Overlooked Reactor Tech: Why Yesterday's Designs are Becoming Tomorrow's Energy Revolution
Nuclear energy is already the greatest source of clean carbon-free power; despite being perpetually shadowed by concerns over safety and waste. Designs built 20-50 years ago are still the safest power generation technology TODAY by the global statistics data. For decades, the dominant image of nuclear power has been the familiar, water-cooled reactor. Yet, as Kirk Sorensen points out, these workhorses of the nuclear world are surprisingly inefficient.
"The heavy water reactor will use about 0.7% of the uranium's energy value, and the light water reactor will use about half of one percent(0.5%)… They both do terrible." - Kirk Sorenson
This inefficiency isn't just a matter of leaving energy on the table; it dictates much of the reactors' design, cost, and inherent challenges.
The most common type of reactor globally is the conventional Light Water Reactor (LWR), they use solid uranium fuel rods clad in zirconium. Sorensen likens the fuel burnup to a campfire: "Stuff on the edges isn't getting burned very good."
To compensate, operators periodically reshuffle the fuel assemblies, moving partially spent fuel towards the periphery. But even then, only a fraction of the potential energy is extracted before radiation damage takes its toll. As shown in the below diagram of a spent fuel pellet, after three years, the vast majority of the original uranium (mostly U-238) remains untouched, alongside a cocktail of highly radioactive fission products and long-lived transuranic elements like plutonium – the very components that drive long-term waste concerns.
"Only a small amount of the energy's been consumed," Sorensen observes, highlighting that the truly 'burned' fraction, the fission products, represents only a few percent of the material.
The real root issue is the use of water as a coolant. Despite being an excellent heat transfer medium its properties under reactor conditions create significant engineering hurdles. "Let me diss on water a few more times," Sorensen quips: “Water boils at a mere 100°C at standard pressure – far too low for efficient electricity generation. To keep it liquid at the necessary operating temperatures (around 300°C or higher), LWRs must operate under immense pressure, typically exceeding 70 atmospheres.”
This high-pressure environment necessitates colossal, thick-walled steel pressure vessels and even larger, heavily reinforced concrete containment buildings. These structures aren't just expensive; they exist primarily to mitigate the "number one accident people worry about," as Sorensen puts it: a Loss-of-Coolant Accident (LOCA). If pressure is lost, the superheated water instantly flashes to steam, expanding its volume "roughly by a factor of a thousand." The containment building, therefore, is sized "precisely to accommodate this event."
Furthermore, water itself isn't chemically inert in a reactor's intense radiation field. Radiolysis can "knock the hydrogens clean off" water molecules (H₂O), creating free hydrogen and oxygen gas. As Dr. Per Peterson of UC Berkeley explains, if the reactor core becomes uncovered and overheats (as happened at Fukushima Daiichi after the tsunami disabled cooling systems), the hot zirconium fuel cladding reacts vigorously with steam, producing even more hydrogen. This flammable gas buildup led to the dramatic hydrogen explosions at Fukushima – events Sorensen wryly notes were often misreported by the media as "nuclear explosions." "No, we didn't," he corrects, "It was a hydrogen gas explosion."
These inherent challenges of high pressure, potential steam explosions, and hydrogen generation weren't lost on nuclear energy's pioneers. Ironically, Dr. Alvin Weinberg, one of the original inventors of the Pressurized Water Reactor (PWR, a type of LWR) patented in 1947, grew increasingly concerned about the safety implications of scaling these designs to large, commercial sizes.
As historical footage reveals, Weinberg worried about the "Faustian bargain", balancing the immense energy potential weighed against the requirement for near-perfect containment under extreme pressure. He noted that while the containment for smaller reactors could be considered "absolute," (like the 60 MWe Shippingport reactor, the first US commercial plant, which did test the use thorium fuel initially) but for large 1000 MWe reactors, "you could not guarantee this [vessel safety]."
Weinberg's concerns led him to champion a different path; one explored extensively at Oak Ridge National Laboratory (ORNL) during the 1950s and 60s: the Molten Salt Reactor (MSR).
"Almost all of the aspects of our nuclear reactors today that we find the most challenging can be traced back to the need to have pressurized water," Sorensen argues.
MSRs elegantly sidestep this primary issue. Instead of solid fuel and high-pressure water, they use fuel dissolved in a liquid molten salt mixture (typically fluoride salts) that acts as its own coolant, operating at high temperatures (600-700°C or more) but near atmospheric pressure.
"Molten salt was one of the best decisions I made, I think," Weinberg reflected later in life. "High temperature is easier than high pressure." His son, Richard Weinberg, confirms this passion: "The Molten Salt Breeder Reactor was one thing that he had a feeling in his heart for."
This low-pressure, high-temperature operation eliminates the need for massive pressure vessels and steam-explosion-proof containment domes. But the advantages run deeper.
Dr. Stephen Boyd characteristically explains:
“Science allows you to look at everyday objects for what they really are, chemically and physically. It really makes you look twice at the world around you.
Your table salt is Frozen. That’s a strange thing to think about your table salt. That it is Frozen…”
Unlike water's covalent bonds, which are susceptible to being broken by radiation, the ionic bonds in salts are remarkably robust.
"A reactor is going to take those guys and just smack 'em all over the place with gammas and neutrons," Sorensen explains, "And the good news is, is they don't really care who they are particularly next to... the big picture is happy."
This chemical stability of salt has profound safety implications. Fluorine, the most electronegative element, is hyper-reactive on its own ("cold fluorine gas blown on iron wool" ignites fiercely). But once it forms a fluoride salt, Sorensen emphasizes, it becomes "incredibly chemically stable and non-reactive." This stability traps volatile fission products like cesium and iodine.
"There is nothing that cesium loves more than fluorine," notes Dr. Peterson. In an MSR, cesium forms Cesium Fluoride, a salt with very low volatility that remains dissolved in the primary fuel salt, drastically reducing the potential "dispersion term" or airborne release compared to LWR accidents.
Furthermore, the liquid fuel allows for continuous online processing – essentially, as Weinberg envisioned, giving the reactor a "kidney" to remove fission products and potentially recycle the fuel. This addresses both the fuel inefficiency and long-term waste problems of solid-fueled reactors.
By continually removing fission products and potentially breeding new fuel (like U-233 from Thorium), MSRs could achieve near-complete utilization of nuclear fuel resources.
The Oak Ridge Molten-Salt Reactor Experiment (MSRE), which ran successfully for over 20,000 hours between 1965 and 1969, demonstrated these principles. It used a graphite moderator (so, liquid fuel, solid moderator – the inverse of an LWR) and proved the fundamental compatibility of the materials.
A key safety feature, also demonstrated, was the "freeze plug": a section of pipe at the bottom of the reactor kept frozen by external cooling. In a total loss of power, the cooling stops, the salt plug melts, and the liquid fuel passively drains into a subcritical, passively cooled drain tank.
"If something happens where that fuel drains away from that graphite, criticality is no longer possible... fission stops," Sorensen explains. "You can't do this in solid fuel. If you do this in solid fuel, it's called a meltdown." - Kirk Sorenson
Many MSR designs incorporate Thorium. Thorium (Th-232) itself isn't fissile, but when it absorbs a neutron, it transmutes into Uranium-233, an excellent nuclear fuel, particularly in the thermal spectrum favored by many MSR designs.
As Sorensen highlights, Thorium offers remarkable energy density – "a million times the energy density of a carbon-hydrogen bond." He calculates that the small amount of Thorium present in an average cubic meter of dirt contains the energy equivalent of over 30 cubic meters of crude oil.
"Thorium is so abundant, you can take a worthless piece of dirt, from anywhere in the world... extract the thorium and turn it into an energy resource greater than the richest crude oil or anthracite coal... to me that is truly a miracle." - Kirk Sorenson
A huge add to its appeal, Thorium is significantly more abundant than Uranium-235 (the fissile isotope used in LWRs) – about as common as lead. Crucially, as mining expert Jim Kennedy points out, Thorium is often found alongside Rare Earth Elements (REEs), critical materials for modern technology like batteries and electronics.
"There's so much rare earths that we're throwing away because of Thorium," Kennedy laments. He notes that his friend's prospective REE mine in Missouri would produce about 5,000 tonnes of Thorium per year as a byproduct – enough, Sorensen calculates, "to supply the planet with all of its energy for a year." Thorium is already mined as a byproduct; it requires "zero environmental cost to acquire."
Unlike uranium, which requires complex enrichment, natural Thorium (100% Th-232) can be used directly in a breeder reactor context. Chemically, Thorium also behaves differently. As Kim Johnson explains, Uranium tends to oxidize and disperse in the environment when leached by water, forming complex, dilute deposits. Thorium, however, "resists weathering" and remains concentrated, often within REE deposits.
The use of Thorium in an MSR fuel cycle also promises to dramatically reduce long-lived nuclear waste. As Sorensen's charts illustrate, while spent fuel from conventional reactors remains hazardous for hundreds of thousands of years due to plutonium and other actinides, a waste stream primarily composed of fission products (possible with efficient MSR recycling and breeding) could return to the radioactivity level of natural uranium ore in just 300 years.
Given these compelling advantages – inherent safety due to low pressure, high efficiency, fuel sustainability with Thorium, and reduced waste – the question arises: why aren't we surrounded by MSRs today?
The answer lies tangled in history, policy, and perhaps institutional inertia. Despite the MSRE's success, the US Atomic Energy Commission (AEC) in the early 1970s, under pressure to choose a path for breeder reactor development, ultimately favored the Liquid Metal Fast Breeder Reactor (LMFBR) and cancelled the Molten Salt Breeder Reactor program.
Richard Engel, a veteran of the MSRE program, recalls being told by AEC management, "Sweep it all off [your desk] and you're finished." Syd Ball, another MSRE alumnus, admitted, "I didn't see it coming." Alvin Weinberg himself lamented the decision but hoped future generations would revisit the technology: "I still think that people will come back to this reactor... I hope that after I'm gone, people will say, 'Hey, these guys had a pretty good idea, let's go back to it.'"
For decades, the MSR remained largely forgotten, its research documents literally gathering dust, some ending up, as journalist Richard Martin discovered, in the Oak Ridge Children's Museum. The limited distribution of the original ORNL reports meant, as Sorensen realized, "Best case scenario, 40 people read what I'm holding in my hand 50 years ago."
Interest began to re-emerge in the early 2000s, partly fueled by Kirk Sorensen's rediscovery and dissemination of the old ORNL documents online, starting around 2006. This sparked a renaissance, leading to international Thorium Energy Conferences and the founding of several MSR startups like FLiBe Energy (Sorensen's own company), Transatomic Power, Terrestrial Energy, ThorCon, Moltex, Seaborg, and Copenhagen Atomics.
Yet, the path to commercialization in the US remains fraught with obstacles. In 2010, President Obama's call for a "new generation of safe, clean nuclear power plants" drew rare bipartisan applause in Congress.
However, as filmmaker Robert Stone pointed out, the subsequent loan guarantees went towards building conventional LWRs (Plant Vogtle). Congressional hearings revealed continued confusion and lack of focus on advanced reactors. Dr. Peter Lyons, then Assistant Secretary for Nuclear Energy, acknowledged Thorium's potential but saw "no compelling reason" to shift away from the established uranium-plutonium cycle, citing a recent OECD-NEA report.
Troublingly, that very OECD-NEA report, when scrutinized, barely evaluated Weinberg's MSR concept. As Sorensen points out, the report's brief mention dismisses the 1 GWe MSBR design studied by ORNL as a thermal reactor requiring complex chemical processing, noting that this "drawback" could be eliminated by using a fast spectrum instead. Sorensen counters by rewording the sentence for clarity:
"This 1 GWe design was a thermal reactor... that avoided the drawbacks of fast-spectrum by removing soluble fission products through the use of chemical fuel-salt treatment."
This subtle but critical misrepresentation, he argues, effectively ignores the core advantages of the thermal spectrum MSR Weinberg envisioned.
Furthermore, current US Nuclear Regulatory Commission (NRC) regulations actively hinder MSR development. As Cavan Stone highlights, specific rules essentially prohibit licensing liquid-fueled reactors over 1 MW thermal without navigating an expensive and unclear process, effectively blocking demonstration projects.
Dr. Leslie Dewan of Transatomic Power confirmed this in a 2014 congressional hearing: "Currently, there is no way for us to build a prototype facility or move beyond the laboratory scale work... we've been forced to... keep an open mind with respect to the other pathways we could take [outside the US]."
This regulatory environment contrasts sharply with other nations. China, as Dr. Jiang Mianheng explained, sees MSRs using Thorium as a strategic imperative for clean energy and resource utilization, launching a major program in 2011. With characteristic metaphor, he asked, "Why is China first to eat the crab?" – implying China is willing to tackle the challenges for the potential reward.
By 2015, China reported having 700 engineers dedicated to their MSR program. India, with vast domestic Thorium reserves, has long pursued a Thorium strategy and is also developing MSR capabilities. Canada, as Terrestrial Energy's Canon Bryan notes, offers a "fundamentally different regulatory environment... very progressive," providing a competitive advantage for advanced reactor development. The Czech Republic, too, has partnered with the US DOE for testing advanced reactor coolants, leveraging their existing expertise.
The situation prompts uncomfortable questions. Is the US inadvertently fostering a "protection of the status quo," as Congressman Rohrabacher suggested, rather than a "bridge to tomorrow"?
Are we repeating past mistakes by neglecting a potentially transformative technology pioneered domestically, only to potentially buy it back from foreign powers later?
As Jim Kennedy observes, regarding the Thorium byproduct from REE mining, "How is it that we created it here... the ultimate gift to humanity, and how is it that China will deliver the system and not the United States?" TMSR, Live Refueling, “Why China is Building TMSR”
The potential of Molten Salt Reactors, particularly fueled by Thorium, seems undeniable: enhanced safety, greater efficiency, drastically reduced waste, and utilization of an abundant fuel resource. Alvin Weinberg's vision of a world powered by clean, abundant, low-pressure nuclear reactors remains compelling.
Kirk Sorensen describes the energy potential locked in Thorium as a "miracle." Yet, decades after the successful MSRE experiment was inexplicably shut down, the path to realizing this potential in the nation that first explored it remains obstructed by regulatory hurdles and perhaps a lack of political will.
Whether the US will overcome this inertia and reclaim leadership in this advanced nuclear frontier, or watch as others "eat the crab," remains THE critical question for North America’s energy future.
