Hello, and welcome back to Beyond NERVA, for our second installment of our blog series on NASA’s new nuclear thermal propulsion (NTP) system.
In the last post, we looked briefly at nuclear thermal rockets (NTRs) in general, and NERVA’s XE-Prime engine, the only time a flight configuration NTR has ever been tested in the US. We also looked at the implications for modern manufacturing and methods that would be used in any new NTR, since we are hardly going to be falling back on 60’s era technology for things like turbopumps and cryogenic storage of fuels. Finally, we looked briefly at a new material for the fuel elements, a composite of ceramic fissile fuel and metal matrix called CERMET.
This post is a deep dive into CERMET itself, including its’ design and manufacture, a little bit of its history during the Rover program, its’ rebirth in the 1990s, the test stands currently used for non-nuclear testing and some current ideas to continue to improve its’ capabilities. This is going to be more of a materials and fuel elements deep dive post, the next post will look at the engines themselves, the hot-fire test options and plans will be covered in the following one, and our last post in the series will look at other low-enriched uranium designs that don’t use CERMET fuels, but instead use carbides.
Fuel elements are where the fission itself occurs, and as such tend to be perhaps the most important part of any nuclear reactor. In the case of nuclear thermal propulsion systems (NTR, NTP to NASA), these come in three broad categories: graphite composite ((GC) such as in NERVA, which we looked at in the last post), CERMET, and carbides (something we’ll look at down the road in this series). Each have their advantages and disadvantages, but all have the same goal: to heat the propellant gas passing through the reactor as much as possible, in order to produce the maximum thrust and efficiency that the engine can provide.
CERMET is a higher-temperature option than the GC elements used during the majority of Rover (although CERMET FEs were tested as part of Rover), and allows for much more control in fabrication thanks to the unique structure of the material itself. In fact, it’s able to provide the possibility of using low enriched uranium for NTR propulsion, which makes it incredibly attractive to NASA.
CERMET composites are used in many different areas of manufacturing and industry, for tooling, bearings, and other materials where hardness, heat resistance, and thermal conductivity are all needed, and the combinations used vary wildly. Different CERMET combinations have different properties, and as such are an incredibly flexible material choice.
Even in the broader nuclear field, there are other CERMET fuel elements being developed, to make more accident-tolerant fuels for terrestrial reactors. These are obviously very different in design (U3O8-Al CERMET fuels are one of the IAEA’s accident tolerant fuels of interest, and are also outside the scope of this blog post), but keep in mind that every time you hear about CERMET nuclear fuel, it’s not necessarily flying humans to Mars, it may be coming soon to a nuclear power plant near you!
However, the focus of Beyond NERVA is space, so let’s turn back to the skies. How is it that CERMET will make NASA’s new nuclear thermal rocket work? To understand that, we first need to understand what CERMET is, and why NASA decided to pick it as a fuel type of interest 20 years ago.
CERMET Fuel Elements
CERMET is an acronym for CERamic METal composite, and was one of the first fuel forms tested as part of Project Rover, primarily by Idaho National Laboratory (INL) and General Electric, in the 1960s, and were picked up again in the 1990s as an alternative to carbides for advanced nuclear thermal fuel elements. This fuel form offers increased temperature resistance, better thermal conductivity, and greater strength compared to the graphite fuel elements that ended up being selected for NERVA, but unfortunately they also required much more development. Other options for fuel elements included advanced graphite composite and carbide fuel elements of various types, which are introduced in the NTR-S page and will be examined in their own posts.
CERMET fuel elements are a way to gain the thermal resistance and chemical advantages of oxide fuels and the thermal conduction properties of metal fuels in a single fuel form. In order to have both, uranium oxide (UO2) fuel pellets measured in millimeters or micrometers are suspended in a metal matrix, usually tungsten. To protect the oxide from any potential chemical change, these microparticles of UO2 are usually coated before the fuel element itself is made. Then the metal matrix is made, usually using a hot isostatic press (HIP), where the powdered material is placed in a mold, then pressed and cooked, although other techniques are possible as well.
There is another characteristic that makes CERMET fuel attractive in the west: it offers the possibility to use low-enriched uranium instead of highly-enriched uranium by carefully selecting the metals that the matrix is made out of to maximize the amount of moderation available from the fuel elements themselves. Low enriched uranium (LEU) offers one major advantage: a lowering of the security burden required to handle nuclear material needed to test reactor components. The vast majority of NTR systems that have been proposed over the years have been fueled with highly enriched uranium (HEU), which is over 95% 235U. This isn’t quite to bomb-grade 235U, but it’s close, and relatively easy to complete the final few steps of isotopic enrichment needed to be able to construct a weapon. (There are many other safeguards in place that make the loss of HEU unlikely, not the least of which is that the reactor won’t even be on the planet anymore, but nuclear non-proliferation is a serious concern that must be addressed in depth – just not here! For a good, in-depth look into non-proliferation I recommend (among many others), the Nuclear Diner blog, most especially the posts on the Iran nuclear treaty, from a technical-policy point of view.) Due to this increased cost (security, permitting, site re-licensing, etc.), the vast majority of institutions are unable to assist NASA and the DOE with their testing of NTR components. This is a problem, because much of the experimental engineering testing work is often done by Master’s and Doctoral students working on their dissertations. Without access to the materials used in construction, this isn’t an option, leaving the testing to NASA and DOE personnel (who are far more expensive and busy), and slowing up the whole development process. By using LEU, these institutions (that are mostly already certified to work with LEU, and many even have research reactors) are able to more fully participate in the development of the next generation of NTRs.
Often, the assumption is that HEU is superior to LEU, because the majority of LEU is fertile, not fissile: it can absorb a neutron (becoming 239U), then go through two beta decays (239Np, 239Pu), and then become fissile plutonium 239, and then can undergo fission. Why not bring along only the stuff that can split already? Breeding is a far messier process in real life than on paper, after all, and the neutronic environment is far more predictable with (mostly) only one isotope of uranium present. However, breeding occurs in all fuel elements, to the point that by the time fuel is removed from a reactor in the current fleet, the majority of the energy isn’t coming from fissioning 235U, but 239Pu. The amount of breeding that occurs is called the breeding ratio, a ratio of 1:1 means that exactly as much fissile material is being produced as is being burned. Generally speaking, this ratio is higher than 1, in order to account for the buildup of fission byproducts (or poisons) produced over the course of the fuel element’s life. The breeding ratio for this type of reactor is likely not much above 1 (most aren’t, unless it’s meant to either fuel other plants or to produce weapons, neither of which is a goal with a rocket engine); one nuclear engineer of my acquaintance suggested a back-of-envelope guess of about 1.01 for the breeding ratio, but this will largely depend on the details of the fuel element that is finally selected, the reactor core geometry, and the amount of propellant being used (among other factors). With this being the case, assuming careful management of the reactor’s neutron budget (how many neutrons are bouncing off/being absorbed/causing fission/being generated, compared to what’s needed to ensure stable operation), the majority of the “useless” 238U can in fact be burned. A paper by Vishal Patel et al (sorry about the paywall, I try and avoid them but they’re very common in nuclear engineering) suggests that the overall system could actually mass less for the same power output, which would mean that it would be better from an engineering perspective to use LEU rather than HEU. These results were for one particular reactor geometry, but the PI did mention in private correspondence that this isn’t necessarily a difficult thing to achieve, as long as the designers don’t remain tied to one particular fuel element geometry, and so could apply to many different reactor architectures.
CERMET Composition and Manufacture
CERMET fuels have many different components to them, and as such many different physical and chemical properties that have to be accounted for. However, the primary concern from a materials point of view tends to be the thermal limitations of the materials used in the FE.
As with any composite material, there are quite a few steps to making CERMET fuels. This will be a shallow but reasonably thorough look at the manufacturing challenges on each step of the way.
In order to construct a CERMET fuel element, first the fissile fuel granules need to be made. This is not too different from the process used to make terrestrial fuel elements, which are uranium oxide (UO2) based, the main difference is the size of the resulting fuel: instead of having fuel in a pellet the size of the last joint of your finger, it’s a roughly spherical granule ~100 um in diameter.
There are relatively few suppliers for this form of UO2, and the most common one (BWXT) does not offer it at the price that NASA can work with. Y12 has plenty available in the right size, but they’re angular and irregular in shape; this is a problem because the release of neutrons and fission products is difficult enough to calculate when the beads are spherical, due to their distribution in the overall matrix, if they aren’t spherical enough that will affect the direction and spectrum of the resulting neutron flux, and therefore the behavior of the reactor as a whole. NASA, fortunately, has the capability to spherize these too-angular granules, though (due to their experience and equipment for plasma spray coatings in the Plasma Spheroidization System in the Thermal Spray Laboratory), and both Oak Ridge NL and the Center for Space Nuclear Research are working on gellation processes that allow for these small particles to become spherical.
After the sphere is made, it (usually) has to be coated with a cladding material for three reasons: first, the hot hydrogen propellant will attack the oxide very aggressively; second, the metal matrix surrounding the fissile fuel is unable to completely trap the fission products in the fuel element, leading to irradiated exhaust; and finally the UO2 in the fuel particles tends to break down, so the clad keeps the now-crystallized U in basically the same place as it was before FE thermal damage. The first coatings experimented with were pyrolitic graphite, the same as is used in TRISO fuel. However, this still has a reasonably low melting temperature (for something in an NTR), so tungsten was experimented with next. Attempts to solidify W powder around the UO2 particles led to inconsistent or relatively poor quality results, and so other options have been explored. These include chemical vapor deposition (CVD, for a long time the preferred method), plasma deposition, and other options. In the last couple years, a new technique has been shown to offer better results, which uses fine grains of tungsten rather than the CVD spray. While not as consistent in its coating, it offers advantages to fission fragment capture and overall coating consistency that make it superior to the CVD coatings.
After the fuel particles themselves are manufactured, it’s time to make the fuel element itself. This is done by pouring (at carefully selected ratios, and in this case in particular locations) the powdered tungsten and fuel particles into a mold (usually niobium), placed on a vibrating table to settle the particles, then compressed at high temperatures for extended periods of time. This process is known as Hot Isostatic Press (HIP) sintering, and continues to be used in many fuel element designs. However, the size of the granules, the amount of pressure and temperature applied for how long, and many other factors play into HIP sintering, and especially in a field where crystalline phase can be a major determining factor of if your reactor will work or not (in fuel, moderator, and even some structural components), having a consistent and high-quality matrix around the fuel particles is essential. Again, there are processes that have been proposed in recent years that offer benefits such as lower temperature and shorter time, but we’ll go into those below.
Initially, the result of these processes was a squat cylinder with coolant channels, which would then be milled and assembled into a fuel element. As time went on, and both techniques and materials understanding improved, the fuel elements began to be cast in longer and longer single units.
Finally, the external clad is applied to the fuel elements. Both chemical vapor deposition and milled inserts have been used over the years for the propellant channel clad, with bubbling in the early tests and differences with the thermal expansion coefficient of the different materials (the clad and the fuel element it’s bonded to would swell at different rates, leading to a number of materials problems) led to the use of milled inserts being used from an early stage. These inserts (usually tungsten or niobium) are then welded to end plates and external clad sheets, also usually niobium.
The Beginnings of CERMET Fuels
Originally developed by Argonne National Labs (ANL) and General Electric(GE) in the 1960s, what were then called composite fuel elements (CFEs) are a type of fuel that gained attention for NTRs in the early to mid 1990s due to the increased thermal conductivity that the metal matrix offers to the FE as a whole. GE developed what would ultimately become the GE710 fuel element from 1962 to 1968, using HEU. After over 300,000 hours of in-environment testing, this program collected a significant amount of data.
According to Gordon Kruger (of General Electric at the time of his presentation to the joint NASA/DOD/DOE Nuclear Thermal Propulsion workshop in 1990, the “seed” source as it were for this section), there were two different ANL designs: one was a 100 klbf, 2,000 MWt NTR, with a thrust-to-weight ratio of 5:1 and offering 850 s of specific impulse, the second was a smaller, 200 MWt design. This was (as with most CERMET designs) a tungsten-uranium oxide (W-UO2) fuel element. The fuel particles themselves were chemically stabilized by doping them with gadolinium, and the clad for the fuel particles was W doped with Rhenium. The fuel element developed in this process is now called the ANL-2000 CERMET FE, and remains a popular one for NTR designers. It has a very high number of propellant channels (331 per FE) to allow for greater cooling capability of the fuel.
The GE design, on the other hand, was meant to be more versatile The base design was for a high temperature gas cooled reactor (HTGR), with helium as a working fluid, designed for a 10,000 hour life. Those same fuel elements, in a different core geometry, could instead burn much faster, and much hotter, for use as an NTR (with cryogenic H2 propellant), but the harder use (and harsher chemical environment) correspondingly shortened the life of the fuel elements. This is the GE 710 fuel element, which in a slightly modified form – known as the GE 711 – is still a strong contender for NTR designs, and was the front-runner for the LEU NTP that NASA is working on. With 64 propellant channels of larger diameter, this FE offers a trade-off of easier manufacture (due to the larger, less numerous boreholes) with the potential for greater thermal differences in the FE due to the greater distance between the channels.
Both these designs have many things in common, such as the hexagonal prism shape, and information sharing between the groups was a regular thing. As such, techniques used for the different stages of manufacture was common as well.
Both designs used spheres of UO2. These can still be manufactured by two places in the US (Oak Ridge National Labs and BWXT), but there are challenges to getting the pieces to be spherical when they’re that small, so the price is correspondingly high. This indicates at least something of a learning curve when it comes to this stage of manufacture, both for ensuring homogeneity of fissile fuel load (if it’s poorly mixed, hot spots and dead zones can form, leading to very bad things – or nothing at all), and for size and shape consistency. Because of the extreme temperatures, both during manufacture and operation, the gadolinium (Ga) doping experimented with at ANL became essential to stabilize the UO2, and to prevent the dissociation of the oxygen and uranium. Nursing the dissociation temperature up was a consistent effort throughout this process.
The clad on the fuel pellets is a challenge another way, as well: applying an even coat of tungsten across the tiny spherical oxide pellets is a major technical challenge, and one that was addressed at the time with chemical vapor deposition (CVD), where the tungsten is liquefied and then sprayed (under a certain set of conditions) over the oxide spheres. Because the droplets are small, they have a high relative surface area, so they are able to coat a material that wouldn’t normally be able to resist the temperature of the molten substance (in this case tungsten, doped with rhenium to lower the melting point). This can lead to a very even coating, if the two substances are chemically compatible, and if the conditions are just right enough for the droplets to be able to spread out enough, and spread evenly enough across the surface. This is a very large challenge, and one that took a lot of time and energy from the teams designing the fuel elements. A competing process, pressure bonded cladding, was also examined for both the fuel particles and the clad for the fuel element itself.
Once the fuel particles were fabricated, the metal matrix of the fuel element could then be fabricated. Hot isostatic press sintering (HIP) was the preferred method of manufacture for the fuel elements. This led to complications stabilizing the UO2 in the fuel (which isn’t able to stand the temperatures of molten tungsten, hence the sintering) used by both groups, hence the Gadolinium doping of the fuel pellets. The trade-off was always how to increase the density of the tungsten (and therefore the energy density and strength of the FE as a whole) while decreasing the amount of decay in the UO2, either by lessening the temperature or the time that the material is cooked, or by chemically stabilizing the oxide itself. Once sintering was complete, the mold is set aside to cool, then the CERMET plug is removed.
The result of this exercise was known as a compact. This was then machined to drill propellant holes and do final shaping, and its fissile fuel load was assessed. It was labeled, and set aside until a sufficient collection of machined compacts had been completed. These were then stacked according to fissile fuel load, and then the tungsten fuel element end plates, external clad and propellant clad tubes were welded into place to form the overall hexagonal prism shape. These are then assembled in a number of different ways for either an HTGR or an NTR.
The most mature designs to come out of this development series was the GE 710 fuel element, with 19 working fluid channels, and the ANL 2000 designs with 312 coolant channels. In many ways, these form a baseline for CERMET fuels as the NERVA XE-Prime serves as a baseline for NTRs as a whole. Many CERMET NTR designs use this as their baseline fuel form, and for good reason. This fuel element was tested for HTGC reactor use in the 1970s, and showed promising results. However, gas cooled reactors were never popular in the US, and production ended.
The Rebirth of the Idea, and the Building of Test Stands
After the cancellation of the GE710 project, CERMET FE design went quiet for a number of decades, until the 1990s, when the idea was revived again after Project Timberwind (and the rest of the Strategic Defense Initiative) got shot down during defense cuts under President H.W. Bush.
In the early 1990s, focus shifted back from the pebblebed and toward other options. While it was acknowledged that graphite composite was better developed, and carbides offered higher-temperature operation, CERMET fuels were seen as a good compromise. At some point after the 1991 Nuclear Thermal Propulsion conference, focus shifted to CERMET fuels as being compatible enough with the legacy NERVA systems and data collected, while also being easier to work with than carbide fuels. A good overview of the decision process to proceed with CERMET fuels can be seen in Mark Stewart’s presentation for NETS 2015, “A Historical Review of CERMET Fuel Development and Engine Performance Implications” (paper and slides).
Many of the best-known designs for NTRs in the last 25-30 years have been the work of either Michael Houts at NASA’s Marshall Spaceflight Center or Stan Borowski, of NASA’s Glenn Research Center. Looking at the systemic implications of not only the rocket engineering side of things, but the mission analysis, development cost, and testing options available to develop NTRs, they firmly established a new baseline nuclear rocket, seen in popular artwork for over 30 years. Many of these designs were based around a smaller Rover-legacy advanced graphite composite fueled reactor known as the Small Nuclear Rocket Engine. Ths idea was to design an engine just big enough to be useful, and if it wasn’t powerful enough, just add another engine! We’ll look at this design more in depth at a later point, but it is important in that it was a mid-1990’s design that could use CERMET fuel, possibly the first modern one, and is in many ways the baseline for what a modern NTR can do.
In order to gather the information needed to develop the nuclear fuel elements, a number of test stands have been built by NASA in recent years to thermally and environmentally test experimental fuel elements, using depleted uranium (DU) and induction heating. The two most commonly used are the Nuclear Thermal Reactor Element Environmental Simulator (NTREES) and the CERMET Fuel Element Environment (CFEET) test stand. Since hot-fire tests were not an option anymore, and the experimental fuel elements still needed to be exposed to the thermal and environmental conditions of an operating NTR, these were seen as the best way to spend what little money had been allocated to nuclear spaceflight over a number of years.
The Nuclear Thermal Reactor Element Environmental Simulator was first proposed by William Emrich of NASA’s Marshall Spaceflight Center in 2008, and was designed to simulate everything but the radiation environment that an NTR fuel element would experience. This was the next best thing possible, short of starting nuclear hot-fire tests again (which neither the regulations nor the budget would allow): many of the other questions that needed to be answered in order to build a new NTR was being addressed in other programs; for example, cryogenic hydrogen was a major challenge in Rover, but research had continued through chemical propulsion systems. The questions that remained mostly had to do with either core geometry or the fuel element itself, and most of those questions were chemical. By substituting other materials (such as ZrO2) with similar properties (thermal behavior, etc) to UO2 in initial tests, and then move on to the more difficult to use depleted uranium (DU) for more promising test runs (as we saw in the KRUSTY post, DU carries a far stricter burden as far as safety procedures and regulation), testing could continue- and be more focused on the last details that needed to be worked out chemically and thermally.
When the test stand was being designed, flexibility was one of the main foci of the design decisions that were made; after all, new equipment for nuclear thermal testing is incredibly rare, and funding for it is virtually impossible to come by, so one piece of test equipment can’t be specialized to just one design, to sit collecting dust on the shelf after that project is canceled and a new one comes along with requirements that make the old equipment obsolete.
NTREES consists of a pressure vessel, an induction heating arrangement for the test article, a data acquisition unit, and an exhaust treatment system. Hydrogen is introduced at the needed pressure and rate into the pressure vessel, where it encounters the test article. Measurements are taken through view ports in the side of the pressure vessel, and then the hot hydrogen is cooled by adding a large amount of nitrogen. This gas mixture is then passed through a mass spectrometer, and then further cooled and collected. The mass spectrometer is designed to be able to detect a wide range of atomic masses, so that uranium-bearing compounds can be detected to measure fissile fuel erosion; with pressure, temperature, and flow sensors they make up the inputs for the data acquisition system.
The bulk of the test stand is the pressure vessel, which is water cooled, ASME code stamped, and has a maximum operating pressure of 6.9 megapascals (MPa). Because of the need for flexibility, NTREES can handle test articles up to 2.5 meters long, and 0.3 m in diameter. A number of sapphire view ports along each side of the pressure vessel are used for instrumentation and observation. Along the bottom are ports for the induction heater used to bring the test article up to temperature (one of these can also be modified for vacuum system use). The induction heater is a 1.2 MW unit, upgraded in 2014, although the upgrade wasn’t immediately able to be fully implemented until later due to having to wait for funding to upgrade the N2 cooling system to handle the power increase.
After the now-hot H2 leaves the test article, it enters a gas mixer, which adds cold nitrogen to cool the H2 rapidly, and to dilute it with a more inert gas to reduce explosive hazards. This sleeve is also water-cooled, which draws out even more heat from the gas. The lessons learned about handling gaseous and liquid hydrogen were well-learned, and multiple safety systems and design choices have gone into handling this potentially dangerous and reactive gas safely. Another example of this is at the hot end interface with the test article: there is more pressure on the nitrogen outside the H2 feed, so that N2 inbleeding prevents any H2 leakage at a seal which would be very prone to failure due to the high temperatures involved.
The mixer is also the first stage of the effluent cleanup system, designed to ensure that no potentially harmful chemical releases occur when the exhaust is released into the atmosphere. The second stage of the cleanup system is a water cooled sleeve that further chills the gas mixture (this system was upgraded in 2014 as well, to allow the system to carry away all the heat generated – and therefore be able to run longer-duration tests at higher temperatures). Finally, a filter and back-pressure system is used to clean the now-cool gas before it is exhausted through a smokestack on the outside of the facility.
After dilution, the gas stream passes in front of a far more flexible spectrometer than usual. Most spectrometers only examine a relatively small band of the periodic table, because they’re only needing to measure particular elements. In this case, the elements that could be in the exhaust stream are spread fairly well across the periodic table, and as such a more versatile spectrometer was needed to be able to accurately assess the effluent stream.
The data acquisition system consists of the mass spectrometer, pressure sensors, gas temperature sensors, flow sensors, thermocouples for general temperature measurements, H2 detectors in the chamber and the room, and pyrometers to measure the temperature of the test article itself, and the associated electronics to collect the information from these sensors.
The design of the facility was safety-oriented from the beginning, with every precaution being taken to handle the GH2 safely. If you’re interested, the systems are looked at more on the NTREES page.
When put together, this facility allows for chemical and thermal testing of NTR fuel elements for extended periods of time in an environment that is missing only one component to mimic the environment of an NTR core: radiation. This means that fuel elements can be easily tested for manufacturing technique verification, clad material choice, erosion rates of fuel element materials, and other questions that are primarily chemical or mechanical rather than nuclear in origin.
There is one other difference between this test stand and the environment that a nuclear fuel element will, and that’s the source and distribution of the heat. In NTREES, the induction heating coil is the source of the heat. Power distribution starts on the outside of the fuel element, and While the coil can be customized to a certain extent to manage the thermal load for different test articles, the spiral pattern will still be there, and the heat will be generated in the fuel element following the rules of inductive heating, not nuclear heating.
In a nuclear fuel element, considerable effort is taken to ensure that there is an even distribution of heat across the fuel element (taking into account all factors), because having a “hot spot” in your fuel element (higher-than desired density of fissile material) can do bad things to your reactor. Because of this, the power density is carefully assessed during manufacture and assembly. In the fuel element, temperature tends to peak around the edge of the fuel element, but otherwise be consistently distributed throughout. This difference can be significant, especially for clad/matrix interfaces where local hot spots can exacerbate thermal expansion differences and clad failure.
The radiation environment in a nuclear reactor will cause additional swelling, and neutron damage, fission product buildup, and other effects will need to be accounted for as well. This difference is something that can be modeled, either through extrapolation from old data sets or from materials analysis in various radiation environments and beamlines in facilities around the world. While verification and validation tests in a reactor environment similar to an NTR core will be needed for whatever fuel elements are selected, this testing allows many of the hurdles to be addressed before this very expensive step is taken.
The CERMET Fuel Element Environmental Test (CFEET) stand was originally proposed in 2012 by David Bradley at NASA’s Marshall Spaceflight Center as a lower-cost alternative to NTREES. One of the consistent problems in engineering is that to make something more flexible the complexity must increase. This increases the cost to both build and maintain the test stand, which results in a higher cost per test. Also, the larger the volume the test stand uses, the more supplies are needed (in the case of NTREES, GH2 and GN2, plus water for the cooling system), which also increases cost.
CFEET is a low-cost, small scale test stand for NTR fuel elements. It also exposes a test article to temperatures and hydrogen environment that they would experience in the core of an NTR, but again the radiation effects aren’t accounted for since this is purely an inductively heated test stand. Rather than have the extensive piping, effluent cleanup, and exhaust systems that NTREES uses, CFEET uses a simple vacuum chamber with a single RF coil for induction heating to test thermal properties and general reactions with the hydrogen (The hydrogen is pumped through the FE during testing, but I can’t find any information about flow rate of the gas).
This means that the majority of CFEET fits on a (large) desktop. The vacuum chamber is only 16.9” tall and 10” in diameter, and it’s the largest component of the system. Rated to 10^-6 Torr, the chamber has a vacuum-rated RF feed-through port one one side, and opposite that port another, sapphire one for pyrometer readings. Additional ports connect the turbopumps and other equipment to the chamber.
The induction heating equipment is rated to 15 kW, with an output frequency of 20-60 kHz. While significantly lower output than NTREES, CFEET is still able to get test articles to reach temperatures over 2400 K. An insulating sleeve (with a hole formed in it to allow pyrometer readings) of various materials is used to minimize heat loss through radiation.
While CFEET is not able to simulate gas flow, as NTREES is, it is able to assess thermal, chemical, and mechanical properties of materials at temperature and in a pure-hydrogen atmosphere. Because the system is far simpler, and takes far fewer consumables to operate, it is far cheaper to use as a test bed.
More info on CFEET is available on the CFEET page!
What Have They Taught Us?
Both NTREES and CFEET have been used to help assess various manufacturing techniques for fuel elements, and also evaluate clad materials and thermal expansion issues. NTREES is able to assess erosion rates (both in mass and in chemical composition). While these aren’t the sexy tests, they have informed decisions about clad materials, manufacturing methods, and the inherent tradeoffs in different designs without having to go through the major expense of designing, building, testing, and then hot-fire testing a nuclear reactor.
Work has continued on investigating different microstructures within the FE, using depleted UO2 (dUO2) for chemical and thermal analysis. These tests have explored many different options as far as fine structure of the fuel forms available, and continue to inform CERMET fuel element design today.
Development Challenges for LEU NTP, and a New Direction
A major change occurred in 2012, however: it was decided by the White House that highly enriched uranium (HEU) would not be used for civilian purposes in the US, in order to reduce the risk of nuclear weapons proliferation, and that low enriched uranium (LEU) would be used for all civilian purposes, including medical and industrial isotope production.. This decision has resulted in thousands, if not tens of thousands, of pages of response, from dry, indifferent technical papers to proponents and opponents of the move screaming and raging in every direction. Because of this decision, NASA’s nuclear programs were forced to look at LEU systems, not the HEU ones that they’d always used. While there are a number of ways to make an NTR out of LEU instead of HEU, the two main options are CERMET and carbide fuel elements. Because CERMET was already under development, and there were ways to use LEU in CERMET fuel, this was the path that was decided by NASA’s management. However, LEU carbide designs (most notably SULEU, the Superior Utilization of Low Enriched Uranium carbide-based NTR) are also an option, and one that offers higher temperature operation as well, but since CERMET fuels are more developed within NASA’s design paradigm they remain the primary focus of NASA’s development.
One of the greatest fears in any development program is the problems that simply can’t be assessed within the budget, the timeframe, or both, of a program. Every program has them, and many engineering fish tales have been made out of solving them. When they haven’t been solved, though, they are the things that often define a program’s schedule… and its cancellation date.
For the LEU NTP program, the main challenge is in the fuel element matrix, and the isotopic purity of the tungsten needed for the metal matrix of the fuel in particular. For an HEU reactor, the isotope of tungsten was less of a concern, because there was a more flexible neutron budget for the reactor due to the higher fuel load. With LEU, the neutron budget becomes tighter, and the more management of the neutron spectrum you can do within the FE, the fewer neutrons are lost to the structural components of the reactor. Isotopic enrichment of reactor components other than fuel elements is relatively common, and so this wasn’t seen as a major challenge.
Most of the analysis up to this point on LEU NTP has focused on this line of development. Tungsten-184 has a small enough neutron capture cross section that it can reflect a neutron many times within the fuel element itself, increasing the likelihood of a capture by the higher-cross sectioned fuel nuclei. In fact, a recent paper by Vishal Patel of the Center for Space Nuclear Research in Idaho Falls, ID (who has kindly answered many questions, often sent at odd hours of the night, while I was researching this post) demonstrated some surprising characteristics that are possible with LEU CERMET fuel… including an overall reduction in system mass! This is an especially surprising result, but he actually went on Facebook to discuss the finding in the first day or two that the paper came out, and the overall conclusion was interesting:
“ So the reason all this ends up working is that you are constrained by thermal design concerns (need enough surface are for heat transfer) rather than neutronic reasons (needing enough volume to go critical). This is typical for reactors of this size and above. At much lower thrusts the neutronics eventually dominates and HEU looks better but no rocket person cares for those lower levels of thrust for this type of system. The idea of this study was to show the systems are comparable, choose whichever one you want (but the obvious first thought is proliferation and economics, so choose the one that fits your constraints).
Unfortunately, tungsten enrichment is a major challenge, and one that we aren’t going to be able to discuss in detail, because 184W is useful in another nuclear technology: explosives. This is because W is a great neutron reflector, and so is used in fission explosives to increase the number of neutrons entering the core during the initial neutron pulse from the initiation of the nuclear detonation. According to NASA, the LEU FEs, as designed, required 90% enriched 184W. It was expected that a 1 mg sample at 50% purity would be available in October of 2016, but a mix of accidents (an inadvertent chemical release is mentioned in the Mid-Year Game Changing Development Status Report for 2017) and technical challenges (which are classified) has forced this requirement into the forefront of everyone involved in the NTP program’s mind.
Alternatives exist, however. BWXT, already a major supplier of experimental fuel elements, has suggested a different core design, where graded molybdenum (Mo) and tungsten can be used instead of (90%) pure 184W. This design is one that is still very new, and because of that (and since it’s being developed by a private company and not a public institution) there’s not much information available. New contracts were signed between NASA and BWXT in 2017 to fund the development of their FE design, and hopefully as time goes on more information will become available. According to one person knowledgeable about the program, hopefully the Nuclear and Emerging Technologies for Space 2018 (to be held in Las Vegas in February) will bring more information. I have been trying to find out more information on this design, but unfortunately there’s not much out there that I can see. I also don’t have the background to determine if the manufacturing techniques described above will be compatible with this particular FE design, or the reasons why they would or wouldn’t be. Being the end of the year, it would be surprising if we heard anything before NETS this year.
Another change that has been floating around since about 2011 is a new process for manufacturing the metal matrix of the fuel element: spark plasma sintering (SPS). This seems to have been most thoroughly explored at Idaho National Laboratory and the Center for Space Nuclear Studies in Idaho Falls, ID. Instead of using HIP sintering, where heat and pressure are used to coax the temperature for a consistent metal matrix down, the individual grains are welded together using electric arcing. This allows a lower sintering temperature to be achieved, allowing for less decomposition of the UO2 in the fuel particles.
This also allows for a new type of clad to be used. Rather than the difficulties that have been experienced with the CVD clad, a binder is used to apply tungsten microparticles. This is one of the newest techniques to be explored for fuel particle coating, and in order to take advantage of it SPS has to be used, because the HIP temperatures are too high. For more info on these developments I recommend this paper by Zhong et al from INL and this presentation by Barnes.
How This Changes the Core
Any time a fuel element is changed, either in composition or enrichment, it can lead to significant changes to the core of the reactor. The biggest change in NASA’s NTP system is that tie tubes have been eliminated from the core. As discussed in the last post, the tie tubes perform many different functions, not just structural support for the fuel elements (which suffered persistent failures due to vibrations in the core), but also provided neutron moderation and supplied power to the turbopumps as well. Because of this, there have been designs for tie tubes for LEU NTR cores, although often these are placed around the periphery of the core rather than spread throughout like was originally planned for in the NERVA core. This changes the power distribution in the core, and makes it so that some reactor geometry design changes are necessary, but those are incredibly specific to the fuel elements used, and the results of extensive modeling of neutronic behavior and reactor physics.
Because the fuel elements are able to withstand higher temperatures, the entire reactor will run at elevated temperatures compared to the XE-Prime engine. This gives an increase in specific impulse over the graphite composite core type, although how much of one will largely depend on the particulars of the fuel elements and reactor power, and therefore core geometry, of the design that is finally tested.
More to Come!
Keep checking back for our next installment, which will look at the various reactor cores and engines themselves, for both the LEU NTP system and the Nuclear Cryogenic Propulsion Stage. We’ll also look at test stands and limitations for hot-fire ground testing, and how those will influence the decisions made for the new engines. Finally we’ll wrap up at a look at the advanced carbide designs that are being looked at (although not too closely on NASA’s part… yet!)
Sources and Additional Reading
- If you only read one reference on this list, make it this one!
My apologies for the delay on posting, the holidays have a way of creating slowdowns in material getting written. Hopefully I will be able to post more regularly soon. Research for the next post (on NASA’s plans for hot-fire test capability at Stennis Spaceflight Center, and the limitations that may place on testing) is underway, as well as research to prepare for results to hopefully be announced at NETS 2018. Sadly, I will not be able to attend, but look forward to all the papers that will be presented on these fascinating engines. I hope to publish on the latest in these new designs shortly after the conference ends. After that, a final post in the series on carbide fuel element LEU NTRs will wrap up this blog series.
At that point, the focus will shift back to trying to get the YT channel going. I haven’t touched Blender in a while, but I don’t think that it will be difficult to do what I need to do, I just need to sit down and learn. The scripts are largely written in draft form, I just need to go back over them for a final edit, then start doing the audio. The search still goes on for video clips to use, especially for Project Rover. Any links to clips that I would be able to use would be greatly appreciated!
Cpoyright 2018 Beyond NERVA. Contact for reprint permission.