Hello, and welcome to the Beyond NERVA blog! This was originally meant to be a post about KRUSTY, the small nuclear reactor that has been in the news a lot recently, but it ended up exploding into a four part series.
In this post, the first in our series, we’re looking at what came before this design, particularly the DUFF experiment, and the organization within NASA that is currently tasked with developing the next generation of in-space nuclear power systems, the Kilopower program within NASA’s Game Changing Directorate.
The second post will be an overview of the Kilowatt Reactor Utilizing Sterling TechnologY, or KRUSTY, including a more detailed look at its design, and a look at the testing that’s been done and is planned for that particular reactor. The more I learn about KRUSTY, the more I like this reactor design: it’s simple, it’s safe, it has very few moving parts, and those parts that do move are well-tested to be maintenance free in space for years, if not decades.
The third post will look at the Fission Surface Power reactor, or FSP, a step bigger than KRUSTY, but still rated for less than a megawatt of electric power (100 kilowatts is the largest this design gets on the books). This reactor was a recent (2010-2012) design, with some interesting features, but due to design and testing limitations (more on that in the blog post), it hasn’t been tested as thoroughly as KRUSTY. This starts the place where significant additional funding into new programs is going to be required to prepare a flight system, and is the start of more long-range planning for NASA.
Finally, in our fourth post, we will look at NASA’s planned reactor for nuclear electric propulsion, in this case Project Prometheus for the Jupiter Icy Moons Observer spacecraft, or JIMO, another recent design for a nuclear-electric spacecraft that we’ll look at later in this blog series. This design has gone through some serious revisions over its lifetime, and we’ll look at what those are and why. This reactor starts with about 100 kilowatts of electric power (kWe, as opposed to kilowatts of thermal power, kWt), and goes into the multi-megawatt range. This is the largest reactor NASA is currently exploring for electricity production, and can be used to send unmanned, and possibly even manned, missions across the solar system.
What we Have (Or Used to Have)
In-space nuclear reactors are far from a new thing. The US launched the SNAP-10A reactor in 1965 from Vandenburg AFB atop an Atlas-Agena D rocket. The reactor was launched successfully, but after 43 days in orbit a voltage regulator on the Agena failed, causing the reactor to shut down at a maximum power of 590 W. It’s still in orbit.
The USSR had a much more successful program, launching a series of Radar Ocean Reconnaissance Satellites, or RORSATs, powered by small nuclear reactors. These were the US-A spacecraft (ironic, that). The active radars that were the primary instrument on board were (and are) major power hogs, so putting a nuclear reactor on one made sense. They flew a total of 33 of these satellites, with two different reactors on board. The first was known by at least three names, the BUK or BES-5 in the USSR and TOPAZ in the West. It was used more than 30 times, and utilized a thermionic power conversion system to minimize the number of moving parts needed for the reactor. The second, often called the TOPAZ-II in the West, is the ENISY reactor, another, more modern thermionic design. We’ll look at these more in depth in the channel. The Russians ended up losing one of these satellites, the KOSMOS 954, in the tundra of Canada, the only time a space-based nuclear reactor has hit land upon reentry, although one other (KOSMOS 1402) fell into the sea. The rest remain in graveyard orbits, where they can be kept out of the way for the indefinite future.
We’re going to look at these reactors in some of the first videos on the channel, as well as one of the most interesting ironies of NASA’s Moon base plans in the 1990s, but I want to mention one thing about a little-known piece of space history. Shortly after the Iron Curtain fell, the US bought a number of these reactors from the Russians, and an exchange program was set up. The US bought these reactors as a “turn-key” setup, which meant that they also had all the necessary testing apparatus. Because of the design of these reactors, they could be fully tested without using fissile fuel, which means that they could be tested and flight-qualified without becoming radioactive. According to the TSET Facility Construction report, the US saved $400 thousand dollars (at 1992 valuation) compared to developing and building their own equivalent test stand. The three-year program ended, and for a while in the mid-to-late 90s, NASA had plans on the books for a Moon base powered by a Soviet-built nuclear reactor.
These reactors were small. This may seem odd to people who are used to terrestrial nuclear power plants, which have much higher power outputs (on the order of hundreds of megawatts to gigawatts). However, for any mission that has been designed in depth (and has a chance of being funded), that amount of electrical power is completely unnecessary, and going to a larger reactor also means you have to launch more reactor mass. These systems are also going to be some of the first we look at in the channel, so I’m going to be brief here. Larger reactors for in-space power use (mainly propulsion) have been designed, and we’ll be looking at those in the channel, but none has ever been built and tested.
Let’s look at power levels from these reactors a bit more: what needs that much power in space? The International Space Station is the biggest power hog that we have in space right now, and it uses about 75-90 kW, with a max power of about 120 kW. To put this in perspective, this orbiting research laboratory requires as much power as 55 American homes.
Now, a space station’s operational power load is much less than, say, a high-powered electric propulsion system, but it’s not that different from the likely power requirements for an early Moon or Mars base. Sure, there are things that we could do if we had more power, but those things also (usually) require more payload mass as well, which is already one of our biggest constraints. So why put money into a program that you don’t need right now, especially considering the incredibly constrained budgets that NASA and the DOE’s in-space power programs operate under?
Kilopower: NASA’s Latest Nuclear Power Reactor Program
At NASA, the Space Technology Mission Directorate (STMD) is where components and systems for space missions are evaluated and tested, and where the mission hardware comes together. Within the STMD, the Game Changing Development Program (GCD) focuses on enabling missions that are currently impossible, but not technically impossible, across the whole range of aerospace mission and component types. In turn, GCD is the home of Kilopower, NASA’s current program to develop advanced in-space nuclear reactors. There are a range of reactors of different sizes and power outputs that have been proposed for the program, ranging from 1-10 kWe reactors for unmanned probes and space missions, to 10-100 kWe reactors for surface operations, to 100 kWe+ reactors for nuclear electric propulsion.
This blog post focuses on the work leading up to this point, so I won’t belabor the point here. DUFF was an excellent test, and set the stage for more work to come.
The next big goal for the program is to build a small reactor. This is the Kilowatt Reactor Using Sterling TechnologY, or KRUSTY. This simple, small reactor uses enriched U235 metal fuel, will have a power rating of 1-10 kWe, and will primarily be used for in-space missions, although surface operations are another possible application.
This isn’t the only reactor within NASA’s current range of possible fission power sources that they’re interested in:
This range of reactors has been described in papers before, with testing regimes carried out on them to a greater or lesser extent. The Fission Surface Power reactor was studied between 2010 and 2012 (the date of the final report), and provides 40-100 kWe of power for surface operations. The largest of the reactors, rated from 130 kWe up, is the design for Project Prometheus, which was chosen for the Jupiter Icy Moons Observer, or JIMO, a nuclear electric spacecraft that we’ll be looking at later on in the channel.
Pre-Existing Conditions, The Budgetary Straightjacket, and the Challenges of Nuclear Reactor Design
NASA, as an organization, is tasked with doing a lot with very little money (Request for 2017). They also have very little flexibility in how the vast majority of the money is spent. While NASA has shown interest in nuclear power in space since the earliest days of the Space Age, the funding levels for this have fluctuated wildly over the years, with a distinct and depressing downward trend.
The thing is, nuclear power isn’t NASA’s balliwick. In the early days, this was the realm of the Atomic Energy Commission (AEC), and now is the domain of the Department of Energy (DOE). Both organizations have/had a general interest in advancing nuclear power in space, and on an individual level the idea is met with optimism and enthusiasm. However, as with NASA, the DOE does not control where its funding goes, and there are other critical missions (like nuclear weapons non-proliferation work worldwide) that hold a lot more of the official attention, and budget.
Unfortunately, even the DOE isn’t allowed to just go and build a test reactor. Since the dissolution of the AEC, the oversight and regulation of all nuclear reactors in the US is the responsibility of the Nuclear Regulatory Commission (NRC). Even with a design that is largely finalized, extensively modeled, and is the result of hundreds of man-hours of careful review, it’s highly unlikely that the reactor itself will ever be built and tested. Instead, each of the components and subsystems are carefully tested in various test stands to determine different properties and operation considerations. In order to actually be able to test a system, so many different components have to go through independent testing to incredibly stringent standards. Therefore, using already-designed components is incredibly attractive. For any new design, the parts that aren’t designed have to be able to fit into the testing equipment, or a new test rig has to be built. These are not cheap systems, most of the time. The conditions inside a nuclear reactor are extreme, and the restrictions on building an experimental reactor that have been put in place by the NRC make “just build it” impossible. Every component has to be tested beforehand, and the push to use presently available test systems is strong (and often this is the only way to go to keep within budget).
Many of these test systems are unique, and while between the DOE and NASA there is a wide variety of different test stands, each has its own limitations. Mainly, each test stand looks at one aspect of a system, so one system will need to be tested on multiple test stands. Thermal testing is separated from nuclear testing, except for fuel element tests which have their own test stands. The two most commonly used are the Nuclear Thermal Rocket Element Environment Simulator, or NTREES (which we’ll look at more when we look at NASA’s LEU NTR design), and the Compact Fuel Element Environment Test, or CFEET. Both of these are geared more toward the extreme conditions experienced by fuel elements in a reactor running as hot as possible, as a nuclear thermal rocket does, rather than a baseload power as seen in an electrical system, so we’ll look at these more then.
The other side to this is that, over the decades, many individual components have been tested, and can be used in a new design. The place that this is most frequently applied is in fuel element design. To make a new type of fuel element, every step in the manufacture and testing has to be modeled extensively before the first test article is made, and a full testing campaign can require dozens of these highly precise and difficult to manufacture objects to ensure all steps of manufacture and use are well understood.
Various national labs develop fuel elements for different purposes. One of these facilities is Y12 outside Oak Ridge, Tennessee. Originally built to enrich uranium for nuclear weapons, like most DOE facilities it also does research and development work in a variety of areas, and the development and testing of specialized fuel elements has been a focus for them since the earliest days of the facility. Other facilities, such as the Idaho National Laboratory, Los Alamos National Laboratory, and others, design experimental fuel elements as well, and in the channel we’ll see some of their work come up in various designs. Y12 is particularly relevant in this post because they developed the fuel element used in KRUSTY: the Uranium-Molybdenum metal fuel. This is a well-understood fuel form, enriched to 93% (by weight) 235U. While this is a far higher enrichment than is seen in terrestrial reactor fuel elements, for the majority of the history of in-space nuclear reactors highly enriched uranium has been the norm. This isn’t necessary, as we’ll see in my next blog series about the low-enriched uranium nuclear thermal rocket currently being developed by NASA.
The development and testing of an in-space nuclear power system goes far beyond just the fuel elements, or the core design that we’ll look at more in the next post. The power conversion system is another component that has been extensively researched, and is available off-the-shelf commercially. In this case we are referring to the Stirling power conversion system built by Sunpower, Inc. Its use here is based on an earlier design integrating it to a larger reactor, the Fission Surface Power (FSP) reactor that we’ll look at more in the third post in this series, but close enough that a lot of the same work that’s been done there will apply to this system as well.
These are just two of hundreds of components that are needed, and while off-the-shelf is easier on the budget in many cases, it also adds engineering constraints on your final system you have to work within (a good case in point, although imperfect, is the Space Launch System). The demanding nature of spaceflight – and the unforgiving nature of reactor physics – means that these systems must be highly reliable, and the regulatory restrictions make this process very difficult and expensive. New testing facilities would be ideal, but for the types of missions on the books, the current facilities can be used for a system that’s just big enough at a far lower price than building new ones.
DUFF, Father of KRUSTY
It’s often easy to overlook the origins of a newly popular piece of technology, and NASA’s new nuclear reactor is no exception. Remember, NASA doesn’t (or wouldn’t) build these reactors, the Department of Energy does. The DOE side of things has been largely ignored in the mainstream media, which focus on the flashier and more PR-conscious NASA, but NASA doesn’t build reactors, the Department of Energy does. On the DOE side, this reactor is the direct result of an experiment carried out in 2012, called the Desktop using Flattop Fissions, or DUFF.
This experiment was conducted by Dr. John Bounds of Los Alamos National Labs, at the Device Assembly Facility (DAF) at the Nevada National Nuclear Security Site (formerly the Nevada Nuclear Test Site). This facility has been a keystone for nuclear technological development of all sorts since the early days of the Atomic Era. Long after the weapons tests, reactor tests, and the US nuclear thermal rocket programs of the 1950s through 1970s they’re most known for to today’s quieter work with actinide irradiation studies, this facility remains a keystone in American nuclear technology development.
In a conference paper for the Nuclear and Emerging Technologies for Space 2013 (NETS 2013), Gibson et al point out that there’s a gap between the designs that have been used for Radioisotope Thermoelectric Generators, (RTGs) and the designs that have been proposed for fission power systems, or FPS. This gap also happens to coincide with the available testing facilities, and the ideas they proposed were not new, just untested. To improve the design, the team chose a simple, fundamental issue with the proposed reactor, and worked with what they had on hand for everything else. DUFF was focused on a novel (for nuclear reactors) heat rejection system: the heat pipe.
The heat pipe is a simple, attractive way of getting rid of waste heat. By using thermal differences and phase changes in a working fluid, a heat pipe can transport a large amount of heat in a simple system that has no moving parts. At the hot end, the working fluid evaporates after coming in contact with the hot casing material, and flows to the cold end. At the cold end of the heat pipe, it condenses onto a wicking material, which then carries the working fluid back to the hot end and completing the cycle. Most of the energy transportation that occurs actually happens in those phase changes, rather than the movement of the working fluid itself. The type of working fluid, and the wicking and casing materials, are defined by the operating environment that it’s going to be in, especially the amount of heat being transported and the temperature of that waste heat. While heat pipes may seem like a peculiar means of rejecting heat, and they’d never been used in a nuclear reactor before this test, most of us use these types of devices every day. The CPU of the computer you’re reading this on is cooled by a heat pipe (even if it’s a phone), as are a huge number of other electronic components. They can operate from very cold temperatures (such as a helium heat pipe), to very high temperatures, such as aluminum. For KRUSTY, the plan is to use sodium potassium eutectic (NaK) for the working fluid.
By replacing pumped coolant with heat pipes, you’re changing more than how the reactor is cooled – you’re also changing how the neutrons in the reactor behave. Any structure in a reactor is going to affect the behavior, or dynamics, of the reactor system. After all, slowing neutrons down isn’t the only way to affect the behavior of neutrons in a reactor, they can be reflected as well. Depending on their chemical and isotopic composition, most materials will either reflect (at a lower energy level), slow, or capture neutrons. This is something that can be modeled with incredible accuracy, but you still have to have the real-world test article to show that it works. This test article was DUFF.
DUFF used an existing test rig for the criticality portion of the experiments. Several were available at the facility, but each required compromises. The chosen test rig was the Flattop criticality test rig, a workhorse of nuclear testing.
Dr. Dennis Beller, a research professor in nuclear engineering at the University of Nevada, Las Vegas, posted this about the choice:
Flattop is one of four old [ed note: built in 1951], much used critical assemblies that were moved from the Criticality Experiments Facility (CEF) at LANL Technical Area 18 to the Nevada National Security Site’s National Critical Experiments Research Center (NCERC) several years ago (at the time of the move it was NTS and CEF, both later renamed). These assemblies (others are Godiva 4, Comet, and Planet) are used for criticality benchmark experiments, cross section measurements, DOE/NNSA’s hands-on criticality safety training courses (I was a recent student), and a variety of other training and research projects. Flattop, which is a uranium-reflected highly enriched (233U, HEU, Pu, or other) sphere, is unique in that it can be operated super critical to produce an internal temperature of about 300 C (not quite what one would expect in a power reactor “prototype”). Flattop also has a hole through its center that permits insertion of experiments or other actinides, in this case a heat pipe that was built specifically for this test (I don’t believe it’s a space power prototype either, but someone from NASA or LANL might disagree [ed note: it’s not a prototypic test article, just a proof of concept]). In addition, the purpose of the heat pipe is not to cool the reactor (LANL’s words, not this authors) although it does remove a tiny amount of the fission power, it is to transfer energy to the Sterling engine so it will produce electric power.
DUFF was a perfect example of the kind of compromises that are taken in an in-space nuclear development campaign: a test article, using different materials than the flight article would (page 9), to demonstrate that the principles of operation were sound, and that nuclear testing could be done more affordably under the current regulatory regime. To get good data, you need a well-characterized system like Dr. Beller describes above, and it’s easier on the budget as a rule to design your test article to the testbed than the other way around. The other advantage to using an existing criticality test rig is that they are usually very heavily studied and very well understood, so that the effects of the particular test equipment can be more easily isolated and studied.
DUFF wasn’t focused on the reactor, remember, it was focused on the heat pipe. Because the power output from Flat-top is so low (700 W, estimated max temp 300 C, steady-state operation at 200 C), they weren’t able to use a sodium heat pipe, as the final reactor would use, because it evaporates at too high a temperature. Instead, they looked at other options that had lower boiling points, something that limited their choices greatly. After testing two options, they settled on water as the working fluid (Dowtherm A was the other option looked at, but it wasn’t able to transfer enough heat). After selecting the working fluid, the casing and wick needed to be decided as well. After testing at NASA’s Glenn Research Center, it was determined that the best option was to use a sintered nickel wick (200-mesh) and a 316L stainless steel casing, although other mesh sizes and casing materials were tried. Since this wasn’t going to be a flight article, the fact that this was made out of steel and nickel didn’t matter: the test stand didn’t care about every ounce of weight like a spacecraft would.
The final part of the system is the Stirling converters, and once again Dr. Bounds and his team used an existing piece of equipment to both increase the certainty of the measurements and decrease the cost of the test. This was a challenge for a number of reasons. While Stirling conversion systems had been researched for in-space use before, the vast majority of the time these were much higher-power units, requiring a minimum of 200 C hot-end temperatures to operate. This was still 50 C higher than the worst-case hot-end scenario for Flattop, so these systems weren’t an option. Instead, they went with one of the only options available, a Buzz convertor. By cooling the cold end down to -50 C, there was enough temperature difference to produce power (although definitely not net-positive electrical power!). As Dr. Marc Gibson noted,
Although the Buzz convertors do not represent the state of the art in Stirling design and performance, they were affordable, available, and compatible with the DUFF test constraints, making them the best choice for this proof-of-concept test.
DUFF passed with flying colors. This proved that a heat pipe waste heat rejection system could be used in a reactor, and also demonstrated a flexibility in thinking among the researchers and designers involved to work within a very limited budget and scheduling constraints imposed across multiple facilities in the DOE. For more info on the challenges leading up to DUFF, I recommend reading through the presentation. The challenges described in critical assembly testing have been enlightening, and the presentation and paper are my main sources of the information about the tests.
Two test runs were made, on Sept. 13th and Sept 18th,, 2012. In the test on the 13th, the reactor power was raised to 2 kWt, and held there for about 5 mins. After reactivity was increased, the Internet connection for the thermal data collection system went out, leaving only the pedestal temperature data available (this is a much lower temperature, possibly the reflector temperature). From here, they decided to fly blind, relying on information from the power conversion system and their models to complete as much testing as possible within the allotted time. Due to pre-test work, it was known that the reactor would have a negative coefficient of reactivity (i.e. the hotter the reactor got, the less neutronically active it would be), so this wasn’t a concern. Limited data collection is a persistent problem in all areas of science, and in astronuclear engineering it has been consistent enough to be ingrained into the researcher’s mindset: some data is better than none. Two minutes later (7 minutes into the test) the heat pipes activated, and more data flowed in. At this point the heat pipe was carrying about 400 W of energy. Over the next ten minutes, the core temperature increased, heating the Stirling engines about 200C. This also kept the core cooler, which in turn adds more reactivity to the core due to the negative thermal reactivity coefficient.
Seventeen minutes in, the Stirling was kick-started (when the hot end was at 225 C) resulting in the production of 24 We. Thermal transfer from the hot end of the Stirling changed the temperatures of the various components significantly over the next minute, to the point that an equilibrium was established and observed, leveling off at 18 We power output. One minute later the reactor was scrammed, and the Stirling engine continued to draw off the residual heat from the various system components. Once the hot end of the Stirling hit ~120 C, it stalled. Four hours later, the team learned the computer that contact had been lost with was still intact, therefore the issue was in communications and not hardware.
After some repairs and adjustments, a second test run was done on Sept. 18th. This was a slower, lower, and longer test than the first, and had some other differences as well. Not only were they going to verify the results from the first test run (hopefully with full experimentation this time), but they were also going to stop and restart the Stirling engines during the test, to see the resulting change in core reactivity and thermal profiles. This is important in a number of ways, but the most important part may be that it allows for better predictions to be made about how a different core would react to the Stirlings either being shut off for maintenance or mission requirements.
After a 9 minute rise to power, the heat pipes activated and the entire system started to approach thermal equilibrium, which was reached about 13 minutes later at ~160C. At this point (22 minutes in), both fission power and system temperature continued to rise at about 5C a minute, while the Stirling remained off. A half hour into the test, the technician turned on the Stirling engine, and heat began to be removed from the system. Once they started the engine, the hot end measured 180C, and electric output was 13 W. Within a minute, the hot head had cooled rapidly, and power output held steady at ~7 W. At the same time, the cooler temperatures increased reactivity, with power output (and temperature) increasing to 11 W. Four minutes later (35 minutes into the test) a final reactor power increase was ordered (reactivity insertion), bringing peak fission power to ~5.5kWt. Over the next five minutes, the negative reactivity coefficient of the reactor kicks in, and the system reached a new equilibrium at about 3 kWt. At this point, the technician changed to a high stroke on the Stirling engine (similar to shifting gears in a car, it changes the torque being utilized by the Stirling engine), increasing the amount of energy produced (from 11 to 17 W), and removing energy from the reactor system. Three minutes later, the technician stopped the engine, and the temperature rapidly increased from 185C to 225C. Two mintues after that, he restarted the engine, reconfirming the results from the first critical run. Forty-six minutes into the test, the reactor was scrammed, and the entire system decayed to thermal equilibrium. The Stirling continued to draw power for about 5 minutes, stalling out when the temperature reached ~115C. With one final gasp of the Stirling 56 minutes after the start of the test, the cooldown period continued through normal radiation of heat energy.
Many firsts were demonstrated with this test, some technical and some organizational. One thing that was surprising to me was that this was the first reactor system developed by Los Alamos to produce electricity. Other organizational notes (that make incredibly depressing reading, to be honest) include: first nuclear space power demonstration since the founding of the DOE (on August 4, 1977), and the first power system operated at the NCERC. More hopeful ones include the first use of a heat pipe power reactor (of any size), and first reactor system to use a Stirling convertor system. Further successes look not to DUFF, but to its’ successor, KRUSTY.
As with many advances in nuclear power, DUFF went largely unnoticed by those outside the nuclear engineering community.
An Interlude into the Humor of Nuclear Engineers
DUFF and KRUSTY are far from the first, or most significant, nuclear experiments that bore the names of cartoon characters (although cartoon beers may be a first). Since the earliest days of nuclear research, lighthearted names have been attached to various testing programs. This, I believe, is partly due to an attempt to lighten the mood for a test that may take years to prepare for but be over in an instant, and partly due to the fact that almost no-one reads the reports!! (Yes, that did deserve an italicized double exclamation point; the amount of information available compared to the number of interested people that have read it continues to astound me!)
Reading through the list of test explosions that the AEC (predecessor to the DOE) conducted, you find names like “Danny Boy” (3/62), “Chinchilla” (2&3/62), “Dormouse Prime” (4/62), (and “Ferret Prime” as well in 2/63), “Gazook” (3/73), and many different bird and fish names from around the world. Some of these were randomly selected by the AEC out of a list of words, but some were named by the scientists and test engineers themselves.
This isn’t limited to just the US, either. At Hokkaido University there’s a program called PIKACHU, for instance.
I hope to be able to write a blog post on just this in the future, but for now… the Simpsons are popular in the nuclear community, nuclear engineers have a sense of humor, and yes, there’s going to be a nuclear reactor named after a cartoon clown, begat by a cartoon beer.