Space fission power has long been identified as a near-term technology that can provide a dramatic leap in our ability to explore and expand into space. The trap that is sometimes fallen into is that “near-term” and “dramatic leap” are not necessarily consistent with each other. If a fission system is to be utilized in the near term, a reasonable, measured first step must be taken. Numerous past space fission power programs have failed because they have tried to do too much, too soon. The ideal development scenario would be a mission where the primary goal is to demonstrate the operation of a space fission system, but this would require a very strong commitment and would likely face many programmatic obstacles. A more likely scenario is to define an adequate mission, or a mission that has enough utility to retain programmatic support, while keeping technical requirements simple. Unfortunately, adequate gets increasingly more difficult with time, as solar and radioisotope technology have steadily evolved via small steps, and continue to raise the bar for the first step in space fission technology. This means that the first step must not only be small, but it should be taken as soon as possible. Fortunately, recent advances in power conversion, radiator, electronics, spacecraft, and thruster technology allow the development of an adequate fission system with a modest reactor—i.e. relatively low power, low temperature, and reasonable mass.
- Design with established materials (infrastructure and performance)
- Design for simplicity at all levels (components, integration, reliability, programmatic, etc.)
- Design with large margins (temperatures, stresses, criticality, etc.)
- Design to simplify control (stuck control elements, transient control, etc.)
- Design with an overly robust safety approach that minimizes program safety effort
- Design to minimize the testing required to verify component and system performance
- Design so that the required test data can be obtained as affordably as possible
The first bullet above is probably most important for a new nuclear system. The easiest way to use established materials is to develop a relatively low temperature system (<1000 K), which for a space reactor is reliant on a “relaxed” mass requirement. Simplicity is the mantra of any good engineer, and for a new reactor, especially one targeting deployment without a conventional nuclear-powered test, neutronic simplicity is very important. A compact, fast-spectrum reactor is the simplest reactor from a neutronics perspective, which is discussed in the Fast_Versus_Thermal reference.
The remaining bullets above are best met be keeping the system power requirement low. Lower power simplifies the entire spacecraft system in terms of development, integration, and thermal management:
Reduced/eliminated material irradiation concerns: Radiation damage (usually swelling and loss of ductility) to most materials does not occur until a certain neutron fluence threshold is reached. For most systems, a power level below 400 kWt will keep radiation damage from becoming significant, and keeping power below 100 kWt can eliminate radiation damage altogether.
Reduced/eliminated nuclear fuel issues: As nuclear fuels are irradiated (“burned”), they generally swell, lose structural integrity, and release fission gas into the fuel pin volume. In general, for 10-year lifetimes, swelling and integrity will not be a problem at powers less than 400 kWt, and fission gas buildup should be manageable. At powers less that 100 kWt, fission gas buildup also becomes negligible.
Reduced material strength concerns: At lower powers, there is considerably less stress on structural materials because of smaller temperature gradients and less fission gas buildup. In addition, other things being equal, peak temperatures will be lower, so materials will retain more of their strength.
Simplified electrical testing: Lower power reactors are more amenable to realistic non-nuclear testing. At higher power levels, it is harder to provide the required power density with electrical heaters, because of physical size limitations for leads and connections, and more difficult heater technology. Lower power should also allow more existing facilities to be available for electrical testing.
Simplified nuclear testing: Lower neutron fluence could provide more flexibility in performing component irradiation testing (more potential facilities and more flux chambers within those facilities), and/or reduce the time required for in-pile tests. More so, if reactor power is low enough, then this type of testing may not be required at all. Lower power would also make a nuclear powered ground test much easier, both from a technical and regulatory perspective, although this type of test is only technically necessary if the gain in reliability is worth the cost and programmatic risk of performing the test.
Simplified qualification process: At lower powers (less neutron fluence) there is a more extensive experimental database, which facilitates design qualification with minimal testing. Also, there is less material property change due to irradiation, so that common/existing analytical tools can be used, minimizing the need for more specialized tools that might have greater uncertainty, less history, and demand significant benchmarking. Most importantly, at lower powers, there is a higher probability that a system can be qualified with non-nuclear testing. There is a point where, at extremely low-power levels, a nuclear-powered ground test effectively becomes a low-cost “zero-power critical” test).
Simplified safety process: Lower power cores require less volume for heat transfer. This allows a more compact system, with smaller voids that could be filled by water in an accident. These factors make accidents easier to mitigate, and can provide more safety margin and flexibility to meet changing safety requirements. This can greatly simplify and expedite the launch approval process. Lower power systems will also have less burnup reactivity loss, so it will be easier to maintain the required criticality safety margin. In addition, lower power systems will have lower total mass, which allows a higher altitude orbit insertion, and potentially an Earth escape trajectory. This should simplify the launch approval process as well.
Simplified handling and deployment: A lower power system will be physically smaller than a higher power system. This could make transport easier by giving more options for handling and transport (cranes, trusses, transport containers, etc.), and more flexibility for launch shroud configuration.
Simplified reactor system component design: The design of several reactor components can be made simpler by lower power and reduced neutron fluence. In higher power systems, increased power deposition in the reflector and shield can dictate more complex designs, less favorable material choices and/or require additional design features to accommodate cooling. In addition, higher fluences can cause degradation of control element bearings and reactor instrumentation.
Simplified reactor control: Lower power reactors will experience less distortion due to temperature gradients. This could significantly reduce the time and effort put into understanding reactor dynamics, or designing more elaborate core structures to control distortion. In addition, compact reactors have numerous control technology options and more potential control element worth. This could make it easier to tailor the control elements to allow a simpler, more robust control system. Lower power reactors will also have less chance of control elements bowing/sticking/failing because of lower fluence and lower thermal distortion and stress. This could eliminate a layer of complexity and/or redundancy in the control system design. Finally, lower power reactors have less burnup reactivity swing, which makes overall control issues simpler.
Simplified decay heat removal: At low power, decay heat removal becomes much easier. Lower power systems (~400 kWt) are more likely to utilize passive decay heat removal. Lowest power systems (~50 kWt) may not even require any special design considerations or complexity to remove decay heat.
Simplified system integration: At higher powers, spacecraft system components will more likely be pushing the envelope of performance, and have less flexibility to meet changing demands set by other components. Reduced flexibility translates to longer development time and lower probability of success. Potential changes in interface requirements could significantly impact the reactor development time and risk. Lower power may also simplify steady-state and transient thermal issues for the entire power system and spacecraft, and reduce the impact of thermal cycles. This may be one the most complex engineering issues of an early-flight system, and lower power will make it easier to model, predict, and test.
Reduced size and complexity of mission: One of the biggest risks of attempting to develop a higher power reactor is that a spacecraft and mission must be designed to utilize it. Higher power adds cost and risk to almost every spacecraft component, including the power conversion, management, distribution, and rejection systems. Also, a mission that can utilize the benefit of increased power is likely to use a more costly and risky payload. In the near term, there is a higher probability of finding missions to utilize lower-power systems.
There are numerous reasons presented why a lower-power fission system could require significantly less development time than a higher-power system. Many of the issues individually may not have significant impact, but as a whole they present a compelling argument. Some of the issues presented could turn out to be non-issues upon further examination, but further examination in itself can increase development time. In general, the reason to develop a low-power system is to reduce the probability that some of the issues above will significantly impact program progress and success.
Upon the investigation of stainless-steel, UO2 entry-level fission systems, it was found that there were numerous knees-in-the-curve that could complicate system development at higher powers. Fortunately, it was found that at a power level of ~200 kWt and a lifetime of 10 years, there was a sweet spot prior to where most of the knees occurred. A discussion of these issues and limits is in the Entry_Level_Sweet_Spot reference.
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