Nuclear reactor development has many interrelated challenges and fundamental physical limitations that must be dealt with. Some of these need to develop simultaneously, some need to be addressed before other experiments can be designed.

Much of the development of an NTR can be encapsulated by the development of the fuel element itself. The fuel element, after all, is where fission happens. How much fissile material is available, and of what type, defines how much moderator is needed, how many neutrons need to be reflected back into the core, how much reactivity is possible. The fuel element, in so many ways, defines the reactor.

At the same time, this is aerospace engineering: the engine has to meet particular demands for mass, for volume, for thrust… so those requirements will feed back to inform and constrain the reactor as well. There’s a feedback system here: an astronuclear reactor designer has to look at the requirements of the mission, or the vehicle, and look at the possibilities for fuel elements – either already-designed, modifications of existing designs, or a whole new fuel element – and the cost of developing that system compared to their budget, and make a compromise between these factors.

Critical Geometry Testing

Critical geometry refers to the arrangement of fissile material, neutron moderator, neutron reflectors, and fission poisons in the correct configuration to produce a sustained nuclear reaction. This doesn’t actually need to be providing enough neutrons to sustain a critical reaction, because a sub-critical test can show all the information that needs to be gained.

Neutronic Design Steps Taub LASL
Rover era critical geometry development



Rover: How This Process Was Done the First Time

Many (most) of these early nuclear tests were carried out in New Mexico, at Los Alamos Scientific Laboratory – specifically the Pajarito Test Area. Here, the critical geometry mockups were made to ensure sufficient reactivity was available.

Honeycomb grainy closeup
Honeycomb, with a KIWI mockup loaded. Image via LANL

The most frequently used 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 initial critical geometry, this test stand was invaluable.

After the initial Honeycomb mockup, a pair of zero-power, warm critical mockups of the reactor are done. These mockups, known as ZEPOs, do not have enough criticality for a susteined reaction, but are able to verify the rough geometry sketched out by the Honeycomb test.

The first, “ZEPO”-rough, mockup, often used materials at hand to save on cost. Assuming everything looked good, and no major problems were discovered in the basic design, a more precise, “ZEPO”-fine mockup was done using custom machined parts. This is where the detailed characteristics of the reactor’s geometry were verified, and where any issues with the amount of criticality available, control requirements, etc. were fleshed out.

the first of these ZEPO-fine mockups, for the KIWI-A reactor, 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. Unlike other ZEPO-fine reactors, this core would be used frequently for irradiation testing, and came to be known as PARKA.

After this, the reactor was ready to be built. Again, this took place at Los Alamos, usually in KIVA-3. Usually, the Honeycomb test stand still had the rough mockup in place, and the ZEPO-fine mockup would be available as well to verify any questions that the assembly team had.

KIWI A Zepo A and Honeycomb shot
KIWI reactor development. Honeycomb can be seen on the left, with the ZEPO-Fine reactor in the back, and the production reactor under assembly (image via DOE)

Modern Techniques

There has been one HUGE change (quite a few, but in this case one matters most) since the days of Rover: computers. Even during the Manhattan Project, mathematical models were critical to nuclear development. Physics is math-based, after all. However, these calculations have a number of limitations: they really only accurately model one particular point in the reactor, at one particular point in time. Then they’re iterated, either in time or location, again and again and again. In the Manhattan Project there were only a limited number of people available for these calculations, so the locations and times had to be carefully selected, and backed up heavily with empirical data from experiments.


Fuel Element Irradiation Development

Rover Irradiation Testing

PARKA in Kiva-3, image courtesy DOE

It is unclear if the first series of fuel elements for Rover underwent irradiation testing in a nuclear reactor prior to hot-fire testing. Later fuel elements would be tested in the PARKA reactor, but the extent to which they were tested remains unclear.

An early, KIWI-type critical assembly test ended up being re-purposed into a test stand called PARKA, which was used to test 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.

Later fuel elements were tested in the Nuclear Furnace reactor at the Nuclear Rocket Development Center in Nevada. This was a full-flow nuclear reactor, designed to test fuel elements that were not yet ready for use in the NERVA engine. A mix of composite fuels and carbide fuels was tested in separate containers within the reactor, and a large moderator island was used to ensure that criticality would be able to be achieved.

Transverse view, Finseth
Image courtesy NASA

Sadly, despite the fact that this reactor was designed to be reused multiple times, and at far lower cost than building a new reactor for each fuel element test, it was only used for one test campaign in 1975 before the program was cancelled, and the reactor was decommissioned.

Modern Testing: SNRE, LEU NTP, and Many More To Come (hopefully)!

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

TRIGA reactor, image courtesy Wikimedia Commons

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.

HFIR core cross section with experiment bays illustrated. Image courtesy Wikimedia Commons

Not only are TRIGA-type reactors common to many universities an option (for LEU designs, at least), 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


Rover testing

Los Alamos Pajarito Site

Los Alamos Critical Assemblies Facility, LA-8762-MS, by R. E. Malenfant,

Thirty-Five Years at Pajarito Canyon Site, LA-7121-H, Rev., by Hugh Paxton

A History of Critical Experiments at Pajarito Site, LA-9685-H, by R.E. Malenfant, 1983

A Review of Fuel Element Development for Nuclear Rocket Engines, Taub 1975


Modern 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: