Development and Testing

NTR Hot Fire Testing 2: Modern Designs, New Plans for the LEU NTP

Hello, and welcome back to Beyond NERVA in the second part of our two-part series on ground testing NTRs. In part one, we examined the testing done at the National Defense Research Site in Nevada as part of Project Rover, and also a little bit of the zero power testing that was done at the Los Alamos Scientific Laboratory to support the construction, assembly, and zero-power reactivity characterization of these reactors. We saw that the environmental impact to the population (even those living closest to the test) rarely exceeded the equivalent dose of a full-body high contrast MRI. However, even this low amount of radioisotope release is unacceptable in today’s regulatory environment, so new avenues of testing must be explored.

NERVAEngineTest, AEC

NRX (?) Hot-fire test, image courtesy DOE

We will look at the proposals over the last 25 years for new ways of testing nuclear thermal rockets in full flow, fission-powered testing, as well as looking at cost estimates (which, as always, should be taken with a grain of salt) and the challenges associated with each concept.

Finally, we’re going to look at NASA’s current plans for test facilities, facility costs, construction schedules, and testing schedules for the LEU NTP program. This information is based on the preliminary estimates released by NASA, and as such there’s still a lot that’s up in the air about these concepts and cost estimates, but we’ll look at what’s available.

Diagram side by side with A3

Full exhaust capture at NASA’s A3 test stand, Stennis Space Center. Image courtesy NASA

Pre-Hot Fire Testing: Thermal Testing, Neutronic Analysis, and Preparation for Prototypic Fuel Testing

Alumina sleeve during test, Bradley


We’ve already taken a look at the test stands that are currently in use for fuel element development, CFEET and NTREES. These test stands allow for electrically heated testing in a hydrogen environment, allowing for testing of he thermal and chemical properties of NTR fuel. They also allow for things like erosion tests to be done, to ensure clad materials are able to withstand not just the thermal stresses of the test but also the erosive effects of the hot hydrogen moving through them at a high rate.

However, there are a number of other effects that the fuel elements will be exposed to during reactor operation, and the behavior of these materials in an irradiated environment is something that still needs to be characterized. Fuel element irradiation is done using existing reactors, either in a beamline for out-of-core initial testing, or using specially designed capsules to ensure the fuel elements won’t adversely affect the operation of the reactor, and to ensure the fuel element is in the proper environment for its’ operation, for in-core testing.



TRIGA reactor core, image courtesy Wikimedia

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.



Irradiation vessel design for ATF, Thody

Design of an irradiation capsule for use with the ATF, Thody OSU 2018

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).

Pre-Hot Fire Testing: In-Pile Prototypic Fuel Testing



High Flux Isotope Reactor (HFIR), Oak Ridge National Lab, image courtesy Wikimedia

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.







Transient Reactor Test (TREAT) at Idaho NL. Image courtesy DOE

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.



Advanced Test Reactor, Idaho NL. Image courtesy DOE

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.

Here’s an example of the properties that would be tested by the time that the tests we’ve looked at so far have been completed:

Fuel Properties and Parameters to Test

Image courtesy Oak Ridge NL

NTR Hot Fire Testing For Today’s Regulatory Environment

It goes without saying that with the current regulatory strictures placed on nuclear testing, the same type of testing as done during Rover will not be able to be done today. Radioisotope release into the environment is something that is incredibly stringently regulated, so the open-air testing as was conducted at Jackass Flats would not be possible. However, there are multiple options that have been proposed for testing of an NTR in the ensuing years within the more rigorous regulatory regime, as well as cost estimates (some more reliable than others) and characterization of the challenges that need to be overcome in order to ensure that the necessary environmental regulations are met.

The options for current hot-fire testing of an NTR are: the use of upgraded versions of the effluent scrubbers used in the Nuclear Furnace test reactor; the use of boreholes as effluent capture and scrubbing systems (either already-existing boreholes drilled for nuclear weapons tests that have not been used for that purpose at Frenchman’s Flat, or new boreholes at the Idaho National Laboratory); the use of a horizontal, hydrogen-cooled scrubbing system (either using existing U-la or P-tunnel facilities modified for the purpose, or constructing a new facility at the National Nuclear Security Site); and the use of a new, full-exhaust-capture system at NASA’s current rocket test facilities at the John C. Stennis Space Center in Mississippi.

The Way We Did It Before: Nuclear Furnace Exhaust Scrubbers

Transverse view, Finseth

NF1 configuration, image from Finseth, 1991 courtesy NASA

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.

Effluent Cleanup System Flow Chart

Image from Nuclear Furnace 1 test report, Kirk, courtesy DOE

Axial view, FinsethThe 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 water was initially used to moderate the reactor itself, and then part of it was siphoned off into a wastewater holding tank while the rest was used for this exhaust cooling injection system. After this, the steam/H2 mixture had a temperature of about 1100 R.

After leaving the water injector system, the coolant went through radial outflow filter that was about 3 ft long, containing two wire mesh screens, the first with 0.078 inch square openings, the second one with 0.095 inch square openings.

Once it had passed through the screens, a steam generator was used to further cool the effluent, and to pull some of the H2O out of the exhaust stream. Once past this steam generator, the first separator drew the now-condensed water out of the effluent stream. Part of the radioactive component of the exhaust is at this point dissolved in the water. The water was drawn off to maintain an appropriate liquid level, and was moved into the wastewater disposal tank for filtering. A further round of exhaust cooling followed, using a water heat exchanger to cool the remaining effluent enough to condense out the rest of the water. The water used in this heat exchanger would be used by the steam generator that was used earlier in the effluent stream as its’ cool water intake, and would be discharged into the wastewater holding tank, but would not come in direct contact with the effluent stream. Once past the heat exchanger, the now much cooler H2/H2O mixture would go through a second separator identical in design to the first. At this point, most of the radioactive contaminant that could be dissolved in water had been, and the discharge from this unit was at this point pretty much completely dry.

A counterflow, U-tube type heat exchanger was then used to cool the effluent even more, and then a third separator – identical to the first two – was used to capture any last amounts of water still present in the effluent stream. During normal operation, though, basically no water would collect in this separator. The gas would then be passed through a silica gel sorption bed to further dry it. A back flow of gaseous nitrogen would be used to dry this bed for reuse. The gas, at this point completely dried, was then passed through another heat exchanger almost identical to the one that preceded the silica gel bed.

Charcoal Trap System

From NFI test report, Kirk, via DOE

After passing through a throttle valve (used to maintain back-pressure in the reactor), the gas was then passed through an activated charcoal filter trap, 60 inches long and 60 inches in diameter, to capture the rest of the radioactive effluent left in the hydrogen stream after being mixed with LH2 to further cool the gas to 250-350 R. Finally, the now-cleaned H2 is burned to prevent a buildup of H2 gas in the area- a major explosion hazard. This filter system was constantly adjusted after each power test, because pressure problems kept on cropping up for a number of reasons, from too much resistance to thermal disequilibrium.

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.

Above Ground Test Option #1: Exhaust Scrubbing

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.

SNTP Test Facility

Image courtesy DOE

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).

Once the exhaust is sufficiently cooled, it is a fairly routine matter to filter out the fission products (a combination of physical filters and chemical reactions can ensure that no radionucleides are released, and radiation monitoring can verify that the H2 has been cleaned of all radioactive effluent). In the NF-1 test, water was used to capture the particulate matter, and the H2O was passed through a silica gel bed to remove the fission products. An activated carbon filter was used to remove the noble gasses and other gaseous and aerosol fission products. After this, depending on the facility setup, it is possible to recycle a good portion of the H2 from the test; however this has massive power requirements for the cryocoolers and hydrogen densification equipment to handle this massive amount of H2.

Saddle Mountain facility diagram

Alternative test facility layout

Due to both the irradiation of the facilities and the very different requirements for this type of test facility, it was determined that the facilities built for the NRDS during Rover would be insufficient for this sort of testing, and so new facilities would need to be constructed, with much larger LH2 storage capabilities. One more recent update to the concept is brought up in the SAFE proposal (next section), using already existing facilities at the Nevada Test Site (now National Nuclear Security Site), in the U-la or P-tunnel complexes. These underground facilities were horizontal, interconnected tunnel complexes used for sub-critical nuclear testing. There are a number of benefits to using these (now-unused) facilities for this type of testing: first, the rhyolite that the P-tunnel facility is cut into is far less permeable to fission products, but remains an excellent heat sink for the thermal effects of the exhaust plume. Second, it’s unlikely to fracture due to overpressure, although back-pressure into the engine itself will constrain the minimum size of the tunnel. Third, a hot cell can be cut into the mountain adjacent to the test location, making a very well-shielded facility for cool-down and disassembly beside the test location, eliminating the need to transport the now-hot engine to another facility for disassembly.

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.

Cost estimates were avoided in the DOD assessment, due to a number of factors, including uncertain site location and the possibility of using this facility for multiple programs, allowing for cost sharing, but the overall cost for the test systems and facilities was estimated to be $500M in 1993 dollars. Most papers seem to think that this is the most expensive, and least practical, option for above ground NTR testing.

The Borehole Option: Subsurface Active Filtration of Exhaust

Many different options have been suggested over the years as to testing options. The simplest is to fire the rocket with its’ nozzle pointed into a deep borehole at the Nevada Test Site, which has had extensive geological work done to determine soil porosity and other characteristics that would be important to the concept. Known as Subsurface Active Filtration of Exhaust, or SAFE, it was proposed in 1999 by the Center for Space Studies, and continued to be refined for a number of years.

SAFE schematic

SAFE concept, Howe 2012, image courtesy NASA

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.

SAFE injector model

SAFE injection system model, Howe 2012

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.

The advantage to doing this test at Frenchman’s Flat is that the ground has already been extensively tested for the porosity (35%), permeability (8 darcys), water content (initial pore saturation 30%), and homogeneity (alluvium, so pretty much 100%) that is needed. In fact, a model already exists to calculate the behavior of the soil to these effects, known as WAFE, and the model was applied to the test parameters in 1999. Both full thrust (73.4 kg/s of H2O from both exhaust and cooling spray, and 0.64 kg/s of H2) and 30% thrust (20.5 kg/s H2O and 0.33 kg/s of H2) were modeled, both assuming 600 C exhaust injection after the steel liner. They found that the maximum equilibrium pressure in the borehole would reach 36 psia for the full thrust test, and 21 psia in the 30% thrust case, after about 2 hours, well within the acceptable pressure range for the borehole, assuming the exhaust gases were limited to below Mach 1 to prevent excess back-pressure buildup.

P-Tunnel setup

Other options were explored as well, including using the use of the U-la facility at the NNSS for horizontal testing. This is an underground set of tunnels in Nevada, which would provide safety for the testing team and the availability of a hot cell for reactor disassembly beside the test point (the P-tunnel facility is also cut into similar alluvial deposits, so primary filtration will come from the soil itself, and water cooling will still be necessary).

INL geology 2

INL geological composition, image courtesy DOE

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, including pahoehoe and rubble basalt, and various types of sediment. Another complication is that 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, the pahoehoe basalt is gas-impermeable, so 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.

A 1998 cost estimate by Bechtel Nevada on the test concept estimated a cost of $5M for the non-nuclear validation test, and $16M for the full-scale NTR test, but it’s unclear if this included cost for the hot cell and associated equipment that would need to be built to support the test campaign, and I haven’t been able to find the specific report.

However, this testing option does not seem to feature heavily in NASA’s internal discussions for NTR testing at this point. One of the disadvantages is that it would require the rocket testing equipment, and support facilities, to be built from scratch, and to occur on DOE property. NASA has an extensive rocket testing facility at the John C. Stennis Space Center in Hancock County, MS, which has geology that isn’t conducive to subterranean testing of any sort, much less testing that requires significant isolation from the water table, and most NASA presentations seem to focus on using this facility.

The main reasons given in a late 2017 presentation for not pursuing this option are: Unresolved issues on water saturation effects on soil permeability, hole pressure during engine operation, and soil effectiveness in exhaust filtering. I have been unable to find the Bechtel Nevada and Desert Research Institute studies on this subject, but they have been studied. I would be curious to know why these studies would be considered incomplete.

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.

Above Ground Test Option #2: Complete Capture

Flow Diagram Coote 2017

Image via Coote 2017, courtesy NASA

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.

Subscale Concept Render

Subscale test stand render, image courtesy BWXT via NASA

There are many questions that need to be answered to ensure that this system works properly, as there are with all of the systems that have yet to be tested. In other to verify that the system will work as advertised, a sub-scale demonstrator will need to be built. This facility will use a hydrogen wave heater in place of the nuclear reactor, and test the rest of the components at a smaller scale wherever possible. Due to the specific needs of the exhaust capture system, especially the need to test complete combustion at different heat loads, the height of the facility may not be able to be scaled down (in order to ensure complete combustion, the gas flow will need to be subsonic before mixing and combustion). Thermal loading on structures is another major concern for the sub-scale test, since many components must be tested at the appropriate temperature, and the smaller structures won’t be able to passively reject heat as well. Finally, some things won’t be able to be tested in a sub-scale system, so what data will need to be collected in the full-scale system needs to be assessed.

One last thing to note is that this system will also be used to verify that high-velocity impacts of hot debris will not be a concern. This was, of course, seen in many of the early Rover tests, as fuel elements would break and be ejected from the nozzle at similar velocities to the exhaust. While CERMET fuels are (likely) more durable, this is an accident condition that has to be prepared for. In addition, smaller pieces of debris need to be able to be fully captured as well (such as flakes of clad, or non-nuclear components). These tests will need to be carried out on the sub-scale test bed to ensure for the regulators that any accident is able to be addressed. This adds to the complexity of the test setup, and encourages the ability to change the test stand as quickly and efficiently as possible – in other words, to make it as modular as possible. This also increases the flexibility of the facility for any other uses that it may be put to.

NTP Testing at Stennis Space Center

SSC overview

Stennis SC test facilities, image courtesy NASA

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.

Diagram side by side with A3

Image from Houts presentation 2017, via NASA

As one of the newer facilities at SSC, the A3 test stand groundbreaking was held in August of 2007, and was completed in 2014. It is the only facility that is able to handle the thrust level (300+ Klbf at altitude, 1,000 Klbf nominal design) and simulated altitude (100 Kft) that testing a powerful upper stage requires. There are two additional facilities designed to operate at lower-than-ambient atmospheric pressures at SSC, the A2 test stand (650 Klbf at 60 Kft) and the E3 test facility (60 Klbf at 100 Kft). The E3 facility will be used for sub-scale testing, turbopump validation, and other tests for the NTP program, but the A2 test stand seems to not be under consideration at this time. The rest of the test stands at SSC are designed to operate at ambient pressure (i.e. sea level), and so they are not suitable for NTP testing.

The E3 facility would be used for sub-scale testing, first of the turbopumps (similar to the tests done there for the SSME), and sub-scale reactor tests. These would likely be the first improvements made at SSC to support the NTP testing, within the next couple years, and would cost $35-38M ($15-16M for sub-scale turbopump tests, $20-22M for the sub-scale reactor test, according to preliminary BWXT cost estimates). Another thing that would be tested at E3 would be a sub-scale engine exhaust capture system, which has been approved for both Phases 1&2, work to support this should be starting at any time ($8.74M was allocated to this goal in the FY’14 budget). From what I can see, work had already started (to an unknown extent) at E3 on this sub-scale system, however I have been unable to find information regarding the extent of the work or the scale that the test stand will be compared to the full system.

A3 under construction

A3 test stand under construction, image courtesy NASA

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.

Notional schedule

Notional Development Timeline

Documents relating to the test stands at SSC show that there has been some funding for this project since FY ‘16, but it’s difficult to tell how much of that has gone to analysis, environmental impact studies, and other bureaucratic and regulatory necessities, and how much has gone to actual construction. I HAVE had one person who works at SSC mention that physical work has started, but they were unwilling to provide any more information than that due to their not being authorized to speak to the public about the work, and their unfamiliarity with what is and isn’t public knowledge (most of it simply isn’t public). According to a presentation at SSC in July of 2017, the sub-scale turbopump testing may start in the next year or two, but initial design work for the A3 test stand is unlikely to start before FY’21.

NTP draft tech demonstration draft timeline

Draft Tech Development Roadmap, image via NASA

According to the presentation (linked below), there are two major hurdles the program needs to overcome on the policy and regulatory side. First, a national/agency level decision needs to be made between NASA, the DOE, and the NRC as to the specific responsibilities and roles for NTP development, especially in regards to: 1. reactor production, engine and launch vehicle integration strategy, and 2. ground, launch, and in-space operations of the NTR. Second, NTP testing at SSC requires a nuclear site license, which is a 9-10 year process even for a traditional light water power reactor, much less as unusual a reactor architecture as an NTR. This is another area that BWXT’s experience is being leaned on heavily, with two (not publicly available) studies having been carried out by them in FY16 on both a site licensing strategy and implementation roadmap, and on initial identification of policy issues related to licensing an NTP ground test at SSC.

Regulatory Burdens, Bureaucratic Concerns, and Other Matters

Originally, this post was going to delve into the regulatory and environmental challenges of doing NTR testing. An NTR is very different from any other sort of nuclear reactor, not only because it’s a once-through gas cooled reactor operating at a very high temperature, but also due to the performance characteristics that the reactor is expected to be able to provide.

Additionally, these are short-lived reactors – 100 hours of operation is more than enough to complete an entire crewed mission to Mars, and is a long lifetime for a rocket engine. However, as we saw during the Rover hot-fire testing, there are always issues that come up that aren’t able to be adequately tested beforehand (even with our far more advanced computational and modeling capabilities), so iteration is key. This means that the site has to be licensed for multiple different reactors.

Unfortunately, these subjects are VERY complex, and are very difficult to learn. Communicating with the NRC in and of itself is a subspecialty of both the nuclear and legal industries for reactor designers. The fact that the DOE, NASA, and the NRC are having to interact on this project just adds to the complexity.

So, I’m going to put that part of this off for now, and it will become its’ own separate blog post. I have contacted NASA, the DOE and the NRC looking for additional information and clarification in their various areas, and hopefully will hear back in the coming weeks or months. I also am reading the appropriate regulations and internal rules for these organizations, and there’s more than enough there for a quite lengthy blog post on its’ own. If you work with any of these organizations, and are either able to help me gather this information or get me in touch with someone that can, I would greatly appreciate it if you contact me.

Upcoming Posts!

For now, we’re going to leave testing behind as the main focus of the blog, but we will still look at the subject as it becomes relevant in other posts. For now, we’re going to do one final post on solid core pure NTRs, looking at carbide fueled NTRs, both the RD-0410 in Russia and some legacy and new designs from the US. After that, we’ll move on to bimodal NTR/chemical and bimodal NTR/thermal electric designs in the next post.

After that, with one small exception, we’ll leave NTRs behind for a while, and look at nuclear electric propulsion. I plan on doing pages for individual reactor designs during this time, both NTR and NEP, and add the as their own pages on the website. As I write posts, I’ll link to the new (or updated) pages as they’re completed.

Be sure to check out the rest of the website, and join us on Facebook! This blog is far from the only thing going on!



In Pile Testing

Technology Implementation Plan: Irradiation Testing and Qualification for Nuclear Thermal Propulsion Fuel; ORNL/TM-2017/376, Howard et al September 2017

DOE Order 414.1D, Quality Assurance; approved 4/2011

10 CFR Part 830, Nuclear Safety Management;

High Flux Isotope Reactor homepage:

Advanced Test Reactor Irradiation Facilities and Capabilities; Furstenau and Glover 2009

Transient Reactor Test Facility homepage:

Al 6061 Matweb page:

300 Stainless Steel; Pennsylvania Stainless,

Grade 5 Titanium Matweb page:

SIGRATHERM, SGL (manufacturer) website:

Nuclear Furnace ECS

Nuclear Furnace 1 Test Report; LA-5189-MS, by W.L. Kirk, 1973

DOE Fact Sheet, Appendix 2

Above Ground Effluent Treatment System

Space Nuclear Thermal Propulsion Final Report, R.A. Haslett, Grumman Aerospace Corp, 1995

Space Nuclear Thrmal Propulsion Test Facilities Subpanel Final Report, Allen et al, 1993

Subsurface Active Filtration of Exhaust (SAFE)

Ground Testing a Nuclear Thermal Rocket: Design of a sub-scale demonstration experiment, Howe et al, Center for Space Nuclear Research, 2012

Subscale Validation of the Subsurface Active Filtration of Exhaust Approach to NTP Ground Testing, Marshall et al, NASA Glenn RC, 2015 (Conference Paper) and (Presentation Slides)

Final Report – Assessment of Testing Options for the NTR at INL, Howe et al, Idaho NL, 2013

Complete Exhaust Capture and NASA Planning

Stennis Space Center Activities and Plans Overview presentation, NASA

Development and Utilization of Nuclear Thermal Propulsion; Houts and Mitchell, 2016 (slideshow)

Low Enriched Uranium (LEU) Nuclear Thermal Propulsion: System Overview and Ground Test Strategy, Coote 2017 (slideshow)

NASA FY18 Budget Estimates:

NTP Technical Interchange Meeting at SSC, June 2017 (slideshow)

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