David LeBlanc at ORNL, May 2010

Update: See this recent interview with David Le Blanc.

Molten Salt Reactor Safety Related Advantages

The molten salt reactor was not exactly intended although Ed Bettis and his associates were concerned with the safety of the original sodium cooled Aircraft reactor design. They just understood that the original reactor had a positive coefficient of reactivity, and that coupled with the sodium cooling was extremely bad news. Their calculations had shown that a fluoride salt cooled reactor would feature a negative coefficient of reactivity. Their choice of fluoride salts was not a fortuitous accident. They worked for the Oak Ridge K-25 plant, which was the largest industrial facility in the world that used chemical processes based on fluoride chemistry, and thus knew that their project would have in house access to chemists who understood fluoride chemistry.

But after the ANP shifted to ORNL in 1950' the scientists involved began to understand that Molten Salt Reactors offered exceptional nuclear safety characteristics. First, reactors are made less safe by dangerous things that you put into their core. Things like water and sodium Water is dangerous because under heat and pressure it turns to steam and can explode. Steam explosions are well known problems with the use of water as a heat to work transfer medium. The literature of the early age of steam features dozens of accounts of boiler explosions. Boiler explosions are not just a curiosity from the past, witness the 2003 boiler explosion aboard the cruse ship Norway while in port in Miami. Light and heavy water cooled reactors are basically boilers heated by nuclear reactors. In order to operate efficiently core coolant water is superheated and kept liquid by high pressure. The presence of critically heated water in the core of water cooled reactors is a fundamental safety issue, that requires special design features to manage. Those features cost money to design and implement.

In contrast Molten Salt Reactors, even while operating at high temperatures, do not produce more than a single atmosphere's pressure, and thus will not produce anything like a steam explosion. Thus MSRs are at a significant advantage as far as nuclear safety costs.

The MSR was designed to cope with a number of safety problems associated with a liquid sodium cooled reactor. While liquid sodium cooled reactors including sodium cooled fast breeders do not operate under high pressures, the chemical nature of sodium, as well as some particularities of sodium flow inside reactor cores, create some safety issues. Sodium is extremely chemically active and will burn in contact with air and water. Thus special care must be taken with sodium cooled reactors to maintain separation between between coolant sodium and air, as well as structural materials which contain water such as concrete. This necessitates special design features which may increase the cost of sodium cooled reactors.

In contrast molten salts used in MSR cores do not burn. In addition the tend to freeze as some as their temperature is dropped by air contact. Thus MSR salt leaks can be expected to self seal. Since the reactor core salt is under only a one atmosphere pressure the frozen salt leak seal can be expected to hold. During the Oak Ridge Molten Salt Reactor Experiment researchers experienced reactor salt leaks on laboratory floors. These were cleaned up without difficulty.

In addition, research on fluid flow inside sodium cooled fast reactors has indicated the existence of flow problems called voids. A void can potentially cause a loss of operator control of a sodium cooled reactor, and lead to reactor run aways with potentially catastrophic consequences. Avoiding the void problem may lead to penalties such as limiting reactor performance and breeding capacity.

A simple Uranium cycle Denatured Molten Salt Reactor (DMSR) would not include such a potential for reactor reactor run away. Single fluid MSRs are extremely stable, and will shut down automatically if they overheat, due to fluid fuel expansion. For this reason there is no reason for control rods, or reactor monitoring by operators. ORNL researchers preparing for the 1960's Molten Salt Reactor Experiment determined that MSR operators would have nothing to do, and so would be board. They chose to design the MSRE without a control room, and ran the reactor without an operator present.

More complex MSRs such as 2 fluid LFTRs would have more complex control issues, but it seems possible through careful design that they can be made as safe as the DMSR.

Reactor safety problems are created by things that are put in reactor cores, as well as things that are created in reactor cores. Solid core reactors are stuck with dangerous fission products, and other materials like plutonium which are created in the reactor core. In the event of a catastrophic accident such materials are seen as a significant menace. Radioactive gases are of continuous concern. Most of the escaped fission products following the Three Mile Island accident were radio active gasses, and voluble fission products. It is possible to bubble radioactive gases out of the carrier salt of a molten salt reactor at low expenses, and the recovery of voluble fission products would not be too expensive. In addition the recovery of other fission products, called nobel metals would not be technically difficult or expensive. Thus many of the most dangerous fission products can be removed from a Molten Salt Reactor core as the reactor operates. By removing radioactive fission products either periodically or as they are produced, the worst case MSR can be rendered significantly less dangerous,

In addition actinides such as plutonium-239 can be simply left in the reactor core until they burn up. In might be desirable to periodically clean the salts of DMSRs or LFTRs, but dangerous materials called actinides can be automatically returned to the reactor core with out ever being accessible to people. This would prevent the diversion of nuclear materials by terrorists, or state based would be nuclear proliferators. The DMSR was designed to be proliferation resistant, and it has many features that would lead a would be nuclear proliferator to chose other routs to produce nuclear weapons.

Molten Salt Reactors, such as the DMSR offer very teal safety advantages over Light and Heavy water reactors as well as sodium cooled fast reactors. These MSR safety features can significantly lower reactor safety related costs, while increasing public confidence in the safety of nuclear generated electricity.

Phoenix Rising

I attended David LeBlanc's lecture at ORNL yesterday (May 18, 2010).

Jess Gehin, our host, took the opportunity to do a set of show-and-tell presentations about molten-salt-related programs at ORNL. It is safe to say, from what I saw yesterday, that the phoenix is rising at ORNL.

David's talk was exciting. David has been in contact with retired ORNL MSR researcher Dick Engel. Dick participated in the ORNL 1980 fling at getting backing for Molten Salt Reactor development, the DMSR. (For documentation of the DMSR concept, see here, here, here and here.) David notes in his Mechanical Engineering article,

The “D” stands for “denatured”—the uranium in the reactor contains too much U-238 to be useful in weapons. The concept also dispenses with processing the salt to remove fission products; the same salt is used throughout the 30-year life of the reactor with small amounts of low enriched uranium added each year to keep the fissile material constant. The amount of uranium fuel needed—about 35 metric tons per GWe year—is only one-sixth of what is used by a pressurized water reactor. . . .

The amount of fissile material needed to start new reactors is also very important, especially in terms of a rapid fleet expansion. The 1 GWe DMSR was designed for 3.5 metric tons of U-235 (in easy-to-obtain low-enriched uranium) which can be lowered if uranium costs go up. A new PWR, by contrast, needs about 5 metric tons, whereas a sodium-cooled fast breeder such as the PRISM design requires as much as 18 tons of either U-235 or spent fuel plutonium. Any liquid fluoride reactor can be started on plutonium as well, but this turns out to be an expensive option, since removing plutonium from spent fuel costs around $100,000 per kilogram.

Reviewing the DMSR from a 2010 perspective, LeBlanc finds many advantages.

The DMSR features a larger, lower power density graphite core than other MSR breeder concepts. So while the graphite would last a full 30 years, the DMSR would still be only a fraction of the size of gas-cooled graphite reactors and would not require a pressure vessel. In fact, the simple thin-walled DMSR containment vessel would be wider but much shorter than those of PWRs and BWRs. The construction of the reactor containment building offers savings as it does not need the huge volume and ability to deal with steam pressure buildup needed for LWRs or CANDU reactors.

The overall thermal efficiency of the plant would be quite high. With a salt outlet of 700 °C and using the latest ultra-supercritical steam cycles or gas Brayton cycles, efficiencies close to 50 percent would be possible.

While up-to-date cost estimates for a molten salt reactor are not available, it is quite simple to see the potential overall advantages. The DMSR needs no capital and O&M costs for fuel processing, and the superior nature of the salts as coolants results in far smaller heat exchangers and pumps. Building and fabrication costs should be lower than conventional nuclear plants, since the design doesn’t put the same sort of stresses on the system.

Among the advantages LeBlanc points out, the potential to lower nuclear costs is the most conspicuous.

It is not unreasonable, then, to assume that capital costs could be 25 to 50 percent less for a simple DMSR converter design than for modern light water reactors. Compared to fast breeders such as the integral fast reactor, which rarely try to claim low capital costs, the DMSR should be even better.

In his ORNL talk, LeBlanc noted the possibility of simply eliminating a Thorium blanket for the DMSR entirely, and running the DMSR as a pure uranium-fuel cycle reactor. While the Uranium fuel cycle DMSR would offer less sustainable technology than the LFTR, it would be a very strong competitor for the current generation of Light Water Reactors. It would offer a very high level of safety, proliferation resistance and nuclear waste control, at a lower cost that current light-water reactor technology. Actinides, the big problem in nuclear waste, could be separated from reactors salts, either periodically or when the reactor is decommissioned. The recovered actinides can be returned to the core of a DMSR where they will be burned as nuclear fuel. Other fission products will essentially disappear after 300 years, if reactor managers chose to treat them as waste, but this is unlikely. Fission products present in "spent nuclear fuel" represent a potential source of valuable materials and noble gases, and the DMSR concept opens the door for the recovery of these minerals.

LeBlanc concluded his Mechanical Engineering essay by declaring,

Molten salt or liquid fluoride reactors will also take a large effort, but every indication points to a power reactor that will excel in cost, safety, long-term waste reduction, resource utilization, and proliferation resistance. As we move deeper into a century that portends financial instability, political uncertainty, environmental catastrophe, and resource depletion, this technology is too valuable to once again place back on the shelf.

Nuclear Green concurs with this view. The DMSR represents a technology that is doable in the year 2010. The technology required to build it exists now, thus developers would not be saddled with huge R&D costs, and and the technological uncertainties that would confront LFTR development. The DMSR would represent a transition, between the traditional solid fuel reactors, and the sustainable LFTR technology. The Phoenix is beginning to rise from its ashes.

David LeBlanc: ORNL and Too Good to Leave on the Shelf

David LeBlanc of the Physics department of Carleton University, Ottawa, Canada and the Ottawa Valley Research Associates, Ltd., is a highly regarded participant in Energy from Thorium discussions and a reactor scientist of considerable note. David is notable because of a significant accomplishment. He has simplified the reactor core to a point beyond which further simplifications are likely to prove impossible. David's reactor core is nothing more than two metal shells, one inside the other. A fluid fuel carrier/moderator/coolant flow into the inner shell, and then out again. A fluid containing fertile thorium flows in a and out the second, outer shell. That is it. The entire core structure is composed of two sheets of shaped metal, one surrounding the other, with openings through which a very hot salt fluid is designed to flow. In the inner chamber, fissionable material that is chemically bonded to the carrier salt becomes critical.

No control rods are control rods are required to control David's reactor, because the inherent properties of the carrier/coolant salt automatically provide feedback that can control reactivity within the core and even shut the reactor down completely.

David is scheduled to give a talk titled, Molten Salt Reactors: An Exploration of Design Space

on May 18, at ORNL.

The abstract David's talk states:

This talk will first review past and current molten salt reactor design principles covering the main development period at Oak Ridge National Laboratory (ORNL) as well as more recent work such as the Thorium Molten Salt Reactor of France (now called the Molten Salt Fast Reactor) and the FUJI concepts of Japan. Two new proposed design routes will then be presented. First a novel but simple core geometry modification to solve the issues that led to the abandonment of ORNL's Two Fluid efforts of the mid-1960's. Two Fluid designs have separate salts to carry the fertile thorium and fissile 233U and which benefit from greatly simplifying fission product removal but previously called for unworkable core architecture. Secondly, the untapped potential of ORNL's late 1970's work on denatured converter reactors termed DMSRs and proposed improvements will be presented. This more conservative route will be shown to also have attractive resource sustainability and long-lived waste reduction while requiring the minimum of development work and maximizing proliferation resistance.

In addition to his pending ORNL talk, David is the author of a new article in the May 2010 issue of Mechanical Engineering, "Too Good to Leave on the Shelf.". The Article offers a brief history of the ORNL Molten Salt adventure, some fascinating pictures,

Alvin Weinberg and the ORNL MSRE at 6000 hours of operation.

and David's own ingenious solution to a vexing problem that frustrated ORNL researchers in the 1960's. David also discusses the highly proliferation resistant DMSR, which has recently occupied his interest. He states

The amount of fissile material needed to start new reactors is also very important, especially in terms of a rapid fleet expansion. The 1 GWe DMSR was designed for 3.5 metric tons of U-235 (in easy-to-obtain low-enriched uranium) which can be lowered if uranium costs go up. A new PWR, by contrast, needs about 5 metric tons, whereas a sodium-cooled fast breeder such as the PRISM design requires as much as 18 tons of either U-235 or spent fuel plutonium. Any liquid fluoride reactor can be started on plutonium as well, but this turns out to be an expensive option, since removing plutonium from spent fuel costs around $100,000 per kilogram.

The DMSR features a larger, lower power density graphite core than other MSR breeder concepts. So while the graphite would last a full 30 years, the DMSR would still be only a fraction of the size of gas-cooled graphite reactors and would not require a pressure vessel. In fact, the simple thin-walled DMSR containment vessel would be wider but much shorter than those of PWRs and BWRs. The construction of the reactor containment building offers savings as it does not need the huge volume and ability to deal with steam pressure buildup needed for LWRs or CANDU reactors.