Molten Salt Reactors

Benefits of Molten Salt Reactors

The advantages of MSRs are abundant, for this reason their durability as an fascinating topic throughout
reactor history. We break them down by subject here.


Sustainability is a procedure of how effectively a system can use natural resources. Most
traditional reactors can just burn about 1% of the uranium on Earth. Many sophisticated reactors,
including MSRs, can do much much better. Here’s why MSRs are excellent in this regard.

  • Online fission item elimination — Because the fuel is liquid, it can be
    processed during operation. This suggests that when atoms split into the smaller sized atoms (fission
    products), those little atoms can be gathered and pulled out of the core really quickly. This
    prevents those atoms from soaking up neutrons that would otherwise continue the chain
    reaction. This enables extremely high fuel efficiency in MSRs.
  • Good utilization of Thorium — As
    mentioned above, the MSR chemical plant can continuously get rid of fission items and other
    actinides throughout operation. This suggests that when Thorium takes in a neutron and ends up being
    Pa-233, the Protactinium can be eliminated from the core and permitted to decay to U-233 in
    peace, without any threat of causing parasitic neutron losses. While this is not the just way
    to burn Thorium, it is maybe the most sophisticated.
  • No neutron losses in structure — Because there is no structure like
    cladding, fuel ducts, grid spacers, and so on, there are no neutron losses in these. This helps
    fuel effectiveness and therefore sustainability.


Though economics are not genuinely known till a system is in industrial operation, there are reasons
to think MSRs would have favorable low cost.

  • Online refueling — Where regular reactors have to shut down to move fuel
    around or put new fuel in, MSRs can do all this while at full power. You simply dump in a new
    chunk of fuel into the vat (carefully, of course). This enables high capability aspects,
    improving economics. The reactors may still have to shut down to do maintenance, but they
    likely will have much better uptimes.
  • No fuel fabrication — Any business fuel fabricator will tell you that
    it is expensive to build fuel assemblies, fuel pellets, cladding tubes, core assistance
    structures, flow orifices, etc. MSRs are basically simply vats of fuel, so they are much
    simpler and less expensive in this regard.
  • High temperature levels possible — Molten salts were very first looked at for their
    ability to go to very high temperature levels. At high temperatures, power cycles transform heat to
    electricity with much less loss, giving you more cash for a provided amount of heat.
    Additionally, numerous industrial processes require state-of-the-art heat, and these reactors could be
    used to that while producing electrical energy. Best of all, MSRs can do high temperature without
    a pressurized coolant (as needed in gas-cooled reactors).
  • Smaller containment — Given that the system pressure is low and the heat
    capacity is high, the containments can be smaller and thinner.


The most important aspect of a nuclear reactor is security. Here’s the excellent news for

  • Very low excess reactivity — Because they can be constantly refueled,
    there is no requirement to load extra fissile product to permit the reactor to run for a long
    time. This implies that it is tough to have something happen (like an earthquake) that
    could cause a shift in geometry that inserts reactivity and triggers a power spike.
  • Negative temperature level coefficient of reactivity — In general, if the fuel
    heats up, it broadens and ends up being less reactive, keeping things stable. Note that this is not
    always true in graphite-moderated MSRs.
  • Low pressure — Given that the fuel and coolant are at climatic pressure, a
    leak in a tube doesn’t automatically result in the expulsion of a lot of fuel and
    coolant. This is a significant safety benefit that makes it possible for passive decay heat removal
    (preventing things like what occurred at Fukushima). The salts typically have exceptionally high
    heat capacity as well, so they can soak up a lot of heat themselves. On the other hand, their
    thermal conductivity is about 60 x even worse than liquid metal salt.
  • No chemical reactivity with air or water — The fuel salt is typically not
    violently reactive with the environment. So where LWRs have hydrogen explosions and SFRs
    have sodium fires, MSRs do well. Of course, MSR leaks are still major due to the fact that it’s
    not simply coolant … it’s very radioactive fuel.
  • Drain tank failure system — If something goes incorrect in a MSR and the
    temperature begins going up, a freeze plug can melt, pouring the entire core into
    subcritical drain tanks that are intimately linked to an ultimate heat sink, keeping them
    cool. This is an intriguing mishap mitigation function that is possible just in fluid fuel

Problems with Molten Salt Reactors

All those fantastic advantages can’t perhaps come without a multitude of issues. Lots of individuals
promote these reactors without acknowledging the concerns, but not us! A reactor idea has to stand
on its two feet even in the face of disadvantages (and we think the MSR can do this). Let’s go
through them.

Mobile fission items

The main issue with MSRs is that the radioactive fission products can get all over. They
are not in fuel pins surrounded by cladding, however are just in a huge, sealed vat. You can put a
double-layer containment around it, sure, however it is still challenging to keep them all accounted
for. Where some of these fission items and actinides are radioactive, others have chemical
effects that can consume away at the containment. The ramifications of this are numerous.

  • Material Degredation — with half the regular table of the components
    dissolved in salt and in contact with the containment vessel, there are lots of deterioration
    and related concerns. Noble metals will naturally plate out on cold metal surface areas. In a
    power reactor, a heat exchanger will be the coldest metal around, and so the heat transfer
    surfaces will need regular replacement. At MSRE, Tellurium triggered breaking of the
    Hastelloy-N material. This was reduced with chemistry, however comparable problems may show up in
    long-lived power reactors.
  • Tritium production If lithium is used in the salt, tritium will be produced,
    which is radioactive and extremely mobile (since it is small, it can go through metal like a
    hot knife through butter). ORNL used a special sodium fluoroborate intermediate salt to
    capture most of it, however a big amount still left to the environment.
  • Remote maintenance The chemical plants will requirement routine maintenance, but all
    of the devices will be extremely radioactive. Pricey remote upkeep will be needed.
    If graphite moderator is used, its replacement will likewise be remote and pricey.
  • Complex chemical plant — Some of the fission item elimination is basic,
    such as the gas sparging to remove Xe and Kr, and worthy metal plateout. However to do the more
    serious fission product (or actinide) separation, complex procedures are needed, such as
    the liquid Bismuth reductive process, volatilization, or electoplating. These have actually been
    studied in information, however are complex adequate to be a disadvantage. Don’t make us post a
    process flow diagram. (You can find one from the MSBR on page 8 of ORNL-TM-6413.)


The primary political barrier to MSRs is their viewed bomb-factory abilities. If you talk to
non-proliferation people, they will inform you that as
soon as the (solid) fuel pins are cut open, a technology is considered proliferative. The issue
with MSRs, then, is that the fuel is already entirely cut open and melted. You’re midway to
a bomb currently, they think. Here’s what they are worried about.

  • Protactinium-233 decomposes to pure, weapons-grade U-233 — Lots of Thorium-cycle
    MSRs have to capture Pa as it is produced, getting rid of it from the system while it decomposes to
    U-233 and then reinserting it into the reactor. They have to do this because otherwise the
    Pa-233 absorbs too lots of neutrons to preserve a reproducing cycle. The issue here is that that
    ex-core U-233 is essentially pure weapons-grade U-233 which might be used to make a bomb. It
    usually comes with Zr, however separating Pa from Zr is basic. Not many common reactors need
    such a proliferative action in their fuel cycle. Numerous MSR principles do not do this, however LFTRs
    require it. Therefore, the owner of a LFTR could be producing bombs on the side. Lots of of the
    ideas for alleviating this problem (such as U-232 contamination and denaturing) just help
    against diversion by a nefarious 3rd celebration. The owners of the plant might side-step these
    kinds of fixes easily, and that’s actually what matters.
  • Inventory tracking is hard — Because a lot of products plate out in the
    reactor and in the chemical plant, it is difficult to keep specific track of all of your
    actinides. The IAEA puts safeguards in reactors to make sure that all the actinides are
    accounted for (to validate that no one’s making bombs on the side) however it will be
    difficult for the IAEA to differentiate plate-out losses from real proliferative losses.

Other minor concerns

There are a couple of other concerns, however these most likely have practical options

  • Unknown waste type — It’s not clear what nuclear waste from
    MSRs will appearance like. The salt itself is not consisted of enough to be put in a
    repository so somebody will have to come up with a stable waste kind.
  • Electrical heating units are needed to stay liquid — in a prolonged power
    outage, the chillier parts of the heat transfer loop might solidify. This might cause
    temperatures to rise over in the core (which will of course still be self-heated liquid).

Exotics in the salt

For MSRs to breed in a thermal spectrum, the lithium in the salt must be enriched to very pure
Li-7. Li-6 is a strong neutron poison and becomes tritium, the pesky mobile radiation source. Likewise,
Beryllium in Flibe is a managed substance. It has some weapons applications and is a very
dangerous product in terms of biological inhalation threats. In chloride salts, you need to enrich to
have pure Cl-37. Otherwise, Cl-35 has a strong (n,proton) threshold reaction that toxins the
reactor. Likewise, the activation item Cl-36 is a long-lived, water soluble, difficult beta emitter that
complicates waste disposal. These enrichment requires increase the cost and complexity of MSR fuel
cycles. Possibly we can discover a more ideal salt at some point.


There are more specific issues with more particular types of MSRs, however you get the basic concept
here. Generally, MSRs are underdeveloped and need a lot more research study (especially in deterioration)
before they can surely take off as the world’s fleet of power plants. Personally, we think (as
did Alvin Weinberg and Edward Teller) that these reactors have a shining location in the mid-future.
Right now, we have to keep studying them! Fortunately, much work is continuous.

History of Molten Salt Reactors

Nuclear  powered  aircraft.  This  one  is  in  Idaho  at  the  EBR-I  museum.  It  wasn’t  in fact  molten  salt  cooled  however  it  reveals  that  they  were  severe.

The  Airplane  Reactor  Experiment,  the  very first  molten  salt  reactor. (NSE,2,  pp  804 -825,  1957)

The first fluid sustained reactors were built throughout the Manhattan task. They were Aqueous
Homogeneous Reactors (AHR), meaning they were solutions of uranium or plutonium in water. These
small reactors were mostly utilized to research study plutonium. Later (concurrently to MSR advancement),
several AHR test reactors were developed at Oak Ridge National Laboratory (these were the Homogeneous
Reactor Experiments HRE-1 and HRE-2). Molten salt fuels were very first developed of in the late 1940 s,
when people started thinking of nuclear powered planes! The concept was to have very long range
bombers in the air at all times. To heat air to high enough temperature levels to supply thrust, such a
nuclear reactor would requirement extremely high temperature levels and very high power densities. The Aircraft
Nuclear Propulsion program was a huge effort, and most of the early research studies of molten salts were
done under its direction. The first molten salt reactor, the Airplane Reactor Experiment (ARE)
operated in 1954 at ORNL with a Na-Zr-U salt for 100 hours. This reactor in fact had fuel pumping
through tubes in obstructs of Beryllium. When inter-continental ballistic rockets came along, this
program was immediately canceled.

After the cancellation of the ANP job, the individuals at ORNL wanted to usage their MSR abilities to
build a business power plant. In 1956, ORNL director Alvin Weinberg worked with a person called MacPherson
to form a group at ORNL to develop such a thing. To be effective, they needed a fuel salt that
absorbed really few neutrons, dissolved much uranium and thorium, was thermally stable, and was
chemically compatible with structural products. They chose the fluorides were finest, but
UF4 had a very high melting temperature level (1035 C). Mixed with BeF2, the melting
temperature ended up being useful, but the viscosity was too high. Mixing in some LiF brought the
viscosity down. Therefore was born UF4-LiF-BeF2, now understood as Flibe. Building
of a test reactor called the Molten Salt Reactor Experiment (MSRE) started in 1962 utilizing this salt
(with a bit of ZrF4), and the plant started operation in 1966. In 1968, all the uranium was
extracted and replaced with U-233, the fissile nuclide of the Thorium cycle. Hence, MSRE was the
first reactor to operate on U-233. The reactor was shut down on schedule in 1969 as financing was set
to go towards designing and building a bigger model MSR called the Molten Salt Breeder Reactor

But it would not be so! The Atomic Energy Commission (the ancestor of the Dept. of Energy) could
not be convinced to put more cash into the molten salt job. For one thing, they were currently
over spending plan on a extremely big and expensive fast-breeder
project. Besides, there were some big issues at MSRE that some individuals were not persuaded might be
solved. ORNL director Alvin Weinberg put much energy into keeping the program going, but it was not
destined to be. Funding was mostly canceled and MSRs lost their momentum.

MSR work continued at a low spending plan. The ORNL specialists came up with most likely solutions to most of the
problems raised at the MSRE. When expansion issues about the MSR were brought up in the 1980 s,
ORNL developed (on paper) something called the denatured MSR, or DMSR that was not fissile
self-sufficient, however still offered favorable performance in contrast to conventional reactors.
Others throughout the world worked on salt chemistry problems and developed lots of other designs,
including some chloride salt quick breeder MSRs. Much later, in 2001, the MSR was chosen by the
Generation IV forum as one of the 6 Gen-IV ideas. This jump-started work on MSRs and engineers
and chemists have actually produced much excellent work on them given that then.

Salt-cooled reactors

The step-sister reactor of the MSR is the fluoride cooled high temperature reactor (FHR). These
reactors have solid fuel in the form of TRISO particles (like the high temperature gas-cooled
reactor), but rather than pumping high-pressure helium coolant through, they pump low-pressure
liquid salt (usually Flibe) through. The idea here is to get the high-temperature and high-burnup
benefits of the gas reactors without the danger of a depressurization mishap. Salt coolant has
incredible heat capability (but low thermal conductivity), so it does a good job of managing
accidents. These reactors are currently being studied by the DOE and lots of others (including the
Chinese). Here is a
presentation by one of the PIs at MIT.

Ongoing Molten Salt Reactor Work

There is continuous work in MSRs around the world. The Europeans are working on MOSART, the Japanese
have FUJI, the Americans are focused on the FHR. In 2011, the Chinese Academy of Science began a
project in Shanghai to put substantial resources into MSR innovation. There are 300 -500 individuals on
the project, with a budget plan in the hundreds of millions. The majority of of the work is on FHRs, but there is
definitely MSR work as well. They are manufacturing Hastelloy-N, separating lithium, structure salt
loops, and structure advanced modeling capabilities. That job is most most likely the most
interesting in MSRs since 1969. They strategy on having both an experimental 2 MW FHR and an
experimental 2 MW MSR operational by 2020.

Several start-up companies have emerged to commercialize molten salt reactors. Hailing from MIT,
Leslie Dewan and Mark Massie began Transatomic to
build a MSR sustained mostly by processed nuclear waste from conventional reactors. In Canada, Terrestrial Energy is working on little modular MSRs,
featuring small reactor vessels that can be switched out of a plant every 7 years or so. ThorCon is utilizing its substantial experience in the ship-building
industry to develop MSRs that requirement no new technology and can be constructed in the very near term. In
Alabama, Kirk Sorensen himself runs a business called Flibe
targeting the development of little modular Thorium-MSRs. These companies represent the
modern revival of interest in MSRs and we dream them all the best!

See Also


Specific referrals coming quickly. Nearly all of this details was compiled from the amazing library
of ORNL reports available online
. This stuff is pure gold.


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