thorium, though, is hard to turn into a bomb; not impossible, but sufficiently uninviting a prospect that America axed thorium research in the 1970s.

    The MSR program shutdown had nothing to do with MSR’s ability to make weapons. Sigh.

  • Thorium: a safer nuclear power This one, focused on China and India’s development programs, touches on most of the myths and confuses higher melting point as a safety advantage (only relevant for solid fuel). Whoops!

  • World’s first thorium reactor ready to be built for cheaper, safer nuclear energy This one was sent in with the subject: “There is a staggering amount of wrong in this one.” Very true. They claim no operator intervention, not weaponizable, 100 years of operation, etc. The whole nine yards!

  • Thorium as a Secure Nuclear Fuel Alternative by A. Canon Bryan This one hits on each of the myths, almost in order. It was posted on reddit TIL and was making it to the front page. A link to this page was posted as a comment and made it to the top comment. The TIL mods then flagged the post as misleading and it was taken off the front. While we’re happy to have been a part in reducing the spread of a confusing and misleading causation, it’s also a little sad that the excitement around Thorium and nuclear had to go with it. Thorium is definitely something to be excited about. If only people would write sensational articles about it that are less misleading! It’s hard to balance marketing and truth.

  • Sameer Surampalli Is Thorium the Fuel of the Future to Revitalize Nuclear? (2019-08-13) This article once again conflates thorium as the only way to breed and fails to mention that uranium breeders are also possible. It directly violates Myths 2, 3, and 4 and has a variety of other inaccuracies, such as claiming Thorium MSRs require highly-enriched uranium. They don’t. They use low enriched uranium.

  • Kirk Sorensen’s TEDxYYC Talk (2011-04-15) In this talk with over 600k views Kirk Sorensen presents the capabilities of breeder reactors in general, but he tells the story as if the only breeder reactor possible is the thorium breeder and does not so much as even mention that there are also uranium breeders that have the same capabilities. It appears that this may be at the foundation of all the confusion we’ve seen. Having met Kirk on several occasions, it’s clear that his technical knowledge is solid, so it’s extremely unlikely that he’s ignorant of uranium breeders. It’s just unfortunate that the time constraints of the TEDx format didn’t let him fit in the additional detail that the capabilities described come from the concept of breeding rather than the concept of Thorium. In my opinion, this detail would have changed the entire tone and message of the video.


  1. "An Evaluation of the Molten Salt Breeder Reactor," U.S. Atomic Energy Commission, WASH-1222, (1972). [pdf]
  2. J. Kang and F. von Hippel, "U-232 and the Proliferation-Resistance of U-233 in Spent Fuel," Science & Global Security, 9., p 1-32, (2001). [pdf]
  3. Dayah, Michael. Dynamic Periodic Table. 1 Oct. 1997. Web. 22 Mar 2014
  4. World Nuclear - Known Thorium Reserves
  5. World Nuclear - Known Uranium Reserves
  6. Generation IV Nuclear Energy Systems Ten-Year Program Plan - Fiscal Year 2007, Appendix 6, Molten Salt Reactor
  7. Uranium-233 purification and conversion to stabilized ceramic grade urania for LWBR fuel fabrication (WAPD-TM-1422)

Update: The American Nuclear Society has issued Nuclear Technology entitled "The Reemergence of the Thorium Fuel Cycle" that is meant to "provide an even-handed description of its inherent attributes, and identify some of the data gaps that have yet to be resolved."

The papers therein cover a wide variety of the things discussed on this page.

Questions? Corrections? Comments? See someone who needs to be added to the wall of shame? Send us a note.

REDDIT ALERT A fairly lively discussion of the content of this page happened on Reddit in this thread.

See Also