**6. Current and near-term nuclear thermal Propulsion programs**

Several National Aeronautics and Space Administration (NASA) programs and the United States Department of Energy (DOE) focus on developing PBRs for near-term missions and technology demonstrations. The key technology used in the PBRs is the triso fuel. Triso (tri-structural isotropic) nuclear fuel particles are small, spherical fuel elements used in high-temperature gas-cooled reactors (HTGRs) and PBRs. The fuel

particles consist of a central kernel of fissile material, typically enriched uranium or plutonium, surrounded by several layers of materials that provide structural support and containment for the fissile material.

The outermost layer of the fuel particle is made of a ceramic material that provides containment for the fissile material and acts as a barrier to releasing radioactive gases. The ceramic layer is highly porous, which allows gas to flow through it, and is designed to retain its integrity even under extreme conditions, such as high temperatures and pressures.

The next layer is made of a layer of carbon, which acts as a moderator to slow down fast neutrons and increases the probability of a fission reaction. The carbon layer also provides structural support for the fuel particle and helps to prevent it from disintegrating during high-temperature operation.

The innermost layer of the fuel particle is the fuel coating, which consists of a mixture of fissile material and other materials, such as pyrolytic carbon and silicon carbide, that help to retain the fissile material and prevent it from spreading.

Triso fuel particles are used in PBRs because they provide a highly-efficient and safe means of containing and controlling the fissile material. The fuel particles are designed to withstand high temperatures and pressures and to provide a high degree of containment for the fissile material. Additionally, the fuel particles are small and can be easily packaged, making them well-suited for compact PBRs.

NASA, the US DOE, and the Defense Advanced Research Projects Agency (DARPA) are supporting research and development programs to develop, test, and demonstrate the feasibility of the triso fuel and the PBR concept. General Atomics, BWX Technologies, Inc. (BWXT), and Ultra Safe Nuclear are some companies currently developing the PBR concept, with technology demonstrations planned for the 2025–2027 timeframe.

BWXT's TRISO fuel is a type of fuel used in high-temperature gas-cooled nuclear reactors. TRISO stands for "Tri-Isotropic" and refers to the unique microstructure of the fuel particles. The fuel particles are coated with multiple layers of carbon and ceramic materials, which helps to retain the fissile material even in the event of a loss of coolant accident. This makes TRISO fuel ideal for use in advanced reactors, such as high-temperature gas-cooled reactors, requiring fuel that can withstand high temperatures and high radiation levels. TRISO fuel is manufactured through a complex process that involves forming, coating, and sintering the fuel particles. The result is a highly durable fuel that offers superior performance and improved safety compared to traditional nuclear fuel.

### **6.1 The centrifugal nuclear thermal rocket (CNTR)**

Liquid-fuel nuclear rocket engines have been envisioned since at least 1954, and various liquid-fuel NTP design concepts were proposed in the 1960s. Liquid fuel nuclear rocket engine concepts described to date employ one of three basic design approaches: (1) the bubble-through reactor, (2) the radiation reactor, and (3) the particle or droplet reactor. These reactor concepts are illustrated in **Figures 1–3**, respectively. The bubble-through reactor design features a reactor fuel that is rotated at high speed to maintain a layer of liquid fuel annulus around the inner cylindrical surface of the fuel. As the hydrogen propellant is bubbled through this liquid fuel, it is heated to the temperature of the liquid fuel. The hot hydrogen then exits the engine through the nozzle to produce a thrust. The radiation reactor design features a rotating cylinder to maintain the liquid fuel but flows hydrogen axially down the center of
