Kilopower is the latest nuclear power source for space missions, developed by NASA, the Department of Energy, and the National Nuclear Security Administration. For now, this will be a copy of the blog posts on the system that I have written, however, in the coming weeks and months it will be reworked into a full writeup of not just the DUFF and KRUSTY tests, but the fully realized system, potential missions, and developments as the system is optimized, missions selected, and (hopefully) deployment of the first successful test of a unique reactor design for any purpose in more than 40 years in the United States.

This page offers a general overview of the Kilopower reactor. If you’re interested in more in-depth information on the development and testing of this reactor, check out our blog posts on DUFF, KRUSTY, and the criticality testing that occurred in early 2017, the biggest hurdle to having this incredible reactor design ready to fly!

For information on the two rounds of major testing that went into developing Kilopower, check out the pages for those tests:

Kilopower: The Basics

Reactor Core and Fuel

Kilopower’s 1 kWe nuclear reactor (the one to be directly tested with KRUSTY) uses metal fuel cast as a single cylinder 11 cm in diameter, with a 4 cm central hole for the test stand and cutouts to accommodate the heat pipes along the periphery. This is made out of uranium-molybdenum alloy, 92% uranium by weight (enriched to 95% 235U), and 8% molybdenum (also by weight). Y12 has extensive experience dealing with this particular fuel form, so it’s well understood both from a reactor physics perspective and from a fabrication and manufacture perspective.

The geometry of the reactor means that there’s a strong negative temperature reactivity coefficient, meaning that this reactor is largely self-regulating and the control rod is only needed for startup, shutdown, or major power level changes. Concepts for increasing the power level of this reactor have heat pipe channels inside the U-Mo fuel as well, but we’ll look at that more later.

There are minor changes to the geometry of the core for the test, but mainly they are to accommodate the reflector being raised around the core rather than a control rod being used to start the fission reaction.

Neutron Reflector

The reactor has two axial neutron reflectors and one radial reflector, all made of beryllium, totaling 70.5 kg. The radial reflector is a frustrum, or truncated cone, with an overall diameter of 27 cm at its widest point, and a cutout to accommodate the core and heat pipes running axially down its center.

This is the part of the reactor that underwent the biggest change from flight configuration to testing configuration. In short, for the KRUSTY test, the reflector is separated from the rest of the reactor, and will be lifted around the core to initiate the fission reaction. 

Primary Coolant

This is one of the new and exciting things about this reactor. Most reactors previously have relied on cooling loops driven by pumps, either mechanical or electromagnetic for some ferrous fluids. Often this working fluid has been sodium, which has been extensively tested for the Liquid Metal Fast Breeder Reactor (lately the Integral Fast Reactor or IFR), as well as for military and civilian power projects around the world. Here on Earth, combustibility concerns due to violent reactions with water severely limit its use, but this isn’t nearly as much of a problem in space, where there’s no water or atmosphere to cause problems.

Sodium is still the working fluid for this reactor, but the way it’s moved has changed. The heat pipe doesn’t require any moving parts to function, instead relying on convection and wicking action, and is dirt simple in construction: no pumps, and very little in the way of painstaking welding of different sections of pipe. As long as evaporation and capillary action are balanced in the heat pipe, it’s happy.

In this case, NASA is using sodium heat pipes made out of Haynes 230 alloy (Nickel-chromium- tungsten-molybdenum). These heat pipes have an outer diameter of 1.59 cm, and an internal diameter of 1.4 cm, and mass 4.1 kg.This gives it an operating temperature of 500 to 1100 C, and have operated for over 20,000 hours without sign of degradation. The contractor to build the heat pipes for KRUSTY and Kilopower is Advanced Cooling Technologies, or ACT.

Radiation Shielding

In order to shield the rest of the spacecraft, including the power conversion system and the radiators, from the reactor core, stacked depleted uranium and lithium-hydride plates are placed in between the reactor and everything else. In total, 40.4 kg of LiH and 45.3 kg of DU are used for shielding the reactor.

Power Conversion

Here’s the other exciting part of this reactor: the power conversion system (PCS). Stirling engines are simple, reliable, and can theoretically reach high energies, but have never been used in real-world applications. However, space has unique challenges and demands, and simplicity is one of the biggest requirements for a system. Materials conversion options offer no moving parts, but also low efficiencies, and the Rankine and Brayton cycle options are complex and heavy. So, NASA turned to the Stirling engine as a simple way to gain more efficiency while minimizing the amount of complexity and number of moving parts.

Being NASA, and being leery of any unnecessary moving parts, they’ve tested these Stirling convertors for over 30,000 hours as part of their Advanced Stirling Radioisotope Generator and Fission Surface Power programs. Manufactured by Sunpower, Inc., these eight free-piston Stirling engines will produce from 1 to 100 kilowatts of electricity (kWe).

Thermal Management

Kilopower’s radiators for the 1 kWe space design are made out of titanium-water heat pipes, with panels of carbon fiber to protect the heat pipes. Radiators for surface operations have also been designed, working off data and design lessons from the FSP program. As with the Ha230-K heat pipes, ACT, Inc is the contractor to supply these heat pipes

A New Family of Reactors

This is just the first step for the program, however. The 1 kWe design is the smallest one that is considered practicable for a fission power system, but the basic design concept can be extended up to 40 kWe. While the basic reactor is able to produce up to about 4 kWe, any additional power increase starts to exceed the heat transfer limits of the heat pipes. In order to increase the amount of heat transferred from the core to the power conversion system, changes are needed to the heat pipe system.

The first option is to move the heat pipes from the periphery of the core to the interior. This is harder than it looks at first glance. The different components of the heat pipe affect the reactivity of the reactor in a number of different ways, which are best assessed “in the wild” with the 1-4 kWt design before moving on to this change.

The advantage to this is that a greater surface area of the heat pipe is exposed to the fuel element generating the heat, so more heat can be transported using the same heat pipes, or even by using smaller pipes (in this case, there’s a reduction in major diameter from 3/8” to 1/2”, and an increase in number to 12). By moving the heat pipes from the periphery to the center of the fuel element, power can be boosted to 13 kWt using only two more kg of 235U, and only 24 kg more in reactor mass.

The larger sizes continue this trend, increasing the number of heat pipes in the core to increase the amount of heat removal. Because the reactor has a strong negative thermal reactivity coefficient, it has a corresponding tendency to increase reactivity as heat is more completely removed, increasing the power output of the reactor. The configuration and size of the heat pipes is based on Monte Carlo and thermal conductivity modeling to ensure that the temperature gradient across the fuel is acceptable, even with heat pipe failure.

With the testing of KRUSTY, enough information will be gained both in reactor engineering and in fuel element manufacturing to enable the internal heat pipes. Additional expansions of the test area at the Nevada site will allow for this expansion of reactor sizes to allow for ground testing of these larger reactors (to test KRUSTY, a site regulation had to be changed to account for the amount of reactivity being inserted into the reactor).

At this point, the fuel that is used in KRUSTY, the UMo metal fuel, can’t be used anymore. There are issues of critical density of fuel, power levelling across a monolithic fuel element, and other issues mean that metal fuel can’t be used for a larger reactor. Metal fuel is relatively rare in reactor designs on Earth, oxide fuels being more common. This is also an option for a heat pipe cooled reactor, and this is a very attractive small modular reactor in its own right.

This is the Megapower concept, a concept being explored by the Department of Defense for forward operating bases, disaster relief, and other missions where the lack of a supply chain for electrical power is critical.

Kilopower and Low Enriched Uranium

As we’ve discussed in the low enriched uranium nuclear thermal propulsion (LEU NTP) posts, the US government has endeavored since 2008 to eliminate the use of highly enriched uranium in all civilian, and many military, reactors. Both KRUSTY and the baseline Kilopower reactor use highly enriched uranium (HEU), but as McClure and Poston point out in a paper from November of last year (available here), the core can be redesigned to use low enriched uranium (LEU) – but there’s a price. Not only will the reactor become significantly larger and massier, but there are additional thermal limitations on the core as well. These limitations would make a failed heat pipe (a challenge that KRUSTY handled brilliantly) a much more significant challenge. As McClure pointed out in an email: “The thermal issues can be dealt with (we have designs) but it is not nearly as elegant as the HEU designs.” The plan for the smaller reactors in the Kilopower group is to continue the use of HEU (and therefore the fast neutron spectrum) for simplicity’s sake. The larger reactor designs (like the ones that would be used to manufacture rocket fuel on Mars) may end up utilizing a moderator in order to soften the neutron spectrum. Apparently, they’ve been looking at a metal moderator (yttrium hydride, YH) for the larger designs, but hydrogen leakage is an ever present concern (as we saw during Project Rover, and is a major component of the propellant tank design for the LEU NTP stage – that’s exactly what I’m writing about on the NTP stage now!), because this leads to a loss of moderation, and therefore reactivity. In addition, this induces a new thermal limitation to the core, and the thermal gradients within the core will be different as well.

These issues will require additional research and testing, probably including fission powered testing along the lines of the successful test announced today, although possibly more extensive (to test hydrogen migration over longer time periods at different power levels). Fortunately, since the design would use LEU, the testing could be done at a non-DOE facility, significantly reducing the cost and regulatory hurdles of the test (a key point in the LEU NTP program as well).

Other LEU options exist as well, and were examined in that paper.

Various Options for low enriched uranium vs highly enriched uranium, ractor core and reflector size implications. Poston and McClure 2017

The first simply expands both the core and the reflector of the current U7Mo fueled reactor, the second replaces the molybdenum with natural uranium, and the third uses uranium zirconium hydride (UzrC) as the fuel element matrix. Each ahs a different impact on the mass of not only the reactor core and reflector, but also the radiation shield and other components.

Poston and McClure, 2017
Poston and McClure, 2017

There are some minor differences between surface and space reactor designs. Here’s the cutaways and masses of the equivalent 10 kWe Mars surface systems:

Private Companies and Kilopower

This leads directly into private companies, rather than the DOE, continuing development, and deploying, Kilopower-derived reactors for space missions. There has been some speculation about SpaceX (or some other private company) using Kilopower for in situ resource utilization (ISRU) techniques on Mars or some other extraterrestrial body.

Due to the restrictions on HEU, any private company looking to develop and use this technology basically has to use LEU. It may be possible for a private company to use HEU (BWXT does this for naval reactor fuel elements, for instance), but the company does not have full control of the design of the reactor (for national security reasons), and any portion of the reactor build involving HEU would have to be done at a DOE facility, increasing the cost and lengthening the development timetable due to the limited resources of the DOE.

An additional concern, for a long-term crewed base, would be non-proliferation. HEU us considered special nuclear material, something that the international nuclear community watches closely. However, even at these higher enrichment levels, the necessary enrichment to go from HEU to weapons grade uranium is, in fact, the hardest (and most equipment-intensive) step in the enrichment process, so this is more of a regulatory straw man than a legitimate concern.

To my knowledge, though, there aren’t any private companies looking to license this technology right now, and for SpaceX in particular, other than a couple of tweets, there hasn’t been any interest shown by the company in nuclear technology of any sort.

The Future of Kilopower at NASA

Currently, NASA and the DOE are examining possibilities for a technology demonstration mission, which is the first step toward widespread deployment of this system. According to McClure, this process is still in the early phases of mission definition. One possibility, a lunar mission (either on the surface or possibly, but far less likely, with the LOP-G, the international lunar space station formerly known as the Deep Space Gateway). However, it is still unclear what the future of the upcoming lunar mission proposals is, and NASA is still waiting for a private company to develop a lunar lander that would be able to complete the missions that NASA is interested in.

Current efforts are focused on a lunar surface mission for the first flight mission of Kilopower. One major complication is that there isn’t a lander currently under development that’s able to deliver the reactor to the surface. NASA is currently going through the process of commissioning such a lander, but the timeline on the program is still very much in the air.

Additional work is being done on a common orbiter bus for the reactor, similar to how Cassini and Galileo or Curiosity and Mars 2020 use most of the same components, with different experimental payloads. This is an area I hope to do more research on in the near future, hopefully by early 2019 I’ll be able to provide more information on this concept.


Nuclear power is the key to enabling more effective autonomous and crewed exploration, and eventually colonization, of the solar system. Kilopower is the first in a range of nuclear reactors for electricity production that NASA is looking to deploy on future missions. We will look at the others in the next few posts; the next will be on Fission Surface Power (reading up on that system somewhat delayed this post, as did a family reunion and changing jobs), followed by Project Prometheus, and finally the drive system for the Jupiter Icy Moons Observer (JIMO), the final design selected out of Prometheus.

If you have any comments, questions, or corrections, please leave them below.


A wonderful resource for those interested in the beginnings of Kilopower is Dr. David Poston’s personal blog, SpaceNuke (, mostly written before the DUFF experiment. There’s a lot of insight into the design philosophy behind the reactor, and also into the difficulties of developing nuclear fission systems for in-space use. I can’t recommend it highly enough.

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