UIUC professor discusses thorium MSR
A professor at the University of Illinois Urbana-Champaign has posted a video on thorium reactors asking whether it’s “something everyone should go and embrace, or an interesting idea that doesn’t matter.” It’s a fair question, and he answers parts of it well. But his conclusion rests on an incomplete picture — both of the competitive landscape for conventional nuclear and of the full value proposition of the technology he’s evaluating. Let me go through it carefully.
What He Gets Right
The professor’s nuclear physics is solid. He correctly describes the neutron capture chain from Th-232 through Pa-233 to U-233 — the same sequence we cover in detail in our fuel cycle video. He correctly identifies the molten-salt reactor as a natural fit for the thorium cycle, explains the freeze plug passive safety mechanism accurately, and understands that the breeding advantage of thorium over U-238 comes from the higher thermal neutron absorption cross-section of Th-232. That cross-section advantage — roughly four times higher, as he notes — is exactly why thorium breeding works well in a thermal-spectrum reactor when plutonium breeding from U-238 does not.
His treatment of nuclear waste is also genuinely good. He correctly distinguishes between fission products, which decay to background levels in two to three hundred years, and transuranics — the elements heavier than uranium — which can remain hazardous for a hundred thousand years and define the true long-term waste challenge. And he correctly shows that the thorium fuel cycle produces orders of magnitude fewer transuranics than the uranium-plutonium cycle. This is one of the most important and underappreciated arguments for the thorium MSR, and he makes it clearly.
His conclusion on proliferation is also well-taken: “governments make nuclear weapons, and any big enough government can figure out how to do it… the solution to them is not in this technological realm but rather in the political realm.” Agreed. The thorium fuel cycle is highly proliferation-resistant, but nuclear security ultimately rests on political decisions, not reactor physics.
The Pa-233 Concern — And What We’ve Done About It
The professor raises a legitimate concern: Pa-233, with its 27-day half-life, could in principle be chemically separated from the reactor, allowed to decay, and then processed to yield U-233 that is relatively free of the U-232 contamination that makes in-reactor U-233 self-protecting. As he puts it: “what if we take this fuel and just separate out the protactinium… wait 27 days later, you’ve got half of it turned into U-233 which you can chemically separate.”
This is a real concern worth taking seriously, and we have taken it seriously. Flibe Energy has worked this problem in depth with Oak Ridge National Laboratory under a GAIN voucher, and we have developed novel engineering strategies to address it — strategies that go well beyond simply relying on U-232 contamination as the sole barrier. We discuss the significance of U-232 and its decay daughters in our U-232 video. The answer is not to dismiss the concern, nor to accept it as a fatal flaw — it is to design around it, as good engineers do.
Thorium vs. Uranium-238: A Critical Distinction
The professor lists as a negative that “we’re not going to run out of uranium-238 — it might be three or four times less abundant than thorium but there is an enormous amount of it on the planet.” This is true, but it obscures a critical physical distinction between the two breeding options that his own cross-section chart should have made clear.
Consuming U-238 as a fuel requires a fast-spectrum reactor. This is not a design preference — it is a nuclear physics requirement. The thermal neutron absorption cross-section of U-238 is too low to sustain efficient breeding in a thermal reactor. Thorium, by contrast, can be consumed as a fuel in a thermal-spectrum reactor — most effectively in a molten-salt reactor. This distinction has profound practical consequences that the professor’s framing glosses over.
Fast reactors require 10 to 20 times more fissile material inventory per unit of power output compared to thermal reactors, simply because all neutron interaction cross-sections — including fission cross-sections — are dramatically lower at fast-neutron energies. That excess inventory is capital tied up in fuel that is not producing power, and it is a major contributor to the economics problem that has plagued every fast breeder reactor program ever attempted. Thorium breeding in a thermal MSR avoids this penalty entirely. The professor’s own cross-section chart showed why — the higher thermal absorption cross-section of thorium relative to U-238 is precisely what makes the thermal thorium cycle easier to implement than the fast U-238 cycle. He just didn’t follow that thread to its conclusion.
He also notes that “there are passively safe ways to fission U-235” and that the molten-salt reactor concept “might work a little better with thorium” but will also work with uranium. Both points are true, but they miss the larger argument: it’s not that the MSR doesn’t work without thorium, it’s that thorium doesn’t work optimally without the MSR. The two technologies are natural partners in a way that uranium and the MSR are not.
The Economic Argument: He’s Watching Different News Than I Am
Here is where the professor’s analysis is most disconnected from current reality. He argues: “we’ve had 60 years of experience operating the U-235 commercial reactors… we have figured out how to do this, we can do it at scale, we understand it… this has been done at a competitive price.”
I genuinely wonder what news he is watching.
Our existing reactor fleet is in full retreat. The last serious attempt to build gigawatt-scale pressurized water reactors in the United States — Vogtle Units 3 and 4 in Georgia — came in roughly $17 billion over budget and years behind schedule, nearly breaking the utility that built them. Since that project broke ground, uranium spot prices have roughly tripled and uranium enrichment prices have roughly quadrupled. The industrial base capable of building large conventional reactors in the United States has atrophied to the point that we now import components and expertise we once exported to the world.
The professor says that “starting over doesn’t have enough compelling economic argument to justify the costs.” But we are already paying the costs of starting over — in the form of an industry that cannot build what it used to build at any price that makes sense. The question is not whether transition will be expensive. It already is. The question is whether we transition toward a technology with a better cost trajectory, or keep pouring money into one whose costs are moving in the wrong direction.
The molten-salt reactor, precisely because it operates at atmospheric pressure and eliminates the massive pressure vessel and emergency core cooling infrastructure that dominate the capital cost of a conventional PWR, has a fundamentally different — and better — cost structure. That argument was always true in principle. Today, with uranium and enrichment prices where they are, it is urgent in practice.
The Missing Piece: Medical Isotopes
The professor concludes that the economic case for thorium MSR “doesn’t have enough compelling economic argument to justify the costs.” He reaches this conclusion because he is thinking only about electricity generation — and electricity generation alone.
He is missing an enormous part of the value proposition.
A properly operating thorium MSR produces isotopes — particularly actinium-225 and bismuth-213 — that are transforming oncology through targeted alpha therapy. These isotopes are desperately needed and currently cannot be obtained in anything close to sufficient quantities from existing sources. Global supply of Ac-225 is measured in doses per week; clinical demand is for doses per day. The gap is not marginal — it is orders of magnitude. A thorium MSR is not merely a power reactor that happens to produce useful byproducts; it is a medical isotope production platform of extraordinary potential that also generates electricity.
When you add that value stream to the economic analysis, the picture changes substantially. The professor’s framework is too narrow to capture it, and it leads him to a conclusion that doesn’t reflect the full opportunity.
So: thanks for the video, professor. The nuclear physics fundamentals are largely right, the waste argument is well-made, and the proliferation framing is sensible. But the conclusion that the thorium MSR lacks a compelling economic argument rests on an outdated picture of conventional nuclear’s cost competitiveness and a framework that ignores the medical isotope value proposition entirely. The world has changed substantially since those 60 years of U-235 reactor experience accumulated — and not in ways that favor staying the course.