Sunday, August 26, 2012

Nuclear Power for Lunar Settlements

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 electricity.

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 bomb-making material.
  • 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.