Basic Design Principles
The simplicity of SpaceNukes reactors starts with the fuel. Low-power designs (1 to 30 kWe) use a cast uranium solid core (alloyed with ~7 w/o molybdenum to improve material properties). Metal fuel has the highest possible uranium atom density of any fuel (U235 is the preferred fissile material for near-term reactors). As depicted in Figure 1, an HEU uranium metal fuel has the highest possible uranium 235 atom density (16.4 g/cc), followed by LEU uranium metal fuel (3.5 g/cc). This means that compared to other fuels (example the currently in vogue TRISO fuel is less than 0.2g/cc), a metal fuel will always allow for the smallest and lightest core possible depending on other design factors (use of moderation, etc.). It also has excellent thermal conductivity. Metal fuel is second only to oxide fuels in terms of available data and experience base. Metal fuel is an easy fuel to manufacture and the infrastructure for making uranium metal fuels is pre-existing at the National Nuclear Security Administration (NNSA) Y-12 facility (they produced the fuel for KRUSTY). Existing infrastructure is one of the primary means of keeping manufacturing costs low and time to delivery short. As the designs move to a higher power, the designs move to a higher temperature and burnup fuel such as Uranium Oxide or Uranium Nitride. As with metal fuels, Uranium atom density and the ability to manufacture the fuel will be the primary discriminator for which fuel is used.
Figure 1 - Uranium 235 Atom Density in Fuels
When most people think about nuclear reactors, they mostly imagine large complicated 1000+ megawatt light water reactors that provide electrical power to the commercial power grid or the power systems used in naval ships. These systems are extremely complex and involve tens of thousands of components and subsystems. As Figure 2 shows, this is not the case for SpaceNukes reactors, which are far less complex than a traditional nuclear power plant. The design philosophy employed is "integrated simplicity",which is the simplest path . As can be seen in Figure 7, the SpaceNukes Kilopower reactor is about three times less complex than previous space reactors, (such as SP-100) designed in the 1980s. The keep it simple mantra is a primary theme in SpaceNukes designs.
Furthermore, SpaceNuke reactors require minimal human involvement. The reactors self-regulate and require no maintenance for decades. The reactors include informational data on output and temperature plus fault detection, e.g. failing Stirling Engine. In the unlikely instance a Stirling fails – the other Stirlings can have their stroke increased to replace electrical production from the failed Stirling.
Figure 2 - Comparisons of Reactor Subsystems for Various Designs
Fast Neutron Spectrum Physics
Perhaps the most important aspect of the “keep it simple” mantra is the choice of using fast-neutron spectrum physics. Others will tout the potential for decreasing fuel mass by moderating (slowing down) neutrons, but it substantially increases the complexity of the physics and system dynamics. The use of a hydride moderator increases uncertainty due to hydrogen diffusion, hydrogen migration, material phase changes, doppler broadening, etc. A simple metal system using fast neutron physics is the simplest type of reactor possible. It also means the system is well behaved and very predictable. The system will, in fact, be self-regulating. Figure 3 depicts the self-regulating phenomena of a metal fueled fast neutron reactor. The reactor will essentially adjust the power output to the power demand of the power conversions system. The boron carbide rod in the system acts as a thermostat to set the reactor temperature. As the power conversion system demands more or less power, the reactor automatically adjusts to the demand using simple physics. Figure 4 shows the proof of this principle, as it was actually demonstrated in the KRUSTY reactor test.
Figure 3 - The Self-Regulating Effect of SpaceNukes Fast Neutron Spectrum Physics Designs
One design principle that keeps the reactor simple is the use of sodium-filled heat pipes for the low power systems. Heat pipes are a simple closed-loop system for effectively transferring heat using the physics of boiling, condensation, and capillary forces. A diagram of a sodium-filled heat pipe is shown in Figure 4. SpaceNukes uses heat pipes because it eliminates the use of complex loops, pumps, thawing heaters, accumulators, etc. found in more complex reactor systems. SpaceNukes pioneered the use of heat pipes in reactors and have built the only heat pipe cooled reactor to date.
As power increases (say to the megawatt range), SpaceNukes has looked at both heat pipe cooled and gas-cooled reactors. For terrestrial applications, heat pipe cooled reactors offer superior safety performance. As a space reactor, the weight and complication of a secondary heat exchanger slightly favor a gas-cooled reactor.
Figure 4 – Simple Example of a Sodium-Filled Heat Pipe (courtesy Wikipedia Commons)
At low powers, the power conversion of choice is the Stirling convertor; i.e. a Stirling engine coupled to an electric alternator to produce electricity. The Stirling engine was developed in the 1800s by Presbyterian minister, Dr. Robert Stirling, as a simple heat engine that works off of temperature difference. A schematic of a Stirling convertor is shown in Figure 5. Stirling engines allow system simplicity because they provide high efficiency (thus low reactor power and a small radiator). Integration is simplified because no fluid is exchanged between the reactor and power conversion. Heat pipes are readily attached to the hot end of the Stirling engine to deliver heat from the reactor core. Conversely, a water heat pipe or coolant loop attached to the cold side of the Stirling engine can reject heat to radiator panels.
Stirling engines have been shown to last for decades during longevity testing by industry and NASA. Commercial engines are available, although in specific sizes. Tailoring of engines may be required.
At higher power levels (> 50 to 100 kWe), the system will move to a Brayton gas power conversion system. A Brayton system uses a hot gas through a turbine to produce power. Commercial Brayton systems are also available and can be readily adapted for use in a space reactor system.
Figure 5 – Example of a Free Piston Stirling Engine with Alternator
Higher Power Designs
The simplest path to achieving higher powers is to maintain the reactor dynamics and operation of KRUSTY and first-generation SpaceNukes flight reactors (to avoid the need for nuclear-powered ground testing). Three design changes to different, but established technologies will be required to move from kilowatts to megawatts.
The first change would be to move from the solid metal Uranium-Molybdenum fuel block to a stainless-steel block with traditional fuel pins, most likely Uranium Dioxide but Uranium Nitride fuel pins would also be considered. The monolith block core would create the same established fast neutron physics (material-expansion-based feedback) and core heat transfer (conduction to heat pipes).
The second change would be to move to gas-Brayton power conversion over Stirling engines. Brayton is preferred at higher powers because the turbomachinery scales better on a mass basis than Stirling. Also, Brayton is the simpler system integration option at higher powers (a heat-pipe-to-gas heat exchanger becomes simpler as the number of heat pipes and Stiling convertors increases). The downside to Brayton is the potential for a single point failure in the gas boundary, but initial systems could use independent redundant Brayton loops coupled to a heat pipe reactor.
The third change would be to move from a heat-pipe-cooled reactor to a gas-cooled reactor. This would occur if/when heat-pipe reactor manufacturing becomes too cumbersome as the number of heat pipes moves from hundreds to thousands. The gas-cooled reactor would use the same monolithic block and core physics as the heat pipe reactor, but with gas-flow holes instead of heat pipes. However, this option will only be preferred over the heat-pipe reactor if Brayton systems are shown to reliably avoid gas leaks (or an ability to repair leaks is established).
Cooling Enhancements for Waste Heat
Power conversion systems are not 100% efficient and there will always be waste heat that must be removed from the system. The heat can be removed in several ways, with the standard way being radiative heat transfer for most space systems. This is the preferred and perhaps is the only way for deep space systems.
For surface systems, heat can be removed and used for productive means. These could include a pumped liquid system to:
‣ Heat a habitat;
‣ Used in thermal or chemical process uses; or
‣ In water mining to melt water.
If the planet has an atmosphere, such as Mars, then large fans can be used for heat removal. All of these techniques can be used to minimize mass.
Most reactor designs include shielding alternatives to minimize mass delivered to space. For deep space reactors, a “shadow” shield will be used that shields the spacecraft and critical electronic components. For surface reactors, a full shield (referred to as 4pi) needs to be used to shield in all directions (to varying degrees) to prevent ground scatter. If the site permits, the reactor can also only include a top shield with the rest of the reactor buried in the regolith to provide shielding.
For the first set of reactors that will be used in space, it is almost a certainty that designs will be tailored for specific applications and improved in the next version. Because of this SpaceNukes see the early space reactor market as consisting of “Bespoke” reactors. SpaceNukes will customize any reactor concept for the specific needs of the customer. But, one point needs to be made. There are “knees in the curve” for many of the properties of materials used in reactor development and design. In some cases, natural breakpoints will exist for some components and materials. SpaceNukes will always point out when a design could be made more efficient by slight changes to the desired requirements.
Factors to be decided include reactor size, fuel type, cooling method, electrical conversation method, heat rejection method. All these factors have trade-offs. SpaceNukes has tools to quickly produce concepts based on customer requirements.