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Proof of concept: The Molten Salt Reactor Experiment in Nuclear News

By late 1960, when the U.S. Atomic Energy Commission authorized plans to build a Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory, the lab already had about 13 years of experimentation with molten salt reactors under its longest-serving lab director, Alvin Weinberg. The MSRE operated from 1965 to 1969, proving that molten salt reactors could operate reliably, and with alternatives to uranium-235 too.

Nearly 65 years later, molten salt reactors—at least experimental- and demo-scale MSRs—are back. Four reactor developers have either submitted applications for MSRs or are in preapplication discussions with the Nuclear Regulatory Commission (other designs are in the works as well): Kairos Power is pursuing a fluoride salt–cooled pebble bed reactor and has a construction permit to build Hermes (a nonpower 35-MWt demo reactor—the first advanced reactor construction approval from the NRC) near ORNL. Abilene Christian University’s NEXT Lab is building a 1-MWt graphite-moderated, molten fluoride salt–cooled research reactor in partnership with Natura Resources. Terrestrial Energy USA , developer of the Integral Molten Salt Reactor, a 195-MWe fluoride salt–cooled thermal spectrum reactor, is in preapplication discussions with the NRC. Only one of the four—the Molten Chloride Reactor Experiment (MCRE) planned at Idaho National Laboratory by TerraPower, Southern Nuclear, and partners to demonstrate the feasibility of a utility-scale molten chloride fast reactor—would use a chloride salt as a liquid fuel carrier.

Plans to follow the MSRE with commercial-scale molten salt breeder reactors—tempting in their fuel economy and versatility—received inconsistent support in the 1970s, as liquid-metal fast breeder reactors pulled ahead before President Jimmy Carter decreed a shift away from breeders in 1977. But it’s not overstating to say that every MSR planned today is predicated at least in part on the proof-of-concept delivered by the MSRE. In this Throwback Thursday, we look at the MSRE as it was described in the pages of Nuclear News in the 1960s and 1970s.

Alvin M. Weinberg (Photo: ORNL)

ANS president: Alvin M. Weinberg served as ANS’s fifth president in 1959–1960. And when he had the opportunity to address an assembly of ANS members, he often spoke about the molten salt experiments he was involved in at Oak Ridge. In June 1960 , Weinberg delivered the first formal farewell address by an ANS president: “Some Aspects of Fluid Fuel Reactor Development.” He reviewed “those aspects of fluid fuel reactor development with which he has been closely associated during the past 13 years at ORNL” and discussed “mainly the aqueous-homogeneous and molten-salt systems with which he has been directly connected.” The full text of Weinberg’s farewell speech to ANS members was reproduced in ANS’s Nuclear Science and Engineering journal.

MSRE is a go: In October 1960, the Atomic Energy Commission gave the go-ahead for the MSRE to be built in an existing building previously used for the Aircraft Reactor Experiment. NN reported the plans in the December 1960 issue:

The AEC will build a reactor experiment of the molten salt type at ORNL as a part of the Commission's effort to investigate advanced reactor concepts having potential advantages for production of electrical power. . . .

The major objectives of the reactor experiment are to demonstrate the dependability, serviceability, and safety of the molten salt reactor concept for civilian power purposes, providing confirmation of earlier experimental work and information on components needed for a large reactor. . . .

The reactor experiment is being designed as a 10,000 thermal kilowatt reactor although routine operation will be at about 5,000 kilowatts. No electrical power will be produced. Instead, the heat will be dumped to the atmosphere through a secondary heat exchanger. . . .

The reactor experiment is planned as a single region reactor for operation at 1,225° Fahrenheit. It will have a cylindrical graphite core about 4 ½ feet in diameter and 5 ½ feet high. Columns of graphite will extend the full height of the core. The molten salt fuel, a solution of fluorides of lithium-7, beryllium, uranium, zirconium, and thorium, will be pumped through some 600 channels in the graphite columns. The heat will be removed from the fuel solution in a primary heat exchanger that will use a mixture of lithium and beryllium fluorides as the coolant.

Costs of the experiment are estimated at about $4,100,000, exclusive of research and development. Construction completion is scheduled for late 1962 and operation is planned for the spring of 1963.

MSRE and HFIR in lockstep: It wasn’t until June 1, 1965, that the MSRE began operating, just two months before the High Flux Isotope Reactor also began operating at ORNL. NN reported in March 1966 :

The U.S. reactor program took an important step forward at the end of January 1966 when the MSRE and the HFIR begin to operate at significant power levels almost simultaneously. The Molten Salt Reactor Experiment and the High Flux Isotope Reactor had been under construction for five years, and had achieved initial criticality in mid-1965. The intervening time has been used to prepare the reactors for operation at high power.

The MSRE is the final step in an attempt to achieve power breeding in the thorium system. MSRE reached a level of 1 MW on January 25, and is expected to achieve the design power level at 10 MW during March.

“The world’s first U-233 reactor”: By May 1968 , when NN reported on a planned outage at the MSRE for refueling with U-233, appreciative enthusiasm for the experiment’s reliability—and the potential for molten salt breeder reactors of the future—was evident.

Oak Ridge National Laboratory’s Molten Salt Reactor Experiment was shut down March 26, ending a six-month period of operation. When the reactor resumes operation in July, it will be fueled with uranium-233, the isotope that will be produced eventually in the breeder reactors for which MSRE is a forerunner. This will make the MSRE the world's first U-233 reactor.

During the run just completed the molten fuel was in the core continuously for 188 days, and the reactor was actually producing nuclear power 98 percent of the time. This long run underscored the reliability attained in the MSRE, demonstrated in 1967 when the reactor was available for experimental purposes 82 percent of the year and was critical for 74 percent of the time. . . .

ORNL claims the MSRE experience has demonstrated that a molten salt reactor can operate successfully at high temperatures without significant corrosive attack on either the metal or the graphite parts of the system, that reactor equipment can operate satisfactorily under these conditions, and that radioactive parts of the system can be repaired or replaced.

Seaborg at the controls: By November 1968 , NN could report that “the Molten Salt Reactor Experiment at the Oak Ridge National Laboratory has become the world's first reactor to operate on U-233. Fueling began in September and involved the gradual addition of U-233 until the design loading of 33 kg was reached. The reactor achieved a self-sustained chain reaction on October 2 and was brought to power (100 kWt) on October 8, when AEC Chairman Glenn Seaborg (a co-discoverer of U-233) threw the switch in the control room. The experimental reactor will be brought to its full power (8000 kWt) at a later date.”

To mark the occasion, Seaborg gave a speech. According to NN , “Chairman Seaborg praised the reactor’s concept and predicted the world would one day see commercial power reactors utilizing the thorium-uranium cycle. Despite these glowing words of praise, AEC watchers see no indication of any shift from the top U.S. priority on the LMFBR [liquid metal fast breeder reactor] concept.”

Another hint that proponents of molten salt breeder reactors were seeing themselves in a competition with LMFBRs for federal support was apparent later in the same issue, in a report from the Northeastern New York ANS Local Section on a dinner meeting that featured Weinberg as guest speaker. His topic? “Reactors Without Fuel Assemblies.”

According to the report:

Dr. Weinberg discussed the molten salt breeder reactor concept of which he has been the leading exponent and which is being developed at ORNL. In the MSR the fuels are fluoride salts, and criticality occurs when the molten salts flow through passages in the graphite blocks that form the core. . . . Because the MSR’s fuel is in a liquid state, Dr. Weinberg said, it can readily be diverted and reprocessed on site, thus requiring a much smaller fuel inventory tied up in reprocessing than other breeder types require, and, in terms of total fuel inventory, the MSR is competitive with fast breeder systems.

Then AEC chair Glenn Seaborg operates the controls of the MSRE—running on U-233 fuel—on October 8, 1968. (Photo: ORNL)

Review and future: In June 1970 , NN reported that “the molten salt reactor experiment’s goals had been met when nuclear operation of the MSRE ended with a planned shutdown, for budget reasons, on December 12, 1969. The reactor was running well, and operation was still producing useful information.”

In September 1974 , when the article “MSBR: a review of its status and future” by L. E. McNeese and M. W. Rosenthal was published, MSRs were once again under development at ORNL:

In February 1973, the U.S. Atomic Energy Commission development program for molten salt reactors was terminated for budgetary reasons, and efforts to develop molten salt breeder reactors seemed likely to come to an end. However, the outlook for MSBR’s brightened recently when the AEC program was reinstated at the beginning of this year and work has resumed at Oak Ridge National Laboratory. . . .

The 7.4-MWt MSRE became critical at Oak Ridge in 1965 and, after a very successful operating history, was shut down in late 1969 after circulating fuel salt at around 1,200°F for a total of two and a half years.

The MSRE experience was of major importance to the molten salt concept. Although the usual startup problems were encountered, sustained power operation provided a remarkable demonstration of operability. Starting in late 1966, an uninterrupted one-month run was made, then a three-month run, and finally a six-month run. Next, by means of a small fluoride volatility plant connected to the reactor, the original, partially enriched U-235 fuel was removed from the salt and replaced by U-233. The MSRE then operated the final year on U-233, which makes it the only reactor to have operated on this fuel; for a period, plutonium was used as the make-up fuel.”

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Molten salt reactors may use molten salts as a coolant and/or fuel. (Image: J. Křepel, Paul Scherrer Institute)

Achieving net zero carbon emissions by 2050 is a daunting challenge, and will require a significant expansion of clean energy sources, including nuclear power . In the short term, the bulk of nuclear new build projects are expected to be light water reactors, the same reactor type that drove the initial nuclear power deployment boom in the 20th century. But other designs under development, including those that use molten salts as both the fuel and the coolant, may play a role as well.

In many ways, molten salt reactors (MSRs) are not so different from conventional nuclear power reactors. Like the pressurized and boiling water reactors that have been industry staples since the early days of nuclear power, MSRs leverage controlled fission reactions to produce electricity. But unlike water-cooled reactors, MSR cores are cooled with salts, a design feature which may confer numerous advantages in terms of efficiency and make MSRs especially suitable for non-electric applications.

The origins of MSRs can be traced to the Oak Ridge National Laboratory (ORNL) in the United States. Initially developed as part of the Aircraft Reactor Experiment in the 1950s, ORNL then ran a trial known as the Molten-Salt Reactor Experiment (MSRE) from 1965 to 1969, operating an experimental 7.34 MW (th) MSR. The project established proof of concept for reactors powered by liquid fuel and cooled by molten salts.

“While MSRs were first conceived of and tested several decades ago, this reactor type has yet to see commercial deployment, though this may change in the near future,” said Tatjana Jevremovic, the Acting Head of the IAEA’s Nuclear Power Technology Development Section. “Molten salt coolants have exceptional capacity for heat absorption, which could allow MSRs to operate at the very high temperatures needed to produce high-grade heat to drive industrial processes including hydrogen production .”  

MSRs may use molten salts as a coolant and/or fuel. Most designs are based around liquid fuels dissolved in the molten salt-based coolant. Others are powered by the more traditional solid fuel rods, with the molten salts only serving as the coolant.

A new publication in the IAEA’s Technical Report Series, Status of Molten Salt Reactor Technology , outlines the current status of MSR technology around the world. It reviews the history of MSRs and takes a look at the current research and development activities taking place. The advantages of this technology, including a smaller high level waste footprint and passive safety features, as well as some of the technical challenges, such as developing components capable of operating in very high temperature environments, are detailed.

“Once sufficient experience will be collected, MSRs have the potential to be the most economical reactor type for closed fuel cycle operation,” said Jiri Krepel, a Senior Scientist in the Advanced Nuclear Systems Group at the Paul Scherrer Institute and Chair of the MSR Working Group in the Generation IV International Forum. “Several designs, utilizing thorium-232 and uranium-238, could provide an unprecedented combination of safety and fuel cycle sustainability.”

MSR designs under development

Several MSR designs are currently under development and approaching deployment readiness. In Canada, a molten salt-based small modular reactor (SMR) concept passed a crucial pre-licensing vendor design review in 2023, the first such review completed for an MSR. And other projects, including in China and the US, continue to make progress, with the hope that MSRs could begin to see deployment as soon as the mid-2030s.

“MSRs can help improve the sustainability of nuclear power, including by contributing to the minimization of nuclear waste, and enhance proliferation resistance,” said Kailash Agarwal, an IAEA Fuel Cycle Facilities Specialist. “MSRs, particularly those powered by fuel composed of U-233 and thorium salts, can also assist in conserving natural uranium resources.”

While optimism abounds for deployments in the relatively near future, key challenges remain to be addressed. Standards in design safety and fuel salt transportation have yet to be developed, and supply chains for MSR-specific reactor components do not yet exist. Analyses of potential accident scenarios unique to MSRs also remain to be conducted.

“We know that MSRs are a viable option to support nuclear power expansion plans, but there is still much work to be done before commercial deployment,” said Jevremovic. “Licensing new reactor technologies requires a lot of thorough evaluation, particularly with regard to safety analysis. It’s also important for interested countries to consider the specific role they envision MSRs playing in their energy systems.”

Support to MSR development

In addition to publications, the IAEA supports MSR development and deployment through a range of other initiatives including technical meetings and workshops. Last October, the IAEA and the Organisation for Economic Co-operation and Development’s Nuclear Energy Agency (OECD-NEA) jointly organized the International Workshop on the Chemistry of Fuel Cycles for Molten Salt Reactor Technologies in Vienna. The IAEA’s Nuclear Harmonization and Standardization Initiative (NHSI), established in 2022, is looking at how to speed up the deployment of advanced reactors, including MSRs, through harmonize regulatory approaches and industrial standardization. The Agency also maintains the Advanced Reactors Information System (ARIS), a web platform that collates information, including technical data and other characteristics, on all advanced reactors currently in development.

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Time Warp: Molten Salt Reactor Experiment—Alvin Weinberg’s magnum opus

The Molten Salt Reactor Experiment achieved its first self-sustaining nuclear reaction on June 1, 1965. Three years later, on Oct. 8, 1968, it became the first reactor ever to run on uranium-233.

MSRE was noteworthy in at least three respects. Beside running on U-233 and acting as an economic proof of concept for nuclear power, the reactor was fundamentally unlike most modern designs. The fuel did not sit in the reactor core while coolants circulated through; rather, the molten salts acted both as a carrier for the fuel and as a coolant. 

Still, liquid-fuel reactors were nothing new; the concept dates from before the end of World War II. Postwar reactor projects were primarily the domain of Chicago’s Argonne National Laboratory, but Weinberg met with Argonne’s director, Walter Zinn, to carve out a role for the Tennessee lab. That experience included the Homogeneous Reactor Experiment and the Aircraft Reactor Experiment, two fluid-fuel reactors developed in the 1950s that paved the way for MSRE. 

During the 1968 event, Weinberg dubbed MSRE the “Mighty Smooth Running Experiment.” As he addressed a gathered crowd, he motioned to nearby barrels containing processed salt carrier and spent fuel.  The barrels had no radiological protection and needed none.

The reactor went on to log more than 13,000 hours at full power* during its brief run.  It was shut down ceremoniously in 1969, having achieved all that was asked of it. The graphite bars that lined the reactor core as its moderator showed little to no damage, whether from heat, radiation or chemical corrosion.

The molten salt program ended in 1973, with the Atomic Energy Commission deciding to focus on other designs. Both government and industry are now reevaluating molten salt technology as an answer to the global energy challenge. It’s a conversation built on what Weinberg considered one of ORNL’s greatest technical achievements.

*This post has been updated to reflect a larger number of operating hours.