Hello, and welcome to Beyond NERVA!
Today, we’re looking at a different kind of fuel element than the ones we’ve been examining so far on this blog, one that promises higher operation temperatures and therefore more efficient NTRs: carbide fuel elements. We’ll also look at a few different options for NTR designs using carbide fuels: the first one being from Russia (and the only NTR to be tested outside the US), the RD-0410/0411 architecture (two different sizes of a very similar reactor type); the second is the grooved ring tricarbide NTR (a modern US design involving a unique fuel element geometry); and, finally, the SULEU reactor (Superior Use of Low Enriched Uranium, another modern US design with many unique reactor architecture and safety features).
So, to begin, what are carbides? Carbides are a solid solution of carbon and at least one other, less electronegative element. These materials are known for very high temperature melting points, and are often used in high speed tooling. Tungsten carbide, for instance, is used for both high-speed wood and metal bits, blades, and other tools.
In the NERVA reactors, niobium carbide and zirconium carbide were used as fuel element cladding, to prevent the fuel elements from being aggressively eroded by the hot hydrogen propellant. By the time of the XE-Prime test, the fuel particles suspended in the graphite matrix of the fuel element were uranium carbide, individually coated with zirconium carbide.
These are monocarbide compositions, though. There are other options: tricarbides (with three electronegative components, leading to a different lattice structure, as well as different mechanical and thermal properties) and carbide nitrides (a composite material containing both carbide and nitride structures; nitrides being a similar concept to carbides, but with N instead of C) – a possibility that is apparently of great interest to Russian NTR designers, but more on that later.
Even during Rover, however, the advantages of making the fuel elements themselves out of carbides were known, and research on the fuel elements began as far back as the 1960s in the US. This research included two of the test chambers in the nuclear furnace tests (examined in the Hot Fire Part 2 blog post to a small extent); but these were considered a more advanced follow-on technology, while the graphite fuel elements with encapsulated fuel particles were the ones that were intended to be used for the planned Mars missions.
Carbides have many advantages over many other materials. One example is that carbides are able to be built up with many different processes, most notably chemical vapor deposition (CVD), where a series of chemical precursors are used to deposit the different components in the carbide structure at much lower temperature than the melting – or decomposition – point of the carbide. Another advantage is that they tend to be relatively dimensionally stable when under high heating, meaning they don’t swell that much.
The USSR, on the other hand, decided very early on to commit to using carbide fuel elements for their NTR, and came up with a novel reactor architecture to both take advantage of the high temperatures of the carbide fuel elements, and to deal with the problems that they posed.
One major disadvantage to carbides is that they are prone to cracking… to a rather severe degree. This means that any cladding material needs to be able to handle this cracking. This was seen in the fuel elements in the NF-1 test, where every (U, ZrC)Carbide fuel element had a great deal of splitting; and was one of the reasons that this fuel was not considered the best option for early NTRs, until these issues were worked out.
Another disadvantage to carbides is the difficulty in manufacturing a consistent carbide, especially if multiple different types of electronegative components are used. Often there will be clusters of different monocarbides in what is supposed to be a tricarbide solution, meaning that the physical properties (notably, the fissile properties of the fuel itself) vary at different points in the fuel element. This can be made even worse if the fuel element is exposed to the hot hydrogen propellant stream as the H2 strips away the carbon (forming CH4, C2H2, and a number of other hydrocarbons); it also changes the chemical properties of the solution, sometimes allowing droplets of metal to form at well above their melting point, resulting in various other problems.
Oxides: The Familiar Fissile Chemical Composition
Carbides have been used for nuclear fuel elements for a very long time. The fuel pellets in later Rover and NERVA engines were encapsulated carbide beads spread in a graphite matrix. This allowed the fissile fuel itself to become hotter before decomposition occurred. To understand the advantages, though, we have to compare them to the other uranium-bearing compound that is more frequently used: uranium oxide.
In the oxide fuel pellets, the O2 would separate from the U, causing metallic crystals to form in the fuel pellet, changing its neutronic and chemical properties. To make matters worse, the O2 could then migrate outside the pyrocarbon or ZrC coating, causing chemical reactions in the surrounding graphite. All of this can occur below its melting point of 2,865 C (3,138 K). This changes the neutronic behavior within the fuel elements in different amounts at different locations within the reactor, causing control issues for the operators, and requiring more design work from the engineers to ensure the reactor can deal with these problems.
Another problem with UO2 is that it has very poor thermal conductivity. Temperature gradients of more than a thousand degrees C are seen in terrestrial fuel pellets of UO2 roughly the thickness of a pencil. There are many ways around this,the latest being the use of CERMET fuels, which use very small pellets of UO2, surrounded by refractory metals that are much better thermal conductors; but these metals themselves also limit the temperature the fuel element can operate at (with the new reactor designs that use beryllium for its’ moderation properties, the relatively low, 1,287 C melting point of Be determines the maximum specific impulse of the rocket).
The Advantages to Carbide Fuel Elements
(Chemistry warning! I’ll keep it as light as possible, but…)
Carbides, on the other hand, have some of the highest melting points known to humanity. Tantalum hafnium carbide (Ta4HfC5) has a melting point of 3942 C, the highest known melting point. How high the melting point is depends on a number of factors, including what materials are used and the ratio between those elements in the structure of the carbide itself.
Unfortunately, both tantalum and hafnium have fairly high neutron absorption cross sections, so they are not ideal materials for carbide nuclear fuel elements. These are typically made out of some combination of uranium carbide and either niobium carbide, zirconium carbide, or both.
Another advantage to using carbide fuel elements is that this allows the actual fissile fuel to be more evenly spread throughout the fuel element, creating a more homogeneous (i.e. consistent) fission power profile across the fuel element. This is an advantage to reactor designers, since the more heterogeneous the reactor, the more headache it is for the designer to ensure stable fission behavior in the fuel element. The more consistently the fissile material is spread, the more controllable it is, and the more evenly the power is produced, making the behavior of the reactor more predictable. This has been known since the beginning of nuclear power, and is why later Rover fuel elements were moving away from the coated pellets mixed into graphite style of fuel and toward a composite fuel element, where the uranium carbide fuel was spread in a webwork throughout the graphite matrix of the fuel element.
The Complications of Carbide Fuel Elements
What the actual melting temperature is for a given material is… complicated, though, for a number of reasons.
The first depends on what proportion everything is in, and this is difficult to get consistent. As noted in a recent paper on a unique NTR geometry (which we’ll look at in the next post), getting the perfect stoichiometric ratio (i.e. the ratio between carbon, uranium, and any other elements present) is virtually impossible, so compromises need to be made. Too much carbon, and the temperature drops slightly. Too little carbon, and the material doesn’t mix as well, causing areas that have lower melting points, or higher thermal conductivity, or a number of other undesirable properties.
The second problem is in mixing: a fuel element designer wants to have a material that’s consistent all the way through the fuel element, not discrete little clumps of different materials as one moves through the fuel element. Because of the way that carbide fuel elements are made (DC sintering, a similar process to spark plasma sintering that’s used for CERMET fuel elements), the end result is grains of NbC, ZrC, and UC2 side by side, rather than a mixture (a solid solution, to be precise) of (Nb, Zr, U)C; and each grain has different thermal, neutronic, and chemical properties. It is possible to heat the fuel element, and have the constituents become this ideal solid solution, as was discovered using CFEET for carbide fuel element testing (more on that in the next post as well). This offers hope for more consistent mixing of the elements in the fuel itself, but establishing the correct ratios remains a problem.
There’s one more big problem with carbide fuel elements, though: hydrogen corrosion. Unlike in graphite composite or CERMET fuel elements, the carbon that is stripped away by hot hydrogen is actually chemically bound to the uranium, zirconium, and niobium in the fuel element, not as a material matrix surrounding the chemical components that support fission in the fuel element. This means that if there’s a clad failure, the local ratio of carbon will change, causing free metal to form, either as a pure metal or an alloy, unevenly across the fuel element. This means that hot spots can develop, or parts of the fuel element will melt far below the melting temperature of the carbide the fuel element was originally made of. Flecks or droplets of metal can be eroded into the hot hydrogen stream, potentially causing damage downstream of the fuel element failure. In a worst case scenario, uranium could collect in areas of the reactor that it’s not meant to, creating a power peak in a spot that could be… inconvenient, to say the least.
These are challenges that carbide fuel element designers have always faced, and continue to face today. Careful chemical synthesis will definitely help, but there are limits to this. Preheating the fuel elements after sintering to ensure a more consistent solid solution is already showing considerable advantages in composition, and in material properties as well. Cladding the fuel element with carefully selected clad materials (often ZrC, which is already a component of the carbide fuel element, with a similar coefficient of thermal expansion and good modulus of elasticity), and ensuring consistent high quality application (usually through chemical vapor deposition these days, which has increased in quality and consistency a lot since the days of Project Rover) of the coating will eliminate (or at the least minimize) the erosion effects of graphite.
Another option that I’ve seen mentioned, but have been unable to find much information on, is an idea mentioned in Russian papers about their RD-041X engines: carbides and nitrides (which have a similar chemical composition, but with electronegative components ionically bonded with nitrogen, rather than carbon) in a solid solution. This leads to a more complex chemical structure, and may allow for less erosion of the carbon from the fuel element. Unfortunately, this literature is hard to find; and, when it is available, it hasn’t been translated from Russian. However, according to the most commonly available paper (linked in the references), adding a nitride component to the fuel element may boost the maximum fuel element temperature.
The Other Fuel: Plutonium Carbides
We don’t talk about plutonium much on this blog (yet), but plutonium carbides have been investigated to a certain degree as well. They may not be as attractive as uranium carbide for a number of reasons, but as a potential fuel element, they may show some promise.
Why are they less attractive? First is the neutron fission cross section of Pu is skewed much more to the fast spectrum in Pu than in U. This means that the more moderated the neutron flux, the more likely it is that when a neutron interacts with a nucleus of 239Pu, it won’t fission but continue up the transuranic chain. Many of these elements are also fissile, but again much more so in the fast spectrum. This means that more and more neutron poisons can build up in your core, requiring more and more reactivity to overcome. This also means that when it’s time to decommission the core, it will be much more radioactive than a similar U-fueled reactor (on average, there are of course a lot of factors that go into this). Finally, this means that the core has to have more fuel in it; and, unlike with uranium, there’s no “Low Enriched Plutonium,” the fraction of 238Pu (used in RTGs) or 240Pu (which is gamma-active, and a headache) is very low. This is convenient if you’re making fuel elements, but a very different regulatory game than LEU, with huge restrictions on who can work with the fuel element materials for development of an NTR.
Second, 239Pu is illegal to use in space, in accordance with international treaty. Now, LEU235 is also illegal, but that is more likely to change, since it involves having less concentrated fissile material in space, unlike the use of Pu, which is considered a major nuclear proliferation risk, even if it’s out in space. The treaty was written to prevent nuclear weapons in space sneaking in the back door, and Pu has been (in the public’s mind) intimately tied to nuclear weapons development from day 1.
Mixed carbide fuels (containing both uranium and plutonium) have been investigated as an alternative to MOX (mixed oxide) fuels for fast breeder reactors, either in the (U, Pu)C or the (U, Pu)2C3 phases. The usual benefits of carbides over oxides apply to this fuel form: higher metal density and better thermal conductivity being the main two. Due to a number of challenges, including very low oxygen requirements for fabrication, minimal experience with fabrication of mixed carbide fuels, and the general lack of information on the chemistry of PuC, this is a largely unknown field, but research is being conducted to extend our knowledge of these areas.
At present, these materials are a curiosity, although they could lead to advanced fuels for terrestrial use. Until their chemistry and materials properties are better known, however, it is unlikely we’ll see an NTR powered with mixed carbide fuel.
How are Carbides Used in NTRs?
“Traditional” Carbide-fueled NTRs
In Rover, carbide fuel elements were researched that had a very similar form factor to the fuel elements. These were hexagonal in cross section, about 33 cm long, and clad in NbC. The main difference was that there was a single large hole, rather than nineteen small holes. An NTR was in the early concept design, but was never put through the reactor geometry refinement process.
Designs have been proposed over the years using hexagonal prism fuels similar to Rover carbide fuel elements, but none are currently under development, as far as I can see. This doesn’t exclude their use, even with LEU, but NASA and the DOE are currently pursuing other fuel element geometries.
The Other Tradition: Russian NRE
Russia has been in the nuclear thermal rocket business for as long as the United States, but their design philosophy is hugely different from the American one. Just like NASA and the DOE don’t use the term “nuclear thermal rocket” (NTR), instead preferring “nuclear thermal propulsion” (NTP), Roscosmos and Rosatom (who work together to develop the Russian program) use the term “nuclear rocket engine”, or NRE.
The design changes start with the fuel element design, extend through the basic geometry of the reactor and beyond, and have major implications for testing and materials options with this system.
First, let’s look at the fuel elements. One of the considerations for fuel element design is the amount of surface area that can be contacted by the propellant. Thermal transfer is determined by the thermal emissivity of the fuel element material, and the thermal conductivity and transparency of the propellant. The more surface area, the more heat is transferred, given those previously mentioned factors are equal. Rather than using a fuel prism as American NTP has done, with increasing number of holes through a hexagonal prism, the Russian NRE uses what is commonly known as a “twisted ribbon” design, where a rectangular prism (or any number of other designs, such as a cluster of rods, square prisms, or other shapes- see the image above for the variations that have been tested) is rotated along its long axis. A cluster of these fuel elements are placed in a tube (known as a calandria, similar to the design used in CANDU reactors, but with different geometry and materials), ending in a nozzle at the end of the bundle.
Unlike with the American NTP designs, there isn’t a single fuel element cluster running down the center of the NRE. In fact, there’s NO fuel at the center of the reactor. The Russians don’t use a homogeneous reactor design, either for neutronic power or thermal energy. The center of the reactor, rather than containing fuel, contains moderator. Since the fuel elements (and therefore all the sources of heat for the reactor) are spread around the periphery of the reactor core, rather than being evenly distributed in the core, this means that a moderator with much lower melting temperatures can be used in the design (both zirconium and lithium hydrides are mentioned as options, neither of which would be able to withstand the temperatures of a homogeneous core NTR). This also means that a bimodal design (known in the Russian program as a “nuclear power and propulsion system,” or NPPS, rather than BNTP as NASA calls it) can integrate the working fluid channels more easily into the design without a complete redesign of either the fuel element or the header and footer support plates. We’ll cover BNTRs in a later post, including the NPPS, but it’s worth mentioning that this design offers more design flexibility than the traditional, hexagonal prism NTP fuel elements used in American designs.
Finally, due to the fact that a number of fuel element bundles are radially spread across the reactor, an individual fuel bundle can be tested on its’ own in a prototypic neutronic and thermal environment, rather than needing to test the entire NTP core in a hot fire test, as is required for the American designs. This testing has been conducted both at the EWG-1 research reactor [with ten consecutive restarts, a total testing time of 4000 s (although how much was at full power, and what sort of transient testing was done, is unknown), at a maximum hydrogen exhaust temperature of 3100 K, achieving a theoretical specific impulse of 925 s and a power density for the system of 10 Mwt/L] and at the rocket test stand in Semipalatinsk (although those test results are still classified). The Russians have also done full-scale electric heating tests of NRE designs, settling on two: the RD-0410 (35 kN thrust, for unmanned probes – and possibly for proof-of-concept mission use) and RD-0411 (~392 kN of thrust, for crewed missions). Statistics for the RD-0410, based on these electrically heated tests, can be seen below:
Sadly, there isn’t much more information available about the current NRE designs and plans. We’ll come back to its’ variant, the NPPS, when we look at bimodal designs in the future.
Grooved Ring NTR: Not All American Designs are Hexagonal
This is a new NTR design, designed around the use of a (Zr, Nb, U)C fuel element of a very different shape than the traditional hexagonal prism, currently under development at NASA and the University of Tennessee. Just as with the twisted ribbon fuel elements, the fuel element geometry for this NTR has been changed to maximize surface area, and allow for more heat to be transferred to the propellant. This both maximizes the specific impulse and minimizes the amount of propellant needed for cooling purposes (however, H2 remains the best moderator available, and a minimum amount for neutronic reasons will always be needed, even if not for cooling the fuel elements).
The fuel elements are radially grooved discs of uranium tricarbide (Nb, Zr, U)C, although hafnium and tantalum were also investigated (and eliminated due to the much higher neutron absorption rates). The hydrogen flows from the outside of a stack of these fuel elements, separated with beryllium spacers, and then flows down a central channel.
Due to the unique geometry of this fuel element design, much optimization was needed for the groove depth, hydrogen flow rates, uranium density in the fuel element (in the initial design, 95% enriched HEU was used for ease of calculation, however with additional optimization and research into stoichiometric ratios of U with the other electronegative components, the authors believe less than 20% enrichment is possible), and other factors.
Thermal testing, including hot hydrogen testing using CFEET, has been carried out at Marshall SFC, using vanadium as a surrogate for depleted uranium. The team hopes to continue to refine such factors as manufacturing consistency, improved mixing of the solid solution of the carbide, and other manufacturing issues in carbide fuels, before hopefully moving on to electrically heated carbide tests using depleted uranium (DU) to optimize the carbide chemistry of uranium itself.
This NTR offers the potential for 3000 C exhaust temperatures at 4 psi. Unfortunately, due to the preliminary nature of the work that has been carried out to date (this reactor design is less than a year old, unlike the designs that have gone through decades of development of not just the fuel elements themselves but also the engine system), thrust and theoretical specific impulse using this reactor design has not been determined yet.
This novel fuel element form offers promise, though, of a new NTR fuel element geometry that allows for better thermal transfer to the propellant, and the team are performing extensive material fabrication and optimization experiments to further our understanding of tricarbide fuel element performance and manufacture, in addition to developing this new fuel element form factor.
Tricarbide Foam Fuel Elements: You REALLY Want Surface Area? We Got It!
This is a very different carbide fuel form, with novel manufacturing practices yielding a truly unique fuel element.
Most solid core fuel elements are chunks of material, no matter what form they take (and we’ve seen quite a few forms in this post already), with the propellant flowing around or through them; either through holes that are milled or drilled, the surface of the twisted ribbon, or through grooves cut in a disc. That’s not the case here, however!
The team at Sandia National Laboratory, Ultramet, Inc., and the University of Florida have come up with a new take on carbide manufacture, utilizing chemical vapor deposition (CVD, a common method of carbide manufacture) on a matrix that starts life as open-pore polyurethane foam. This foam is then pyrolized (baked… ish) to form a carbonized skeleton of the foam structure. This is then heated, and CVI (chemical vapor infiltration, a variation of CVD) processes are used to impregnate the carbonized skeleton with uranium, zirconium, and niobium; turning the structure’s outer surfaces to (U, Zr, Nb)C carbide (a number of factors affect the depth of the penetration). Then, CVD is used to coat the new carbide structure with ZrC or NbC to clad the more chemically fragile tricarbide, and protect it from the H2 propellant that will flow through the open pores remaining after this carbidization and CVD coating process.
This concept has been tested using tantalum as a surrogate for uranium (a common choice for pre-depleted uranium electrically heated testing of carbide fuel elements), with two foam densities, 78% and 85%; leading to the discovery that there’s a trade-off: the 78% had better thermal transfer properties, but the 85% offers more volume for the fissile material, meaning that lower enrichment was possible.
The team members at Sandia made a preliminary MCNP model of an NTR for use with these fuel elements, with a number of unique features. This was a heterogeneous core (meaning uneven fuel distribution), with 60% porosity foam fuel, using yttrium hydride for the moderator (which has to be maintained below 1400 K by circulating hydrogen between it and the fuel), and with a Be reflector. For these initial modeling calculations, 93.5% enriched HEU was used. It was discovered that a 500 MWt NTR was possible using this fuel form, but due to the unoptimized, preliminary nature of this design, values for thrust and specific impulse are still up in the air.
INSPI at the University of Florida will be conducting electrically heated hot hydrogen tests on DU-containing tricarbide fuel foams in the temperature range of 2500-3000 K, as these fuel foams become available, although the timeline for this is unclear. However, research is continuing in this truly novel fuel form, and the possibilities are very promising.
Carbides: Great Promise, with Complications
As we’ve seen in this post, carbide fuel elements offer many advantages for designers of nuclear thermal rockets. Their high melting point allow for higher propellant exhaust temperatures, improving the specific impulse of an NTR. Their ability to have their properties manipulated by changing the composition and ratio of the components allows a material designer to optimize the fuel elements for a number of different purposes. Their strength allows for truly novel fuel forms that give an NTR designer a lot more flexibility in design. Finally, their similar coefficient of thermal expansion, and often good modulus of elasticity, make them important materials for use in all NTRs, not just those fueled with fissile-containing carbides.
However, the chemical and materials properties of these substances, manufacturing processes required to consistently produce them, and modes of failure (including the implications for these types of failure in an operating NTR) show that there’s still much work to be done in order to bring carbide fuel elements to the same level of technological maturity currently enjoyed by graphite composite fuel elements.
The promise of carbides, though, makes developing the chemistry of fissile-bearing carbides of all forms, perhaps most especially uranium tricarbides, a worthy goal for the advancement of nuclear power in space. This research has been ongoing for decades, continues worldwide, and is bearing fruit.
Uranium Dioxide Wikipedia page: https://en.wikipedia.org/wiki/Uranium_dioxide
Thermodynamic and Transport Properties of Uranium Dioxide and Related Phases, IAEA 1965 http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/24/071/24071477.pdf
Thermal Conductivity of Uranium Dioxide, IAEA 1966: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/34/065/34065217.pdf
Nuclear Thermal Propulsion Carbide Fuel Corrosion and Key Issues; Pelaccio et al 1994
Evaluation of Novel Refractory Carbide Matrix Fuels for Nuclear Thermal Propulsion; Benensky et al 2018
Ultra High Specific Impulse Nuclear Thermal Rocket, Part II; Charmeau et al 2009
Study of a Tricarbide Grooved Ring Fuel Element for Nuclear Thermal Propulsion; Taylor et al 2017
Plutonium Tricarbide Isomers: A Theoretical Approach; Molpeceres de Diego, 2015 https://uvadoc.uva.es/bitstream/10324/13556/1/TFM-G413.pdf
Mastery of (U, Pu)C Carbide Fuel: From Raw Materials to Final Characteristics, Christelle Duguay, 2012
Rover Carbide Fuel Elements
Nuclear Furnace-1 Test Report, LA-5189MS; Kirk et al 1973
Performance of (U, Zr)C-Graphite (Composite) and of (U, ZR)C (Carbide) Fuel Elements in the Nuclear Furnace 1 Test Reactor, LA-5398-MS; Lyon 1973
Nuclear Rocket Engine
Russian Nuclear Rocket Engine Design for Mars Exploration Zakirov et al 2007 https://www.researchgate.net/publication/222548572_Russian_Nuclear_Rocket_Engine_Design_for_Mars_Exploration
Ticarbide Grooved Ring NTR
Grooved Fuel Rings For Nuclear Thermal Rocket Engines tech brief; MSFC 2009 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090008640.pdf
Multiphysics Modeling of a Single Channel in a Nuclear Thermal Propulsion Grooved Ring Fuel Element; Barkett et al 2013 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130011208.pdf
Study of a Tricarbide Grooved Ring Fuel Element for Nuclear Thermal Propulsion; Taylor et al 2017 Conference paper: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180002033.pdf Presentation Slides: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180002060.pdf
Tricarbide Foam Fuel Element
A Tricarbide Foam Fuel Matrix for Nuclear Thermal Propulsion, SAND-2006-3797C; Youchison et al 2006