Sustainability considerations of nuclear power in space transportation and infrastructures

doi:10.21203/rs.3.rs-4824628/v1

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Sustainability considerations of nuclear power in space transportation and infrastructures

https://doi.org/10.21203/rs.3.rs-4824628/v1

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Nuclear thermal propulsion (NTP) and nuclear electric propulsion (NEP) systems are considered to be potential enablers for exploring Mars and beyond. The fission surface power can provide continuous heat and electricity, which complements solar power on the Moon and Mars. Nuclear propulsion and power are attracting spacefaring nations and stakeholders again after the Space Race era due to their advantages and envisioned applications. This paper examines space nuclear power from a few sustainability perspectives. First, from the space logistics and transportation perspective, we identify potential operational regimes where nuclear space propulsion could complement or outperform chemical rocket propulsion. Second, we identify self-engineering technologies that may enhance the reliability of nuclear power systems in orbit or on the surface. Lastly, we discuss extraterrestrial infrastructures, inspired by the design approaches and technologies adopted in the architecture, engineering, and construction (AEC) industry, which could enhance the sustainability of nuclear power in space and contribute to the cislunar circular economy.

More than half a century ago, ground tests during Project Rover demonstrated capabilities of re-igniting and clustering nuclear thermal rocket (NTR) engines. The initially envisioned application of NTR engines was to power the upper stages of intercontinental ballistic missiles. After NASA took over the project from the United States Air Force in 1958, Project Rover became part of the Nuclear Engine for Rocket Vehicle Application (NERVA) program. Potential applications of nuclear rocket engines were lunar shuttles and early manned interplanetary round-trip expeditions, as envisioned in NASA roadmaps comprising a round-trip to Mars by 1978, a perpetual settlement on the lunar surface by 1981, and the follow-up missions to outer planets. Those plans could not be realized due to the cancellation of the NERVA project in 1973. In the meantime, the RD-0410 engine was developed by the Soviet Union from 1965 through the 1980s, whose application was also crewed missions to Mars.

Despite the lack of flight records for nuclear engine rockets, there have been continued efforts to study nuclear rocket engines because they are more than twice as efficient as chemical rocket engines. Because the reduced propellant mass can increase the cargo capacity, nuclear propulsion can shorten the interplanetary transit time during which crews are subject to space radiation that is difficult to shield. In 2023, the Defense Advanced Research Projects Agency (DARPA) and NASA announced that they would collaborate in assembling and testing NTR engines from as early as 2027 [1]. The Demonstration Rocket for Agile Cislunar Operations (DRACO) engine will first transport cargo to the Moon, in preparation for crewed missions to Mars.

The advent of mega-constellations and the increasing occurrences of conjunction events would necessitate extreme care throughout the lifecycle of spacecraft with nuclear propulsion systems onboard. At the same time, the increased use of nuclear power propulsion could open up opportunities for novel missions in orbit or on the surface of the Moon and Mars. The improved fuel efficiency will lower the transportation of space logistics, thereby contributing to the circular economy on Earth and in space. The organization of this paper is as follows; Section 2 illustrates the near-future application of nuclear propulsion technologies by comparing their fuel economy with that of chemical propulsion in the Earth-Moon system. In Section 3, we introduce the concept of self-engineering and its applicability to nuclear propulsion systems. In Section 4, we investigate terrestrial architecture and nuclear infrastructures as a reference to establish sustainable habitation on the Moon and beyond.

2. Where nuclear propulsion would work

As mentioned above, the early NASA roadmap for NTR engines did not consider cis-lunar transits as its application, whereas DRACO aims at using NTR engines in cislunar space. Both plans are examples of nuclear thermal propulsion where the reactor heat is used for gas expansion. Another nuclear-powered option is nuclear electric propulsion (NEP). The term ‘nuclear electric rockets’ is less favored over NEP since the thrust does not directly originate from the reactor heat; instead, the thermal energy from its nuclear reactor is converted to electricity for powering electric thrusters. While rockets with nuclear thermal engines have not flown in orbit yet, nuclear electric propulsion has spaceflight records dating back to the US experimental satellite SNAP-10A, which marked the first operation of a nuclear reactor and an ion thruster in 1965. The follow-up NEP missions include the SP-100 program in 1983 and Project Prometheus/Prometheanin in 1993, neither of which was implemented in hardware. The Soviet Union ground-tested NEP (“Romashka”) in 1965 and deployed the next generation NEP system (“Buk” or “Bouk”) on over 30 reconnaissance satellites flown in the 1970s [2]. These satellites used the thermoelectric effect, also called the Peltier–Seebeck effect, in their thrusters where the temperature difference is directly converted to the voltage difference. The thermionic effect is another effect adopted in NEP systems to convert heat to electricity. A thermionic generator generates electricity from electrons released from an electrode (Edison effect) at high temperatures, whose efficiency is greater than that of thermoelectric generators. Examples of thermionic thrusters are TOPAZ and YENISEI reactors, which were produced in the Soviet Union and ground-tested in the US. First ground tested in 1971, TOPAZ (or TOPAZ-I) was flown in 1987 onboard the Soviet satellite, Kosmos 1818, which later experienced a coolant leakage problem [3]. YENISEI (or TOPAZ-II) was considered a successful collaboration between the U.S. and Russia, where British and French specialists also participated in ground testing; no flight plans followed despite possible applications of powering spacecraft radars and propulsion systems [4, 5, 6]. To evaluate the potential of nuclear propulsion, its performance relative to chemical propulsion could be analyzed in terms of orbital mechanics (“delta-V”) and scaling laws.

2.1 Cis-lunar transfers

NTR engines and NEP engines can be classified into different categories of rocket propulsion. NTR engines are similar to chemical propulsion (high-thrust) and have a high specific impulse (Isp) of approximately 1000 seconds. Their thrust level might enable take-off under limited low-gravity conditions. On the other hand, NEP engines have higher efficiencies (Isp = 7000s) but much lower thrust levels, thus limiting their application to in-orbit operations. As shown in Eq. (1) and Eq. (2), the same rocket equation may be written for high-thrust (HT) engines and low-thrust (LT) engines [7].

\(\:{\left(\frac{{m}_{pl}}{{m}_{0}}\right)}^{HT}=1-{\left(\frac{{m}_{f}}{{m}_{0}}\right)}^{HT}=exp\left(-\frac{\varDelta\:{V}^{HT}}{{I}_{sp}^{HT}{g}_{0}}\right)\) Eq. 1

\(\:{\left(\frac{{m}_{pl}}{{m}_{0}}\right)}^{LT}=1-{\left(\frac{{m}_{f}}{{m}_{0}}\right)}^{LT}=exp\left(-\frac{\varDelta\:{V}^{LT}}{{I}_{sp}^{LT}{g}_{0}}\right)\) Eq. 2

In the equations above, the ratio of the payload mass to the total initial mass decreases for a larger delta-V and increases for a larger specific impulse. When the payload ratio is subtracted from 1, we obtain the ratio of the propellant (fuel) mass to the total initial mass. The specific impulse value is for low-thrust propulsion, but the low-thrust delta-V tends to be also higher than the high-thrust counterparts. As shown in Eq. (3), a generalization can be made for a hybrid rocket by multiplying the payload ratios of HT phases and LT phases:

\(\:{\left(\frac{{m}_{pl}}{{m}_{0}}\right)}^{hyb}={\prod\:}_{i}^{}exp\left(-\frac{{\sum\:}_{j}^{}\varDelta\:{V}_{j}^{i}}{{I}_{sp}^{i}{g}_{0}}\right)=exp\left(-{\sum\:}_{i}^{}\frac{{\sum\:}_{j}^{}\varDelta\:{V}_{j}^{i}}{{I}_{sp}^{i}{g}_{0}}\right)\) Eq. 3

where \(\:\varDelta\:{V}^{HT}\) and \(\:\varDelta\:{V}^{LT}\) are represented by \(\:\varDelta\:{V}_{i}^{j}\) instead, with i for flight legs and j for propulsion types. While the above equations would apply to deep space transfers, we focus on cislunar space, i.e. the space within the Moon’s orbit around Earth, where nuclear propulsion will be tested and operated first.

2.1.1 Lunar surface to Earth-Moon-Lagrange points

The delta-V requirements for high-thrust transfers between the Moon and its vicinity are summarized in Table 1. A low-lunar orbit (LLO) is a Moon-bound orbit with an altitude of 100 km (in this case) or below. Landing on the lunar surface from an LLO or launching from the lunar surface to an LLO has a delta-V requirement of 1900 m/s. The flight legs between an LLO and an Earth-Moon Lagrange (EML) point tend to have lower delta-V requirements. Altogether, those requirements translate into the payload proportion of 42.4% out of the total rocket mass if only chemical propulsion, with a specific impulse (Isp) of 300 seconds, is used from the lunar surface to an EML. The same trip would yield a much higher payload fraction of 77.3% if only nuclear thermal propulsion is used, with a higher Isp value of 1000 seconds. However, this NTR-only option means that nuclear rockets must lift off from or land on the lunar surface, which would be exposed to radioactive emissions.

Table 1

delta-V requirements (m/s) for high-thrust transfers near the Moon
From\To	Lunar surface	LLO	EML1/2	EML4/5
Lunar surface	X	1900	2520	2580
LLO	X	X	640	980

For the same scenario, chemical propulsion (from lunar surface to LLO) and NTR (from LLO to EML1/2) might be combined to avoid contaminating the lunar surface with radioactive emissions from NTR engines. Compared to the chemical-propulsion-only option, the payload fraction increases in the chemical-NTR hybrid to 49.1% from Eq. (3). The increase is not drastic because the flight leg from/to the lunar surface accounts for most of the delta-V to overcome the lunar gravity. If we hybridize chemical propulsion with nuclear electric propulsion, the resulting payload fraction is still around 51%, whose value is not sensitive to the range of Isp in NEP engines (7000 to 10000 seconds). The higher efficiency of NEP is offset by the increased delta-V requirement for low-thrust propulsion, which is approximately twice the high-thrust counterpart in this example. Overall, constrained operations of nuclear rockets to Moon-bound orbits and their vicinity, excluding the lunar surface, will not make significant improvements in the fuel efficiency of a mission or campaign.

2.1.2 Low-Earth-orbit to Earth-Moon-Lagrange points

In the next scenario, the operations of nuclear propulsion are extended to Earth-Moon transfers. During early phases of lunar settlements, materials and equipment should be carried from Earth; once habitation is established, materials may be sourced from the Moon to EML points to support exploration to Mars and beyond. Table 2 summarizes high-thrust and low-thrust delta-V requirements from the low-Earth orbit (LEO) and the geosynchronous equatorial orbit (GEO) to the select destinations in Table 1. Each entry in this table has two delta-V values; high-thrust (HT) values are calculated from Hohmann transfers, and low-thrust (LT) values are approximated using Edelbaum’s approach [8].

Table 2

delta-V requirements (m/s) for high-thrust (HT) and low-thrust (LT) transfers between the Moon and Earth [8]
From\To	LEO	EML1	EML4/5	LLO
LEO	X	3770 (HT) 7000 (LT)	4000 (HT) 7500 (LT)	4040 (HT) 8000 (LT)
GEO	3900 (HT) 4800 (LT)	1400 (HT) 3000 (LT)	1700 (HT)	X

If nuclear propulsion is allowed in LEOs for Earth-Moon transfers, its advantages in terms of fuel efficiency are clear. Taking a LEO-EML1 transfer as an example, the payload ratio of 27.7% in chemical propulsion increases to 68.1% in NTR engines and 90.3% in NEP engines. If nuclear rocket operations are only permitted at higher altitudes, around a GEO for example, its advantage is less pronounced; the payload ratio of 62.1% in chemical propulsion increases to 86.7% in NTR and 95.7% in NEP.

If a LEO-LLO transfer is considered, the payload ratios are 25.3% for chemical engines, 66.2% for NTR engines, and 89.0% for NEP engines, as shown in Fig. 4. For scenarios where nuclear engines are banned in lower altitudes (LEOs), one way of dividing a LEO-LLO transfer is dividing into a sub-flight from a LEO to an EML1 (chemical) and a sub-flight from an EML1 to a LLO (nuclear) as a first-order approximation. The resulting increases in payload fraction are minimal, 26.0% for the chemical + NTR option and 27.5% for the chemical + NEP option, meaning that starting nuclear engines near an EML point does not provide fuel-saving advantages. Starting nuclear engines earlier would offer more advantages but is more challenging to characterize. Dividing a flight leg from a LEO to an EML into a sub-flight from LEO to GEO and a sub-flight from GEO to an EML based on Table 2 is not valid due to excessive delta-V requirements for arriving (re-circularizing the orbit) at and departing from the geosynchronous orbit. The diversity of LEO-LLO transfer trajectories poses challenges in analyzing HT-LT hybrid options comprehensively in their tradespace. While we will leave the details of design and optimization as future work, HT-LT hybridization might be an economically competitive alternative in space logistics and space domain awareness. Another option is to use solar electric propulsion (SEP) closer to Earth and NTR elsewhere, but this architecture may have too high complexity relative to its fuel-efficiency gain.

2.2 Unconventional orbits

Nuclear propulsion has relative advantages over chemical propulsion in terms of fuel efficiency, as analyzed above, and in terms of thrust time as well. The total duration of the trajectory correction maneuvers (TCMs), or burn time during which thrust is generated, is 20–40 minutes for an Earth-to-Mars transfer with chemical propulsion and is expected to be around 80 minutes with NTR (Microsoft Word - Mission Spacecraft and Instrument.docx (nmsu.edu)). Using the rocket equations, NEP for the same mission will yield a burn time of 460 minutes for Isp = 7000s. Another example, the Prometheus 1 Spacecraft in the Jupiter Icy Moons Orbiter (JIMO) mission envisioned a 3-year thrust duration with its NEP system on for its trip from Earth to Jupiter. These examples illustrate the potential for increased thrust duration. Furthermore, the scaling law analysis in this section suggests that the levels of thrust and acceleration would scale up as the size of a nuclear propulsion system increases. As a result, adopting nuclear propulsion might realize novel types of orbits, previously seen as infeasible due to perturbation forces, in cislunar space missions as well as in deep space exploration.

Comparing the specifications of several flown and proposed nuclear fission systems, the system specific mass (kilogram per watt) appears to be decreasing as the system power (watt) increases. Figure 5 describes this relationship in a logarithmic scale; in the figure, the specific power (watt per kilogram) is used instead, which is increasing with the system power [9, 10]. The data points, from the left to the right, represent SNAP-10A, RORSAT, TOPAZ, SP100, JIMO, Mars Cargo, Mars Human, and Longhurst’s concept [11, 12, 13]. Although the larger spacecraft missions are unimplemented prototypes, strong linear dependency is consistent in their modeling, continued from the data of smaller reactor sizes implemented in the past.

Table 3

Specifications of nuclear propulsion systems in Fig. 5 [9, 10, 11, 12, 13]
Mission	Meaning or other names	Power (kWe)	Year
SNAP-10A	Systems for Nuclear Auxiliary Power, Space Nuclear Auxiliary Power Shot, OPS-4682	0.5	1965
RORSAT	Radar Ocean Reconnaissance Satellites	1.8	1980
TOPAZ	TOPAZ-I	5.0	1987
SP-100	Space reactor Prototype (concept)	100	1994
JIMO	Jupiter Icy Moons Orbiter (concept)	200	(concept)
Mars Cargo/Human	-	1000/2500	(concept)
Longhurst	-	50000	(concept)

Denoting the specific power (kg/W) and the power (W) by s and P, respectively, the linearity between log(s) and log(P) yields a power law that can be expressed as

\(\:s={k}_{1}{m}^{{k}_{2}}\) Eq. 4

where k₁ and k₂ denote the scaling coefficient and the exponent of a power law, respectively. By definition, the power is a product of the specific power (s) and the reactor mass (m).

\(\:P=sm={k}_{1}{m}^{{k}_{2}+1}\) Eq. 5

Given the fitted value of k₂ = 1.975 in Fig. 4, it can be said that the specific power is roughly proportional to the square of the reactor mass in Eq. (4) and that the power is proportional to the cube of the reactor mass in Eq. (5). The thrust of a rocket engine is represented as

\(\:F=\frac{2P\eta\:}{{I}_{sp}{g}_{0}}=\frac{2\eta\:{k}_{1}{m}^{{k}_{2}+1}}{{I}_{sp}{g}_{0}}=ma\) Eq. 6

where eta is efficiency, Isp is specific impulse, and g0 = 9.8 m/s². Therefore, the thrust is proportional to the cube of the power. Dividing the thrust by the reactor mass, the acceleration is proportional to the square of the mass. Eq. (7) also shows that the acceleration is proportional to (reactor power)^0.66 where the exponent is evaluated from k₂/(k₂ + 1).

\(\:a=\frac{F}{m}=\frac{2P\eta\:}{{I}_{sp}m{g}_{0}}=\frac{2{k}_{1}{m}^{{k}_{2}+1}\eta\:}{{I}_{sp}m{g}_{0}}=\frac{2{k}_{1}{m}^{{k}_{2}}\eta\:}{{I}_{sp}{g}_{0}}={\left({k}_{1}P\right)}^{\frac{{k}_{2}}{{k}_{2}+1}}\frac{2\eta\:}{{I}_{sp}{g}_{0}}\) Eq. 7

It should be noted that this acceleration formula considers the reactor mass only. Using the spacecraft mass when dividing the thrust, the resulting spacecraft acceleration will be less than the acceleration in Eq. (7) as it is. The spacecraft mass is the sum of the propellant mass and the dry mass without propellant. The dry mass is calculated by scaling up the reactor mass. Assuming a scaling factor of k₃ multiplied to convert the reactor mass to the spacecraft mass (dry mass), the reactor mass term at the denominator of Eq. 7 is first replaced by k₃m. After that, the rocket equation is used to add propellant mass to the dry mass; the resulting gross mass (wet mass) is thus the k₃m term multiplied by\(\:(1+{e}^{\varDelta\:V/{I}_{sp}{g}_{0}}){g}_{0}\) as shown in Eq. 8:

\(\:a=\frac{2P\eta\:}{{I}_{sp}{k}_{3}m(1+{e}^{\varDelta\:V/{I}_{sp}{g}_{0}}){g}_{0}}={\left({k}_{1}P\right)}^{\frac{{k}_{2}}{{k}_{2}+1}}\frac{2\eta\:}{{I}_{sp}{k}_{3}(1+{e}^{\varDelta\:V/{I}_{sp}{g}_{0}}){g}_{0}}\) Eq. 8

As a reference, the acceleration of the NEP spacecraft is around 5×10^− 5 m/s² for Jupiter Icy Moons Orbiter (JIMO) whose reactor mass, spacecraft mass, and gross mass are 6 tonnes, 12 tonnes, and 20 tonnes, respectively. This acceleration magnitude is similar to that of the J2 effects and is greater than contributions from higher-order nonspherical harmonics (3×10^− 7 m/s²), solar radiation pressure (1×10^− 7 m/s²), and third-body effects from the Sun and the Moon (5×10^− 6 m/s²). If this acceleration can be continuously or repeatedly applied, space missions with unconventional orbits, deemed inappropriate for chemical propulsion, would become possible as the acceleration benefits from the reactor size effect. Among the envisaged orbits with continuous propulsion is the so-called ‘cylindrical orbit’ depicted in Fig. 6 [14]. The coordinate system is rotating around the Earth’s center such that the x-axis is always coinciding with the geostationary point of interest, marked by a black dot in Fig. 5. Instead of being placed at the geostationary point, a satellite with continuous propulsion could be placed above or below the geostationary point. While the vertical displacement will result in small circular motion, this concept will greatly expand the usability of geosynchronous orbits which is mostly limited to the geostationary points on the Earth’s equatorial plane. The proposed cylindrical orbit concept relies on solar sails for thrust generation, but nuclear propulsion may provide an alternative or complementary option, whose investigation is left for future work. example, very-low-lunar-orbits [15]. Whether conventional or unconventional, orbital operations of nuclear power space vehicles should comply with the global norms such as the International Safety Framework for Nuclear Power Source Applications and other prior space treaties [2, 16].

Nuclear propulsion opens new opportunities for space missions that have been considered infeasible and adds extra degrees of freedom to cislunar activities by commercial and governmental players. The cislunar volume, far greater than near-Earth space, will pose several challenges in terms of space domain awareness (SDA). Furthermore, adopting nuclear propulsion for SDA in the cislunar region will complicate safety assurance and other regulatory issues. The lack of transparency in nuclear assets, either inevitable due to remoteness or by intention, might lead to catastrophic results. One such example is the Cosmos-954 satellite, a nuclear reconnaissance satellite launched by the Soviet Union in the 1970s, which had a crash incident in northern Canada. To compare safety and performance of various space nuclear systems, the French ERATO space nuclear power program carried out simulation studies for gas-turbine (closed Brayton cycle, CBC) and low-temperature thermoelectric (TE) options. The CBC option had satisfactory results in compactness, weight, and power source adaptability, while the TE option was shown to provide high reliability because of passiveness, high redundancy and extensive spaceflight experience (e.g. Buk). Contingency scenarios considered in the study included meltdown scenarios during normal operations, loss-of-flow scenarios, and loss-of-coolant scenarios. If the coolant does not flow or is leaked, the fluctuations of power and temperature levels inside the reactor should be avoided or at least be predictable, which was the main theme of the study [17]. Nuclear fission reactors in orbit must be resilient to failure or deviation from normal operating conditions in harsh and remote environments where direct access is difficult for repairing and refurbishing. Self-engineering could be one of the approaches to enhance the safety and reliability of space nuclear systems under the aforementioned requirements and constraints.

3.1 Self-healing

Most self-healing material research has focused on polymer-based materials which are not directly useful in nuclear reactors, Stirling engines, or Brayton engines. Some success has been seen with multi-layered self-healing thermal coatings for aerospace engines [18]. These coatings often work by creating a new oxide layer protecting the substrate but require ambient air for oxidization. Self-healing and life-elongation processes via pore-filling have been demonstrated in aluminum alloys and steels with internal cracks [19]. However, the mechanisms require particular stress, strain or temperature conditions. Ceramic and metal matrix composites have been investigated for critical, complex systems in air-breathing engines, gas turbines, nuclear reactors as well as solid oxide fuel cells [20]. In particular, nano/multicomposites based on ceramic, nickel or silver are other candidates to enable self-healing in engine parts and electronic components under high working temperatures and low oxygen environments [21]. The concept of heterogeneities, as seen in biological examples where hard segments and soft segments are interconnected, may also enhance the self-healing performance of metals or composites [22].

Other potential research directions include the use of different phases or size scales. Galinstan is a liquid metal composed of gallium, indium, and tin that can be used to make self-healing electrical wires [23]. Solidification of Galinstan below − 19°C would necessitate adequate thermal controls, but its application is envisioned for deep space missions utilizing thermal-to-electrical conversion such as thermo-acoustically driven liquid metal magnetohydrodynamic generators [24]. Homogeneous nanocrystalline particles from copper and gold were reported to self-heal radiation-induced defects [25]. Through the ‘loading-unloading’ process, defects are shown to be absorbed and eliminated more easily in ultrafine grains than in larger-grain counterparts [26]. The latest research shows that the migration speed of ultrafine grain boundaries, acting as defect sinks, is decreased after irradiation but increased under mechanical stresses. More simulations and experiments with increased scales would be necessary for those novel self-healing concepts to be considered in prolonged nuclear propulsion, such as NEP with closed-cycle gas turbines (Brayton) which was once considered for NASA’s Project Prometheus.

3.2 Self-sealing

Self-sealing components could prevent leaks in mechanical components such as Stirling engine cylinders or heat pipes. The difference between self-healing and self-sealing is not definitive, but the latter relates to structural integrities against impact or puncture while the former might involve other functionalities such as the electric conductance of wires. Existing self-sealing examples in terrestrial applications include armor, pressure vessels, hydraulic pipes and gas valves up to 500 bar pressure [27, 28, 29, 30]. NASA’s high-velocity impact tests investigated polymers and reactive liquid monomers, which could apply to pressurized crew cabins because the self-sealing process requires projectile energy and ambient oxygen [31]. Two properties, viscosity and elasticity, are of interest and need to be balanced; the material’s viscosity contributes to sealing after its elasticity establishes fast contact between the sealant and the damaged substrate. Prompt, autonomous sealing can preserve the performance of engine components against vacuum and space radiation [32].

A wider definition of self-sealing may include self-retaining features commercialized in military-grade bolts and nuts for aerostructure applications. Thread-end release bolts offer a passive safety feature with self-locking beyond the thread; impedance bolts contain spring-loaded balls to produce a detent effect. Self-retaining adjustable-diameter bolts and captive nuts offer a more active mechanism where expanding elements compensate for wear due to vibration [33]. These self-retaining techniques will become more effective if combined with automated loose-bolt monitoring [34].

3.3 Self-repairing

Self-repairing has functions overlapping with self-healing or self-sealing, but it may be distinct from the two; for example, tightening a bolt with rotational forces or torques is not identical to sealing or healing in our context. Self-tightening bolted joints have been demonstrated in lab experiments using piezoelectric-based health-monitoring techniques (self-sensing) with shape memory alloy washers (self-actuating) to tighten loosened bolts [35]. Two major classes of shape memory alloys are nickel-titanium-based alloys and copper-based alloys. The latter is cheaper but more fragile than the former [36]. Both classes of self-repairing, superelastic bolts can be manufactured via the Plasma Skull Push-Pull process in combination with 3D printing [37]. Shape memory alloys require external heating since they have little internal resistance and cannot incur sufficient Joule heating for actuation.

Within engines, wear can occur between moving parts. Self-repairing engine systems have been investigated which monitor for wear debris in the lubrication oil [38]. Once the debris is detected, lubricating oil additives could extend the service life if disassembly or overhaul is not feasible [39]. One example is the addition of micro-particles to bearing lubrication to fill in wear damage [40]. Solid lubrication systems with self-adapting and self-repair capabilities have been highlighted in high-temperature applications. Conventional liquid-based lubricants undergo degradation in internal combustion engines and metal processing machinery with operating temperatures as high as 1000°C, making solid lubricants a potential alternative [41]. Biomimic high-temperature coatings for aerospace blades are envisioned, where 4D printing technologies could program material behaviors upon external stimuli [42].

In this section, self-engineering has been investigated in the context of enhancing reliability of space nuclear propulsion. Self engineering also has potential synergy with other approaches, such as biomimicry [43] or 3D printing [44], to make nuclear spacecraft design and operation more technically feasible. However, long-term sustainability and economy of nuclear propulsion will depend on the infrastructures supporting nuclear propulsion. These infrastructures also provide functions of habitation, in-situ mining, and off-Earth manufacturing or recycling, whose holistic framework is highlighted in the following section.

Historically, the popular application of nuclear power sources has been the power source for rovers and landers on lunar or planetary surfaces. Those missions used decay heat for thermal control (radioisotope heater unit, RHU) or electricity generation (radioisotope thermoelectric generator, RTG). Due to political/environmental issues, NTR engines have not advanced beyond ground test phases. Similarly, NEP hardware prototypes after SNAP-10A were limited in scale, as shown in the Safe Affordable Fission Engine (SAFE) and Kilopower projects, during which the intended usage shifted from in-space propulsion toward surface power supply applications. Although the decay heat from radioisotopes was the only viable solution when solar panels alone could not generate sufficient power, crewed surface missions would require power scales beyond the capacity of RHUs and RTGs. The use of nuclear fission reactors will be inevitable in lunar and Martian surface settlement. By maximizing the design commonalities in engines (e.g. closed-Brayton cycle) or electric power systems between the transportation side and the infrastructure side, nuclear-powered spacecraft could complement the power demand of surface habitation when landed. A similar concept on Earth is a floating nuclear power plant that can be relocated for refuelling or refurbishing. As a barge structure, this off-shore platform is not necessarily self-propelled; vehicle-to-grid (V2G) is similar to the combination of nuclear spacecraft and habitation with this respect since self-propelled vehicles provide electricity to the stationary infrastructure.

For comparison with terrestrial counterparts, the IAEA classifies small modular reactors (SMRs) as nuclear reactors with a power capacity per unit up to 300 MW. All space nuclear reactors plotted in Fig. 4 fall into the SMR category. Regarding space propulsion applications, perhaps the only example exceeding this SMR range would be the Phoebus-2A reactor which ran during the later phases of Project Rover. During its ground tests, the Phoebus-2A reached the power of 4,000 MW, comparable to the Space Shuttle’s overall power of 12,000 MW at take-off. Technically, all reactors in Fig. 4 except Longhurst’s concept are also classified as microreactors as their capacity is less than 10 MW.. This level of add-on power would be still useful, especially at early settlement stages. The surface habitat may consume the reactor-provided energy on the spot or convert it to chemical energy (e.g. batteries) and/or materials for energy storage (e.g. hydrogen) for later use. More terrestrial analogies could be drawn to enable the co-development of nuclear-powered space transportation and the off-Earth infrastructures that support nuclear space transportation. This chapter discusses three aspects that could be considered: prototyping, standardization, and circular economy. Their applications are roughly in chronological order from short-term to long-term.

4.1 Design Approaches and Prototyping Platforms

The extreme environments of the Moon, characterized by lower gravity, lack of atmosphere, radiation, and risks of asteroid impacts, necessitate a novel design approach to building structures and façade design. These challenges demand innovative structural and material solutions to ensure the safety and viability of habitation in extraterrestrial settings.

4.1.1 Innovative Architectural Solutions for Extraterrestrial Habitation

Various prototypes have been proposed to address these challenges, including subterranean structures, 3D printing, and inflatable enclosures. A theoretical proposal for the Mars colony design competition showcased a habitat made using a subtractive construction method similar to ancient rock-cut architecture around the rim of a crater [45]. This method provides natural protection from radiation and other dangers. The idea is inspiring as it could be applied to lunar surfaces, suggesting that new forms of habitation may not always follow prevailing contemporary construction methods on Earth. 3D printing technology, an emerging innovation on Earth, also holds significant potential for lunar habitation. Recently, NASA awarded ICON, a US-based construction technology company, a contract to develop 3D-printed habitats capable of withstanding the harsh conditions of lunar environments [46]. For the light-weight enclosure system, The European Space Agency (ESA) collaborated with MIT and SOM to develop a prototype for lunar habitation featuring an inflatable multilayer shell designed for pressurization to support human occupancy [47]. For further protection from radiation, a multidisciplinary team of architects and scientists proposed an innovative H2O-based enclosure system for a Martian habitat. This design utilizes 3D printing technology to create an ice layer, effectively forming a protective barrier against radiation [48]. This idea is particularly applicable to lunar regions with the presence of ice, where similar techniques can be employed to construct protective enclosures.

4.1.2 Modular construction, Design for Manufacturing and Assembly (DfMA), Design for Disassembly (DfD)

Innovative design approaches from the terrestrial architecture, engineering, and construction (AEC) industry can also be adapted for extraterrestrial environments. Globally, there is a rising interest in modular construction due to its benefits of reduced construction time and savings in material and waste generation, contributing to the goal of a circular economy [49]. Similarly, Design for Manufacturing and Assembly (DfMA) and Design for Disassembly (DfD) have been actively investigated by building industry leaders as strategies to reduce material usage, labor costs, and enhance the lifecycle of buildings [50].

These design approaches streamline construction, ensure efficient use of resources, and even consider the reuse of building components. Thus, adopting these concepts as primary construction methods in space can significantly enhance the sustainability and feasibility of building habitats and structures in extraterrestrial environments. The actual application can range from larger-scale modular pod designs for easier assembly to reusing structural metal components or finish materials such as plastic or glass.

4.1.3 Artificial intelligence (AI) and large language models (LLMs)

In the architectural design industry, artificial intelligence (AI) and large language models (LLMs) are employed across all phases, from preliminary design to structural and façade development, incorporating both objective factors and subjective preferences [51]. For lunar habitation, a new set of variables specific to the extraterrestrial environment—including gravity, solar exposure, protection from asteroids, and special mechanical, electrical, and plumbing (MEP) —can be introduced to explore and optimize design. AI and LLMs can also be implemented during the design phase of a nuclear power plant, improving overall safety in operation, and providing cost-effective design solutions and the plant’s whole life cycle [52]. AI and LLMs can further facilitate comprehensive knowledge management in rapidly advancing lunar environment research, as well as ensuring adherence to nuclear industry standards for radiological protection, radioactive waste management, and safety analysis.

4.2 Standardization of Components

Given the limited resources transportable from Earth due to energy constraints, construction of infrastructures on the Moon must rely on the compatibility and interoperability of building components. This necessitates standardization in both the design and production stages. The need for standardization extends beyond architectural components to include the nuclear modular reactors and transportation systems. A similar approach can be found in the Metabolism movement of the mid-20th century, which envisioned architecture and urban design that mimic the metabolism of an organism, with its tissues constantly growing, renewing, and decaying, as depicted in Fig. 7 [53]. In this utopian scenario, modular building parts are plugged into skeletal tubes and are replaced as the system expands into a larger self-sufficient ecosystem. Despite the theoretical nature of this movement, it highlights the importance of compatibility and interdependence of different parts within a closed system, which is inspirational for long-term habitation on the Moon and can be manifested in the following examples.

4.2.1 Modular Construction

Modular construction, depicted in the Metabolist vision as a key element of standardization, is making a return in the contemporary construction industry in response to the global housing shortage and a focus on sustainable development [54]. The industry is witnessing an increase in projects and research fueled by the many known benefits of this technique, including speed, cost-effectiveness, safety, and environmental friendliness [55]. Modular construction is particularly advantageous for standardized and repeatable programs, such as housing [56], suggesting its potential application for lunar construction given the apparent need for housing units. A large settlement can be efficiently constructed using highly standardized modules, ensuring scalability and interoperability. Transportation challenges persist for modular construction due to the innate separation between the manufacturing and assembly site [55]. Utilizing nuclear modular reactors for transportation can potentially address these challenges, enhancing the feasibility of modular construction for extraterrestrial habitats.

4.2.2 Interoperability of Nuclear Modular Reactors

In addition to addressing transportation challenges, modular reactors can play a crucial role in facilitating the construction process and supporting sustained habitation. The UK Space Agency has backed research into a space micro-reactor model, exploring nuclear power as a sustainable energy source for spacecraft, planetary surface exploration, and habitation [57]. This model suggests that micro-reactors can be conveniently integrated into various construction phases, from transportation and assembly to occupancy, much like the individual pods envisioned in Peter Cook’s Plug-in City depicted in Fig. 7 [58]. To achieve interoperability across different uses and ensure safety in various operations, standardization is essential. The recent EU policy on a single charging solution for electronic devices serves as a good precedent for standardization [59]. Ensuring compatibility among architecture, transportation, and energy generation will significantly enhance efficiency in construction in outer space.

Efforts are being made in the architecture, engineering, and construction (AEC) industry to standardize components across various disciplines, and this could be a good reference to achieve maximum interoperability. The UK government has set guidelines for a platform approach to standardize components and interfaces within the construction industry [60], collaborating with industry players to develop standards and classification down to the level of fittings and fixtures. This process involves digitization and building information modeling (BIM) [61]. Ultimately, the principle of standardization will enhance interoperability and sustainability beyond Earth, paving the way for future advancements in outer space exploration. This universal approach will enable lunar settlers to overcome the unique challenges of the harsh extraterrestrial environments.

4.3 Circular Economy in Extraterrestrial Development

Many government bodies and organizations now acknowledge climate change as a top-priority challenge, resulting in various global initiatives and policies. The European Commission recently presented a vision for climate neutrality that includes action plans to achieve net-zero goals. In addition to decarbonization and the use of renewable energy, developing a circular economy is a key strategy. This involves practices such as material recycling and recovery, and replacing energy-intensive materials ultimately to reduce greenhouse gas (GHG) emissions [62]. This strategy is highly inspiring for lunar habitation, as it addresses the challenges of limited resources, reduction of waste and extended life cycles within a closed-loop system to create further value [63].

4.3.1. Life Cycle Assessment (LCA) as a Framework

Life Cycle Assessment (LCA), standardized in ISO 14040-series, is a technical framework designed to assess circular economy strategies and support decision-making in practice [64]. The framework defines the life cycle stages of a product’s life from raw material acquisition, transportation, and construction to deconstruction, waste processing, reuse, and recycling. LCA is generally deemed suitable for the built environment, where it assesses various environmental impacts such as global warming, ecotoxicity, and resource depletion [65].

Similarly, the model can be implemented for circular economy strategies in lunar environments to manage resources sustainably. Figure 8 illustrates the energy and resource use in lunar construction compared to terrestrial construction, taking into account all LCA phases and sustainable measures. In doing so, it is important to recognize the different goals and variables compared to the Earth-based cases. For instance, carbon, typically seen as greenhouse gas (GHG) on Earth, can serve as a valuable insulator for lunar habitation. Additionally, carbon may need to be introduced into the lunar ecosystem for food production and the synthesis of materials and fuels. Therefore, redefining the variables and goals for lunar habitation is imperative to assess circular economy strategies for extra-terrestrial logistics.

4.3.2. Local Material Sourcing

As mankind discovers more available resources on the Moon, ideas around local extraction of essential resources, often referred to as In Situ Resource Utilization (ISRU), are being actively investigated. Recent ESA-funded experiments have shown promising prospects for extracting metal and oxygen from lunar regolith [66]. Additionally, carbon dioxide has been confirmed to exist in the polar regions of the Moon. It is suggested as a potential source for producing essential resources such as steel and fuel once successfully extracted [67].

Beyond life-sustaining resources, building materials can also be sourced locally. A group of UK scientists is investigating a concrete-like lunar construction material using lunar regolith and starch [68]. While precise structural analysis and field testing are still required, this approach eliminates the need to transport composite materials from Earth. Further exploration and discovery of the Moon's raw materials will enhance local sourcing efforts, reducing the energy required for sourcing and transportation, and leading to a more sustainable and circular economy.

4.3.3. Closed-loop Active Systems

To further increase efficiency in a closed-loop system, active mechanical systems can be implemented. NASA has developed the Environmental Control and Life Support System (ECLSS), which includes multiple active components that recover valuable life-supporting resources from waste. For example, the water recovery system reprocesses wastewater and condensate into clean water [69].

Similar concepts have been applied to net-zero energy buildings on Earth, where buildings solely rely on solar energy for their building operation and incorporate active mechanical systems for self-sufficiency. For instance, heat recovery ventilation systems are commonly integrated into heating, ventilation, and air conditioning (HVAC) systems to extract residual energy from exhaust air, thereby drastically reducing the heating and cooling demands [70]. Although these systems are not completely closed due to ventilation, they highlight the potential of active systems that can contribute to the circularity of lunar habitation.

4.3.4. Recycling and Repurposing Materials

There is increasing interest in the architecture, engineering, and construction (AEC) industry for recycling and repurposing materials from existing structures to achieve circularity. The U.S. Environmental Protection Agency (US EPA) promotes the reuse of construction and demolition materials, ranging from brick and concrete to insulation, hardware, and fixtures, as part of their sustainable materials management approach [71]. This idea, which aligns with the aforementioned Design for Disassembly (DfD) approach, is highly applicable to lunar construction where energy-intensive materials such as metals, polymers, and composites are considered emerging materials for space missions [72] [73]. Creative reuse of these energy-intensive materials will contribute to a more circular economy.

The reuse of aggregate materials, such as cast-in-place concrete, is also being explored for circular economy applications. Moving beyond the conventional practice of reusing crushed concrete aggregates, a novel approach involves repurposing concrete blocks by incorporating post-tensioned cables as a complementary structural element. This method has been shown to have a significantly lower global warming potential (GWP) compared to other conventional materials used in bridge construction [74]. This study demonstrates a potential method for reusing and repurposing materials like regolith-based concrete to support long-term development and habitation in extraterrestrial environments.

Recycling is not limited to conventional building materials and extends to waste management. There are numerous case studies of recycling everyday refuse on Earth—such as plastic, paper, glass, and organic waste—into building materials that would otherwise end up in landfills [75]. These terrestrial cases highlight the need for innovative waste management methods in lunar environments to achieve circularity.

4.3.5. Circularity on a Macro Scale

The circularity, inspired by the latest terrestrial examples, should not be merely construed as an isolated concept within a single habitation unit in extraterrestrial implementation. In discussing energy efficiency in net-zero buildings, the ultimate goal is autonomy in an urban scale where energy balance is improved by individual buildings exchanging resources through a smart grid network [70]. Similarly, while achieving self-sufficiency at the habitat level is an immediate task, circularity can occur on a much larger scale through the exchange of resources across varied scales: within the unit, within the village, or even across different planets, as exemplified in Fig. 9, depending on the available resource and environmental goals.

As ideal as it sounds, macro circularity and integrated resource management face significant challenges, particularly in the transportation system. A sustainable and cost-effective transportation infrastructure is essential to support surface missions and interplanetary travel. Among all energy sources, solar and nuclear energy appear to be the most viable options. A standardized small modular reactor could significantly reduce energy demand and mitigate environmental impacts, thus enabling frequent transportation between different systems. Additionally, nuclear energy as a power source will benefit further exploration of deep space and create extended circularity across forthcoming habitats such as Mars and other planets.

Nuclear power sources have been employed by several space missions in different forms. In-space propulsion and planetary surface power could benefit from nuclear fission to meet energy requirements that are difficult to achieve with other methods. Despite some technical or political issues, the use of nuclear power would be inevitable in executing exploration campaigns and expanding human habitation on the Moon and Mars.

Of recent interest are designing nuclear thermal rockets and testing them in cislunar space, as well as deploying nuclear fission reactors on the lunar surface [76]. While the cislunar space currently acts as a sandbox or testbed for nuclear power, this work shows that some operations in this regime would still prefer nuclear propulsion due to fuel efficiency and scaling effects.

To harness the unique advantages of nuclear power, safety measures must be taken. Aeronautical systems and nuclear power plants on Earth could provide a reference, but extra layers of consideration would be necessary for off-Earth settlements and transportation. On a broader scale, the nuclear power infrastructures could become part of the architecture, engineering, and construction (AEC) industry in space, in support of a circular economy spanning Earth and the Moon.

The authors hereby declare that they have no conflict of interest.

Funding

This work was carried out without external funding. Open-access funding was provided by City University of London.

Author Contribution

S.W.P., E.S.P., S.B. and R.R. wrote the main manuscript text. S.W.P. prepared figures 1-6 and E.S.P. prepared figures 8-9. All authors reviewed the manuscript.

Acknowledgments

The authors would like to thank comments from anonymous reviewers.

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Sustainability considerations of nuclear power in space transportation and infrastructures

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Version 1

Abstract

Figures

1. Introduction

2.1 Cis-lunar transfers

2.1.1 Lunar surface to Earth-Moon-Lagrange points

2.1.2 Low-Earth-orbit to Earth-Moon-Lagrange points

2.2 Unconventional orbits

3. Self Engineering for Nuclear Power

3.1 Self-healing

3.2 Self-sealing

3.3 Self-repairing

4. Nuclear Power Infrastructures for Circular Space Economy

4.1 Design Approaches and Prototyping Platforms

4.1.1 Innovative Architectural Solutions for Extraterrestrial Habitation

4.1.2 Modular construction, Design for Manufacturing and Assembly (DfMA), Design for Disassembly (DfD)

4.1.3 Artificial intelligence (AI) and large language models (LLMs)

4.2 Standardization of Components

4.2.1 Modular Construction

4.2.2 Interoperability of Nuclear Modular Reactors

4.3 Circular Economy in Extraterrestrial Development

4.3.1. Life Cycle Assessment (LCA) as a Framework

4.3.2. Local Material Sourcing

4.3.3. Closed-loop Active Systems

4.3.4. Recycling and Repurposing Materials

4.3.5. Circularity on a Macro Scale

5. Conclusion

Declarations

Funding

Author Contribution

Acknowledgments

References

Additional Declarations

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