Nuclear energy provides energy densities not available from any other readily available source of power and heat. Nothing else even comes close. And nuclear fuels such as uranium and thorium are relatively cheap and readily available. If used wisely, Earth’s supplies of nuclear fuels could safely last for hundreds of thousands of years. That is why human ingenuity must be channeled toward these forms of energy — which can be used safely and affordably on and off planet.
One pound of uranium fuel can produce as much usable energy as about 3 million pounds of burnable coal.
The question is not whether humans must turn to nuclear power to light and drive their future. The question is: “What is the cleanest, safest, most affordable path to exploiting nuclear fission, fusion, and other ways of tapping the nuclear forces?”
A Promising New Fission Reactor Prototype
The Los Alamos National Lab has developed a new scalable (between 1kW and 10kW) nuclear fission small reactor prototype which promises to fill crucial niches in powering outer space missions, human space colonies, and even terrestrial habitats and settlements in remote locations — such as Antarctica and remote islands. NASA, the US Department of Energy, and Los Alamos Labs have teamed up to develop this clever new approach to small scale nuclear power.
How Does it Work?
First, a very basic description of the reactor system:
Together, NASA and [US DOE NNSA] have designed and developed a 1 kWe reactor prototype with technology that is relevant for systems up to 10 kWe. It consists of a highly enriched uranium core built by NNSA, heat pipes provided by Advanced Cooling Technologies
through a NASA Small Business Innovation Research contract, and Stirling generators provided by Sunpower, Inc. The core is a solid block of a uranium alloy, and heat pipes are clamped around the core to transfer heat to Stirling power conversion units to generate electrical power. Much smaller than terrestrial nuclear plants, Kilopower systems are small enough to be demonstrated here on Earth in existing facilities at the Nevada National Security Site.
For human exploration, multiple 10kWe Kilopower systems could provide the 40 kWe initially estimated to be needed by NASA’s preliminary concepts for a human outpost, with the ability to add power as the outpost grows. For robotic exploration, 1kWe Kilopower units enable abundant, reliable power for science and communications, and the potential to field deep space missions based on science return while conserving the limited supply of radioisotope fuel for its best opportunities. Characteristics of fission power that make it so beneficial for Mars outposts and deep space robotics also apply to other space missions. Nuclear fission systems could be scaled up to power nuclear electric propulsion vehicles to efficiently transport heavy cargo beyond Mars, and they could potentially shorten
crewed trip times to Mars and other distant planets. __ NASA
The reactor is expected to provide safe, stable, low power output for roughly 15 years using the initial fuel load.
A more technical description:
The baseline material for the core has been chosen to be 93% highly enriched U235 alloyed with 7% Mo by weight, and is expected to produce an optimum balance between neutronic, thermal, and metallurgical properties . Figure 1 shows the current LANL design of a 4kWt flight core, which provides 1kWe with Stirling power conversion.
Beryllium Oxide has been selected as the radial reflector material for its high neutron reflectance throughout the required temperature and energy spectrum as well as its mechanical and thermal properties. The reflector design is monolithic and does not incorporate control drums typical of higher power reactors, highlighting another significant and simplified design feature. For criticality safety throughout ATLO, only a single control rod of B4C is needed in the center of the core to keep keff at a safe level during all expected operations and hypothetical accidents. When the reactor reaches its startup location in space or on the surface of another planet, a control mechanism will slowly remove the poison control rod and allow the reactor to start up. This benefit of controlled startup at the mission destination allows BOM power levels and overall power system life expectancy to be directly coupled to mission timeline requirements. The flight system could utilize a number of control rod options that could be specifically tailored to mission requirements. For instance, the simplest control rod design, as baselined, would perform only one movement at the beginning of the mission allowing the reactor to start up and load follow the power conversion system. This method allows the natural degradation of the core temperature as fuel is spent and neutronic behavior
changes over time. Conservative thermal degradation of the 4kWt core using this method are estimated to be 3 K/yr with 0.1% fuel burnup over 15 years. An alternative approach is to use an active control rod that adds reactivity as needed throughout the mission to keep the reactor temperature and power constant. Using active control, the reactor can provide constant power for several hundred years at the 4kWt level due to very little fuel burnup and needed reactivity insertion. These simple control rod and startup methods are an important design feature needed for missions that may require the reactor to startup under minimum power at a location where solar energy is limited and battery power is necessary. Initial estimates of the required power for startup is 1 amp hour at 28VDC.
Thermal energy from the core is transported to the power convertors via sodium heat pipes. Heat generated from the fission reactions is conducted through the core, into the heat pipe evaporator and vaporizes the sodium liquid. The sodium vapor travels up the heat pipe where it can be accepted by the Stirling convertors at the condenser interface. As the sodium vapor releases its latent heat and condenses back to the liquid phase, the wick pumps it back to the evaporator where the cycle continues. Alloy 230 is the baseline envelope material for the heat pipes because of its known compatibility and prior experience with sodium as well as its high temperature strength and creep resistance. This passive thermal transport operates solely on thermal energy and requires no electrical power for pumping. This is an important design feature, which reduces the parasitic losses of the power system and simplifies system startup and control.
High efficiency free piston Stirling convertors have been baselined for the initial designs to increase system performance and provide high specific power. Their use benefits from existing flight development of the Sunpower Inc. 80 We Advanced Stirling Convertor (ASC) as well as recent successful technology demonstrations of both 1 and 6 kWe convertors developed by Sunpower Inc. for NASA under the current Nuclear Systems Program. The Stirling engine heat acceptor is conductively coupled to the sodium heat pipe condenser and uses the thermal energy from the reactor to thermodynamically drive the power piston and linear alternator. The Stirling convertors in both the 1 and 10 kWe Kilopower designs are arranged in the vertical dual opposed configuration allowing easy power scaling while minimizing the shield half angle and mass. Thermoelectric conversion has been studied as an alternate power conversion technology that offers simplicity and additional redundancy but requires significantly more thermal power from the reactor due to its lower efficiency. NASA studies are currently looking at these two power conversion options for future development and mission use. The preliminary baseline design (figure 1) uses 8 125We ASC style convertors in a dual opposed configuration with mating hot ends. Coupling the hot ends of all 8 convertors is a conduction plate that allows redundant heat paths in the event of a heat pipe or convertor failure. This hot end assembly increases reliability but also reduces mass using a unified insulation package and fewer components. Future Stirling designs will likely incorporate a shared expansion space between engine pairs similar to the current 6kWe Sunpower Inc. design.
The Stirling engines must reject their waste heat to space at the optimum temperature in order to establish a balance between the conversion efficiency and radiator mass. This optimum temperature is not equal for the 1 and 10 kWe systems but does fit well within the operating range for water based heat pipes. Titanium water heat pipes are baselined for the Kilopower systems and have already been through a significant development cycle with numerous successful designs and tests. The Ti/H2O heat pipes will transfer the waste heat from the Stirling engines to a radiator fin where it can then be rejected to space.
Kilopower, NASA’s Small Fission Power System for Science and Human Exploration (PDF Download Available). Available from: https://www.researchgate.net/publication/269208033_Kilopower_NASA%27s_Small_Fission_Power_System_for_Science_and_Human_Exploration .
Much more at the researchgate.net link above.
Scaling Up Kilopower to Megapower
Los Alamos National Lab is scaling up the Kilopower prototype for higher power reactor applications, working toward a “Megapower” fission reactor in the MW ranges.
Lessons learned from the kiloPower development program are being leveraged to develop a Mega Watt class of reactors termed MegaPower reactors. These concepts all contain intrinsic safety features similar to those in kiloPower, including reactor self-regulation, low reactor core power density and the use of heat pipes for reactor core heat removal. The use of these higher power reactors is for terrestrial applications, such as power in remote locations, or to power larger human planetary colonies.
The MegaPower reactor concept produces approximately two megawatts of electric power. The reactor would be attached to an open air Brayton cycle power conversion system. A Brayton power cycle uses air as the working fluid and as the means of ultimate heat removal.
MegaPower design and development process will rely on advanced manufacturing technology to fabricate the reactor core, reactor fuels and other structural elements. Research has also devised methods for fabricating and characterizing high temperature moderators that could enhance fuel utilization and thus reduce fuel enrichment levels. __ Los Alamos Monitor via NextBigFuture
The program is proceeding step-by-step, starting small and building to larger systems as each concept is proven and each systems suite is demonstrated and optimised:
E. Highly Enriched UMo Core and the KRUSTY Test
At completion of the NASA thermal vacuum testing of the DU core, a final design review will be held to determine if the HEU core is ready to begin fabrication. This final design review will accumulate all design revisions throughout the testing program and release the final design drawings to Y-12 for fabrication. When complete, the HEU core will be shipped to the DAF facility where it will integrate into the test assembly at the proper time. The full scale nuclear testing will be performed with the Comet criticality machine at the Device Assembly Facility (DAF). The flight prototypic assembly for the KRUSTY test will be identical to the DU core test leaving only the nuclear design to be verified. Figure 5 illustrates the integrated test assembly with Comet depicting the two extreme reflector positions using the lift table. Comet will make the test assembly go critical by raising the radial reflector around the core and provide the necessary reactivity to create the 4kWt steady state thermal power. The test assembly will incorporate a custom vacuum chamber specifically designed to fit on top of Comet and provide the relevant space environment throughout the nuclear testing. The KRUSTY test will verify and/or
1. Reactor Startup Operations
2. Excess reactivity needed to meet Kilopower thermal power and temperature requirements
3. Integral nuclear cross sections and temperature dependence
4. Reactor load following to Stirling convertor demands
5. ATLO assembly procedures
6. Steady state and transient differences between electrical and nuclear heat sources
7. Temperature feedback mechanisms and dynamic response
8. Operational stability for follow-on engineering-unit nuclear tests
9. Nuclear design tools such as FRINK and MRPLOW
Developing a small fission power system for NASA’s science and human exploration is an endeavor worth taking with potential to open up a new class of missions not currently achievable with radioisotope and solar power sources. An affordable approach to addressing many engineering risks of a future flight development program have been proposed to take approximately three years and ten million dollars using a test plan that progresses through increasing levels of hardware fidelity leading up to a full nuclear ground test nicknamed KRUSTY. This development project of Kilopower will provide extensive science and engineering data not attained in the last five decades of U.S. space reactor programs.
Starting with the lower power 4kWt reactor core for the first nuclear demonstration is extremely important to keeping development costs at an affordable level. Nuclear testing costs are directly proportional to reactor thermal power and the 4 kWt design allows the testing to take place at existing facilities under current regulations and licensing at the Nevada Test Site. By design, the lower power demonstration offers a subscale test of a 10 kWe capability, adding considerable value to both science and human exploration needs and paving the way for future higher power systems. Successful nuclear testing of the Kilopower reactor will help fill the existing technology gap of compact power systems in the 1-10 kWe range enabling new higher power NASA science and human exploration missions.
Kilopower, NASA’s Small Fission Power System for Science and Human Exploration (PDF Download Available). Available from: https://www.researchgate.net/publication/269208033_Kilopower_NASA%27s_Small_Fission_Power_System_for_Science_and_Human_Exploration [accessed Jan 19 2018].