Elina Charatsidou is a Greek PhD student at KTH Royal Institute of Technology in Stockholm, Sweden, where she researches accident-tolerant solid fuels for lead-cooled fast reactors — specifically uranium nitride and SIMFUEL pellets. Before her PhD she completed a master’s thesis internship at Westinghouse Electric, working on machine learning approaches to nuclear fuel fabrication. She runs a YouTube channel under the name “Your Friendly Nuclear Physicist,” and in December 2022 she posted a video titled “Nuclear Physicist EXPLAINS — What are Thorium Reactors?” It has since accumulated over 519,000 views, making it the second most popular video on her channel. Given that reach, and given the number of significant errors it contains, I think a careful response is warranted.

To be clear: Elina is not an opponent of nuclear energy. She is an advocate for it, and her channel does genuine good work demystifying nuclear physics for a broad audience. Her video on thorium reactors even gets several things right, and I will acknowledge those. But her treatment of the LFTR specifically is shaped, I think, by the professional world she inhabits — one built around solid fuel fabrication and fast-spectrum reactor concepts — and that shapes her conclusions in ways that lead her viewers to an overly pessimistic picture of where the thorium molten-salt reactor actually stands.

Her complaints against the LFTR fall into several categories. On proliferation, she argues that the reactor’s decay tank contains “pure” uranium-233 — which she calls a “perfect” weapons material — and that this makes the LFTR a serious proliferation risk. On safety, she raises concerns about the handling of hot, radioactive liquid salt and frames the reactor’s passive safety features as less significant than proponents claim. On economics, she argues that the LFTR lacks a compelling economic case compared to the existing uranium reactor fleet and that starting over with a new fuel cycle is not justified. On efficiency, she correctly identifies some genuine advantages of the liquid-fuel approach but then steers the discussion toward fast-neutron reactors as though they are the natural alternative. On waste, she makes several factual errors about the radiological characteristics of thorium reactor spent fuel and the corrosion properties of fluoride salts. And on the current status of the technology, she gives a survey that systematically understates the progress being made — particularly in China — and omits the ongoing work of companies like Flibe Energy entirely.

I will go through each of these in turn. Each section header is a link to the time-stamped location in her video where she makes her arguments, so you can hear what she says before reading my response.

Proliferation

Elina spends 3-1/2 minutes on the issue of proliferation. Really, this is the section where the most egregious errors are perpetrated, and it could have been completely avoided by just attempting to talk to us about our LFTR design. I still find it hard to believe that she would make a video like this without trying to understand things better, but I guess that’s how YouTube works these days. Here’s a specific report with details in it if you want. Elina states that she thinks the LFTR is a big problem for proliferation because it has a decay tank with “pure” uranium-233 in it. She calls uranium-233 a “perfect” weapons material. Really, Elina? Your background is in solid fuel fabrication and lead-cooled fast reactor development — have you been working in weapons programs as well? Because as far as I can see in every direction are nuclear weapons built with highly-enriched uranium (essentially uranium-235) and with plutonium. No one has uranium-233 weapons in their stockpile. There was a single test (MET) of a “composite” plutonium/uranium-233 explosive core as part of Operation Teapot series in Nevada in April 1955, and by all accounts it was a “dud”, with a yield nowhere close to expectations. The description in Wikipedia says:

The MET was the first bomb core to include uranium-233 (a rarely used fissile isotope that is the product of thorium-232 neutron absorption), along with plutonium; this was based on the plutonium/U-235 pit from the TX-7E, a prototype Mark 7 nuclear bomb design used in the 1951 Operation Buster-Jangle Easy test. It produced a yield of 22kt (comparable to the Fat Man plutonium-only weapon that exploded over Nagasaki), but significantly less than the expected amount. Since it was a military effects test, the DoD specified that the device should have a calibrated yield within 10% of ratings. However, weapon designers at Los Alamos substituted the experimental core without notifying the DoD. The unexpected lower yield, 33% less than the DoD expected, ruined many of the military’s tests.

How much of that test was plutonium and how much was uranium-233? No one knows. It’s undoubtedly classified. But the whole thing was an expensive failure. We have no knowledge of any other U-233-based explosive tested by the US ever again. They certainly never integrated it into their nuclear arsenals. And notably, it would appear from unclassified sources that no one else did either. Not Russia or China or the UK or France, or even Pakistan or Israel or North Korea. India has been developing uranium-233 for decades. There is some indication that they made a test or two with the stuff, and they never went any further with it. And that’s a country that has a national goal to produce uranium-233 and has adversaries on every side of them, adversaries armed with nuclear weapons. Even the Indians stuck with plutonium.

There’s a good reason for all of us. Let’s assume for a moment that Elina is correct and U-233 is a “perfect” material for a nuclear weapon. You still have to develop it. You still have to test it. You still have to figure out how to integrate it into an arsenal that should be at the ready for many years, even decades. These are not things that just happen in a weekend. A major country had to make a major commitment to that approach and follow through with fabrications and tests and manufacturing. And as I mentioned, not one nuclear power has ever done this, even the Indians, who probably have a greater motivation and ability to use uranium-233 above all others. No one has ever done this and there’s good reason to believe no one ever will. Because there are simpler, cheaper, and more reliable ways to build nuclear weapons if you want to do so.

Now let me go beyond the historical record and engage the technical substance of the proliferation concern more directly, because it deserves a rigorous answer rather than just an appeal to precedent.

Is U-233 a capable weapons material in principle? Yes, it is. It has a critical mass of roughly 5–6 kg in a beryllium-reflected sphere — comparable to plutonium-239 and considerably lower than the 15 kg or so required for U-235 in similar geometry. Its spontaneous fission rate is very low, which means it does not suffer from the pre-initiation problem that makes reactor-grade plutonium difficult to use in a gun-type weapon. A declassified 1966 Los Alamos memorandum described U-233 as “highly satisfactory as a weapons material.” I am not going to pretend that the physics are bad. They’re not.

But “capable in principle” is not the same as “easy in practice,” and this is where Elina’s analysis — calling it “perfect” — badly overstates the case. The reason is U-232.

U-232 is an unavoidable co-product of the thorium fuel cycle. It cannot be chemically separated from U-233 because they are isotopes of the same element — isotope separation of uranium requires the same gaseous diffusion or centrifuge technology used to enrich U-235, and it is enormously more difficult to separate U-232 from U-233 (which differ by only one mass unit) than to enrich U-235 from U-238 (which differ by three). The U-232 decays through a chain that culminates in thallium-208, which emits a 2.6 MeV gamma ray — one of the most energetic gamma-emitting nuclides in any nuclear material. And it grows back. Even if you chemically purify the U-233 to remove the Tl-208 decay products, within weeks the gamma field is re-established as new daughters grow in from the U-232 that remains inseparably mixed with the fuel.

The practical consequences for a weapons program are severe. A weapon-grade implosion device requires U-232 contamination below 50 parts per million. Above that threshold the U-233 is formally classified as “low grade” — the weapons-design analogue of reactor-grade plutonium. A 5 kg sphere of U-233 containing 0.4% U-232 — a level typical of thorium reactor production — would produce a gamma-ray dose rate of roughly 13 rem per hour at one meter, one year after chemical purification, rising to 38 rem per hour ten years later. At those dose rates, assembling a weapon from such material without lethal radiation exposure to the workers requires remote handling infrastructure of the kind that nation-state weapons programs spend decades building. It also means the weapon is easy to detect: the 2.6 MeV gamma signature of Tl-208 penetrates most shielding and is immediately apparent to radiation detection equipment. You cannot move this material covertly. You cannot handle it in a garage. It announces itself.

Compare this to the alternatives. Highly-enriched uranium emits so little gamma radiation that the workers who assembled the Little Boy bomb at Los Alamos handled it with minimal shielding. Weapons-grade plutonium-239 (containing around 0.3% Pu-240) is an alpha emitter — dangerous to ingest but requiring only modest shielding for external exposure. These are the materials that have actually been weaponized by every nuclear state on earth. There is a reason for that. It is not because of moral qualms about thorium.

Elina’s concern about the Pa-233 separation route is more technically interesting, and I addressed it separately. But the point to make here is that the IAEA, the NRC, and the nuclear security community broadly do not consider U-233 from a properly operating thorium MSR to be equivalent in proliferation risk to HEU or weapons-grade plutonium. They treat it as a material of concern requiring safeguards — as they should — not as a straightforward path to weapons that negates the technology’s other advantages. That is the informed view, and it is different from Elina’s.

So let’s say we avoid thorium and uranium-233 entirely because of a concern about proliferation. Is the world now free of danger? Of course not. Today’s reactors rely on uranium enrichment. That’s the key technology needed to make highly-enriched uranium (HEU). Making HEU is just a matter of doing more separative work than making low-enrichment uranium (LEU). The same centrifuge cascades that enrich uranium to 5% for reactor fuel can, with sufficient additional operation, enrich it to 90% for weapons. This is not a theoretical concern — it is precisely the pathway that Iran, North Korea, Pakistan, and others have pursued or are pursuing. The enrichment infrastructure that supports today’s global nuclear power fleet is a far more direct proliferation risk than anything in the thorium fuel cycle, and Elina does not mention this once.

Safety

Elina’s treatment of the LFTR’s safety characteristics manages to both misrepresent the design and understate its genuine advantages. Let me go through her claims carefully.

She correctly identifies the freeze plug as a passive safety mechanism — if temperature rises beyond a threshold, the plug melts and the fuel drains by gravity into subcritical dump tanks. But she frames this as though it is merely a clever trick, one passive feature among many that other reactor designs also possess. What she fails to appreciate is the architectural significance of what the liquid-fluoride design achieves. In a pressurized water reactor, preventing a core melt requires active systems: pumps, valves, electrical power, operator intervention, backup cooling water. Remove power and you have minutes to hours before consequences become severe, as Fukushima demonstrated. In a liquid-fluoride reactor, removing power causes the reactor to shut itself down by draining. There is no core to melt, because there is no solid fuel. The “worst case” scenario for an LFTR is that the fuel solidifies in the dump tanks and sits there safely until operators choose to restart. This is not a minor improvement in safety — it is a qualitative change in the accident physics.

The LFTR also operates at atmospheric pressure. This is worth dwelling on. The entire pressure vessel and containment architecture of a conventional PWR exists because the reactor must operate at roughly 155 atmospheres to keep the water coolant from boiling at operating temperature. That pressure is what turns a loss-of-coolant accident into an explosive event. The LFTR has no such pressure. The fuel salt boils at well over 1400°C at atmospheric pressure — far above any operating temperature — so there is no thermodynamic driving force for the kind of steam explosion that disperses contamination. The reactor building for an LFTR does not need to be the massive, expensive reinforced concrete structure required for a PWR. This has significant implications for construction cost that Elina ignores entirely in her economics section.

Elina also raises concerns about the handling of highly radioactive fuel salt and the challenges of working with materials in such a demanding environment. These are legitimate engineering challenges. But she presents them as though they are unique to the LFTR and absent from conventional reactor operation. They are not. Every operating nuclear reactor handles intensely radioactive materials under demanding conditions. The difference is that the LFTR’s liquid fuel allows online processing and removal of fission products that would otherwise remain in a solid fuel assembly indefinitely, building up neutron poisons and increasing the radioactive inventory that must ultimately be managed. The LFTR’s chemistry is challenging, but it is the chemistry of a system that is continuously cleaning itself — a significant long-term advantage over solid fuel, not a disadvantage.

Economics

Elina’s economic argument is that the thorium MSR is unproven and that conventional nuclear is already economically established. This is the most anachronistic part of her video, and it was already becoming outdated when she posted it in 2022.

Let’s look at the record. The last attempt to build large conventional PWRs in the United States — the Vogtle Units 3 and 4 project in Georgia — came in billions of dollars over budget and years behind schedule, nearly bankrupting the utility that built them. This was with 60 years of accumulated PWR experience, the most experienced nuclear workforce in the world, and the full weight of the American nuclear industrial base behind it. The result was still a financial catastrophe. Meanwhile, uranium spot prices have roughly tripled and uranium enrichment prices have roughly quadrupled since the early 2010s. These are not temporary fluctuations. They reflect structural constraints in the global uranium supply chain and enrichment capacity that are not going away.

Elina argues that the LFTR lacks a compelling economic case because it is unproven technology requiring new investment. But the relevant comparison is not between a proven LFTR and a proven PWR — the LFTR is not yet commercial. The relevant comparison is between the cost trajectory of conventional nuclear and the potential cost trajectory of a technology that eliminates the two largest cost drivers in nuclear construction: the high-pressure vessel and the emergency core cooling system. A reactor that operates at atmospheric pressure and whose passive safety relies on gravity and freezing points rather than pumps and valves has a fundamentally different cost structure. The LFTR’s economics have not been proven — but they have not been falsified either, and the structural argument for lower costs is sound.

There is also a dimension to the LFTR’s economics that Elina doesn’t engage with at all: medical isotopes. A properly operated thorium MSR produces actinium-225 and bismuth-213, isotopes that are transforming cancer treatment through targeted alpha therapy. Global supply of Ac-225 is currently measured in doses per week while clinical demand is for doses per day — an orders-of-magnitude gap that no existing source can close. A thorium MSR is not just an electricity generator that happens to produce some useful byproducts. It is a medical isotope production platform that also generates electricity. When you include that value stream in the economic analysis, the picture changes substantially. This is not a peripheral consideration — it is potentially the most economically significant product of the thorium fuel cycle in the near term.

Efficiency

Thankfully in this section, Elina correctly notes that the LFTR design does not contain the typical neutron-absorbing core internal structures so common in solid-fueled reactors. Cladding, spacers, wires, grids — all kinds of hardware are absent along with their neutron-absorbing properties. She also correctly notes that uranium-233 is a “superfuel” in the thermal-neutron spectrum. It’s the only fuel that will allow you to breed in the thermal spectrum. But then she claims a fast-neutron reactor will have more neutrons. Um, kinda sorta maybe. Betraying how she gets paid a little, she can’t help but try to shift the discussion to fast-spectrum reactors.

Let me explain what she gets wrong here, because it’s an important distinction. It is true that fast-spectrum fission events release slightly more neutrons per fission on average than thermal-spectrum fission events. But “more neutrons per fission” is not the same as “better neutron economy.” What matters for breeding is the eta value — the number of neutrons produced per neutron absorbed in the fuel. For U-233 in the thermal spectrum, eta is approximately 2.28. This is higher than the eta of U-235 or Pu-239 in the thermal spectrum, and it is high enough to sustain breeding with margin to spare even after accounting for parasitic neutron absorption in structural materials, moderator, and fission products. This is why U-233 is the only fissile material that enables breeding in a thermal-spectrum reactor. No other fissile material can do it.

Fast reactors do offer high eta values for plutonium, and this is the basis for the fast breeder reactor concept. But fast reactors carry a severe cost: they require 10 to 20 times more fissile inventory per unit of power output, because fission cross-sections are dramatically lower in the fast spectrum. That inventory cost is capital — it represents fissile material in the reactor that is not producing power, and it must be financed and maintained for the life of the plant. It is one of the primary reasons that every fast breeder reactor program ever seriously pursued has run into severe economic difficulties. The thermal thorium MSR avoids this penalty entirely, and Elina’s hand-waving toward fast reactors as a neutron-rich alternative glosses over this fundamental economic reality.

Waste

But Elina makes a substantial error when talking about the radiotoxicity of spent fuel from a thorium reactor versus a uranium reactor. She correctly notes that thorium reactors tend to produce uranium-232 by a variety of different means, and she also correctly notes that U-232 will decay into thallium and lead daughters that emit strong gamma radiation. But the notion that these gamma radiations are the dominant emission from the spent fuel is a gross error. There are many common fission products that have strong gamma radiations. Cesium-137 is a good example. And these fission products are generated by ANY fissile material in its fission process, be it thorium-based (uranium-233) or uranium-based (plutonium-239) or from natural uranium (uranium-235). The gamma emissions from fission products completely DWARF the gamma emissions from U-232 decay in the spent fuel.

What makes the gamma emissions from U-232 notable is that they cannot be separated from the uranium fuel chemically. Cesium and other fission products can be partitioned from spent nuclear fuel through chemical separations. Uranium-232 cannot be separated chemically from other uranium, so even if you extract uranium from a reactor, if that uranium is contaminated with U-232, in only a few days those gamma emissions will “grow” right back in. You could attempt to purify it again, but in a few more days the same thing would happen again. That’s why any in-growth of uranium-232 makes the resulting uranium radioactively troublesome for the next few centuries. It’s also another reason why thorium-based fuels are really challenging to refabricate into solid fuels. But the notion that gamma emissions are unique to the thorium fuel cycle is just flat-out WRONG.

Elina says that it’s hard to protect the workers and environment from gamma radiations from spent fuel, but we do it all the time. That’s why spent fuel is usually in a deep “cooling pond” of water. Gamma emissions. And that’s why we put spent fuel in a thick “dry cask” of concrete. Gamma emissions are driving both of those shielding situations, not alpha and beta emissions, which are easily shielded. I was actually really surprised that a nuclear physicist like Elina would make such a big mistake in her video, and one that left the viewers with a very wrong impression.

But then the errors continue. She states that the liquid fluoride salt fuel is very corrosive and hard to store. That’s just not the case. The types of fluoride salts we plan to use in these reactors really bind up fluoride ions in very stable salts like lithium fluoride and beryllium fluoride, as well as in uranium tetrafluoride, thorium tetrafluoride, and plutonium trifluoride. All of these salts hold on to these fluoride ions with strong bond energies. What drives corrosion is free fluorine or fluoride ions, and with proper chemistry selection, that just isn’t going on. So we’ve been able to store FLiBe in containers for a long time without corrosion problems by choosing the right container materials, namely high-nickel alloys like Hastelloy-N. The lithium and beryllium want the fluoride ions a whole lot more than the Hastelloy wants them, and you don’t get corrosion. In fact, this is the basic principle of fighting corrosion in any material, in any circumstance: make sure that the chemical configuration you want is more stable than the chemical configuration of the “corroded” scenario. And for fluoride salts and Hastelloy-N that’s pretty straightforward to do.

I spent a lot of time talking to metallurgists and materials scientists who used to work on the old ORNL MSRP, and they would always emphasize to me: Kirk, don’t let them tell you we had a corrosion problem. We didn’t and we don’t. This would have been another thing I could have told Elina about if she’d ever reached out to me about this video.

Then Elina says that these salts should come in contact with water and I would tend to agree, but even if they did they would not have any sort of vigorous chemical reaction. There would be a slow enhancement of the corrosion situation but it would be nothing like what we see with liquid sodium metal coolant, which explodes on contact with water. Nor would it be anything like what we would see when uranium dioxide comes in contact with water and evolves into uranium trioxide, expanding in volume and bursting out of the pressed pellets into which we form it. Neither metal fuel, nor metal coolants, nor oxide fuel is stable in water and none of these materials are suitable long-term chemical forms for the disposal of nuclear material. Elina implies that the fuels we have are suitable for long-term storage. They’re not. We keep them in cooling ponds and in dry casks because their zirconium cladding is intact, and that prevents contact between the water and the uranium dioxide. But that zirconium metal cladding will rust away over geologic time and leave the interior fuel completely vulnerable to oxidation and pulverization. What we have today is not suitable for geologic disposal. More research would have made that point more clear, had Elina chosen to pursue it in greater depth.

Current Status

In the final section of her video, Elina gives a brief survey of the current state of thorium reactor development worldwide, and it is here that her institutional context comes through most clearly. Before getting into it, it’s worth understanding exactly where she sits professionally. Elina did her master’s thesis internship at Westinghouse Electric — one of the world’s largest commercial PWR fuel vendors — working on machine learning approaches to nuclear fuel fabrication. She then moved to KTH Royal Institute of Technology in Stockholm for her PhD, where her research focuses on accident-tolerant solid fuels for lead-cooled fast reactors (LFR): specifically uranium nitride and SIMFUEL pellets, including fabrication, characterization, DFT simulations, and machine learning modeling. In other words, her entire professional formation has run through the solid-fuel supply chain for conventional and next-generation solid-fueled reactor concepts. The liquid-fluoride thorium reactor is not just an unfamiliar technology to her — it is a competitor to the approach she is being paid to develop. That context matters when evaluating her dismissiveness toward the LFTR’s prospects. And this video has now been seen by over 519,000 people, which is why its errors deserve a careful response.

She acknowledges that China is leading the world in thorium MSR development. This is accurate and important. China’s Thorium Molten Salt Reactor program, operating out of the Shanghai Institute of Applied Physics, has been running a test reactor in the Gobi Desert since approximately 2021 — the first liquid-fueled molten salt reactor to achieve sustained operation since the Oak Ridge Molten-Salt Reactor Experiment shut down in 1969. China has announced plans for a 60 MWt demonstration reactor to follow. They did not achieve this by dismissing the technology as economically uncompelling or technically marginal. They read the same Oak Ridge reports we digitized and posted on this blog twenty years ago, recognized what was there, and committed to developing it. The United States invented this technology. China is now leading its development.

Elina does not mention the work being done at Flibe Energy on spent nuclear fuel recycling using fluoride volatility reprocessing — a direct application of the chemistry at the heart of the LFTR fuel cycle. She does not mention the growing legislative interest in the United States in preserving and utilizing the existing uranium-233 stockpile at Oak Ridge, which represents irreplaceable bred material from the MSRE program and the only domestic starting inventory for a thorium fuel cycle. She does not mention the NRC’s evolving regulatory framework for non-light-water reactors, which is slowly creating the licensing pathway that has been absent for decades.

What Elina’s video ultimately reflects is a perspective shaped by where she sits in the nuclear world: a researcher developing solid uranium nitride fuel for lead-cooled fast reactors, trained in the Westinghouse fuel fabrication tradition, working within European nuclear research institutions that have long been oriented toward uranium-plutonium cycles and fast-spectrum systems. From that vantage point, the LFTR looks like a peripheral curiosity with unresolved technical challenges and insufficient economic justification — and one that, if it succeeded, would render her own research direction less relevant. From where I sit — having spent twenty years building the technical and policy case, having worked with Oak Ridge under federal funding, having watched China move from reading our documents to operating a test reactor — it looks very different.

The technical challenges Elina identifies are real. We have never said otherwise. Hastelloy-N needs further qualification. Online reprocessing chemistry is demanding. The regulatory pathway is long. These things are true. What is not true is that they are insurmountable, or that the alternative — continuing to invest in a solid-fueled, high-pressure reactor paradigm whose construction costs are spiraling and whose fuel costs are rising — is the obvious rational choice. It is not. The world has changed since the decisions were made to shelve the Molten-Salt Reactor program in the 1970s, and it continues to change in ways that make the case for the thorium MSR stronger, not weaker, with each passing year.

Elina, if you ever do want to talk — we’re still here. And the offer stands.