KRUSTY: We Have Fission! Kilopower part III

Hello, and welcome back to Beyond NERVA! Today, we return to the KRUSTY test, and the Kilopower system, being presented today by NASA, the Department of Energy, and the National Nuclear Security Site.

As we’ve seen before, KRUSTY is the testbed for the Kilopower reactor, developed by Los Alamos as a small, simple nuclear reactor meant for space missions (although it also has terrestrial uses as well, and two companies have proposed similar, but larger architectures since: Oklo Power and Westinghouse). After an initial proof of concept fission test (DUFF), KRUSTY was designed and built by NASA (at the Glenn Research Center in Cleveland), and the Department of Energy (Y12 in Tennessee fabricated the core, and Los Alamos was the lead design site), and just last month completed fission powered testing at the National Nuclear Security Site (NNSS) in Nevada.

As mentioned in my previous post on KRUSTY (which you can read here), a number of nuclear tests have been conducted over the last 6 months, culminating in the full-power critical test on March 20th and 21st. These initial tests were component criticality tests to ensure the neutronic characteristics of various components (fuel, neutron reflector, shield, and startup rod), cold critical testing (which added the rest of the components of KRUSTY, including the heat pipes, clamps, insulation, vacuum vessel, and more), and warm critical (where heat was generated through fission in the reactor core, but at low power, to verify reactor dynamics) testing were completed at the NNSS.

The warm critical test demonstrated that the reactor dynamics were well within acceptable operational margins, and gave the all clear for full power testing. This test produced fission power for just over 6 hours to wait for the oscillations of the reactor startup to dampen, and the system to stabilize in normal operation. One interesting note on the warm critical test is that so little power was being drawn (100 W), the oscillations of fission power and core temperature after startup were very slow, almost 75 minutes long. This is to be expected at these low energies, and isn’t typical of a full power startup (as we’ll see in the full power test).

60 cent run
Warm critical test results (60 cent reactivity insertion), Image courtesy DOE

Full Power Test

This is the thing that nuclear engineers have been looking forward to since the 1970’s: the first fission-powered full system test of a space reactor since the 1970s, with SNAP-10a. The wait has been long, but the wait couldn’t have been ended with better test results!

On March 20th, KRUSTY’s core was lowered into the neutron reflector on the Comet test stand once again, beginning the 28 hour full power test. The series of reactor dynamics and simulated equipment failure tests conducted was the same as the electrical heating profile used with the depleted uranium core at NASA’s Glenn Research Center [insert lab name], and the test results show that the modeling that the earlier (non-nuclear) test profile very closely matched the results that are being released today.

First, let’s look at the results of the electrical test and fission test side by side, and look at the individual parts of the test:

Full power DU Results
Electrically Heated DU Core Test, NASA GRC (based on modeling of KRUSTY behavior), image courtesy DOE
Full Power test Result Slide
Fission-powered Full Power Test, image courtesy DOE

KRUSTY achieved full fission and Stirling power in the first hour, and the reactor temperature increased to about 850 C. Since the test profile was designed for 800 C (the slightly higher temperatures weren’t a significant issue, but it’s best to be as accurate as possible), the reactivity of the core was adjusted after about 6 hours to meet the target temperature over the course of the next hour.

Eight hours in, they started playing with things: First, the power drawn from the Stirlings was reduced to 60%, resulting in a small (less than 25 C) fluctuation in core temperature and about 750 W fission power reduction in the core. After an hour, the Stirlings were returned to full power, and then an hour later the Stirling simulators were cranked up to 200% power. This resulted in a large (~1200 W) increase in fission power being produced by the core. An hour later, the reactor was once again returned to nominal full power operating conditions.

Now they started (simulatedly) breaking the heat removal and power conversion systems: First, they simulated a single Stirling failure, resulting in a dip in fission power (and, if I’m reading the graph right, a slight increase in the heat pipe temperature, which I suspect is the blue line on the graph – but the temperature points aren’t labelled, so I can’t be sure). After another hour, they proceeded to remove another Stirling from operation, with similar results. In both cases, the reactor temperature only slightly varied from its’ nominal 800 C temperature.

Another hour of nominal operation, and the Stirlings were once again cranked up to 200% power, with effectively identical results to the first time this was done about 3 hours before. After another return to nominal operation, a series of tests to simulate control rod adjustment were done, including what looks to be (simulated) almost full removal of the control rod 18 hours into the test (this would actually be full insertion of the core into the reflector), resulting in a huge (2500 W) spike in power in the reactor core. Once again, the reactor temperature remained well within the acceptable bounds of the test, despite the rather severe adjustments being made to the amount of reactivity in the core.

With another return to normal operation, they killed most of the heat removal, resulting in a 1500 W drop in fission power – a wonderful demonstration of the strong negative thermal reactivity coefficient that makes Kilopower such an appealing design from a reactor physics point of view. Two and a half hours later the heat removal was eliminated as much as was possible. This resulted in a further, but smaller, drop in fission power being produced. An hour later, two of the Stirlings were restarted, and after the power transients dampened down, the last six were restarted as well, with corresponding increases in fission power.

27 hours after the beginning of the test, all heat removal was once again killed for the core, returning the fission power to the ~1500 W that were produced in the earlier simulation of this situation. An hour later, the reactor was scrammed (all reactivity removed), and the reactor was left to cool down.

Based on the test profile that was designed by the NNSS, KRUSTY was then set aside for the shorter-lived (and therefore more dangerous) fission products to decay.

This highly successful test shows that KRUSTY performed exactly as expected, and that Kilopower is ready for the next step in its’ development: the construction of a flight article for the first new astronuclear reactor design in the US for close to 50 years. Considering all the design and testing work for this system has cost less than $20 million dollars, this is nothing short of an epic achievement on the part of Drs. Patrick McClure, David Poston, and the rest of the LANL space nuclear reactor design team, as well as Marc Gibson at NASA’s Glenn Research Center, and everyone else involved in the program.

Further Development of Kilopower

KRUSTY is a major milestone for US in-space reactor development, but Kilopower has a lot more to offer than just the small 1 kilowatt (electric, kWe) reactor that KRUSTY proved the design of.

The first thing the Kilopower program offers is more power. As a system architecture, Kilopower has four different sizes of reactor, ranging from 1 to 40 kWe, for everything from small, electrically propelled deep space probes to in situ resource utilization and power supply for manned missions, both on planetary and orbital missions.




By moving or adding additional heat pipes, upgrading the power conversion system to match, and increasing the reactor core size, much more power can be drawn out of the potential core configurations of this flexible design architecture. Of course, changing the pattern of heat removal affects the thermal gradients (hot and cold spots) of the fuel element. In this case, the entire core is one single fuel element (known as a monolithic core), an unusual arrangement for a nuclear reactor, so the behavior of this type of reactor isn’t as well studied and understood as the more common type of reactor with many separate fuel elements.

However, Patrick McClure, the head of the Kilopower program at Los Alamos National Laboratory, is confident that any additional testing that is needed to verify the thermodynamic behavior of these larger and more complex designs can be done through electrical heating, similar to what was done at NASA Glenn with the depleted uranium dummy core for the electrical heating test (see the previous KRUSTY post for details on that test), without further fission-powered testing. This means that further development of the larger reactors can be done at only a modest increase in program cost.

Another thermal concern that is common in reactors is known as edge heating, where the edges of the reactor core (or individual fuel elements) are hotter than the center. This is often (including for Kilopower) due to the moderated neutrons being reflected back into the reactor core.

Radial core power distribution
Radial core power deposition of Kilopower core, largely reflecting thermal loading

Depending on what materials the fuel elements and core structure are made out of, this can become a limiting factor for heat rejection (and therefore power extraction) in a nuclear reactor. In the case of KRUSTY and the smallest Kilopower reactor, the heat pipes are placed along the edge of the core, where the problem is the worst, but all other designs have the heat pipes internal to the reactor core. Fortunately, Kilopower’s uranium-molybdenum alloy fuel element (U7Mo) has both high thermal conductivity and high thermal limitations, so this isn’t a major concern in this group of reactor cores; however, changes in the fuel element type (for instance, using oxide fuel as is proposed for the Megapower derivative), or the addition of a thermally limited neutron moderator, can make this a much larger issue.

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.

LEU Reactor Cutaways
Some LEU Kilopower reactor core options, 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.

1 kWe LEU masses
1 kWe space-optimized system mass estimates, Poston and McClure 2017
10 kwe LEU masses
10 kWe space-optimized system masses, 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:

10 kwe martian system cross section
10 kWe Mars-optimized reactor cutaways, Poston and McClure 2017
10 kwe martian system masses
10 kWe ISRU Martian demonstration mass estimates, Poston and McClure 2017

It’s unclear right now why the YH moderated core is the current frontrunner for a LEU design at Los Alamos, however I hope to be able to find out, and will update this when that information is available.

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.

At the end of the KRUSTY post, I looked at many of the possible initial missions that could utilize Kilopower. For those that did not see that, I’ll cut and paste it here. Unfortunately, I have not been able to expand on the mission profiles, and I’m certain there have been certain changes in priorities and likelihood of any particular mission coming to fruition, but the variety of missions gives a good idea of the diverse missions that would benefit from this truly game-changing technology

Most of these missions did not incorporate a nuclear reactor as part of their power supply options so often the mission changes from what was originally proposed to account for the reactor. In fact, they were all powered by multiple RTGs, as Cassini was (three MMRTGs), which don’t scale well as a general rule. Even if a mission had planned for a reactor, the specific data about this reactor firms up questions that were left in the original design study.

Titan Saturn System Mission (TSSM)

TSSM Artist's Impression, NASA
Artist’s Impression of TSSM with RTG, image via NASA

This was a design from a 2010 decadal survey design, re-examined by the Collaborative Modeling for Parametric Assessment for Space Systems (COMPASS) in 2014. Originally designed with a 500 W ASRG, a 1 kWe Kilopower reactor was installed instead in the 2014 study. This is a good example of the tradeoffs that are considered when looking at different power supplies: there’s less mass and a shorter trip time for the original, RTG-based electric propulsion spacecraft, but the fission power supply (the reactor) allows for more power for instruments and communications, allowing for real-time, continuous communications at a higher bandwidth while allowing higher-resolution imaging due to the increased power available.


TSSM FPS config
TSSM Kilopower variant, NASA GRC

As with the following concepts, this was a mission that was briefly looked at as an option for a mission to use the Kilopower reactor, not a mission designed with the Kilopower reactor in mind from the outset. The short development time of the reactor (I never thought I’d write those words…), combined with the newness of the capability, caught NASA a bit flat-footed in the mission planning area, so not all the implications of this change in power supply have been analyzed.


TSSM Baseline Mission Graphic
Mission vehicle breakdown, NASA/ESA

The mission as designed is impressive: not only is there an orbiter, but a lander (to be designed by ESA, who have already successfully landed on Titan with the Huygens probe), and as a buoyant cherry on top, a balloon for atmospheric study as well.


These low-power missions are where any new in-space power plant will be tested, to ensure a TRL high enough for crewed missions. Because of this, I’m going to be adding mission pages to the website over time, with this being the first, looking at these nuclear-powered probes is the best way to see what could be coming down the pipeline in the near future.

TSSM Mission Summary Timeline
Chart courtesy ESA

Here’s the published papers on the mission:


Chiron Orbiter

Chiron Orbiter FP configThis design is for a flyby of 2060 Chiron, a Centaur-class asteriod. Originally proposed in 2004 as part of a study of radioisotope electric propulsion across the solar system, it was re-examined in 2014 by the COMPASS team.

This is a mission that I’m very interested in, but unfortunately not much has been published on it:

Kuiper Belt Object Orbiter (KBOO):

KBOO on orbit, NASA
Kuiper Belt Object Orbiter, MMRTG variant, image via NASA

A close cousin to the Chiron Observer, the KBOO was originally a RPS-powered mission which used an incredible 9 ASRGs, with a total power output of a little over 4 kWe, to examine an as-yet undetermined target in the Kuiper Belt. Having access to nuclear power is a requirement that far into the solar system, and with Kilopower not only is the mission not power-constrained, but is able to increase the amount of bandwidth available for data, and the power will allow for radar surveys of the objects that KBOO will do flybys of.


KBOO Components, NASA

Jupiter-Europa Orbiter

JEO Artists Rendering
Jupiter-Europa Orbiter MMRTG variant, NASA

A predecessor to the Europa Clipper, the JEO was originally designed with 5 MMRTGs (the equivalent of 1 ASRG, 500 We). However, the design could have double the available power, and much higher data return rates and better data collection capabilities, if a 1 kWe reactor was used. This would increase the power plant mass (at 260 kg for the MMRTGs) by an additional 360 kg, but this would also eliminate the need for Pu-238, which remains very difficult to get a hold of.


JEO Planned Payload
Note: This is initial planned payload, not accounting for the higher power from Kilopower. Image via ESA

The Europa Clipper is based on a more economical version of this mission, the Europa Multiple Flyby Mission, and has some of the same hardware.

Here’s the published papers on the mission:

Human Exploration Missions

While this is certainly smaller than the power requirements for many crewed surface missions, Kilopower has been designed with crewed surface missions in mind. The orientation of the heat pipes has already been tested, and will be tested more thoroughlly at NNSS (when held vertical in a gravity field, the heat pipe acts as a thermosyphon, increasing how much heat the pipes can reject). This reactor could certainly be used for manned space missions as well, but only for what’s known as “hotel load,” not for providing large amounts of electrical power for an electric drive system (we’ll get to that in a couple blog posts). As such, it’s typically seen being used in crewed missions as a modular power unit, with more reactors added as the base grows to keep up with increased power demand.

Kilopower vs FPS


Kilopower integrates fairly easily into NASA’s Design Reference Architecture 5.0, the “Handbook for Getting to Mars” as it were. This is a multiphase program.

Phase 1 launches before humans ever leave Earth, for ISRU, and will either be solar or fission powered. The trade-off between the systems mass and time required for refueling: more fuel and water can be extracted faster using Kilopower, but it masses more than solar panels (after factoring in the full power production system). Phase 2 is the beginning of crewed missions. In this case, a NASA study showed significant mass savings due to energy storage costs over solar.

Surface deployment, Mars NASA


The fundamental advantage on the Moon for fission power systems is the lack of energy storage requirements for the lunar night. The Fission Surface Power program was, in fact, primarily oriented at use with manned Lunar (and later Martian) missions. Kilopower will be able to operate well in these environments, if only offering up to 40 kWe of power (which is where FSP takes over). The study above looks at Lunar mission options and requirements as well.



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.

If an image doesn’t have credit, it’s from NASA or the DOE, from one of the sources below.

I’m going to break this up into KRUSTY and Kilopower sections, organized chronologically. The KRUSTY papers tend to be focused more on the reactor physics and hardware testing side, and are a great source for more detailed information about the reactor. The Kilopower papers and presentations are bigger-picture, and focus more on missions and policy.


KRUSTY Experiment Nuclear Design, presentation by Poston et al, Los Alamos NL, July 2015

Kilowatt Reactor Using Stirling TechnologY (KRUSTY) Demonstration: CEDT Phase 1 Preliminary Design Documentation, Sanchez et al, Los Alamos NL, Aug 2015

KRUSTY Design and Modelling, presentation by Poston for KRUSTY Program review, Los Alamos NL, Nov 2016

NCERC Kilowatt Reactor Using Stirling TechnologY (KRUSTY) Experiment Update: March 2017, presentation by Sanchez of LANL, Mar 2017

Electrically Heated Testing of the Kilowatt Reactor Using Stirling TechnologY (KRUSTY) Experiment Using a Depleted Uranium Core, Briggs et al, NASA GRC, July 2017


Design and Testing of Small Nuclear Reactors for Defense and Space Applications, presentation for American Nuclear Society Trinity Section, McClure and Poston, Los Alamos NL Sept 2013

Development of NASA’s Small Fission Power System for Science and Human Exploration, conference paper by Gibson et al, NASA GRC, for AIAA Propulsion and Energy Forum, July 2014

Nuclear Systems Kilopower Overview for Game Changing Development Program, presentation by Palac et al, NASA GRC, Feb 2016

Space Nuclear Reactor Development, McClure et al, Los Alamos NL technical report, Mar 2017

Space Nuclear Reactor Engineering, Nuclear Engineering Capability Review, presentation by Poston, Los Alamos NL, Mar 2017

NASA’s Kilopower Reactor Development and the Path to Higher Power Missions, Gibson et al NASA GRC, conference paper for IEEE Aerospace Conference, Mar 2017

Use of LEU in a Space Reactor, Poston and McClure, Los Alamos National Lab, White Paper, Nov 2017

Other related sources

Summary of Test Results From a 1 kWe-Class Free-Piston Stirling Power Convertor Integrated With a Pumped NaK Loop, Briggs et al NASA GRC, Dec 2010

High Temperature Water Heat Pipes for Kilopower System, Beard et al, Advanced Cooling Technologies, conference paper IECEC 2017

Considerations for Launching a Nuclear Fission Reactor for Space-Based Missions, Voss et al, Global Nuclear Network Analysis LLC, conference paper for AIAA SPACE Forum, Sept 2017

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13 Responses

  1. This is a great result. One interesting thing worth noting about low enriched uranium fuel is that it might be illegal to use under UN space law. UN space law states that space based nuclear reactors are only allowed to use highly enriched uranium 235 as fuel:
    This could change though, the original intent of this rule was to prevent space based reactors from using plutonium. This came about as the drafters were not aware that other nuclear fuels existed or what uranium enrichment was.

    1. Interesting! I hadn’t actually read through this before.

      I agree that this is something that is likely to be changeable, and possibly might end up not being enforced due to the IAEA focus on reducing the use of HEU worldwide… but one thing you don’t really play close to the edge in international law is nuclear material.

  2. Awesome post! I’m excited about Kilopower. You mentioned that simething called “FSP” would take over for mission designs greater than 40 kilowatt hours electric. What is FSP?

    1. The Fission Surface Power System was an earlier, and quite similar, heat pipe cooled reactor, but it required a multi stage radiator system that ate the project’s entire budget to half test. The lessons learned directly informed Kilopower design.

      Check out Dave Poston’s blog for some links, and NASA TDRS for additional info (I’m not at my computer, or I’d link them).

  3. Very fascinating report! It occurs to me that the use of heat pipes being adopted in more powerful micro-reactors like eVinci are partly a spinoff of this development process. Given the simplicity, reliability and self-regulating safety features of this system, it must be mass-producible. Instead of worrying about creating 10 Megawatt class reactors for later generations of much larger Moon bases, why not just have a large bank of these smaller, reliable ones. For example, for a large lunar base, why not have a bank of 100 of these things lined up?

    1. Because fission scales down poorly, but it scales UP really well. A reactor 2x the power is going to mass far less than 2x as much, and a reactor with 100x the power may only be 20% massier… depending on an insane number of variables at least.

      You also get power balancing and control advantages going up… to a certain extent at least.

  4. Very cool! I’m pretty new to all of this… do you the passive heat transfer systems would have trouble in null-g?

    1. None whatsoever. I’m not sure if you’re talking about the heat pipes or the decay heat removal, so I’ll address both.

      Heat pipes have been used in space for decades, and if you can turn whatever you’re reading this on upside down they work in -1 gee (and in 0 gee). They’ve been used for mission critical components on the Moon, ISS, Mars, and more. You do the math right, and both the primary and secondary heat pipe loops are golden.

      It’s pure thermal conduction and radiation: the rest of it heats up to a greater or lesser extent, and photons are emitted at the relevant energies and wavelengths from the heated components.

      There MIGHT be some issues with restarting a critically damaged power supply (if a meteorite smashed four of the heat pipes, for instance), but for “normal” emergency shutdown, and even for massive mechanical damage, it’s designed to handle the waste heat just from surface area and conductivity. If you can reduce it to a size where it CAN’T radiate the heat fast enough through surface area… well, you got bigger problems than trying to get the thing restarted.

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