Molten Salts in Nuclear Applications
Andrew Hsieh
COSMOS Cluster 2
July 30, 2015
Abstract
Many look to nuclear reactors as the future of global energy. Whether or not one believes in the
effects of greenhouse gases, there is a persistent and undeniable connotation of “clean energy”
inherent to nuclear power. However the current nuclear infrastructure is inefficient, dangerous,
and ineffective due to government policies. In this report, these regulations will be disregarded
and nuclear power will be analyzed to its fullest potential through the review of a nuclear reactor
concept deemed too dangerous to be commercialized. This is the molten salt reactor concept.
1. Necessity of Nuclear Energy
Supplying energy for an increasingly populated and demanding earth has become a
popular point of contention for politicians and scientists alike. With increasing economic growth
in Asia and Africa leading to industrialization, it is estimated that energy demands will increase
by 1.6% annually between 2010 and 2030. The main sources of energy in the status quo are
coal, oil, and natural gas. The problems with “clean energy” such as wind power and
photovoltaics is low energy density, irregular outputs, and financing. This leaves nuclear energy
as the most viable future energy source. As of 2013, there are 433 nuclear power plants
producing 371 GWe. There are also 63 plants under construction, capable of producing 62 GWe.
To supply enough electricity for total dependency on nuclear power, 20x more nuclear power
plants are required, producing a total of 10,000 GWe. Currently, 1 GWe light water nuclear
reactors require .9 tons of fissile material a year; assuming maximum efficiency, this means
meeting the world’s demand for energy through LWRs would require 9000 tons of 235U.
Only .5% of mined natural uranium is 235U so there simply isn’t enough fissile material to sustain
LWRs as long term sources of energy. In addition, current LWRs produce large amounts of
radioactive waste, mostly minor actinides such as plutonium, which has a 24000 year half-life,
and other actinides like +neptunium which has a longer 2 million year half-life.
2. Nuclear Power in the Status Quo
Currently 80% of nuclear plants are light water reactors which have been in use since the
1950s. These reactors are extremely inefficient, only utilizing .5% of natural Uranium.
This sparked the race for fast breeder reactors which still aren’t being used commercially due to
the fact that they require liquid sodium cooling, which is high cost and low safety, and they
could potentially be used for manufacturing weapon-grade plutonium production.
3. Explaining Molten Salt Reactors
The Molten Salt Reactor (MSR) is one of six concept reactors being studied in the
Generation IV initiative. The Generation IV initiative is a collaborative international project
which aims to develop next-generation nuclear reactors. The MSR uses molten salt as a
moderator for the fissile material in its core, much like the water modulated nuclear reactors of
the status quo. The molten salt must have a low-melting temperature (typically below 823 K),
low vapor pressure (between 873-1000 K), and thermodynamic stability at high temperatures.
There are currently two types of MSRs: the actinide burner and the breeder reactor. The actinide
burner has a fast neutron spectrum. In addition to producing energy, it is also used for
transmuting radioactive isotopes in nuclear waste. The breeder reactor is designed to produce
more fuel than it consumes. In order for the chain reaction to occur in a breeder reactor and for it
to produce more fuel than it consumes, there must be little neutron loss. Compounds like NaF,
KF, and RbF can be added to the fuel to lower the melting point of the cores while, being an
Alkali metal, reducing neutron capture.
4. Placing a Thorium Core in Molten Salt
Alvin Weinberg invented the LWR and patented many related inventions which can be
found used in nuclear submarines and pressurized water reactors. The initial LWR plans
involved using Thorium instead of Uranium because Uranium was scarce, and could be enriched
into weapons-grade plutonium. Weinberg realized the Homogeneous Reactor Experiment
(HRE) which used light water instead of heavy water. Oak Ridge National Laboratory (ORNL)
performed research on nuclear powered aircrafts requested by the military. Since high
temperature levels were required (870 C), solid fuel and water were impractical. As a result, a
liquid fuel reactor cooled with molten salt were used.
Molten salt has very low pressure stability and vapor pressure as well as a high boiling
point. The first MSR was named Aircraft Reactor Experiment. It achieved continuous fission
reactions, but ironically did not generate any electric power. The molten salt fuel consisted of
NaF-ZrF4-UF4 (53-41-6 in mol%) and was extremely successful. The reactor had a single
graphite moderated core, a Hastelloy N vessel and pipes (which are corrosion-resistant), and
molten salt (LiF-BeF2-ZrF4-UF4). The MSRE didn’t contain thorium fluoride because it relied
on a two-fluid breeder design. The two-fluid breeder design has a fuel region containing fissile
material while the surrounding region contains thorium.
Another reactor producing 1000 MWe called the Molten Salt Breeder Reactor was
developed. The MSBR was supposed to use 700 C molten salt but was canceled because large
funding was required, deadlines were met late, supporting voices were weak, and nuclear
weapons technology is too different. The Thorium Molten Salt Nuclear Energy Synergetic
System combines the fission power reactor of a molten salt reactor with an accelerator molten
salt breeder to produce Uranium. Using the THORIMS-NES allows for electrical power to be
produced while making fissile material. Thorium is 5 times as abundant as uranium. It is
estimated that there are roughly 25 million tons of Th in Earth’s crust. This means that using the
previous estimations, in which 10,000 GWe are needed annually to provide electricity on a
global scale, Thorium could serve as fuel for over 2000 years. Thorium is converted into 233U,
234
U, 235U, 236U, 237Np, 238Pu, and finally 239Pu through a long process of repeated neutron
absorptions. This long process means that very little plutonium is produced which means that
thorium reactors are nuclear weapon proliferation-resistant. Furthermore, the 233U produced
from thorium is incredibly hard to handle since it is accompanied by so many different isotopes
of uranium. The uranium is easy to detect and hard to store due to its radioactivity.
Molten salt was determined to be the best liquid fuel candidate by ORNL. It has good
chemical stability, low pressure availability, low vapor pressure, and high boiling point. Because
molten salts have high melting temperatures, the instant a leakage occurs, the salt freezes
preventing any radioactive materials from escaping. The final choice for molten salt is a lithium
fluoride and beryllium fluoride eutectic mixture which contains fertile Th and fissile U or Pu.
MSRs work the same way most normal water nuclear reactors due with the exception of having a
graphite modulator. Liquid fueled reactors, like molten salt, are preferred to solid fuel reactors
because there is no radiation damage, since molten salt is an ionic liquid not affected by neutron
or gamma radiation, the molten salt doesn’t have to be replaced and radioactive gases produced
by the reaction can be removed from liquid unlike solids. The gas is removed by a charcoal
filter. These gases include xenon and krypton, which have a relatively short half-life.
Furthermore, MSRs can be operated at 5 atm due to their high boiling temperature and
low vapor pressure as opposed to the 150 atm of modern PWRs. Lower pressure means that
reactor vessel pipes will be less prone to breaking. The high temperature at which molten salt
can achieve increases the thermal efficiency of the plant. An MSR operates at 700C while a
PWR operates at 330C. Molten fluoride salts are not corrosive with containment alloys because
of its greater thermodynamic stability. While beryllium is quite toxic, in the case of a nuclear
reactor melt down, there is no release of Be.
Molten Salt works well as a heat transfer medium because of low pressure, high boiling
temperature, high heat capacity, appropriate viscosity, and suitable Prandtl number. It allows for
fuel reprocessing due to its ability to act as the solvent in dry processing. Because molten salts
are not organic liquids, there is less risk of a critical accident. In liquid fuel reprocessing, the
fissile material must be diluted in order to prevent critical accidents. Molten Salt allows for
more compact solid processing.
FREGAT was developed by Czechoslovakia and the Soviet Union in the 1980s which
can reprocess LWR fuels. The concept is based on fluorination flame reactor technology which
uses a fluoride volatility process to separate Uranium and Plutonium. An accelerated molten salt
breeder is necessary for the breeder reactor concept because it produces 20 to 30
neutrons/protons per reaction as opposed to the 2 or 3 neutrons per reaction in a light water
reactor. There are flaws with the AMSB, for example, it produces ten times less power than a
normal fission reactor. The AMSB, also, isn’t practical for continuous operation which means
that it would not work well as a power plant. It would be most practical to have two separate
reactors working together, a fuel breeder to produce fuel and a fission reactor to produce energy.
MSRs were initially designed to be small plants which could be deployed internationally since
the system doesn’t require high pressure.
5. Small Molten Salt Power Reactor FUJI
The thermal efficiency of FUJI reactors is higher than LWR at 44% instead of 33%. The
reactor vessel is graphite moderated and salt flows upwards though the core via a pump. This
salt then enters a chamber where it temperature exchanges with a secondary salt. The heat from
the secondary salt is then transferred to steam through a supercritical steam generator. This
produces electricity. This reactor is expected to last 30 years in which the moderator does not
have to be replaced. In the case that the fuel overheats, the fuel salt emergency drains into a tank
where it freezes. The same happens in a case where a valve or pipe bursts. Because it is easier
to remove radioactive xenon, krypton, and tritium from molten salt reactors, it is constantly
being done to reduce catastrophe impact and to increase reprocessing efficiency. The MSR
would produce .03% of MA and Pu produced by a boiling water reactor.
6. A Larger Super-FUJI
A design was created in which a “super-FUJI” could produce 1000 MWe and 2272 MWt.
There are three levels of safety containment. Level 1 consists of Hastelloy N pipes. Level 2 is a
high-temperature containment vessel. Level 3 is the concrete shielding and Hastelloy N
shielding.
85% minor actinides produced by LWRs can be transmuted by a MSR-FUJI over the course of
30 years. The super-FUJI has a core composed of hexagonal graphite rods with hollow cores.
The fuel salt passes through these holes.
7. Define Hastelloy N
Hastelloy N is 71% nickel, 16% molybdenum, 7% chromium, 5% iron, and 1%
unspecified.
Because there is no water or oxygen in molten fluoride salt, only .02 mil/year of corrosion
occurs.
8. Graphite as a Moderator
Graphite must be irradiation stable since when irradiated, it shrinks then expands. This
means that the fuel salt cannot penetrate the graphite moderator.
9. Safety of MSR-FUJI
MSR-FUJI reactors are surprisingly safe due to its unique build. It is not prone to the
dangers which occurred at Fukushima due to a series of safety precautions. The primary and
secondary loop operates at roughly 5 atm which is relatively low in pressure preventing any
ruptures or leaks. Molten salt is chemically stable and inflammable. Then, because molten salt
is an ionic liquid, radiation damage and fuel failure cannot happen. Also, there is no water in the
molten salt system meaning that evaporation cannot occur. To further safety precautions, an
emergency drain tank in the core prevents critical accidents since molten salt fuel can only go
critical where there is graphite. The liquid fuel allows for radioactive gasses to be trapped easily
through coal filters. However, what makes FUJI different from Fukushima is that the graphite
fires cannot occur since there is no oxygen inside the molten salt reactor, MSR is safe during
electricity loss because of natural circulation cooling, and the drain tank runs a cooling system
which does not use electricity.
10. A Brief Look at Future Applications
A proton accelerator, molten-fluoride target system, and heat transfer/electrical
generation system make up an Accelerator Molten Salt Breeders (AMSB). Salt fills the moltenfluoride target system bath forming a 1m vortex. A proton beam is injected near the bottom of
the vortex to prevent neutron leakage. Because the system is subcritical, there is no radiation
effects. This makes heat removal easier, turning thorium into 233Uranium. PuF3 may be added
as initial fissile material. Then, a 1GeV x 300 mA proton beam is fired into the vortex. The
electricity is recovered by fission energy found in the salt. If there is an 80% load factor of the
accelerator, 800kg of 233U is produced annually.
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circulating fuel.
4. Rosenthal, M.W. (2009). An account of Oak Ridge National Laboratory’s thirteen nuclear
reactors.
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Molten-Salt Reactor with Thorium- Uranium Fuel.
6. Haynes Inertnational, Inc. (2002). HASTELLOY N alloy;
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