The Future Should Be Through The Viewport On An Asteroid

The Future Should Be Through The Viewport On An Asteroid
Asteroid P/2010 A2 | Credit: NASA, ESA, and D. Jewitt (UCLA)

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Thursday, March 5, 2015

Space Exploitation: Powering Space II - Thorium Molten Salt Reactors

Diagram showing the internals of the Molten-Salt Reactor Experiment
Oak Ridge National Laboratory | Public Domain

The expanded era of space exploitation will require large amounts of energy, especially in the form of electrical power. Determining, considering and evaluating the various options available for power in space is essential, especially for large-scale space-based endeavor. One such power source is the Molten Salt Reactor, a class of nuclear fission reactor, in which molten salt functions as coolant, fuel or both.

Molten Salt Reactor
US Department of Energy Nuclear Energy Research Advisory Committee | Public Domain

MSRs vary in design, but the basic concept is that the MSR operates a higher temperatures than water-cooled reactors, such as PWRs, thereby achieving a higher thermodynamic efficiency, while maintaining relatively low pressure within the reactor. Research for the MSRs has been tentatively ongoing, with sudden starts and halts, from its early beginnings in 1954, through the Molten-Salt Reactor Experiment (MSRE) in the 1960s, to the present.

Molten FLiBe Salt
Oak Ridge National Laboratory | Public Domain

One near-future, Generation IV reactor design is a molten salt fueled and cooled reactor, with the initial reference design intended for 1,000 Megawatts of relatively continuous generation, although designs for MSRs vary in terms of fueling and cooling. One of the best known multi-use molten salts is made from Lithium Fluoride and Beryillium Fluroide, the mixture of which is known as FLiBe. FLiBe is both a coolant and solvent for fertile or fissile material, and served this purpose in the MSRE. FliBe is very useful, because it reach reach high temperatures, without also reaching a high vapor pressure, and it also does not react violently with either water or air.

Molten Salt Reactor Experiment (MSRE)
Oak Ridge National Laboratory | Public Domain

Proponents of MSRs argue that they offer many distinct advantages over conventional light water reactors (LWRs). MSRs rely on passive components to generate energy, with some designs making use of the temperature coefficient of reactivity to improve design effectiveness. MSRs operate at low pressure, which reduces the risk of complications from issues related to depressurization, and also simplifies design factors, as pressure containment is much less of an issue. There is no requirement for fuel rod manufacturing or maintenance, as no fuel rods are present in the design. Certain designs may be configured so as to burn transuranic elements, which can present issues in conventional nuclear reactors.

MSRs may react to load changes in less than 60 seconds, which is an issue in conventional solid-fuel nuclear power methods, which may lead to Xenon poisoning in such nuclear power plants. MSRs operate at higher temperatures than conventional nuclear power plants, which yields higher efficiencies for the purposes of power production. MSRs, unlike LWRs, can be turned on and off once initial criticality is achieved for the MSR. Evidently, it is matter of acquiring the energy necessary remelt the salt, and engage the pumps which flow the molten salt through the channels. Also, contemporary MSR designs are set so the MSR levels off energy production if temperatures exceed operational thresholds, permitting the MSR to function effectively without achieving catastrophic temperatures.

MSRE Air Cooled Heat Exchangers
Oak Ridge National Laboratory | Public Domain

An interesting permutation of MSRs is the Liquid Fluoride Thorium Reactor (LFTR), which is a kind of Thorium Molten Salt Reactor (TMSR), and a type of thermal breeder reactor. This reactor relies on the Thorium Fuel Cycle to achieve criticality, and thereby engage the power production cycle. As with MSRs, LFTRs use nuclear fuel contained in the form of a molten salt mixture. Thorium and Uranium-233 are dissolved in carrier salts, which in turn forms a liquid fuel. The liquid is channeled between the reactor core and an external heat exchanger, in which the thermal energy is shifted to a nonradioactive, secondary salt. This secondary salt is then used to transfer heat for power generation, such as to a steam turbine system, or to a closed-cycle gas turbine system. The LFTR makes use of the multi-function FLiBe salt, which acts a carrier salt for the dissolved Thorium and Uranium-233 fuel mixture.

Map of Thorium Concentrations | USGS Open-File Report 2005-1413 | Public Domain

The advantages of a LFTR are many, especially as it is a TMSR, and thus relies on Thorium over Uranium as its primary input. LFTR designs have inherent safety features, implying that there is a low degree of risk even during malfunctions. Thorium absorbs additional neutrons if it overheats, which leaves fewer neutrons available to continue the chain reaction, which reduces power. There is also a graphite moderator present in the LFTR , which contributes to temperature regulation, absorption and dispersion. Moreover, if the fuel overheats, it will expand rapidly and considerably, which cause the liquid fuel to spill out of the active core region, and into the channel, which which reduces the capacity for a chain reaction.

The molten fluoride salts are very chemical stable, and relatively resilient against radioactive exposure. The salts will not decompose to other elements, explode violently or burn during high temperature or high radiation conditions, relative to that of a functioning nuclear reactor, which is fairly extreme as compared to prosaic conditions. The molten fluorides do not react violently with water or air, which is an issue for sodium coolantsMolten fluorides do not typically have problems with hydrogen production, which is a combustion risk, and is characteristic of water coolant methods. Molten salts, however, are susceptible to radiolysis at low temperatures, such as below 100 degrees centigrade. In such a case, production of contaminants becomes a risk, although it would seem that molten fluoride salts are fairly resistant to such buildups.

Molten fluoride salts remain liquid at high temperatures, with LFTR cores intended to function at very low pressures, such as 0.6MPa, which is roughly the pressure in a drink water system. As there is relatively little increase in volume during core failure, the containment structure for the reactor core cannot explode or blow up. The lack of water or hydrogen in the reactor core further reduces issues for pressure increases or explosions in the reactor core, which are a typical risk during catastrophic events for conventional reactors.

LFTRs may be designed with a fail safe for the reactor core, in which a plug is actively cooled by an electrical cooling system. If the electrical cooling system fails, such as during a disaster or catastrophe, the plug melts, with the fuel draining away to a subcritical, passively cooled storage unit beneath the reactor. The reactor can also be designed to sit in a larger container, which functions like a kitchen sink, permitting any disastrous leaks to drop down into the storage unit.

Comparison: LFTR against LWR
ShotmanMaslo | Public Domain

Perhaps most significant is the potential operational differences between a LFTR and a conventional LWR. An LFTR may be used to "breed" Thorium into Uranium-233 fuel. Thorium is approximately as abundant as lead, and is more abundant in the Earth's crust than tin, mercury or silver. Conventional reactors use approximately less than 1% of mined uranium, with the rest leftover as waste product. A reprocessing LFTR may consume up to 99% of its Thorium fuel source. LFTRs, if combined with supercritical steam turbines, would operate at 45% Thermal-To-Electric Efficiency (TTEE), with innovations potentially allowing for up to 54% TTEE. Contemporary LWRs function at approximately 33% TTEE.

Power Conversion System: Space Molten Salt Reactor (SMSR)
Michael Eades

Most significantly, it may be feasible to develop MSR technology for space-based endeavor. This may be done by leveraging the Brayton cycle; specifically, the Closed Brayton cycle. The Brayton cycle is a thermodynamic cycle which is used to describe a constant pressure heat engine. A closed Brayton cycle recirculates the working fluid. The LFTR could be used in conjunction with an Space MSR (SMSR) design, thereby taking advantage of the utility of the LFTR, along with the closed-system design of the SMSR.

Ion Thruster Comparison
Ad Astra Rocket Company (AARC)

The lessons of the Aircraft Nuclear Propulsion (ANP) program may be of great value to such an endeavor, especially if combined with exploratory efforts to use an MSR for warship propulsion, at least for sealing and shield purposes, as the use of seawater would not be available in the Near-Earth environment. The Aircraft Reactor Experiment (ARE) demonstrated the viability of MSR, with a 6 engine system exhibiting a specific impulse of 28, and a thrust of 240,204 Newtons, with a thermal efficiency of 29%. Of course, this applies for a turbojet tested in the early 1950s, but nevertheless, this shows that even early MSRs were quite powerful. A contemporary LFTR will be even more so, and would be valuable for powering advanced ion engines, as well as for VASIMR.

Of course, much as with PBRs, any nuclear reactor will be unusually large and massive, at least as compared to any typical piece of space-based equipment. A particular LFTR would need to be built in space, with most of the parts assembled on Earth and shipped to space. an LFTR would be valuable for human endeavor in space, especially in the field of space exploitation, most especially in the later era of asteroid mining. Replacement parts, or parts which are frequently used, may also be 3D printed in space for the LFTR.

Optimal Energy Technology By Duration Of Use and Electical Output | Solar Flux As Function of Distance Away From the Sun
Michael Eades (Original: NASA)

Although solar cells are most definitely the immediate future of space exploration, as well as the very near future of space exploitation, the later era of expanded space exploitation will be enabled through nuclear power, especially as space exploitation takes place further and further away from our furious Sol. PBRs and MSRs are a possible technology permutation of nuclear power in space, and should be considered for assembly and use in the era of expanded space exploitation, especially during the later era of asteroid mining. For it is through innovationcost-reductions and radical technologies that humanity will achieve the dream of a space-based civilization, first in the Near-Earth environment, and then onward to the rest of the Solar system.