The Moon is often regarded as the next logical step in the expansion of human activities into space and it also contains resources which can be exploited for such purposes. Energy is required for these activities and to sustain human settlements on the lunar surface. A lunar settlement will have the same basic needs as any community on Earth, but it will have a number of unique constraints. The absence of coal, natural gas, petroleum, an atmosphere and any lakes or rivers severely limits the number of options available to provide power for a lunar settlement. Solar energy will be a tough option because a night on the Moon lasts 2 weeks and storing 2 weeks worth of energy will be a problem. Only lunar settlements at the poles of the Moon can benefit from solar energy as collectors can be erected on top of strategic mountain peaks at the poles where the Sun rarely sets.
It seems that nuclear energy is the only
feasible option to power lunar settlements and to support the expansion of
activities on the Moon. However, almost all commercial nuclear reactors used
around the world today are uranium-fuelled light water reactors (LWRs) and the
numerous disadvantages associated with such reactors make them unsuitable to
power lunar settlements. As a result, a different type of reactor known as a
liquid fluoride thorium reactor (LFTR) comes in as an attractive choice as it
does not have the problems associated with uranium-fuelled LWRs.
In LWRs, U235 is the primary fissile
material that is burnt to produce energy. LWRs use solid fuel rods that are
arranged into fuel assemblies within the reactor core. The uranium in the fuel
rods is enriched with 3 percent U235 and the rest is U238. Some fission energy
is also generated from the fissioning of Pu239. Pu239 is produced when U238
absorbs a neutron. LWRs use ordinary water as both the coolant and moderator in
the reactor core. Water boils at 100 degrees Centigrade at atmospheric pressure
and this is insufficient to carry away the heat that is generated from the
fission process in the reactor core. Therefore, water in a LWR needs to be
pressurised up to over 150 times atmospheric pressure in order to bring up its
boiling temperature for it to become an effective coolant. As a result, a LWR
has to be designed as a pressure vessel and it has to be placed within a
massive containment building to keep the high pressure steam from escaping in
the event of an accident.
Named after the Norse god of thunder, thorium
is a silvery-white metal that is slightly denser than lead. It is about 4 times
more abundant than uranium in the Earth’s crust and it frequently occurs as a
by-product from the mining of rare earth metals. All thorium in nature is found
as Th232 which alpha decays with a very long half-life of 14.05 billion years.
Within the Earth, the decay of radioactive uranium (U235 and U238), thorium
(Th232) and potassium (K40) is responsible for generating most of Earth’s internal
heat. Like on Earth, the Moon also contains abundant surface deposits of
thorium which can be exploited to power LFTRs.
Figure 1: Global map of elemental
thorium on the Moon. Credit: NASA.
A LFTR is a type of molten salt reactor
(MSR) where the nuclear fuel is in the form of a fluoride-based molten salt
mixture. In such a reactor, U233 is the fissile material while Th232 is the
fertile material. The production of nuclear energy originates from the
fissioning of U233. When a U233 nuclei absorbs a neutron, it fissions and
produces an average of just over 2 neutrons. One neutron continues the chain
reaction by causing another U233 nucleus to fission while the excess neutrons
are used to create more U233 from Th232. U233 is created by exposing Th232 to
neutrons. In this process, Th232 absorbs a neutron to become Th233 and after a
couple of beta decays, U233 is produced. In such a fuel cycle, slightly more fissionable
U233 is produced than consumed. Therefore, in the operation of a LFTR, all
Th232 can be converted into fissionable U233 to produce energy.
A typical design for a LFTR consists of
a core which contains fissile U233 and an outer blanket which contains fertile
Th232. In the outer blanket, Th232 absorbs neutrons produced from the
fissioning of U233 in the core and transforms into U233. The U233 that is
produced in the outer blanket can be chemically separated continuously using a
small adjacent chemical plant and then fed into the core as fission fuel. Since
molten salts are used, a LFTR can operate at atmospheric pressure or lower. The
heat produced during the fissioning of U233 in the reactor core mostly comes
from the kinetic energy of the resulting fission fragments. The heated molten
salt mixture is pumped from the core to a primary heat exchanger. Here, heat is
transferred to a second loop of molten salt mixture which is pumped through an
intermediary heat exchanger where it heats a working fluid. A typical working
fluid is water which is heated to drive a turbogenerator to generate
Figure 2: Layout of a molten salt reactor
(MSR). Credit: Generation IV International Forum (GIF).
To get a LFTR running, an initial load
of fissile material will be required. Besides U233, U235 can also be used as
the initial start-up material. Since a LFTR breeds slightly more U233 than it
consumes, the excess U233 can be used to start-up new LFTRs. The technologies
required to construct a LFTR were largely addressed successfully during the
1960s and 1970s. In fact, most of the technologies were tested in the Molten-Salt
Reactor Experiment (MSRE) led by American physicist Alvin Weinberg at Oak Ridge
National Laboratory (ORNL). The centrepiece of the MSRE was a fluoride-based
molten salt reactor which employed U233 as the fissile material. The reactor
went critical in 1965 and it operated until 1969 which at that time set the
record for the longest continuous operation of a nuclear reactor.
Figure 3: Energy extraction comparison
between a uranium-fuelled LWR and a LFTR.
Advantages of LFTRs over LWRs:
- For LFTRs, no reprocessing of naturally
occurring Th232 is required since all of the Th232 can be converted into U233
and be burnt in the reactor to generate energy. Whereas for LWRs, fissile U235
makes up only 0.71 percent of naturally occurring uranium and it has to be
enriched to about 3 percent through a complex process of isotope separation
before being used as nuclear fuel. In LFTRs, all of the U233 can be burnt to generate
energy. However, in LWRs, only a small fraction of the fuel in the fuel rods is
burnt before the fuel rods become spent and must be replaced. As a result,
LFTRs can produce up to a factor of three hundred times as much electrical
power per unit mass of raw fuel ore than LWRs.
- Since LFTRs are basically tubs of molten
salt, fuel fabrication is not needed at all. In the case for LWRs, the enriched
uranium fuel needs to be fabricated into solid fuel rods before being inserted
into the reactor. This is an expensive and lengthy process which imposes a much
higher operational cost for LWRs. The simplicity of LFTRs is a huge plus point
for powering lunar settlements since the facilities for enrichment and fuel
fabrication are entirely unnecessary.
- Comparatively, LFTRs produce many times
less radioactive fission products than LWRs. Furthermore, the fission products
from LFTRs decay to background levels in less than 300 years but those from
LWRs take over 10,000 years. This makes it much easier to have a repository to
store nuclear waste from LFTRs. However, a lot of the “nuclear waste” from
LFTRs have novel applications and are likely to be extracted for use rather
then be tucked away in a repository.
- LFTRs offer much greater resistance to
proliferation than LWRs. Although U233 in LFTRs is a fissile material, it is
not an attractive bomb-making material since it contains small amounts of U232
which decays into products that emit highly energetic gamma radiation. Also, virtually
all of the plutonium produced in LFTRs is Pu238 which is not a fissile material
and cannot be employed in bomb-making. In comparison, the technology involved
in the enrichment of U235 for LWRs can be extended to produce highly enriched
weapons grade U235 for bomb-making. Additionally, fissionable Pu239 produced in
LWRs from the absorption of fast neutrons by U238 is also a conventional
- Unlike LWRs, LFTRs are not pressurized
and do not need to be designed as a pressure vessel. This allows LFTRs to take
on a much lighter design which makes them more feasible for space applications
as it is a lot less costly to deliver a lighter reactor. Since LFTRs are not
pressurized, they cannot explode or fail from overpressure which is a huge
safety advantage over LWRs.
- During any fission process, large
amounts of xenon and krypton gases are produced. In LWRs, these gases build up
to high pressures within the cladding of the solid fuel rods and it can pose a
serious problem during a heating transient or an accident. In LFTRs, these
gases are continuously removed from the molten salt mixture and there are no
confine spaces where these gases can build up to high pressures.
- The fluoride-based molten salt mixture
employed in LFTRs is chemically stable and impervious to radiation. In LWRs, an
overheating anomaly can dissociate water to produce combustible hydrogen gas
which can accumulate and lead to an explosion as seen during the Fukushima-Daiichi
nuclear accident. Since water is not present in the core of a LFTR, a hydrogen
explosion is impossible for such a nuclear reactor. Finally, a fluoride-based
molten salt mixture has a slightly higher volumetric heat capacity than water
and this allows it to absorb more heat during heating transients.
- LFTRs can operate with overall thermal
to electrical efficiencies that exceed 50 percent. In comparison, LWRs have
overall efficiencies of only 30 to 35 percent.
- LFTRs do not experience downtime during
refuelling since the nuclear fuel is in the form of a fluoride-based molten
salt mixture and new fuel can be continuously fed into the reactor. This allows
LFTR to produce power continuously. In comparison, LWRs will experience
downtime during refuelling since the reactor must be shut down before the spent
fuel rods can be taken out and replaced by new ones.
- The reactor core of a LFTR is fail safe
since it contains a freeze plug at the bottom which has to be actively cooled
using a small electric fan. If the cooling fails because of a power outage or
an emergency, the freeze plug melts and the fuel gravitationally drains from
the reactor core into a passively cooled storage facility which rapidly shuts
down the reactor. Since the drained fuel does not require active cooling to
keep it from overheating, an incident like the Fukushima-Daiichi nuclear
accident is impossible to occur for a LFTR. Once the power outage or emergency
is over, the drained fuel can be fed back to the reactor core and it is
business as usual for the LFTR.
- Unlike a LWR, it is impossible for a
LFTR to experience a nuclear meltdown since the fuel in the reactor core is
already molten in normal operation.
With a LFTR, a lunar settlement can be
entirely self-sufficient. Energy produced from a LFTR can be used to power a
wide range of activities which include dissociating water to produce rocket
fuel, growing food on the Moon even during the 2 week lunar night, processing
lunar material, HVAC (heating, ventilation and air conditioning), life support
systems, lighting, communications and the recycling of water, air and waste
products. In fact, to power any settlement on any planet or moon in the Solar
System, nuclear power generation systems, especially LFTRs, will be well suited
for such purposes. This is because nuclear systems can provide power during the
night, are not affected by the Sun’s proximity or orientation, can operate in
dusty environments, are compact, have a high specific power, can be scaled to
very high power levels, can potentially have very long lifetimes and can serve
as a source of heat in addition to electricity generation.
Figure 4: This is a split image of
Shackleton with elevation map (left) and shaded-relief image (right).
Shackleton is a 21 kilometre diameter crater located adjacent to the lunar South
Pole. Its interior is permanently shadowed and large deposits of frozen water
are known to exist within it. Credit: NASA/Zuber, M.T. et al., Nature, 2012.
If the LFTR is such an attractive means
of provide power to lunar settlements, they should also be very useful here on
Earth as a cheap, clean, safe and reliable means of energy generation. In July
2001, the Generation IV International Forum which consists of a dozen or so
governments was established to explore the feasibility and performance
capabilities of the next generation nuclear energy systems. Listed are a number
of competing technologies. Most of them are advances to existing technologies
and only the molten salt reactor (MSR) is truly different from the rest. The
LFTR is a type of MSR and its huge benefits have fuelled a renewed interest
worldwide. There is sufficient easily accessible thorium on Earth to provide
carbon-free energy to meet the world’s energy needs for many thousands of
years. To sum up, LFTRs can deliver what fusion promises but without the
numerous difficulties that plague conventional uranium-fuelled reactors.