Non-Nuclear Thermal 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).

What To Test, and How

Chemical Properties

There are many questions that need to be addressed before a candidate fuel element can be tested in a nuclear environment. Some of these questions are thermodynamic, some are mechanical, and some are chemical. In order to get a research institution to allow a test sample to be placed in their reactor, they want to have a high degree of confidence that you aren’t going to be screwing up their reactor, so dozens of questions need to be answered.

In addition, due to the unique nature of nuclear reactions, many of the combinations of materials used are very different from those that would be selected from a purely thermal or chemical point of view. For instance, no one would choose to combine graphite with hot hydrogen if they could get away with it, since the graphite will be quickly eaten away by the H2, creating a host of hydrocarbons (mostly CH4 and C2H2, but others show up as well), but the graphite is incredibly useful as a fuel element matrix material, and is a well understood nuclear material. So, designers on Project Rover added first niobium carbide (NbC) and later zirconium carbide (ZrC) cladding to protect the graphite. When this was done, though, they needed to check a couple things: how strongly the NbC or ZrC reacted with the H2 gas at temperature, and how much it reacted with the graphite at temperature. A nuclear reactor is a very hot place (often around 2500K), so many things that normally wouldn’t react in fact do because of the very high temperatures. Add in the fact that this isn’t a simple block of graphite, but that it has uranium oxide or carbide (UO2 or UC) fuel particles, either unclad or covered with a clad material (pyrolitic carbon, ZrC, or other cladding materials) spread through it at a particular particle density that can change depending on the fuel loading, adds another set of chemical reactions to control for.

Mechanical Properties

An important property for nuclear materials is their modulus of elasticity. This is actually a set of different properties that describe elastic deformation capacities of a material, depending on if the deformation is tensile (stretching), shear (tearing), or bulk (swelling), and other properties. In short, they describe how much a material can be deformed in particular ways while still being able to return to its original size and/or shape.

Another thing that must be tested, and can be done most cheaply by doing so in a non-nuclear environment, is the durability of the materials to be used. Broken fuel elements due to not only the large pressure drop from the cold side of the reactor to the hot, but also due to the intense vibrations experienced in the engine, were a major and consistent problem in Rover. Being able to figure out exactly what those limits are for the various materials, and either find a material workaround or a mechanical structure to solve the problem (in this case, the tie tube) is just as important as knowing what the neutronic behavior of the fuel elements themselves.

Thermal Properties

Another major concern in any high-temperature application, but especially for nuclear thermal rocket components, is how much each material will swell under the heat. This is known as the coefficient of thermal expansion, or CTE, and can be found for most materials. The amount will vary based on how much the material is heated, of course, but the CTE allows for a simple multiplicative factor to be used based on the temperature. If there are two materials with very different CTEs that are supposed to be next to each other, then either one will swell so much that it ends up damaging the other, or one will separate from the other, leaving a gap that will either cause heat to transfer poorly across (through radiation rather than conduction), or allow unwanted gasses such as H2 to get in the gap and cause all sorts of problems, both chemical and in the gas flow dynamics.

As an example of a challenge that must be dealt with, if a material has a high enough CTE, but a low bulk modulus, then it will end up swelling to the point that the material will have microfractures throughout its structure, weakening the material and leaving it swollen even after the material has been cooled down.

Another example of the effects of the combination of a CTE mismatch (as its’ known) and too low of a modulus of elasticity would be with a fuel element clad in a material. If the shear modulus is too low for the amount that the fuel element is going to swell, the clad will crack, split, or flake, causing imperfections that can allow the hot hydrogen to erode the fuel element, release fission products, and cause the flakes of the clad to be carried downstream, causing erosion damage and potentially opening up more fuel element material to erosion.

Finally, but also of absolutely critical importance, is the thermal conductivity properties of various components in the reactor. This is most important in the fuel elements themselves, where the amount of heat that can be transferred in a given time, for a given surface area, defines how much propellant you can pump through the reactor at any given time (and how much you can heat any given volume of gas, as well). However, the H2 is also used to cool the nozzle of the engine, the reflectors, the control drums, and many other components as well, and the amount of H2 in the core will greatly impact the neutron spectrum. So, this thermal conductivity testing is of absolute importance to be able to give the neutronicists on the design team the information that they need to properly design the fuel elements and control systems.

Over time, some fuels (especially carbides) may undergo changes in their chemical structure, mostly due to H2 erosion, but sometimes due to fission product buildup as well if the reactor is run for long enough. Due to the complex chemical nature of many of these fuels, such as (U,Nb,Zr)C, this can change local thermal conductivity and melting/vapor temperatures, causing localized problems in the fuel elements that can cause a cascade of issues. These effects can only be truly understood in an irradiated fuel element sample, but they may be able to be simulated, or with less-than-ideally manufactured test articles that have an uneven distribution of the various carbides in a solid solution. These properties are difficult to study, and every experiment has its’ surprises, but as much as CAN be understood before a candidate fuel element is placed in a research reactor, SHOULD be understood.

But… Why Not Just Irradiate Them?

Research reactors are expensive pieces of equipment, costing hundreds of millions of dollars. The institutions that have them are under a heavy safety and regulatory burden concerning all operations. Any accident can cause contamination of the reactor vessel, which is difficult and expensive to clean up – gunk on the walls is NOT acceptable in a research reactor core.

Because of this, test articles must be placed in special containers, and loaded into specific parts of the reactor. No reactor operator, and especially not the site manager, will allow a poorly-understood material to be placed in their reactor: they just don’t know HOW it could be screwed up.

Finally, once an item is irradiated, it’s going to be, well, radioactive, and in the case of a fuel element, it may be gamma-active for quite a long period of time (for research purposes, at least). This means that a glove box and/or hot cell will be needed. These are expensive, time consuming to maintain, and difficult to acquire (although not quite as difficult as the reactor itself). Why deal with that if you don’t have to?

Test Stands

Project Rover Hot Gas Furnace

Hot Gas Furnace

Hot gas furnace at LASL, image courtesy LANL

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, as did the fuel particles.

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.

Compact Fuel Element Environmental Test (CFEET)

CFEET Diagram, BradleyIn 2012, a new test stand was proposed, because NTREES (see below), while perfect for its task, was also expensive to operate. Therefore, in 2012 the Compact 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.

CFEET is a great tool for initial testing of materials, thermal cycling, and basic chemical reaction analysis.

You can find out more at the CFEET page.

Nuclear Thermal Rocket Environmental Effects Simulator (NTREES)

Chamber installationThis 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. This is important, because the experimenters need to be able to identify if clad materials or surrogate chemicals for fission products or fuel are being released in testing.

One of the most valuable things about NTREES is that it’s able to simulate long-duration, full-flow conditions at temperature. Mass erosion from the H2 gas passing over the clad materials is potentially a significant problem, especially over the course of multiple thermal cycles – as an NTR will experience. NTREES is the best available testbed for these types of test campaigns available anywhere in the world.

You can read more about it on the NTREES page.