Nuclear power is one of the most mundanely challenging things to develop. It’s challenging, because in many ways this is where the rubber meets the road when it comes to quantum mechanics becoming an inherent part of engineering; at the same time, it’s become mundane in many ways over the last near-century as the quirks and difficulties have become known and understood, and additional safeguards and regulations have codified this knowledge into a body of wisdom that is able to prevent the vast majority of accidents (if followed properly).
Because of the inherent challenges in reactor physics, much testing is required before a fuel element can be placed in an operating nuclear reactor. The thermal transfer and chemical properties of the fuel element and clad material must be assessed, as well as the effects of manufacturing differences in otherwise identical fuel elements (during the Rover program, mass loss from “identical” fuel elements could vary by as much as 9% of total FE mass). Especially with the changes that are occurring today in the field of CERMET fuel manufacture, the ability to understand the impact of manufacturing variations, and why they’re different, is critically important to the operating of the future rocket engine.
In order to ensure that any in-space nuclear system is a safe, reliable, and efficient source of either thrust or electricity (or both), extensive testing is required. There are two broad categories of nuclear testing: non-nuclear testing, and then nuclear testing.
Non-nuclear testing is the (primarily) thermal and chemical analysis of proposed reactor components. While many different testbeds have been used over the years, many of them were designed as one-off test stands for one particular type of fuel element, and so were discarded as each project was cancelled, and replaced with new ones that were able to meet the needs of the next astronuclear design (sometimes a decade later).
There was a pair of hot gas furnaces (one at LASL, one at WANL) for electrical heating of fuel elements in an H2 environment that used resistive heating to bring the fuel element up to temperature. This became more and more important as the project continued, since development of the clad on the fuel element was a major undertaking. As the fuel elements became more complex, or as materials that were used in the fuel element changed, the thermal properties (and chemical properties at temperature) of these new designs needed to be tested before irradiation testing to ensure the changes didn’t have unintended consequences. This was not just for the clad, the graphite matrix composition changed over time as well, transitioning from using graphite flour with thermoset resin to a mix of flour and flakes, and the fuel particles themselves changed from uranium oxide to uranium carbide, and the particles themselves were coated as well by the end of the program.
Despite the valuable data these furnaces developed, they were difficult to maintain and expensive to operate. So, with the end of Project Rover, the furnaces were decommissioned, with only small scale test beds used on occasion for particular projects.
This changed in 2008, with the building of the Nuclear Thermal Rocket Environmental Effect Simulator, or NTREES. This full-flow, inductively heated test stand is able to handle a wide range of fuel element types, environmental adjustments, and has an extensive data collection system including one of the most versatile laser mass spectrometers in the world. You can read more about it on the NTREES page.
In 2012, a new test stand was proposed, because NTREES (while perfect for its task) was also expensive to operate. Therefore, in 2012 the CERMET Fuel Element Environmental Test (CFEET) stand was proposed, and built in the next year. Rather than being a large setup that takes up a good chunk of the room, CFEET fits on a (large) desktop. While it still provides H2 flow for chemical analysis, and uses induction heating of the test article, it also doesn’t have nearly as much data recording capability as NTREES does. This test stand is mainly for duration and destruction testing before moving to NTREES for erosion modelling and other, more advanced testing. You can find out more at the CFEET page.
Critical Geometry and Zero Power Testing
Project Rover and the Aircraft Nuclear Propulsion program
For Project Rover, nuclear testing of potential NTR designs started at Los Alamos Scientific Lab, and the Pajarito Canyon facility in particular. In time Kiva 3 (a kiva is an underground religious chamber for many of the Pueblo tribes in the Southwest; in this case it was an underground criticality laboratory, of which three were eventually constructed. Kiva 3 had a number of test stands, and was the site for rough core mockups, fine-tuned mockups, and zero-power testing of NTR designs before being sent to the Nuclear Rocket Development Center in Nevada.
The first of these was known as Honeycomb, due to its use of square grids made out of aluminum (which is mostly transparent to neutrons), held in large aluminum frames. Prisms of nuclear fuel, reflectors, neutron absorbers, moderator, and other materials were assembled carefully (to prevent accidental criticality, something that the Pajarito Test Site had seen early in its’ existence in the Demon Core experiments and subsequent accident) to ensure that the modeled behavior of possible core configurations matched closely enough to predicted behavior to justify going through the effort and expense of going on to the next steps of refining and testing fuel elements in an operating reactor core. Especially for cold and warm criticality tests, this test stand was invaluable, but with the cancellation of Project Rover, there was no need to continue using the test stand, and so it was largely mothballed.
After the initial critical assembly was tested in Honeycomb, a crude mockup of the core was constructed, a ZEPO-rough assembly, to further refine the neutronic behavior of the proposed core configuration. Once this general configuration was proven to have the right balance of reactivity, neutron moderation, and other characteristics, a ZEPO-fine assembly was constructed. This is when systems like control drums were added, and their effects on reactivity were then demonstrated.
This concluded with a final criticality check of the test article, which would then be loaded onto a custom rail car, and with a contingent of engineers and technicians from both Los Alamos and the Nuclear Rocket Development Center, would be sent to the NRDC for hot fire testing.
In-Pile Irradiation Testing
During Project Rover, a modified KIWI-A reactor, which used a low-pressure, heavy water moderated island in the center of the reactor to reduce the amount of fissile fuel necessary for the reactor to achieve criticality. This reactor, known as Zepo-A (for zero-power, or cold criticality), was the first of an experiment that was carried out with each successive design in the Rover program, supporting Westinghouse Astronuclear Laboratory and the NNTS design and testing operations. Later it ended up being re-purposed into a test stand called PARKA, which was used to test both NTR and liquid metal fast breeder reactor (LMFBR, now known as “Integral Fast Reactor or IFR, under development at Idaho National Labs) fuel pins in a low-power, epithermal neutron environment for startup and shutdown transient behavior testing, as well as being a well-understood general radiation source.
Modern In-Pile Testing
A number of reactors could be used for these tests, including TRIGA-type reactors that are common in many universities around the US. This is one of the advantages of LEU, rather than the traditional HEU: there are fewer restrictions on LEU fuels, so many of these early tests could be carried out by universities and contractors who have these types of reactors. This will be less expensive than using DOE facilities, and has the additional advantage of supporting additional research and education in the field of astronuclear engineering.
The initial fuel element prototypes for in-pile testing will be unfueled versions of the fuel element, to ensure the behavior of the rest of the materials involved won’t have adverse reactions to the neutronic and radiation environment that they’ll be subjected to. This is less of a concern then it used to be, because material properties under radiation flux have been continually refined over the decades, but caution is the watchword with nuclear reactors, so this sort of test will still need to be carried out. These experiments will be finally characterized in the Safety Analysis Report and Technical Safety Review documents, a major milestone for any fuel element development program. These documents will provide the reactor operators all the necessary information for the behavior of these fuel elements in the research reactor in preparation for fueled in-pile testing. Concurrently with these plans, extensive neutronic and thermal analysis will be carried out based on any changes necessitated by the in-pile unfueled testing. Finally, a Quality Assurance Plan must be formulated, verified, and approved. Each material has different challenges to producing fuel elements of the required quality, and each facility has slightly different regulations and guidelines to meet their particular needs and research guidelines. After these studies are completed, the in-pile, unfueled fuel elements are irradiated, and then subjected to post irradiation examination, for chemical, mechanical, and radiological behavior changes. Fracture toughness, tensile strength, thermal diffusivity, and microstructure examination through both scanning electron and transmission electron microscopy are particular areas of focus at this point in the testing process.
One last thing to consider for in-pile testing is that the containment vessel (often called a can) that the fuel elements will be held in inside the reactor has to be characterized, especially its’ impact on the neutron flux and thermal transfer properties, before in-pile testing can be done. This is a relatively straightforward, but still complex due to the number of variables involved, process, involving making an MCNP model of the fuel element in the can at various points in each potential test reactor, in order to verify the behavior of the test article in the test reactor. This is something that can be done early in the process, but may need to be slightly modified after the refinements and experimental regime that we’ve been looking at above.
Another consideration for the can will be its’ thermal insulation properties. NTR fuel elements are run at the edge of the thermal capabilities of the materials they’re made out of, since this maximizes thermal transfer and therefore specific impulse. This also means that, for the test to be as accurate as possible, the fuel element itself must be far hotter than the surrounding reactor, generally in the ballpark of 2500 K. The ORNL Irradiation Plan suggests the use of SIGRATHERM, a soft graphite felt, for this insulating material. Graphite’s behavior is well understood in reactors (and for those in the industry, the fact that it has about 4% of the density of solid graphite makes Wigner energy release minimal).
Once this extensive testing regime for fuel elements has been completed, a fueled set of fuel elements would be manufactured and transported to the appropriate test reactor. Not only are TRIGA-type reactors common to many universities an option, but three research reactors are also available with unique capabilities. The first is the High Flux Isotope Reactor at Oak Ridge, which is one of the longest-operating research reactors with quite a few ports for irradiation studies at different neutron flux densities. As an incredibly well-characterized reactor, there are many advantages to using this well-understood system, especially for analysis at different levels of fuel burnup and radiation flux.
The second is a newly-reactivated reactor at Idaho National Laboratory, the Transient Reactor Test (TREAT). An air cooled, graphite moderated thermal reactor, the most immediately useful instrument for this sort of experiment is the hodoscope. This device uses fast neutrons to detect fission activity in the prototypic fuel element in real time, allowing unique analysis of fuel element behavior, burnup behavior, and other characteristics that can only be estimated after in-pile testing in other reactors.
The third is also at Idaho National Lab, this is the Advanced Test Reactor. A pressurized light water reactor, the core of this reactor has four lobes, and almost looks like a clover from above. This allows for very fine control of the neutron flux the fuel elements would experience. In addition, six of the locations in the core allow independent cooling systems that are separated from the primary cooling system. This would allow (with modification, and possible site permission requirements due to the explosive nature of H2) the use of hydrogen coolant to examine the chemical and thermal transfer behaviors of the NTR fuel element while undergoing fission.
Each of these reactors uses a slightly different form of canister to contain the test article. This is required to prevent any damage to the fuel element contaminating the rest of the reactor core, an incredibly expensive, difficult, and lengthy process that can be avoided by isolated the fuel elements from their surrounding environment chemically. Most often, these cans are made out of aluminum-6061, 300 series stainless steel, or grade 5 titanium (links in the reference section). According to a recent Oak Ridge document (linked in references), the most preferred material would be the titanium, with the stainless being the least attractive due to 59Fe and 60Co activation leading to the can to become highly gamma-active. This makes the transportation and disposal of the cans post-irradiation much more costly.
Hot Fire Testing
Currently, there are no operating nuclear test stands in the West, although there are some areas where some low-powered nuclear tests have been done in the last year. The Russians continue to use Semipalatinsk as their test area, although I don’t know of any hot-fire tests since the early 90s. In the US, much work has been done at the National Nuclear Test Site, both for Project Rover in the 50s to 70s, and for the DUFF and KRUSTY tests in 2014 and 2017.
Other astronuclear testing was conducted at various locations throughout the country in the heyday of Rover. Los Alamos Scientific Lab and Oak Ridge National Lab led the development of the graphite composite fuel element used in NERVA. Both Argonne National Laboratory and Westinghouse Astronuclear Laboratory were heavily focused on CERMET (then called composite) fuel elements, which are the lead candidate for NASA’s new NTR. Lawrence Livermore Lab, Batelle Research Institution, and others focused on carbide fuel element manufacture (as did the Russians).
Project Rover Test Stands
Hot fire testing of NTRs during Project Rover occurred at the Nuclear Rocket Development Center, now part of the National Nuclear Security Site (although not currently in operation), near Jackass Flats, NV.
These tests were broadly divided into two categories: research reactors – the KIWI-A, KIWI-B, PHOEBUS, PEWEE, and NUCLEAR FURNACE reactors – and development reactors – the NERVA NRX-A, NRX-EST, and NE-PRIME reactors. These tests ranged from a single firing of an engine to a whole series of tests.
These test articles would arrive at the E-MAD facility, pictured above, for final checkout and installation of various instruments, as well as preparation to hook up both LH2 coolant and LN2 purging and cooling gas. They would remain on the rail cars for ease of transport around the facility, and out to the test stand itself, where final connections would be made.
Next would be a cold-flow gas test, to ensure the flow of propellant through the engine didn’t cause any unforseen flow instabilities or harmonic issues within the engine. Assuming this checked out, a hot-fire test, where the reactor was operated at various power levels and coolant flow rates, would be conducted. The exiting H2 gas had oxygen injected into the flow, and was then combusted, to minimize the explosive risk caused by the large volume of highly flammable gas.
Early hot fire tests would happen only once or twice in a reactor, often due to the extremely high rates of damage to the fuel elements of the reactor. It was not uncommon to see power levels oscillate wildly, and then observe sparks flying out of the nozzle as broken pieces of fuel elements were shot into the air out of the nozzle! As reactor architecture improved, and fuel elements broke or eroded less, test campaigns would become longer, with more tests and longer duration tests being performed on the same reactor.
After testing, the reactor would be disconnected, and wheeled out onto a shunt line laid into the desert, to allow the shorter-lived – and therefore more radiologically dangerous – fission products to decay. After a period of time that varied depending on operation time, fuel burnup, and radiological characteristics of the reactor’s components, it would be returned to E-MAD, only this time to a hot cell – an enclosed room with remote manipulators used to disassemble and examine the reactor.
Chemical analyses were made of the fuel elements to examine fission product ratios, which would be used to verify power levels in the reactor during operation, refine models of fission product ratios, and assess the damage done to the fuel elements during operation. Connecting components would be examined for mechanical issues, and mechanical components would be assessed for neutron activation and embrittlement issues.
After this post-mortem analysis, the reactors were buried in a waste disposal facility on-site.
This facility is, sadly, unusable at this point, due to the changing requirements for testing, activation and irradiation of many key facilities, and safety concerns. It is currently being decommissioned by Bechtel Nevada, and environmental remediation is under way as well.
The NF-1 test, the last test of Project Rover, actually included an exhaust scrubber to minimize the amount of effluent released in the test. Because this test was looking at different types of fuel elements than had been looked at in most previous tests, there was some concern that erosion would be an issue with these fuel elements more than others.
The hydrogen exhaust, after passing the instrumentation that would provide similar data to the Elephant Gun used in earlier tests, would be cooled with a spray of water, which then flashed to steam. This was then run through a set of condensers and filters to contain any fission products in the exhaust stream before the remaining hydrogen was flared off.
So how well did this system do at scrubbing the effluent? Two of the biggest concerns were the capture of radiokrypton and radioxenon, both mildly radioactive noble gasses. The activated charcoal bed was primarily tasked with scrubbing these gasses out of the exhaust stream. Since xenon is far more easily captured than krypton in activated charcoal, the focus was on ensuring the krypton would be scrubbed out of the gas stream, since this meant that all the xenon would be captured as well. Because the Kr could be pushed through the charcoal bed by the flow of the H2, a number of traps were placed through the charcoal bed to measure gamma activity at various points. Furthermore, the effluent was sampled before being flared off, to get a final measurement of how much krypton was released by the trap itself.
Looking at the sampling of the exhaust plume, as well as the ground test stations, the highest dose rat was 1 mCi/hr, far lower than the other NTR tests. Radioisotope concentrations were also far lower than the other tests. However, some radiation was still released from the reactor, and the complications of ensuring that this doesn’t occur (effectively no release is allowed under current testing regimes) due to material, chemical, and gas-dynamic challenges makes this a very challenging, and costly, proposition to adapt to a full-flow NTR test.
Modern Hot Fire Testing
The most detailed analysis of this concept was in support of the Space Nuclear Thermal Propulsion program, run by the Department of Energy – better known as Project Timber Wind. This was a far larger engine (111kN as opposed to 25 kN) engine, so the exhaust volume would be far larger. This also means that the costs associated with the program would be larger due to the higher exhaust flow rate, but unfortunately it’s impossible to make a reasonable estimate of the cost reduction, since these costs are far from linear in nature (it would cost significantly more than 20% of the cost estimated for the SNTP engine). However, it’s a good example of the types of facilities needed, and the challenges associated with this approach.
The primary advantage to the ETS concept is that it doesn’t use H2O to cool the exhaust, but LH2. This means that the potential for release of large amounts of (very mildly) irradiated water into the groundwater supply are severely limited (although the water solubility of the individual fission products would not change). The disadvantage, of course, is that it requires large amounts of LH2 to be on hand. At Stennis SC, this is less of an issue, since LH2 facilities are already in place, but LH2 is – as we saw in the last blog post – a major headache. It was estimated that either a combined propellant-effluent coolant supply could be used (~181,440 kg), or a separate supply for the coolant system (~136,000 kg) could be used (numbers based on a maximum of 2 hours burn time per test). To get a sense of what this amount of LH2 would require, two ~1400 kl dewars of LH2 would be needed for the combined system, about ¾ of the LH2 supply available at Kennedy Space Center (~3200 kl). After the gas has passed through a length of tunnel, and cooled sufficiently, a heat exchanger is used to further cool the gas, and then it’s passed through an activated charcoal filter similar to the one used in the NF-1 test. This filtered H2 will then be flared off after going through a number of fission product detectors to ensure the filter maintained its’ integrity. The U-la tunnels are dug into alluvium, so we’ll look at those in the next section.
One concern with using charcoal filters is that their effectiveness varies greatly depending on the temperature of the effluent, and the pressure that it’s fed into the filter. Indeed, the H2 can push fission products through the filter, so there’s a definite limit to how small the filter can be. The longer the test, the larger the filter will be. Activated charcoal is relatively cheap, but by the end of the test it will be irradiated, meaning that it has to be disposed of in nuclear waste repositories.
In this concept, the engine is placed over an already existing (from below-ground nuclear weapons testing) 8 foot wide, 1200 foot deep borehole, with a water spray system being mounted adjacent to the nozzle of the NTR. The first section of the hole will be clad in steel, and the rest will simply be lined with the rock that is being bored into. The main limiting consideration will be the migration of radionucleides into the surrounding rock, which is something that’s been modeled computationally using Frenchman’s Flat geologic data, but has not been verified.
The primary challenges associated with this type of testing will be twofold: first, it needs to be ensured that the fission products will not migrate into groundwater or the atmosphere; and second, in order to ensure that the surrounding bedrock isn’t fractured – and therefore allows greater-than-anticipated migration of fission products to migrate from the borehole – it is necessary to prevent the pressure in the borehole from reaching above a certain level. A sub-scale test with an RL-10 chemical rocket engine and radioisotope tracers was proposed (this test would have a much smaller borehole, and use known radioisotope tracers – either Xe or Kr isotopes – in the fuel to test dispersion of fission products through the bedrock). This test would provide the necessary migration, permeability, and (given appropriate borehole scaling to ensure prototypic temperature and pressure regimes) soil fracture pressures to ensure the full filtration of the exhaust of an NTR.
Further options were explored in the “Final Report – Assessment of Testing Options for the NTR at the INL.” This is a more geologically complex region, and INL is on the Snake River plain, and above an aquifer, so the site will be limited to those places that the aquifer is more than 450 feet below the surface. However, if a site can be found that has a layer of this basalt below the borehole but above the aquifer, it can provide a gas-impermeable barrier below the borehole.
One advantage to these options, though, which cannot be overstated, is that these facilities would be located on DOE land. As was seen in the recent KRUSTY fission-powered test, nuclear reactors in DOE facilities use an internal certification and licensing program independent of the NRC. This means that the 9-10 year (or longer), incredibly expensive certification process, which has never been approved for a First of a Kind reactor, would be bypassed. This alone is a potentially huge cost savings for the project, and may offset the additional study required to verify the suitability of these sites for NTR testing compared to certifying a new location – no matter how well established it is for rocket testing already.
In this NTR test setup, the exhaust is slowed from supersonic to subsonic speeds, allowing O2 to be injected and mixed well past the molar equilibrium point for H2O. The resultant mixture is then combusted, resulting in hot steam and free O2. A water sprayer is used to cool the steam, and then passes through a debris trap filled with water at the bottom. It is then captured in a storage pool, and the remaining gaseous O2 is run through a desiccant filter, which is exhausted into the same storage pool. The water is filtered of all fission products and any unburned fuel, and then released. The gaseous O2 is recaptured and cooled using liquid nitrogen, and whatever is unable to be efficiently recaptured is vented into the atmosphere. The primary advantage to this system is that the resulting H2O can be filtered at leisure, allowing for more efficient and thorough filtration without the worry of over-pressurization of the system if there’s a blockage in the filters.
This last testing concept seems to be the front-runner for current NASA designs, to be integrated into the A3 test stand at NASA’s Stennis Space Center (SSC). SSC is the premier rocket test facility for NASA, testing both solid and liquid rocket engines. The test facilities are located in the “fee area,” a 20 square mile area (avg. radius 2.5 miles) surrounded by an acoustic “buffer zone” that averages 7.9 miles in radius (195 sq mi). With available office space, manufacturing spaces, and indoor and outdoor warehouse space, as well as a number of rocket engine test stands, the facility has much going for it. Most of the rocket engines being used by American launch companies have been tested here, going all the way back to the moon program. This is a VERY advanced, well-developed facility for the development of any type of chemical engine ever developed… but unfortunately, nuclear is different. Because SSC has not supported nuclear operations, a number of facilities will need to be constructed to support NTR testing at the facility. This raises the overall cost of the program considerably, to less than but around $850M (in 2017 dollars). A number of facilities will need to be constructed at SSC to support NTR testing, for both E3 and A3 test stands.
The A3 facility has the most that needs to be added, including facilities for power pack testing ($21M); a full-flow, electrically heated depleted uranium test (cost TBD); a facility for zero power testing and reactor verification before testing ($15M); an adjacent hot cell for reactor cool-down and disassembly (the new version of the EMAD facility, $220M); and testing for both sub-scale and full scale fission powered NTP testing (cost to be determined, it’s likely to be heavily influenced by regulatory burden). This does not include radiation shielding, and an alternate ducting system to ensure that the HVAC system doesn’t become irradiated (a major headache in the decomissioning of the original E-MAD facility). It is unlikely that design work for this facility will start in earnest until FY21, and construction of the facility is not likely to start until FY24. Assuming a 10 year site licensing lead time (which is typical), it is unlikely that any nuclear testing will be able to be done until FY29, with full power nuclear testing not likely until FY30.