**5. Technology considerations for space and aero-nuclear propulsion**

Nuclear reactors are an essential component of nuclear propulsion systems and play a crucial role in generating heat and power for spacecraft. Over the years, several nuclear reactor designs have been developed and tested, each with its own unique features and capabilities. Below, we will take a closer look at six of the most significant nuclear reactor designs: KIWI, N.R.X., Phoebus, PEEWEE, XE-PRIME, and NF.

KIWI (Kiwi Reactor) was a prototype nuclear reactor developed by the United States in the late 1950s. It was one of the first reactors developed specifically for use in spacecraft and was designed to be lightweight, compact, and highly efficient. KIWI used a solid core fuel design and employed a unique cooling system that used sodium as the coolant. The reactor was highly successful, and its design served as the basis for several other reactors developed later in the space program.

NRX (Nuclear Reactor Experiment) was a research reactor developed in the United States in the mid-1950s. It was used to test various reactor components and to study the behavior of materials in a nuclear environment. NRX was a small reactor that used a liquid core fuel design and was designed to be highly flexible, allowing for a wide range of experiments. The reactor was highly successful, and its design was later adapted for use in other reactors, including the KIWI and Phoebus reactors.

Phoebus (Phoebus Reactor) was a nuclear reactor developed in France in the mid-1960s. It was designed for use in space propulsion systems and was the first European reactor specifically developed for this purpose. PHOeBUS used a solid core fuel design and employed a unique cooling system that used helium as the coolant. The reactor was highly successful, and its design was later adapted for use in other reactors, including the XE-PRIME and NF reactors.

PEWEE was a small prototype reactor developed in the United States in the late 1950s. It was designed to be highly compact and lightweight and was used to test various components and materials for use in space reactors. PEEWEE used a solid core fuel design and was designed to be highly flexible, allowing for a wide range of experiments to be performed. The reactor was highly successful, and its design was later adapted for use in other reactors, including the KIWI and XE-PRIME reactors.

XE-PRIME (Experimental Prime Reactor) was a research reactor developed in the United States in the late 1960s. It was designed to test various components and materials for use in space reactors and to study materials' behavior in a nuclear environment. XE-PRIME used a liquid core fuel design and was designed to be highly flexible, allowing for a wide range of experiments to be performed. The reactor was highly successful, and its design was later adapted for use in other reactors, including the Phoebus and NF reactors.

NF (Nuclear Furnace) was a nuclear reactor developed in France in the late 1970s. It was designed for space propulsion systems and was one of the first reactors to employ a new type of fuel known as particle bed fuel. NF used a solid core fuel design and employed a unique cooling system that used helium as the coolant. The reactor was highly successful, and its design was later adapted for use in other reactors, including the PHOEBUS and XE-PRIME reactors.

These six nuclear reactor designs represent some of the most significant advancements in the field of nuclear propulsion. We describe each in further detail. **Table 1** shows the comparison of the different designs.

### **5.1 High-temperature materials**

The core of a high-temperature nuclear reactor for nuclear thermal propulsion is where the fission reactions occur. It must be made of materials that can withstand the extremely high temperatures and radiation levels generated by these reactions. The melting point of the materials used in the reactor core is an essential factor in determining the safety and performance of the reactor. We will examine the melting points of several materials commonly used in high-temperature nuclear reactor cores. High-performance and temperature liquid-fueled systems are often proposed


### **Table 1.**

*Key design parameters for post-NERVA conceptual NTP reactors.*

that use propellant flow to keep all core structural and moderator materials at reasonable temperatures (<800 K) while still allowing molten fuel to heat the propellant to a very high temperature before expansion through a nozzle. One potential concept is the Centrifugal Nuclear Thermal Rocket (CNTR) described in [5, 6].

For traditional solid-fuel NTP engines and the structural and moderator components of liquid-fueled engines, the first material we will consider is graphite, which has been used as a moderator in some high-temperature reactors. Graphite has a high melting point of around 3600°C and is an excellent thermal conductor, making it an ideal material for high-temperature reactors. However, graphite is also highly flammable and can become highly reactive in the presence of oxygen and high temperatures, making it less attractive for air-breathing high-temperature reactors.

Another commonly used material in high-temperature reactor cores is beryllium, which has a melting point of around 1278°C. Beryllium is a good thermal conductor with a high thermal expansion coefficient, making it well-suited for high-temperature reactors.

Tungsten is another material that is sometimes used in high-temperature reactors. Tungsten has a melting point of around 3410°C, making it an ideal choice for hightemperature reactors. Tungsten is also a good thermal conductor and is highly resistant to thermal shock, making it a good choice for high-temperature reactors. However, tungsten is also a very dense material, which can make it challenging to handle and can increase the weight of the reactor. Tungsten is also a strong neutron absorber, making it difficult to use in moderated systems fueled by high assay low enriched uranium (HALEU).

Hafnium is another material that is sometimes used in high-temperature reactors. Hafnium has a high melting point of around 2227°C and is highly resistant to thermal shock, making it well-suited for high-temperature reactors. However, hafnium is a highly reactive material that can be difficult to work with and has a high neutron

absorption cross-section. Hafnium is also expensive, making it a less attractive choice for high-temperature reactors.

Molybdenum is another material that is sometimes used in high-temperature reactors. Molybdenum has a high melting point of around 2620°C and is a good thermal conductor, making it well-suited for use in high-temperature reactors. Molybdenum is also highly resistant to thermal shock, making it a good choice for use in hightemperature reactors. However, molybdenum is also a very dense material, which can make it difficult to handle and can increase the weight of the reactor.

The thermal properties of materials are critical in determining their suitability for use in high-temperature nuclear reactors. We will compare the thermal properties of five materials commonly used in high-temperature reactors: uranium dioxide (UO2), uranium carbide (UC), carbon, niobium carbide (NbC), and tungsten.

Uranium dioxide (UO2) is a commonly used fuel in nuclear reactors and has a melting point of around 2800°C. UO2 has a low thermal conductivity, meaning it does not conduct heat well and can lead to overheating in high-temperature reactors. Additionally, UO2 is highly reactive and can become unstable at high temperatures, which can pose a safety risk in high-temperature reactors [7].

Uranium carbide (UC) is a relatively new material that is being investigated for use in high-temperature reactors. UC has a high melting point of around 2900°C and higher thermal conductivity than UO2. However, UC is also highly reactive and can become unstable at high temperatures, which can pose a safety risk in hightemperature reactors [8].

Carbon is a common material used in high-temperature reactors as a moderator and reflector. Carbon has a high melting point of around 3600°C and is a good thermal conductor, making it well-suited for use in high-temperature reactors. However, carbon is also highly flammable and can become highly reactive in the presence of high temperatures, which can pose a safety risk in high-temperature reactors.

Niobium carbide (NbC) is a refractory material that is being investigated for use in high-temperature reactors. NbC has a high melting point of around 3300°C and is highly resistant to thermal shock, making it well-suited for use in high-temperature reactors. However, NbC is also a highly reactive material that can be difficult to work with, which can pose a challenge in high-temperature reactors [9].

Tungsten is another material that is commonly used in high-temperature reactors. Tungsten has a high melting point of around 3410°C and is a good thermal conductor, making it well-suited for use in high-temperature reactors. Tungsten is also highly resistant to thermal shock, making it a good choice for use in high-temperature reactors. However, tungsten is also a very dense material, which can make it difficult to handle and can increase the weight of the reactor.

In conclusion, the thermal properties of the materials used in high-temperature reactors are critical in determining their suitability for use in these reactors. Uranium dioxide, uranium carbide, carbon, niobium carbide, and tungsten all have unique thermal properties that make them suitable for different applications in hightemperature reactors. The choice of material for use in a high-temperature reactor will depend on the specific requirements of the reactor, including the desired thermal conductivity, resistance to thermal shock, and ease of handling.

The choice of material for use in a high-temperature reactor will depend on the specific requirements of the reactor, including the desired thermal conductivity, resistance to thermal shock, and ease of handling.

**Table 2** shows the melting points of some high-temperature reactor materials of interest.


**Table 2.**

*Melting points of some nuclear reactor materials of interest in nuclear thermal propulsion.*

## **5.2 The ROVER program**

The Rover Nuclear Rocket Engine Program was a research and development program aimed at developing a nuclear-powered rocket engine for space exploration and interplanetary missions. The program was run by the US Atomic Energy Commission (AEC) and the National Aeronautics and Space Administration (NASA) from 1955 to 1973. It was part of a larger effort to develop new technologies for space exploration [10].

The main objective of the Rover program was to develop a nuclear-powered rocket engine that could provide a high specific impulse (a measure of fuel efficiency) and high thrust, enabling spacecraft to reach high speeds and travel longer distances than was possible with chemical propulsion systems. The program was focused on developing a new type of engine, known as a nuclear thermal rocket engine, which used nuclear reactors to heat a propellant to provide thrust.

The Rover program consisted of several research and development phases, including laboratory experiments, component testing, and full-scale engine testing. During the laboratory phase, researchers conducted experiments to study the behavior of various materials and components in a nuclear environment, including the heat transfer and cooling systems, the fuel elements, and the reactor core [11].

During the component testing phase, individual components of the engine were tested to validate their performance and to identify any problems. This phase included testing the heat exchangers, the pumps, and the fuel elements.

The full-scale engine testing phase involved the development and testing of prototype engines to validate the performance of the engine and to demonstrate its feasibility. The tests were conducted at various facilities, including the Nevada Test Site and the Marshall Space Flight Center. The engines were operated at full power during these tests, and the performance was measured and analyzed.

The Rover program was highly successful and produced several important breakthroughs in the field of nuclear propulsion. The program demonstrated the feasibility of nuclear thermal rocket engines and showed that they could significantly improve specific impulse and thrust over chemical propulsion systems. The program also produced a wealth of data and information on the behavior of materials and components in a nuclear environment, which has been invaluable for future research and development in this field.

However, despite its many successes, the Rover program was eventually terminated in 1973 due to a change in priorities and a shift in focus toward other areas of space exploration. The program was never fully operational, and no nuclear-powered spacecraft were ever built or flown as a result of the Rover program. **Table 3** shows the timetable of the nuclear thermal propulsion tests.

The Rover Nuclear Rocket Engine Program was a highly successful research and development program that made significant contributions to the field of nuclear propulsion. The program demonstrated the feasibility of nuclear thermal rocket engines and produced valuable data and information for future research and


### **Table 3.**

*Timeline of nuclear thermal propulsion tests.*

## *Nuclear Propulsion DOI: http://dx.doi.org/10.5772/intechopen.110616*

development. Although the program was eventually terminated, its legacy continues to influence the development of new technologies for space exploration.

**Table 1** shows key design parameters for the tested nuclear thermal propulsion systems. These reactors were fueled with UC2 particles with a diameter range of 50– 150 μm. The fuel elements were hexagonal in shape, and the propellent was H2. The matrix material in the fuel elements was graphite.

## **5.3 Conceptual designs of nuclear thermal propulsion reactors**

Many conceptual designs were created during and after the NERVA program. A summary of some of those reactor concepts is provided here [12].

ENABLER (Economical Nuclear Auxiliary Booster Launch Engine for Reentry): this was a conceptual design for a smaller NTP that could be used as an auxiliary engine for space missions. The design was based on a liquid-core concept, which was more compact than the solid-core design used by NERVA-1.

SMALL ENGINE: this was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. The design was based on a liquid-core concept and was intended to be smaller and lighter than NERVA-1 [13].

SNRE (Space Nuclear Rocket Engine): This was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. Like SMALL ENGINE, it was based on a liquid-core design [14].

710: This was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. The design was based on a liquid-core concept and was intended to be smaller and lighter than NERVA-1.

CERMET (Ceramic-Metal) Nuclear Rocket Engine: This was a conceptual design for an NTP that would use ceramic-Metal fuel instead of traditional nuclear fuel. The design was based on a liquid-core concept, and the fuel was intended to provide a higher power density than traditional nuclear fuel.

PBR #1 (Phoebus-1 Reactor): PBR #1 was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. The design was based on a liquidcore concept and was intended to be smaller and lighter than previous NTPs [15].

PBR #2 (Phoebus-2 Reactor): PBR #2 was a conceptual design for an improved version of PBR #1, with a higher power density and improved efficiency.

PeBR (Phoebus-Electron Beam Reactor): PeBR was a conceptual design for an NTP that would use an electron beam instead of a traditional nuclear reactor. The design was based on a liquid-core concept and was intended to be more efficient than traditional NTPs.

LPNTR#1 (Low-Power Nuclear Thermal Rocket): LPNTR#1 was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. The design was based on a liquid-core concept and was intended to be smaller and lighter than previous NTPs [16].

LPNTR#2 (Low-Power Nuclear Thermal Rocket 2): LPNTR#2 was a conceptual design for an improved version of LPNTR#1, with a higher power density and improved efficiency [16].

MARS WIRE CORE: MARS WIRE CORE was a conceptual design for an NTP that would use a wire-wrapped fuel element instead of traditional fuel rods. The design was based on a liquid-core concept and was intended to provide a higher power density than traditional NTPs.

**Table 1** shows a comparison of the various conceptual designs for NTPs.
