High-temperature plumbing and advanced reactors

None of the advanced coolant concepts are truly new; in fact, nearly all were proposed and explored for fission power purposes during the 1940s and 1950s.^4,5 Originally formulated to enable breeder technology and limitless fuel, the technical need and financial stomach for advanced fluids were surpassed sometime in the 1980s.

Forty years later, those searching for methods to impact climate change have once again turned to high-temperature fluids and the new power generation methods they may enable. These futurists are faced with a monumental challenge and are starting from the ground up. While salts and liquid metals were a near-mature technology 50 years ago, they were cut off from intergenerational technology transfer during the abrupt decline of the United States’ advanced nuclear program in the early 1990s.^6–8 Many of the field’s leaders have long since retired or passed away, forcing a new crop of engineers to revitalize high-temperature fluid technology from old papers and re-created laboratory experiments.

It is this author’s opinion that for these new engineers to be successful, they must emulate the key behaviors and conditions of the field’s founders. Distilled to basics, an adventurous, pioneering spirit with a tolerance for mistakes must be established. This culture must then be leveraged with the necessary budget to fabricate, test, and perfect the hardware. Lastly, the knowledge gained by engineers and scientists during these efforts must be recorded, disseminated, and taught so that poor practices are not repeated. While other paths to the successful implementation of high-temperature fluids can surely be realized, an undeniably true solution is to simply replicate the core practices and circumstances established by the field’s founders.

The deeds of our high-temperature progenitors still seem like science fiction 50 years later. Lithium was pumped through piping systems at speeds greater than 48 meters per second—nearly 100 miles per hour—while glowing white-hot at 1,073°C.⁹ Sodium-cooled reactors were operated with closed fuel cycles.⁷ Boiling potassium Rankine cycles were experimentally explored.¹⁰ Molten salt reactors operating with cherry-­red heat were chemically defueled and refueled by dissolution in the same manner in which sugar is dissolved in water.^11,12 More than 10 different styles of electromagnetic pumps—liquid metal pumps with essentially no moving parts—were theorized and prototyped to convey liquid metals, and they were used on one of the first two nuclear submarines.^13,14 New alloys were developed from scratch and reactors were built with them.¹⁵ Large pumps with almost 40-inch impellers were tested in 593°C sodium, conveying up to 34,000 gallons per minute.¹⁶

The staggering loss of experience can be quantified. It has been estimated that more than 2 million hours of molten salt and liquid-metal experiments were performed at Oak Ridge National Laboratory by 1972, with a quarter of those being mechanically pumped.^17,18 All these achievements involved unique, objective risks associated with toxic, flammable, and hot fluids that trailblazers acknowledged and accepted as part of their job. Leadership recognized this, took ownership of these risks, defended their necessity to enable advanced energy, and created the incubator for visionary engineers.

Today, cognitive dissonance exists concerning the hazards of these fluids and the goals of advanced power plants. Modern engineers admire the achievements of their predecessors and understand the utility of past work but are fearful of undertaking these tasks themselves. This is evidenced by the considerable development effort that goes toward simulation, modeling, surrogate work, and benchtop tinkering. While the root trouble of advanced nuclear power—and hence the implementation of these high-­temperature fluids—is consistently blamed on oppressive regulation and high costs, one cannot ignore that the pioneering, hardware-­focused spirit of the 1960s has also been lost along the way.^19,20

As theoretical physicist Freeman Dyson aptly said, “The adventurers, the experimenters, the inventors, were driven out, and the accountants and managers took control.”²¹ Once filled with boundary pushers, the field of nuclear engineering has adopted a rigid culture with no margin for failure. While this approach is warranted for commercial systems where public safety must be held above all, it is not a productive nursery for research-level systems. A conservative, one-size-fits-all nuclear culture focused on minimizing risk is detrimental for the field’s future. If it was unacceptable for a SpaceX rocket to explode on the launchpad, there would never be any iterative development, no Darwinian design process, and no product. Without their fast-paced culture, SpaceX would almost certainly not attract as many forward-­thinking engineers who would be willing to work long hours. An appetite for failure must be acquired; otherwise, there will not be any meaningful mechanical systems from which a bright young engineer can learn.

Conversely, how can advanced power generation systems be made if there are no battle-worn engineers? Obviously, the consequences of failing a reactor are much higher than a rocket, but it has been done many times before at nuclear’s “Cape Canaveral”—the high desert outside of Idaho Falls.²² There must be a sensible allowance for leaks, fires, and mistakes (lessons from most of which have been previously learned and forgotten). The severity of these risks must be reassessed, and safety culture’s grasp on progress must be carefully examined. If the risk is too high, the system should be scaled down until confidence is gained.

If cultural reform can be implemented, a second set of hurdles must be overcome—those of budget and timeline. Key players are making decisions solely based on the immediate necessity of bringing a product to market to combat climate change and reactor aging. These developers are looking for the fastest solutions using the current state of the art and minimal research and development. Many are currently following paths that aggressively hop from idea to commercial reactor with one or two large-scale systems in between as part of the effort to succeed commercially. While this type of effort may certainly succeed by luck and hard work, it is more likely that it will be met with considerable obstacles. For example, the incremental technology development (chemistry, pumps, materials) for the predecessor to the Molten Salt Reactor Experiment and the Aircraft Reactor Experiment cost $880 million ($9.6 billion in 2025 dollars) and took around seven years.¹² In 1982, approximately $500 million of government funds ($1.7 billion in 2025 dollars) were appropriated for liquid metal work, with $285 million allocated to the technology program and test facilities alone.²² The Connecticut Advanced Nuclear Engineering Laboratory complex was built for high-temperature alkali metal work at a cost of $65 million in 1957 ($743 million in 2025 dollars); in 1963, its budget for its 2,500 employees was $40 million ($415 million in 2025 dollars).²⁴ These are only portions of three programs out of the many that existed at the time.

While some modern institutions are exploring advanced fluid technology with large investments, it is assuredly not with the same vigor as was seen in the past. Unfortunately, it seems that our current circumstances do not allow the same hardware-based, gradual advancement as that of our predecessors. Today’s aggressive schedules and overpromises result in repetitive failures and underperformance, serving to reinforce the misconception that molten salts and liquid metals are too difficult to master. This is especially disappointing considering the hundreds of millions—perhaps billions—of dollars poured each year into space flight, which is difficult to rationalize when the benefits of space exploration are much more abstract. If the United States seriously considers energy prosperity to be part of its future, the money must be there to enable it. If the proposition of large-scale altruistic investment in physical power systems is culturally unacceptable to the United States, then the technology will be developed and utilized by other nations that understand the long-term value of a diverse energy portfolio.

Perhaps the only reason this field still draws interest is because our predecessors documented their work and design decisions with excessive detail. A great deal of credit is owed to the librarians who have painstakingly digitized this work. Interviews have been conducted with many prominent scientists and engineers of the era to preserve their oral histories, and those transcripts and videos are available online.²⁵ Historical videos have even been posted on YouTube.²⁶

Browsing these sources quickly shows that at its peak, thousands of American high-temperature fluid engineers and scientists were pushing in the same direction at each national laboratory and through private industry giants such as General Electric and Rockwell International.^13,27,28 The attendance sheet of this era can be gleaned in the organization charts of Oak Ridge Quarterly Progress Reports and Sodium NaK Handbook bibliographies.^29,30 Even documentation of large-scale collaboration between multiple private and public interests on fundamental sodium technology can be found—something that’s hard to believe in today’s litigious environment.³¹

Today’s thermal hydraulics experts are isolated and scattered across academia, private industry, start-ups, and national laboratories. There is no central brain trust, and this leads to repetitive mistakes and hardship. Even worse, the largest-scale advanced fluid research today is being performed by private companies that have the desire for eventual profit through intellectual property. While in pursuit of long-term profits, each of these companies keep trade secrets and common knowledge from the others, forcing each of them to reinvent basic techniques, practices, standards, requirements, and skill base. All of this results in waste and hardship, which will prevent intellectual property from being developed in the first place.

A sure way to avoid reinventing the wheel is to have an open and honest discussion of difficulties among peers. Best practices could be documented by committee, not unlike that of the American Society of Mechanical Engineers’ Boiler and Pressure Vessel Code, so that foundational knowledge is preserved and disseminated. This would greatly increase the deployment odds of this technology. While the “nuclear renaissance” is often seen solely as a period of rebirth and growth, its broader parallels—such as the emulation of classical ideals, fiscal benevolence, and social reform—are frequently overlooked.

If we agree this technology is worthwhile, we must recapture the pioneering spirit of the past and secure the financial means necessary to regain those two million hot hours documented by ORNL. We will have to accept that there are nonnegligible risks in our goals that we will try as engineers to minimize, but not at the expense of unregulated fear. We can cooperate with each other and share information to more safely and cheaply develop this technology.

If this is too large of an ask for the private sector, it may be a path forward for national laboratories to reclaim their old mission as executors of “things too difficult or too risky for private industry to undertake,” rebuilding their old, hard-earned institutional knowledge, as Alvin Weinberg said in The First Nuclear Era.¹² Perhaps success in a fresh field like fusion will provide the activation energy to reinvigorate high-temperature fluid competency.

Whatever way it is realized, the benefits of advanced fluids are likely not an engineering mirage. These are valuable tools spanning the periodic table that can be used to enable and reinvent many forms of energy generation. The failure to re-master them is to deny humanity of another lever in the fight against climate change. On a more personal note, the inability to release the promise of molten salts or liquid metals will nullify the careers of thousands, relegating their progress as no more than a forgotten moment of American ingenuity, creativity, and greatness.

Brian Kelleher is a thermal fluids engineer with 15 years of laboratory experience in molten salts and liquid metals.

References

1. B. N. Sorbom et al., “ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets.” Fusion Engineering and Design 100, no. 3 (2014): 378; doi.org/10.1016/j.fusengdes.2015.07.008.
2. M. Mehos et al., “Concentrating solar power Gen 3 demonstration roadmap.” Technical report, DOE Office of Energy Efficiency and Renewable Energy (2017); doi.org/10.2172/1338899.
3. C. W. Forsberg, “Sustainability by combining nuclear, fossil, and renewable energy sources.” Progress in Nuclear Energy 51, no. 1 (2009): 192; doi.org/10.1016/j.pnucene.2008.04.002.
4. W. Holman, “Materials for liquid metal systems.” Lecture presented at the Atomic Industrial Forum’s Third Course on Reactor Materials, Stanford University, July 9–19, 1957; doi.org/10.2172/4273872.
5. H. G. MacPherson, “The molten salt reactor adventure.” Nuclear Science and Engineering 90, no. 4 (1985): 374; doi.org/10.13182/NSE90-374.
6. C. P. Lesperance, “Closure of the Fast Flux Test Facility (FFTF): Current status & future plans.” Presentation at the 11th International Conference on Environmental Remediation & Radioactive Waste Management, Burges, Belgium, September 2–6, 2007; osti.gov/biblio/908291.
7. C. E. Till and Y. I. Chang, Plentiful energy: The story of the Integral Fast Reactor, the complex history of a simple reactor technology, with emphasis on its scientific basis for non-specialists (published by the authors, 2011).
8. C. Westfall, “Vision and reality: The EBR-II story.” Nuclear News, Feb. 2004, p. 25.
9. L. G. Hays, “Corrosion of niobium—1 percent zirconium alloy and yttria by lithium at high-flow velocities.” Technical report, Jet Propulsion Lab (1967); osti.gov/biblio/4504388.
10. H. C. Young and A. G. Grindell, “Summary of design and test experience with cesium and potassium components and systems for space power plants.” Technical report, Oak Ridge National Laboratory (1967); osti.gov/biblio/4361858.
11. P. N. Haubenreich and J. R. Engel, “Experience with the Molten-Salt Reactor Experiment,” Nuclear Applications and Technology 8, no. 2 (1970): 118; doi.org/10.13182/NT8-2-118.
12. A. M. Weinberg, The first nuclear era: The life and times of a technological fixer (Springer, 1994).
13. R. S. Baker and M. J. Tessier, Handbook of electromagnetic pump technology (Elsevier, 1987).
14. R. G. Rhudy and J. P. Verkamp, “Electromagnetic alkali metal pump research program final report.” Contractor report (1966); ntrs.nasa.gov/citations/19660008603.
15. H. E. McCoy, “The INOR-8 story.” Oak Ridge National Laboratory Review 3, no. 2 (1969): 35;osti.gov/biblio/4766202.
16. T. Boardman, “CRBR prototype pump test program in SPTF: Final report.” Technical report, Rockwell International Corp. (1984); osti.gov/biblio/707545.
17. A. P. Fraas, “Design of high-temperature liquid metal systems.” Technical report, Oak Ridge National Laboratory (1972); osti.gov/biblio/4687515.
18. A. G. Grindell, W. F. Boudreau, and H. W. Savage, “Development of centrifugal pumps for operation with liquid metals and molten salts at 1100–1500°F.” Nuclear Science and Engineering 7, no. 1 (1960): 83; doi.org/10.13182/NSE60-A25701.
19. A. Stein and T. Nordhaus, “Advanced nuclear energy is in trouble: NuScale cancellation should be wake-up call for advocates,” The Breakthrough Institute, Nov. 28, 2023; thebreakthrough.org/blog/advanced-nuclear-energy-is-in-trouble.
20. J. C. Wagner, “A call to action,” Nuclear Newswire, Jan. 17, 2022; ans.org/news/article-3580/.
21. F. J. Dyson, Disturbing the universe (Harper & Row, 1979).
22. R. O. Haroldsen, The Story of the BORAX nuclear reactor and the EBR-I meltdown (published by the author, 2008).
23. U.S. Department of Energy, “Program summary for the civilian reactor development program.” Technical report, DOE Office of Nuclear Energy (1982); osti.gov/biblio/6746377.
24. Pratt and Whitney Aircraft, “Second generation portable nuclear powerplant: Volume V, Contractor qualification and management data—proposal.” Technical report, Pratt and Whitney Aircraft (1963); osti.gov/biblio/4061398.
25. “Oral history of Sam Beall and Paul Haubenreich,” interview of Paul Haubenreich by Stephen H. Stow (2003), Center for Oak Ridge Oral History; cdm16107.contentdm.oclc.org/digital/collection/p15388coll1/id/259/rec/1.
26. Oak Ridge National Laboratory, The Molten-­Salt Reactor Experiment, film (1969); youtube.com/watch?v=tyDdp5HRs0o.
27. L. E. Donelan, “Helical induction pump design.” Technical report, North American Aviation, Atomics International Division (1970); osti.gov/
    biblio/4174708.
28. J. W. Gahan, P. T. Pileggi, and A. H. Powell, “Primary loop electromagnetic pump design.” Contractor report, General Electric (1970); ntrs.nasa.gov/citations/19700022538.
29. A. W. Savolainen, ed., “Aircraft nuclear propulsion project quarterly progress report for period ending June 10, 1955.” Technical report, Oak Ridge National Laboratory (1955); osti.gov/biblio/4146223.
30. O. J. Foust, Sodium–NaK engineering handbook. Vol. IV: Sodium pumps, valves, piping, and auxiliary equipment (Gordon and Breach, 1978).
31. H. W. Savage, “History of the AEC sodium components development program.” Technical report, Oak Ridge National Laboratory (1966); osti.gov/biblio/4433577.