KRUSTY: First of a New Breed of Reactors, Kilopower Part II

**UPDATE: full power fission test is compete! If you’re looking for a description of the reactor, this is the post to read. If you want to know how the test went (except for it went perfectly), check out part 3 here:

Hello, and welcome back to the Beyond NERVA blog, and the second installment in our series on NASA’s current plans for in-space nuclear reactors. Last time, we looked at the experiments leading up to the development of NASA and the Department of Energy’s newest reactor. Today, we’re looking at the reactor that will be tested later this year (2017), and the reactors that will follow that test. We have two more installments after this, on larger power systems that NASA has planned and done non-nuclear testing on, but can’t continue due to testing and regulatory limitations. These are the Fission Surface Power program and Project Prometheus.

As we saw in the last post, in-space nuclear reactors have been flown before, mainly by the USSR, and their development in the West has stalled in terms of testing since the 1970s. However, a recent (2012) test at the National Nuclear Security site by scientists and engineers from the Department of Energy (DOE) and NASA, the Desktop Using Flattop Fission test (DUFF), has breathed new life into the program by demonstrating new heat transport and power conversion techniques with a nuclear reactor for the first time.

Now, the results of this experiment are being used to finalize the design and move forward with a new reactor, the Kilowatt Reactor Utilizing Stirling TechnologY, or KRUSTY. This is an incredibly simple small nuclear reactor being developed by Los Alamos National Laboratory (LANL) for the DOE, and Glenn Research Center (GRC) and Marshall Spaceflight Center (MSFC) for NASA.

Since we’ve seen what’s been planned and tested in the past, let’s look at the next big step for in-space nuclear reactors!

KRUSTY: The Little Reactor that Could

KRUSTY cutaway, image courtesy LLNL

As we saw in the last post, there are many hurdles to getting a new nuclear reactor developed and funded. One of the biggest is lack of interest (and therefore funding) from the DOE and NASA. Between the size limitations of current test stands, the expense of new stands, and the regulatory and safety limitations on nuclear testing, development of new nuclear reactors has always been under major constraints. Then, engineers at Los Alamos realized that there was a hole in the range of operational and planned in-space power systems, between the Advanced Stirling Radioisotope Generator (1kWe) and the Fission Surface Power reactor (40kWe),. Because of the new reactor’s small size, it could be tested in current facilities, and there are plenty of missions that fit into that power level. Larger units can easily be built based on the data gathered from the test of the 1 kWe design.

What’s so special about this reactor? Well, in summary, it’s a small reactor that uses heat pipes to transfer the heat out of the reactor core instead of a pumped liquid, and this heat runs to a Stirling power conversion unit. We discuss the theory behind this in the previous post, and we’ll look at the application more in a little while, but this is the first reactor that uses this common, but not well-known, technology, both for heat transport and power conversion.

Let’s look at the basics of the flight reactor concept before we get into the work that’s already been done, and the work that will be done by the end of 2017:

The Core

LANL Kilopower screencap core highlight

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.

The Reflector

LANL Kilopower screencap Core Reflector highlight

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, the reflector is separated from the rest of the reactor, and will be lifted around the core to initiate the fission reaction. More on this, and its implications for the test and the flight article, later.

The Heat Pipes

LANL Kilopower screencap Heat Pipes

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 (as detailed in the last post), 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.


The Radiation Shield

LANL Kilopower screencap Rad ShieldIn 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.

The Power Conversion System

LANL Kilopower screencap Stirling closeup cutaway

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).

The Heat Rejection System

LANL Kilopower screencap 3

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. For KRUSTY, heat will be extracted using cryonic gas cooling to simulate the radiator structure.

Kilopower vs. KRUSTY

As we saw in the last post, testing nuclear reactors is difficult, if not practically impossible. Instead, individual components are tested thermally, chemically, and neutronically before an integrated test can be done. This involves thermal vacuum testing of all components, exposure of key components to beamlines at research reactors, and incredibly extensive modelling.

Often, once all this is done, the researchers are stuck for money. Funding for in-space nuclear systems is rare, much of that goes to radioisotope systems that are either already flight-proven or are improvements of the same basic architecture, and therefore low-risk investments. With a limited number of small testing facilities able to accommodate a test reactor, these larger designs were left out in the cold, not because they wouldn’t work, or due to insurmountable engineering challenges, but simply due to the cost of building new testing facilities. There are places that this could be done, such as the National Nuclear Security Site where Project Rover occurred, but scratch-building a facility to do environmental testing in a vacuum chamber for an operating nuclear reactor is far from cheap.

KRUSTY test article 3d cutaway LANL
KRUSTY mounted on COMET, image via LLNL

The difference here is that KRUSTY is small, both in size and in power output. Even accounting for a vacuum chamber’s weight, it is possible for current facilities to be used to test this reactor design. As we saw in the last post, there are critical assemblies that have been used since the 1950s to do benchmarked fission tests. There, we looked at Flattop, the spherical reactor used to prove heat pipe cooling of a reactor could be predictably modeled. This time, we’re going to meet COMET, another of these benchmark criticality test stands. COMET is not a reactor, nor does it have any nuclear components itself. Instead, it is designed to bring two different parts of a test together very precisely. It consists of a table with hydraulic presses and vernier adjustment that can handle having a sizeable test apparatus secured to it, and a central pillar to place the other part of the critical assembly being tested. COMET is even older than Flattop, having been used for criticality testing for the Little Boy gun-type atomic bomb. Since then, it was used at Los Alamos’ Pajarito test site until it was moved for that site’s decommissioning, along with the rest of the critical assemblies, to the National Criticality Experiments Research Center at the National Nuclear Security Site in Nevada.

The biggest difference between KRUSTY and Kilopower lies in the reflector of the reactor: rather than having a control cylinder that moves in a fixed reactor core, KRUSTY’s core, along with everything else but the reflector, is placed in a vacuum chamber, which is then mounted on the upper platen of Comet, and a reflector made out of discs of beryllium oxide (BeO) of various thicknesses will be raised around the core using Comet’s lift systems. By using different reflector thicknesses, how much reactivity is available can be changed, and the data collected can be used to finalize the reflector design for Kilopower.

Building KRUSTY: Prototyping and Non-Nuclear Testing

Compromises for the Budget

One of the impressive things about Kilopower is that so far they have managed to keep costs incredibly low. This is wonderful in that more work can be done for much less money, and it’s far more likely to fly when it’s able to be cheaply tested. As we’ve seen though, this means that compromising between the ideal, prototypic testing regime and the one that can be afforded. A great example of this is the Fission Surface Power program, where not only did they not manage to do a single nuclear test, but even the heat rejection system was only half-tested in a vacuum chamber.

This doesn’t mean that we learned nothing from that program, or that there wasn’t extensive testing completed on individual components. For instance, that program looked not only at Rankine power conversion systems, but carbon fiber radiators and heat pipe thermal management as well. Virtually every component in this reactor is similar: it’s been tested before, just not in this specific application. KRUSTY is that system-level series of tests that ensures the flight reactor will function as advertised

Building off this knowledge base and available, off-the-shelf technology, the team at GRC built a mockup of KRUSTY, using stainless steel (SS) instead of uranium for the core (these tests were detailed in the DU thermal testing report). They then did a number of thermal tests using electric resistance heaters to mimic the nuclear reaction, using the models of reactor behavior that the DOE (LANL and Y12, mainly) have shown to be the most likely behavior.

At the same time, a decision was reached that would save significant money for the development effort, but would lead to a change in the power conversion system for KRUSTY. Rather than purchase the full set of eight 125 W Stirling convertors that Kilopower would have, instead the engineers at GRC decided to reuse two 70 W Stirling convertors that had been built and tested for the ASRG, and replaced the other six Stirlings with thermal simulators that they designed for the purpose. This meant that the test wouldn’t be using a prototypic PCS, but this is less of a concern than for other components, especially the core and heat pipes. The PCS can be further refined and tested without the headaches and difficulties of dealing with a critical assembly.

Initial testing focused on the individual components that needed to be verified. This proceeded on two fronts: materials testing for unavailable or inconclusive data from past research, and subcomponent testing. The materials questions included fuel creep, thermal expansion coefficient of the fuel, and diffusion between the fuel and the heat pipes. The subcomponent tests focused on the heat pipes and their connection to the reactor core, ensuring that there would be enough heat transport available for the power conversion system.

The success of these tests led to a full scale thermal prototype test using the stainless steel core. This took place at NASA Glenn in the second half of 2015. These tests took place in a vacuum chamber at GRC, using electrical heaters to heat the core to a design temperature of 800 C while conducting a variety of tests. Of special note were the tests focusing on the heat pipe thermal transport capabilities. In one test, the cold end of the heat pipe was connected to an evaporator, which then discharged into the vacuum chamber that the test was taking place in. This test demonstrated that the Haynes 230 heat pipe was able to transfer 4 kWt over 1 m of distance, showing that the system was able to handle the thermal load required to keep the reactor cool.

With this latest set of successful tests complete, a new dummy core was made, this time out of depleted uranium (DU), to test for any chemical reactions, and for mechanical and thermal interfaces with the heat pipes and other components. This also marked the end to individual component testing for the components that would be used for the KRUSTY test, as they would be integrated with the modified power conversion system for testing with the new core in the last round of non-nuclear thermal testing.

Regulations Rear Their Head Again… But We Can Work With That!

DU core in hand
DU test article, image via DOE/LANL

DU is a more difficult substance to work with. This particular core simulator was produced at Y12, contained 8% molybdenum by weight, and the only difference between it and the KRUSTY core is the percentage of 235U (<15%). This is important to characterize thermal effects arising from the density and structural properties of the fuel, and also isolates any chemical, mechanical, or tooling issues with the manufacture of the HEU core to be used by KRUSTY later this year. This was a major step forward for the program, dotting the i’s and crossing the t’s before the nuclear test. This also allowed Y12 to make the molds and other tooling required for the HEU core, and verify that there wouldn’t be any issues in the manufacture.

This also forced a design decision on the power conversion team. Two ways had been discussed to mount the Stirling pistons: the first, and the final as well, is what’s called a dual convertor design. Here, the heat pipes are arranged in pairs radially, with their hot ends toward the center. By managing the stroke of the pistons, their actions cancel each other out from an overall inertia point of view. The alternative was to mount the pistons vertically, and run them in parallel. This requires an active structure to counterbalance the inertial force from the pistons, adding complexity. When it was time to finalize the design, the simpler dual convertor design won. Another compromise from the prototypic flight configuration was the heat rejection system: cold nitrogen gas would be used to remove heat from the cold end of the Stirling convertors and simulators. This allowed for a smaller test apparatus, and also allowed the 75 W Stirlings to simulate a system 50% more powerful.

Stirling PCU for KRUSTY, Mason 2011
Image via NASA

Due to its mild radioactivity, DU does require special handling as a nuclear material. While this is often “only” a major headache, in this case the Kilopower team looked upon it as an opportunity to get everyone together for a dress rehearsal before KRUSTY.

One of the reasons that NASA is looking to get away from using HEU is due to the security required for its storage and handling. The required personnel and organizational resources don’t come cheap, so the less time the fuel actually has to sit in the reactor on the ground, the better. Fortunately, since all nuclear spacecraft have the reactor as far away from the rest of the spacecraft as possible, and since the reactor core is a single piece, fueling can be held off until much later in the process of vehicle assembly and integration.

Neutronics is a tricky business. When first assembling a new nuclear reactor, there are many unknowns, and accidental criticality lurks behind seemingly innocuous mistakes. Because of this, it would be nice to run through an assembly process WITHOUT having to worry about having a nuclear reaction occur.

DU Integration pre clamp
Integration of heat pipes and DU core, NASA/LANL

So this is what everyone did. Personnel from NASA’s Glenn Research Center and Marshall Spaceflight Center (MSFC), Los Alamos (LANL), and the Device Assembly Facility (DAF) at the National Nuclear Security Site gathered for the first dress rehearsal for fueling the reactor. This way, any hitches found could be dealt with on this far simpler test, and everyone was able to run through their roles in preparation for the big day. No major problems were discovered, the core was installed, and KRUSTY was ready for its last round of non-nuclear testing.

One other note about the preparations for nuclear testing: using the DAF sets limitations on instrumentation and other conditions, for instance on coolant for the cold end of the Stirling engines. In order to make sure that there were no issues here, all the connections to the test stand at GRC were identical to the ones that would be used for KRUSTY at the DAF.

DU Core w clamps
DU core with integrated heat pipes, courtesy NASA

Final integration allowed for assessment of the DU core’s interface with the rest of the reactor, especially the mechanical and thermal connection with the heat pipes. This is one of those critical areas that can be well estimated and modeled, but unless it’s experimentally verified with flight-like components there will be unanswerable questions. Another is the possibility of chemical reactions. With no major problems discovered, testing moved on to preparing for the nuclear test.

A final benefit gained from this test rehearsal is the ability to better estimate fueling requirements for a flight reactor. It was determined that the reactor could be fueled, instrumented, insulated, and canned in 12 hours, and final assembly of the radial reflector and control rod requires another 8 hours. This was conservatively estimated at four working days. This is good to know, since one of the biggest costs associated with HEU is the security required for transport and the highly enriched “Special Nuclear Material” fuel, so the shorter the time that the fuel has to be integrated to the spacecraft, the less time you have to pay for those expensive nuclear security personnel and procedures.

The Final Non-Nuclear Tests

The last round of non-nuclear testing occurred in 2016 at GRC. In these tests, the heaters’ control software was programmed with the projected behavior of KRUSTY during a number of reactor and PCS states. The test profile that was programmed in was meant to mimic as closely as possible the thermal environment the reactor would experience at various points in the testing process.

This does not mean that the reactor system will experience exactly these conditions. This is a model of predicted thermal behavior based on nuclear modeling of components that have only been thermally tested using non-nuclear methods. Further nuclear testing before the full-power test would refine the model, and the thermal test profile is designed to account for any unknown thermal effects during the actual test.

So what did the test look like? As close to the testing that will be done at DAF as possible, so we’ll look at that test timetable and mention any variations from it as they come up.

The first test at the DAF will be a thermal break-in test, where the HEU core is electrically heated. This is a final verification that all of the components are functioning correctly, heating rates can be easily controlled, and thermal interfaces are functioning properly (especially at the hot and cold ends of the heat pipes) before the reflector is raised around the core for the cold (or zero-power) fission test. This is also the test that was duplicated by GRC with the DU dummy core.

Full power DU Results
Heating profile of DU core, Briggs et al, NASA GRC 2016

After a ~3 hour warm-up period, the sodium in the heat pipes started to boil, and the hot end of the Stirling engines began to warm. Once they reached 650 C, the two Stirlings and six thermal simulators were turned on, dropping their hot end temperatures. The system was then left to reach a steady state thermal equilibrium, which took about 2-3 hours.

Before the test was concluded, though, transient testing was conducted. This was to verify behavior of the reactor in case something failed, such as a piston jamming, reducing the amount of heat that the Stirling convertor would remove. To simulate this, the convertors were stalled, and the simulators were turned up to full power, to verify that a partial loss of power conversion could be accounted for with the rest of the convertors. Then, the convertors were shut down as well, to see the thermal response of the core to a complete loss of cooling. After this, the heater was turned off, the simulators were turned back on to full power, and shutdown occured.

This is identical to the thermal break-in test that will occur in Nevada. From there, the testing at the National Nuclear Security Site’s Device Assembly Facility differs from the tests that can be done at GRC, in much more stringent conditions and with a lot more cost as well, so every lesson that can be squeezed out of this testbed saved the program money and headaches further down the road.

As Dr. Mason mentions in a conference paper on the electrically heated DU core test article, this test was a major milestone.

“Testing of the Kilopower technology demonstration with the DU core, using the test sequence and configuration for the final testing with the HEU core, has reduced the risk of any unexpected issues in fueling, assembly, or test operations at NNSS… The system as it stands is capable of delivering 120 W electric from two ASC convertors with a maximum thermal power draw of roughly 3000 W, which is sufficient to verify neutronics models at the nominal Kilopower operating condition. There were no issues encountered during DU testing that caused unexpected operational issues which would need to be addressed prior to HEU testing.”

As with any well-constructed test, problems were isolated, which led to potential changes in the design. In this case, Dr. Briggs of NASA Marshall points out (in the DU testing final report) one such issue that was isolated, as well as a potential fix:

“Testing of the DU core shows that at nominal operating conditions there is a 200º C temperature drop between the core and the Stirling convertor hot end. The majority of this temperature drop takes place through the conduction plate and throught the ASC heat acceptor, both of which can be eliminated in future design iterations if money comes available for customized convertors and interfaces.”

However, he also notes:

“There were no issues encountered during DU testing that caused unexpected operational issues which would need to be addressed prior to HEU testing.”

Additional advances continue to be made, and there will be more minor changes between KRUSTY and a flight reactor, but these changes are likely to be minor.

Nuclear Testing: Now Things Get Serious

As seen above, nuclear materials are things that are far from taken lightly by NASA. There are arguments to be made both for and against the overabundance of caution that government agencies are required to take (these days), but that’s another topic that we aren’t going to touch today. Instead, what we’re going to look at is what goes into the nuclear testing side of reactor development.

As mentioned in the last post, the critical assembly machines that used to be at the

Comet Critical Assembly ORNL

Pajarito Test Site (in structures known as Kivas) were either decommissioned or moved to the then-new National Critical Experiments Research Center (NCERC) at the National Nuclear Security Site (NNSS) in Nevada. While we were looking at one of the critical assemblies last time, this time we’re looking at what could be called a “critical assembler,” where it integrates critical assemblies without having any nuclear components itself. There are two of these at the NCERC, Comet (which dates back to Little Boy testing), and Planet (which was built to relieve scheduling pressure on Comet, and ended up being a slightly smaller version of the same thing), although there were others in the past that have since been decommissioned.

Why is this important? One of the odd, and difficult, things about nuclear reactions is that they are sensitive to things that we don’t often think of as important. A classic example comes from the dawn of the nuclear age, when Enrico Fermi was still at the University of Cagliari in Italy. Being Italy, marble is a common choice for lab benches, as was wood. When he and his team were participating in early neutronics research, they discovered by accident that the material of the tabletop they were using made a significant difference in the outcome of the experiments they were performing, because the wood tabletop would slow the neutrons more than the granite tabletop would.

Device Assembly Facility, National Nuclear Security Site, Nevada. Image via DOE

This is one example of hundreds that can be offered of the odd effects of neutronics and the condition known as critical geometry, where a self-sustaining fission reaction can occur. All reactors require some form of cooling, and depending on the fissile material, moderators, and relative positions of these components, a reactor can go critical when you aren’t prepared for it to do so. This is called accidental criticality, and is a spectre that hangs in the room of any operation involving fissile material. Accidental criticality reports are made on a regular basis by Los Alamos National Labs, and the lessons from these accidents are then integrated into new handling procedures for nuclear material.

A quick aside on units here, criticality is measured in dollars and cents. One dollar of reactivity is exactly enough to barely sustain a fission chain reaction, and one cent is 1/100 of a dollar. This is also sometimes referred to as keff. When these terms are used, they refer to the reactivity being “inserted” or “removed” from a reactor. In this case, as the reflector is raised around the core, it reflects more neutrons back into the reactor, and therefore inserts reactivity, but removing control rods, rotating control drums, or removing fission poisons all insert reactivity into a reactor as well. There are also a number of ways to remove reactivity, either by inserting control rods, changing moderator configuration, allowing fission product buildup, or in this case lowering the reflector so it’s not reflecting as many neutrons back in the core.

The key to preventing accidental criticality is care in the design of the test, and in the movement of everything in the test area. Sometimes, a known reactor is used (such as Flattop or one of the Godiva reactors), and sometimes a machine is used that is designed to avoid accidental criticality (such as Comet and Planet, but other designs have been used over the years as well). This allows for a level of certainty and precision that takes into account the unique challenges of initial nuclear assembly. To build a critical assembly for the first time takes an incredible amount of forethought, modeling, planning, and preparation.

That being said, I am not a nuclear engineer, merely an enthusiast with a penchant for research and a respect for the limitations of physics and engineering, so explaining the conditions that lead to accidental criticality are not so much off the deep end of my skill set as off the continental shelf. Treat me as the classic “guy on the internet,” I try and provide as many original sources as I can and always love finding more, and as ever if there’s information I’m missing please let me know in the comments!

Comet (and Planet) have been designed to take care of those concerns to a large extent. They are simple machines, with a fixed platen and a movable one. The lower, movable platen on Comet has rough control via a set of hydraulic lifts, and fine control using a screw drive connected to an electric stepping motor. In this case, all of KRUSTY but the core will be placed in a vacuum chamber mounted on the upper platen on Comet. The core will hang down below this assembly, and the lower platen will be used to raise a reflector around the core.

Platen position details
COMET platen position diagram, Potson LANL 2016

A number of things remained untested (although modeled) after the DU tests, all of the significant ones being on the nuclear side. The first is how much reactivity will be needed for the core to operate In order to account for this, the reflector for KRUSTY is modular in design, containing a number of annular discs (think thick washers) of varying thicknesses that could be selected to tweak how many neutrons are reflected back into the core, and therefore how much reactivity is added. According to Monte Carlo (MCNP) modeling, a reactivity of $1.70 was needed for nominal operating temperatures to occur, but there’s a possible margin of error of up to $1.50. This was an issue, because Comet at the DAF was only authorized to handle $0.80 in excess reactivity, but the site permit changes and modifications were authorized for testing to proceed (a facility basis safety change was approved by the DOE, and there are some indications that even higher reactivity insertion limits may be allowed with the construction of new facilities at the NNSS).

KRUSTY startup reactivity, Potson LANL
Modeled startup reactivity requirements, Potson LANL 2016

Before the nuclear testing occurs, the reactor will undergo a thermal break-in test, mimicking the electrically heated DU test precisely. This is to ensure that all of the thermal interfaces and thermodynamic properties observed at GRC with the DU core are the same with the HEU one, and that the integration of the new core is done properly. After that, testing can move on to neutronics requirement refinements.

In order to test the needed reflector geometry a number of cold criticality (also known as zero power) tests were conducted over the summer (cold criticals in June or July, warm criticals in September) to refine the reflector geometry. By using different thicknesses of BeO, different amounts of criticality can be inserted into the test article, and this information will then inform the final design of the reflector. By adding components in a step-wise fashion, the reactivity requirements can be pinned down.

After the cold criticality testing data has been fed back into the models to verify the predicted behavior and make any adjustments necessary, low temperature testing can begin. This is important to refine the models of reactor behavior before full-power testing can occur, and again is done in a step-by-step manner, from $0.15, to $0.30, $0.60, and finally up to $0.80 excess reactivity (added in $0.02 intervals). This maximum number was chosen because it ensures that the reaction is sustained by the delayed neutrons and occurs at a lower temperature than what would be found at full power.

With the successful completion of the cold-critical tests, KRUSTY’s operating conditions will be well-enough characterized that any potential issues or unknowns discovered during the cold critical tests can be addressed before the full-power, high temperature testing.

Full-Power Test: The Day 40+ Years in the Making

The big test comes this year: KRUSTY will undergo full power testing by the end of November 2017. It’s been a long time since the US has done a nuclear-powered ground test on an in-space nuclear reactor. In fact, it’s been so long that the Department of Energy has never conducted a full-power in-space reactor, the last tests were conducted by the AEC! There are a number of reasons for this, some of which we’ve looked at before, and we’ll touch on them more again in the future. For now, let’s look at the test itself, and what we’ll learn. Based on modeling and non-nuclear testing (again, I can find no data on zero power or cold critical testing), all possible required modifications to the test apparatus have been isolated to the reflector, which can be easily reconfigured to allow for as much reactivity is needed.

The test itself will be a long one, starting the same way that the low-power tests did and continuing through a series of tests meant to measure steady-state operation, and simulating failure of cooling or power conversion systems to test reactor dynamics. Because of the strong negative thermal reactivity coefficient, no issues are anticipated in these power transient tests (as they’re known), but the dynamics of the system still need to be verified experimentally. By wisely selecting the tests performed, a great number of other interactions can be simulated to ensure efficient and reliable operation for many different errors or types of damage. In order to ensure that there’s enough excess reactivity to account for a worst-case underestimation of the amount of reflection needed, the reflector will be loaded with sufficient BeO plates to allow for an insertion of up to $2.20, well more than the $1.70 that MCNP modeling predicts in case of unanticipated reactivity loss.

Full Power Temp Run Full Slide, Potson 2016

As with the other tests, this one begins with an insertion of $0.15 in reactivity, and stepping the reactivity insertions up at regular intervals until the desired fuel temperature is reached (850 C). After this, several hours of steady state operation are recorded, verifying thermal equilibrium across the system.

Now things get interesting. The first transient test involves cutting the power draw from the two Stirlings by a factor of 2, and monitoring the reactor as it adjusts itself to the new power level. Once the system achieves a steady state, the power draw is increased again to test the load following capabilities of the reactor. This is followed by several more hours of solid state operation.

Load Following Experiment SlideThe next transient test involves shutting off one of the Stirling engines to simulate a failed heat pipe. The resulting changes in temperatures and power levels will be monitored and used to refine the failure mode prediction modeling for Kilopower. After the system adjusts to this change, power will be brought back online from the stalled Stirling, and the system will reach full power again.

Full Run Power Removal Halt Test Full Slide

Finally, a total loss of cooling will be simulated, by turning off all heat removal from the Stirling engines and the simulators. Based on the modeling of this system, the temperature will increase, decreasing the reactivity in the core to the point that dissipation of heat from the core and the fission reaction reach a balance point. At this point, the reflector is withdrawn, and the reactor is left to cool, both thermally and radioactively, for a few days.

Modelling has been done on other failure modes, such as adjacent heat pipe failures, but these situations won’t be tested because the data collected from these tests will be enough to predict reactor behavior under those conditions.

Typically, a post-mortem analysis of a test reactor is done after a nuclear-powered test, but due to the low power, relatively short burn time, and high enrichment of the fuel, this may not be as necessary as in other reactors. I’ve found no reference to post-mortem inspections in the papers and presentations I’ve gathered, but it seems like a logical set of tests to make, and presumably with the resources of Y12 available the fuel recycling may be easier than in other projects. If anyone has any information on these plans (or the results after the test is complete) please let me know in the comments!

However, the reactor will be left for between 45 and 60 days to cool while on COMET, and then will be set “in a corner” (in Dr. Poston’s words) to cool further.

After Final Run with stowed platen

What We Expect to Learn

According to Gibson and Poston:

“The KRUSTY test will be the first flight prototype nuclear test of a space reactor performed in decades. The results of the KRUSTY test will validate the computer models, methods, and data used in reactor design. In addition, valuable experience in design, fabrication, startup, operation, transient behavior (load following based on reactor physics), and reactor shutdown will be obtained. The ultimate goal of the KRUSTY experiment is to show that a nuclear system can be designed, built, nuclear tested, and produce electricity via a power conversion system in a cost effective manner.”

Nuclear engineering is a field where the lessons learned from one test can be applied to many different systems, even ones with different specific operating conditions (as seen in the DUFF experiment in the last post). Heat pipe cooled reactors are an attractive option, but the single test of this concept in a nuclear reactor was with a very low power reactor, and a heat pipe material that wouldn’t be used in a flight reactor. KRUSTY takes it a step further, using a coolant that would be used in a wide variety of reactor architectures, and operating at considerably higher power (although still only 1 kW) and temperature. Until heat pipe cooled reactor tests are more common, the data from KRUSTY will inform every reactor design using this technology.

Other unusual characteristics of the reactor will also be able to be better modeled. For instance, the monolithic fuel design (where all the fuel for the reactor is in a single piece) is unusual, and additional information about its’ behavior can be gained.

Radial Power Distribution simulation
Radial power deposition model, Potson LANL 2017

The Stirling PCS, while not prototypic, is another subsystem new to nuclear power production, and the lessons gained in this experiment can be applied to future reactor designs (as well as possibly making improvements for the solar thermal designs). Another system that will be directly impacted by this is the Fission Surface Power reactor, the next in our series, which uses Stirling convertors that are very similar to Kilopower. Questions raised there about the effectiveness of this type of PCS can be addressed with this smaller-scale test.

The lessons learned from this reactor will impact many designs, in many different ways, for many years to come, and as with any technological advance, it’s impossible to guess what will have the most impact.

First Order Value of Experiments 1First Order Value of Experiments 2

A Fundamentally Enabling Technology

There is no more difficult field of human endeavor than spaceflight. The difficulties, the costs associated with those difficulties, the extreme distances and environments that missions must endure all present challenges that are rarely paralleled in other fields of human endeavor. In fact, the only other field with comparable costs and constraints of engineering, chemistry, and physics is nuclear power. Despite this, an inventive team has managed to make do with existing technology, facilities, and equipment to design a reactor that not only can but will be tested, for a pittance. As an engineering student, you are constantly told to keep costs of materials and in mind from the first engineering class you take, but this goal is rarely able to be carried over into aerospace OR nuclear engineering. Here, Dr. Poston and his team at Los Alamos have performed miracles of economy, to bring the first electricity generation from a DOE reactor, and the first full-scale nuclear test of an in-space fission power system since the DOE was founded.

Juno solar panel size
Relative size of Juno spacecraft, image via NASA

So why is this important? Many in aerospace consider nuclear power to not be worth it, the costs in terms of both money and procedural burden are high, even for a simple plug of 239Pu for an RTG. However, there are limits to what solar, the most often suggested substitute, can provide. A classic example is the outer solar system: the Juno spacecraft currently in orbit around Jupiter has three solar arrays large enough for a semi tractor-trailer, but is so power-starved it takes almost two weeks to transmit data back to Earth. Communications isn’t the only area that power is important: radar is a power hog (in fact, the nuclear-powered US-A satellites launched by the USSR were Radar Orbiting Reconnaissance Satellites); electric propulsion is another classic example of more benefits directly deriving from more available power.

Another limitation crops up with the use of radioisotope thermoelectric generators, or RTGs, which use decay heat from a radioactive substance (usually 239Pu, but other isotopes are also used on shorter missions): their power is at its peak when the fuel element is assembled, and it will only drop in power from there. The amount of power can’t be changed, and the decay process can’t be slowed. Additionally, engineering practicalities limit how efficient an RTG can be, and moving toward a heat engine-based conversion system (such as the ASRG) can only do so much to increase available power for a given mass with these systems.

A fission reactor can be launched “cold,” be left off until needed (perhaps even years into a mission) with no nuclear degradation of fuel or materials, and can draw more or less power, and even be turned completely off, on command. Combining this capability with the ability to operate independent of local conditions (mostly) and the ability to provide a very dense power source, fission power supplies offer unique capabilities for both unmanned and manned missions throughout the solar system.

Kilopower: A Nuclear Reactor for Higher-Power Missions

TSSM Enceladus
Titan System Survey Mission, ASRG variant, over Enceladus. Image via NASA

For those in the nuclear engineering field, often the reactor itself seems to be an end in and of itself (I am guilty of this, as well). However, no matter how simple, elegant, unique, or original a concept is, it still is a power source for… something; in this case a NASA mission of some sort. These fall into two broad categories: spacecraft (orbiters or fly-by missions) and landers (either fixed or rovers). Both orbiters and landers have been considered for Kilopower, and we’ll look at some options for each.

What missions have been proposed that this reactor makes possible? Remember, NASA has stacks and stacks of missions that it commissions a one-to-three (usually two) year study on, and stacks them up to wait on certain enabling technologies to come about. Often, this enabling technology is the power supply, and these are the missions that stand out for Kilopower.

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 New Family of Reactors

Kilopower 4 kWtThis 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.

12 kWt Kilopower coreThe 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.

21 kWt KilopowerThe 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.

43 kWt KilopowerWith 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.

Reactor Size Cross section diagram
Image via DOE/LANL

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.

2MWe Megapower
Image via Poston, LLNL

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.


All sources are linked in-line, but because I’m currently on a free WordPress account I’m not allowed to add code to tweak the blog’s appearance to allow for that. So, I’m also going to link the important sources here, so that it’s easier for people to find them. However, this isn’t an exhaustive list, if there’s something that should be here but isn’t, let me know in the comments and I’ll add it.

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

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

Channel and Webpage Updates

I hope to have these posts out somewhat more regularly, this last month has been a busy one for me, between switching to a new (overnight) position at work, a week-long family vacation (in Hawaii!), and getting the resources together for web pages that will hopefully be up soon.

The YouTube channel has been on hold for about a month due to these delays, but that should make editing the scripts slightly easier. Work on Blender is proceeding as well, with a 3D model of the Boeing Integrated Manned Interplanetary Spacecraft (Von Braun’s Battlestar Galactica to Mars) pretty much roughed in for the channel, as well as various 3D flow diagrams for the ROVER A6, and LARS proceeding apace. I hope to start releasing videos early next year, but I was hoping to have two or three done by now anyway, so we’ll see.

Again, thanks to everyone that has helped on this so far. Your support and expert assistance are what has made this project possible so far, and will continue to make this possible in the future!
All rights copyright Stuart Graham 2016

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20 Responses

    1. That’s awesome!

      Rover is actually going to be the intro to reactor physics focus when the YouTube channel finally gets going (hopefully this summer). I touched on it a little in the NTR post series, but plan on doing a LOT more!

  1. You mention that NASA has stacks of missions waiting for Technology to catch up. Where can I read about these missions?

    1. That’s tough. I got the list in the post from a couple presentations on KRUSTY, based on multi RTG power supply missions. The Decadal Surveys are a good starting point, though. They tend to be the higher powered missions.

      Eventually, I’m hoping to do writeups on some of them, but unfortunately I’m trying to catch up on the NTR systems still.

    1. Thank you! I’m hoping to add more on this system in the future, but unfortunately delving into the “propulsion” side of nuclear electric propulsion has been eating ALL of my time these days.

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