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Nuclear Thermal Systems Test Stands

Fluid Fueled NTRs: A Brief Introduction

 Hello, and welcome back to Beyond NERVA! This is actually about the 6th blog post I’ve started, and then split up when they ran more than 20 pages long, in the last month, and more explanatory material was needed before I discussed the concepts I was trying to discuss (this blog post has also been split up multiple times).

I apologize about the long hiatus. A combination of personal, IRL complications (I’ve updated the “About Me” section to reflect this, but those will not affect the type of content I share on here), and the professional (and still under wraps) opportunity of a lifetime have kept me away from the blog for a while. I want to return to Nuclear Thermal Rockets (NTRs) for a while, rather than continuing Nuclear Electric Propulsion (NEP) power plants, as a fun, still-not-covered area for me to work my way back into writing regularly for y’all again.

This is the first in an extensive blog series on fluid fueled NTRs, of three main types: liquid, vapor, and gas core NTRs. These reactors avoid the thermal limitations of the fuel elements themselves, increasing the potential core temperature to above 2550 K (the generally accepted maximum thermal limit on workable carbide fuel elements), increasing the specific impulse of these rockets. At the same time, structural material thermal limits, challenges in adequately heating the propellant to gain these advantages in a practical way, fissile fuel containment, and power density issues are major concerns in these types of reactors, so we’re going to dig into the weeds of the general challenges of fluid fueled reactors in general in this blog post (with some details on each reactor type’s design envelope).

Let’s start by looking at the basics behind how a nuclear reactor can operate without any solid fuel elements, and what the advantages and disadvantages of going this route are.

Non-Solid Fuels

A nuclear reactor is, at its basic level, a method of maintaining a fission reaction in a particular region for a given time. This depends on maintaining a combination of two characteristics: the number of fissile atoms in a given volume, and the number and energy of neutrons in that same volume (the neutron flux). As long as the number of neutrons and the number of fissile atoms in the area are held in balance, a controlled fission reaction will occur in that area.

Solid Core Fuel Element, image DOE

The easiest way to maintain that reaction is to hold the fissile atoms in a given place using a solid matrix of material – a fuel element. However, a number of things have to be balanced for a fuel element to be a useful and functional piece of reactor equipment. For an astronuclear reactor, there are two main concerns: the amount of power produced by the fission reaction has to be balanced by how much thermal energy the fuel element is able to contain, and the fuel element needs to survive the chemical and thermal environment that it is exposed to in the reactor. (Another for terrestrial reactors is that the fuel element has to contain the resulting fission products from the reaction itself, as well as any secondary chemical pollutants, but this isn’t necessarily a problem for astronuclear reactors, where the only environment that’s of concern is the more heavily shielded payload of the rocket.) 

This doesn’t mean that a reactor has to use a solid fuel element. As the increasingly well known molten salt reactor, as well as various other fluid fueled reactor concepts, demonstrate, the only requirement is the combination of the number of fissile atoms and the required energy level and density of neutrons to exist in the same region of the reactor. This, especially in Russian literature, is called the “active zone” of the reactor core. This can be an especially useful as a term, since the reactor core can contain areas that aren’t as active in terms of fission activity. (A great example of this is the travelling wave reactor, most recently investigated – and then abandoned – by Terrestrial Energy.) But more generally it’s useful to differentiate the fueled areas undergoing fission from other structures in the reactor, such as neutron moderation and control regions in the reactor. The key takeaway is that, as long as there is enough fuel, and the right density of neutrons at the right energy, then a sustained – and controlled – fission reactor has been achieved.

The obvious consequence is that the solid fuel element isn’t required – and in the case of a nuclear thermal rocket, where the efficiency of the rocket is directly tied to the temperature it can achieve, the solid fuel is in fact a major limitation to a designer. The downside to this is that, unlike solids, fluids tend to move, especially under thrust. Because the materials used in a solid fueled rocket are already at the extremes of what molecular bonds can handle, this means that either very clever cooling or very robust containment methods need to be used to keep the rest of the reactor from destroying itself.

Finally, one of the interesting consequences of not having a solid fuel element is that the reactor’s power density (W/m^2) and specific power (W/kg) can be increased in proportion to how much coolant can be used in theory, but in practice it can be challenging to maintain a high power density in certain types of fluid fueled reactors due to the high rate of thermal expansion that these reactors can undergo. There are ways around this, and fluid fueled reactors can have higher power densities than even closely related solid fueled variants, but the fact that fluids are able to expand much more than solids under high temperatures is an effect that should be taken into account. On the other hand, if the fluid expands too much, it can drop the power density, but not necessarily the specific mass of the system.

Types of and Reasons for Fluid Fuels

Fluid fuels fall into three broad categories: liquids, vapors, and gasses. There are intermediate steps, and hybrids between various phase states of fuel, but these three broad categories are useful. While liquid fuels are fairly self-explanatory (a liquid state fissile material is used to fuel the core, often uranium carbide mixed with other carbides, or U-Mo, but other options exist), the vapor and gas concepts are far less straightforward overall. The vapor core has two major variants: discrete liquid droplets, or a low pressure, relatively low temperature gaseous suspension similar to a cloud. The gas core could be more appropriately called a “plasma core,” since these are very high temperature reactors, which either mechanically hold the plasma in place, or use hydrodynamic or electrodynamic forces to hold the plasma in place.

However, they all have some common advantages, so we’ll look at them as a group first. The obvious reason for using non-solid fuel, in most cases, is that they are generally less thermally limited than solid fuels are (with some exceptions). This means that higher core temperatures, and therefore higher exhaust velocity (and specific impulse) can be achieved.

Convection pattern in radiator-type
liquid fuel element, image DOE

An additional benefit to most fluid fueled designs is that the fluid nature of the fuel helps mitigate or eliminate hot spots in the fuel. With solid fuels, one of the major challenges is to distribute the fissile material throughout the fuel as evenly as possible (or along a specifically desired gradient of fissile content depending on the position of the fuel element within the reactor). If this isn’t done properly, either through a manufacturing flaw or migration of the fissile component as a fuel element becomes weakened or damaged during use, then a hot spot can develop and damage the fuel element in both its nuclear and mechanical properties, leaning to a potentially failed fuel element. If the process is widespread enough, this can damage or destroy the entire reactor.

Fluid fuels, on the other hand, have the advantage that the fuel isn’t statically held in a solid structure. Let’s look at what happens when the fuel isn’t fully homogeneous (completely mixed) to understand this:

  1. A higher density of fissile atoms in the fuel results in more fission occurring in a particular volume.
  2. The fuel heats up through both radiation absorption and fission fragment heating.
  3. The fuel in this volume becomes less dense as the temperature increases.
  4. The increased volume, combined with convective mixing of cooler fuel fluids and radiation/conduction from the surface of the hotter region cools the region further.
  5. At the same time, the lower density decreases the fission occurring in that volume, while it remains at previous levels in the “normally heated” regions.
  6. The hot spot dissipates, and the fuel returns to a (mostly) homogeneous thermal and fissile profile.

In practice, this doesn’t necessarily mean that the fuel is the same temperature throughout the element – this very rarely occurs, in fact. Power levels and temperatures will vary throughout the fuel, causing natural vortices and other structures to appear. Depending on the fuel element configuration, this can be either minimized or enhanced depending on the need of the reactor. However, the mixing of the fuel is considered a major advantage in this sort of fuel.

Another advantage to using fluid fuels (although one that isn’t necessarily high on the priority list of most designs) is that the reactor can be refueled more easily. In most solid fueled reactors, the fissile content, fission poison content, and other key characteristics are carefully distributed through the reactor before startup, to ensure that the reactor will behave as predictably as possible for as long as possible at the desired operating conditions. In terrestrial solid reactors, refueling is a complex, difficult process, which involves moving specific fuel bundles in a complex pattern to ensure the reactor will continue to operate properly, with only a little bit of new fuel added with each refueling cycle.

PEWEE Test stand, image courtesy DOE

There were only two refuelable NTR testbeds in the US Rover program: Pewee and the Nuclear Furnace. Both of these were designed to be fuel element development apparatus, rather than functional NTRs (although Pewee managed to hit the highest Isp of any NTR tested in Rover without even trying!), so this is a significant difference. While it’s possible to refuel a solid core NTR, especially one such as the RD-0410 with its discrete fuel bundles, the likely method would be to just replace the entire fueled portion of the reactor – not the best option for ease of refueling, and one that would likely require a drydock of sorts to complete the work. To give an example, even the US Navy doesn’t always refuel their reactors, opting for long-lived highly enriched uranium fuel which will last for the life of the reactor. If the ship needs refueled, the reactor is removed and replaced whole in most cases. This reticence to refuel solid core reactors is likely to still be a thing in astronuclear reactors for the indefinite future, since placing the fuel elements is a complex process that requires a lot of real-time analysis of the particulars of the individual fuel elements and reactors (in Rover this was done at the Pajarito Site in Los Alamos).

Fluid fuels, though, can be added or removed from the reactor using pumps, compressed gasses, centrifugal force, or other methods. While not all designs have the capability to be refueled, many do, and some even require online fuel removal, processing and reinsertion into the active region of the core to maintain proper operation. If this is being done in a microgravity environment, there will be other challenges to address as well, but these have already been at least partially addressed by on-orbit experiments over the decades in the various space programs. (Specific behaviors of certain fluids will likely need to be experimentally tested for this particular application, but the basic physics and engineering solutions have been researched before).

Finally, fluid fuels also allow for easier transport of the fuel from one location to another, including into orbit or another planet. Rather than having a potentially damageable solid pellet, rod, prism, or ribbon, which must be carefully packaged to not only prevent damage but accidental criticality, fluids can be transported with far less risk of damage: just ensure that accidental criticality can’t occur, chemical compatibility between the fluid and the vessel it’s carrying, and package it strongly enough to survive an accident, and the problem is solved. If chemical processing and synthesis is available wherever the fuel is being sent (likely, if extensive and complex ISRU is being conducted), then the fuel doesn’t even need to be in its final form: more chemically inert options (UF4 and UF6 can be quite corrosive, but are easily managed with current materials and techniques), or less fissile-dense options (to reduce the chance of accidental criticality further) can be used as fuel precursors, and the final fuel form can be synthesized at the fueling depot. This may not be necessary, or even desirable, in most cases, but the option is available.

So, while solid fuels offer certain advantages over fluid fuels, the combination of being more delicate (thermally, chemically, and mechanically) combine to make fluid fuels a very attractive option. Once NTRs are in use, it is likely that research into fluid fueled NTRs will accelerate, making these “advanced” systems a reality.

Fuel Elements: An Overview

Now that we’ve looked at the advantages of fluid fuels in general, let’s look at the different types of fluid fuels and the proposals for the form the fuel elements in these reactors would take. This will be a brief overview of the various types of fuels, with more in-depth examinations coming up in future blog posts.

Liquid Fuel

A liquid fueled reactor is the best known popularly, although the most common type (the molten salt reactor) uses either fluoride or chloride salts, both of which are very corrosive at the temperatures an NTR operates at. While I’ve heard arguments that the extensive use of regenerative cooling can address this thermal limitation, this would still remain a major problem for an NTR. Another liquid fuel type, the molten metal reactor, has also been tested, using highly corrosive plutonium fuel in the best known case (the Liquid Annular Molten Plutonium Reactor Experiment, or LAMPRE, run by Los Alamos Scientific Lab from 1957 to 1963, covered very well here).

Early bubbler-type liquid NTR, Barrett 1963

The first proposal for a liquid fueled NTR was in 1954, by J McCarthy in “Nuclear Reactors for Rockets.” This design spun molten uranium carbide to produce centrifugal force (a common characteristic in liquid NTRs of all designs), and passed the propellant through a porous outer wall, through the fuel mass, and into the central void in the reactor before it was ejected out of the nozzle.The main problem with this reactor was that the tube was simply too large to allow for as much heat transfer as was ideal to take place, so the next evolution of the design broke up the single large spinning fuel element up into several thinner ones of the same length, increasing the total surface area available for heating the propellant. This work was conducted at Princeton, and would continue on and off until 1973. These designs I generally call “bubblers,” due to the propellant flow path.

Princeton multi-fuel-element bubbler, Nelson et al 1963

One problem with these designs is that the fuel would vaporize in the low pressure hydrogen environment of the bubbles, and significant amounts of uranium would be lost as the propellant went through the fuel. Not only is uranium valuable, but it’s heavy, reducing the exhaust velocity and therefore the specific impulse. Another issue is that there are hard limits to how much propellant can be passed through the fuel at any given time before it starts to splatter, directly tying thrust to fuel volume. 

In order to combat this, a team at NASA’s Lewis Research Center decided to study the idea of passing the propellant only through the central void in the fuel, allowing radiation to be the sole means of heating the propellant. Additional regenerative cooling structures were needed for this design, and ensuring the propellant got heated sufficiently was a challenge, but this sort of LNTR, the radiator type, became the predominant design. Vapor losses of the uranium were still a problem, but were minimized in this configuration.

It too would be cancelled in the late 1960s, but briefly revived by a team at Brookhaven National Laboratory in the early 1990s for possible use in the Space Exploration Initiative, however this program was not selected for further development.

Despite these challenges, liquid core NTRs have the potential to reach above 1300 s isp, and a T/W ratio of up to 0.5, so there is definite promise in the concept.

Droplet/Vapor Fuel

Picture a spray bottle, the sort used for household plants, ironing, or cleaning products like window cleaner. When the trigger is pulled, there’s a fine spray of liquid exiting the nozzle, which contains a mix of liquid and gas. Using a similar system to mix liquids and gasses is possible in a nuclear reactor, and is called a droplet core NTR. This reactor type is useful in that there’s incredible surface area available for radiation to occur into the propellant, but unfortunately it also means that separating the fuel droplets from the propellant upon leaving the nozzle (as well as preventing the fuel from coating the reactor core walls) is a major hydrodynamics challenge in this type of reactor.

Vapor core NTR, Diaz et al 1992

The other option is to use a vapor as fuel. A vapor is a substance that is in a gaseous state, but not at the critical point of the material – i.e. at standard temperature and pressure it would still be a liquid. One interesting property of a vapor is that a vapor is able to be condensed or evaporated in order to change the phase state of the substance without changing its temperature, which could be a useful tool to use for reactor startup. The downside of this type of fuel is that it has to be in an enclosed vessel in order to maintain the vapor state.

So why is this useful in an NTR? Despite the headaches we’ve just (briefly, believe it or not) discussed in the liquid fuels section, liquid fuel has a major advantage over gaseous fuel (our next section): the liquid phase is far better at containing its constituent parts than the gas phase is, due to the higher interatomic bond strength. At the same time, maintaining a large, liquid body can be a challenge, especially in the context of complex molecular structures in some of the most chemically difficult elements known to humanity (the actinides and transuranics). If the liquid component is small, though, it’s far easier to manage the thermal distribution, as well as offering greater thermal diffusion options (remember, the heat IN the fissile fuel needs to be moved OUT of it, and into the propellant, which is a direct function of available surface area).

The droplet core NTR offers many advantages over a liquid fuel in that the large-scale behavior of the liquid fuel isn’t a concern for reactor dynamics, and the aforementioned high surface area offers awesome thermal transfer properties throughout the propellant feed, rather than being focused on one volume of the propellant.

Vapors offer a middle ground between liquids and gasses: the fissile fuel itself is in suspension, meaning that the individual molecules of fissile fuel are able to circulate and maintain a more or less homogeneous temperature. 

This is another design concept that has seen very little development as an NTR (although NEP applications have been investigated more thoroughly, something that we’ll discuss the application and complications of, for an NTR in the future). In fact, I’ve only ever been able to find one design of each type designed for NTR use (and a series of evolving designs for NEP), the appropriately named Droplet Core Nuclear Rocket (DCNR) and the Nuclear Vapor Thermal Reactor (NTVR).

Droplet Core NTR, Anghaie et al 1992

The DCNR was developed in the late 1980s based on an earlier design from the 1970s, the colloid core reactor. The original design used ultrafine microparticles of U-C-ZR carbide fuel, which would be suspended in the propellant flow. This sort of fuel is something that we’ll look at more when covering gas core NTRs (metal microparticles are one of the fuel types available for a GCNTR), but the use of carbides increases the fuel failure temperature to the point that structural components would fail before the fuel itself would, leading to what could be called an early pseudo-dusty plasma NTR. The droplet core NTR took this concept, and applied it to a liquid rather than solid fuel form. We’ll look at how the fuel was meant to be contained before exiting the nozzle in the next section, but this was the main challenge of the DCNR from an engineering point of view.

The NVTR was a compromise design based on NERVA fuel element development with a different fissile fuel carrier. Here, the fuel (in the form of UF4) is contained within a carbon-carbon composite fuel element in sealed channels, with interspersed coolant channels to manage the thermal load on the fuel element. While significant thrust-to-weight ratio improvements were possible, and (in advanced NTR terms) modest specific impulse gains were possible, the design didn’t undergo any significant development. We’ll cover containment in the next section, and other options for architectures as well.

Gas Fuel

Finally, there are gas core NTRs. In these, the fuel is in gaseous form, allowing for the highest core temperatures of any core configuration. Due to the very high temperatures of these reactors, the uranium (and in general the rest of the components in the fuel) become ionized, meaning that a “plasma core” is as accurate a description as a “gas core” is, but gas remains the convention. The fuel form for a gas core NTR has a few variants, with the most common being UF6, or metal fuel which vaporizes as it is injected into the core. Due to the high temperatures of these reactors, the UF6 will often break down as all of the constituent molecules become ionized, meaning that whatever structures will come in contact with the fuel itself (either containment structures or nozzle components) must be designed in such a way to prevent being attacked by high temperature fluorine ions and hydrofluoric acid vapors formed when the fluorine ions come in contact with the propellant.

Containing the gas is generally done in one of three ways: either by compressing the gas mechanically in a container, by holding the gas in the middle of the reactor using the gas pressure from the propellant being injected into the core, or by using electromagnets to contain the plasma similarly to how a spherical tokamak operates. The first concept is a closed cycle   gas core (CCGCNTR, or GC-C), while the second two are called open cycle gas core NTRs (OCGCNTR or GC-O), because while the first one physically contains the fuel and prevents fission products, unburned fuel, and the previously mentioned free fluorine from exiting in the exhaust plume of the reactor, the open cycle’s largest problem in designing a workable NTR is that the vast majority (often upwards of 90%) if the uranium ends up being stripped away from the plasma body before it undergoes fission – a truly hot radioactive mess which you don’t want to use anywhere near anything sensitive to radiation and an insanely inefficient use of fissile material. There are many other designs and hybrids of these concepts, which we’ll cover in the gas core NTR series, and will look briefly at the containment challenges below.

Fluid Fuel Elements: Containment Strategies

Fluid fuels are, well, fluid. Unlike with a solid fuel element, as we’ve looked at in the past, a fluid has to be contained somehow. This can be in a sealed container or by using some outside force to keep it in place.

Another issue with fluid fuels can be (but isn’t always) maintaining the necessary density to achieve the power requirements for an NTR (or any astronuclear system, for that matter). All materials expand when heated, but with fluids this change can be quite dramatic, especially in the case of gas core NTRs. Because of this, careful design is required in order to maintain the high density of fissile fuel necessary to make a mass-efficient rocket engine possible.

This leads to a rather obvious conclusion: rather than the fuel element being a physical object, in a fluid fueled NTR the fuel element is a containment structure. Depending on the fuel type and the reactor architecture, this can take many forms, even in the same type of fuel. This will be a long-ish review of the proposed fuel containment strategies, and how they impact the performance of the reactors themselves.

One thing to note about all of these reactor types is that 235U is not required to be the fissile component in the fuel, in fact many gas core designs use 233U instead, due to the lower requirements for critical mass. (According to most Russian literature on gas core NTRs, this  reduces the critical mass requirements by 1/3). Other options include using 242mAm, a stable isomer of 242Am, which has the lowest critical mass of any fissile fuel. By using these types of fuels rather than the typical 235U, either less of the fuel mass needs to be fissile (in the case of a liquid fueled NTR), or less fuel in general is needed (in the case of vapor/gas core NTRs). This can be a double-edged sword in some systems with high fuel loss rates (like an open cycle gas core), which would require more robust and careful fuel management strategies to prevent power transients due to fuel level variations in the active zone of the reactor, but the overall reduction in fuel requirements means that there’s less fuel that can be lost. Many other fissile fuel types also exist, but generally speaking either short half-lives, high spontaneous fission rates, or expense in manufacture have prevented them from being extensively researched.

Let’s look at each of the design types in general, with a particular focus on gas core NTRs at the end.

Liquid FE

For liquid fuels, there’s one universal option for containing the fuel: by spinning the fuel element. However, after this, there’s two main camps on how a liquid fueled NTR interacts with the propellant. The original design, first proposed in the 1950s and researched at least through the 1960s, proposed the use of either one or several spinning cylinders with porous outer walls (frits), which would be used to inject the propellant into the reactor’s active region. For those that remember the Dumbo reactor, this may be familiar as a folded flow NTR, and does two things: first, it allowed the area surrounding the fuel elements to be kept at very low temperatures, allowing the use of ZrH and other thermally sensitive materials throughout the reactor, and second it increases the heat transfer area available from the fuel to the propellant. Experiments (using water as a uranium analog) were conducted to study the basics of bubble behavior in a spinning fluid to estimate fuel mass loss rates, and the impact of evaporation or vaporization of various forms of uranium (including U metal, UC2, and others) were conducted. 

This concept is the radiator type LNTR. Here, rather than the folded flow used previously, axial flow is used: the H2 is used as a coolant for reactor structures (including the nozzle) passing from the nozzle end to the ship end, and then injected through the central void of each of the fuel elements before exiting the nozzle. This design reduces the loss of fuel mass due to bubbling in the fuel, but adds an additional challenge of severely reducing the amount of surface area available for heat transfer from the fuel to the propellant. In order to mitigate this, some designs propose to seed the propellant with microparticles of tungsten, which would absorb the significant about of UV and X rays coming off the fuel, and turn it into IR radiation which is more easily absorbed by the H. At the designed operating temperatures, this reactor would dissociate the majority of the H2 into monatomic hydrogen, increasing the specific impulse significantly.

In all these designs, there is no solid clad between the fuel itself and the propellant, because this means that the hottest portion of the fuel element would be limited by how high the temperature can reach before melting the clad. Some early LNTR designs used a mix of molten UC2 and ZrC/NbC as a fuel element, with the ZrC meant to migrate to the upper areas of the fuel element and not only provide neutron moderation but reduce the amount of erosion from the propellant. It may be possible to use a liquid metal clad as a barrier to prevent mass erosion of the fissile fuel in a metal fueled reactor as well, and possibly even add some neutron moderation for the fuel element itself. However, the material would need to have not only a very high boiling point, high thermal conductivity, low reactivity to both hydrogen and the fuel, and low neutron capture cross section, it would also need to have a high vapor pressure in order to prevent erosion from the propellant flow (although I suppose adding additional clad during the course of operation would also be an option, at the cost of higher propellant mass and therefore lost specific impulse).

Droplet/Vapor FE

Now let’s look at the vapor core NTR.

NVTR fuel element, Diaz et al 1992

Containing the UF4 vapor in the NVTR vapor core NTR is done by using a sealed tube embedded in a fuel element, which is then surrounded by propellant channels to carry away the heat. Two configurations were proposed in the NTVR concept: the first used a large central cavity, sealed at both ends, to contain the vapor, and the second design dispersed the fuel cylinders in an alternating hexagonal pattern throughout the fuel element. The second option provides a more even thermal distribution not only within the fuel element itself, but across the entire active zone of the reactor core.

Droplet core NTRs are very different in their core structure. Rather than having multiple areas that the fissile fuel is isolated in, the droplet core sprays droplets of fissile fuel into a large cylinder, which is spun to induce centrifugal force. The fuel is kept away from the walls of the reactor core using a collection of high-pressure H2 jets, injecting the propellant into the fuel suspension and maintaining hydrostatic containment on the fuel. The last section of the reactor core, instead of using hydrogen, injects a liquid lithium spray to bind with the uranium, which is then carried to the walls of the reactor due to the lack of tangential force. The fuel is then recirculated to the top of the reactor vessel, where it is once again injected into the core.

This hydrostatic equilibrium concept is very similar to how many gas core NTRs operate (which we’ll look at below), and has proven to be the biggest Achilles’ Heel of these sorts of designs. While it may be theoretically possible to do this (the lower temperatures of the droplet core allow for collection and recirculation, which may provide a means of fissile fuel loss reduction), many of the challenges of the droplet core are very similar to that of the open cycle gas core, a far more capable engine type.

Gas Core

Gas core containment is possibly the most complex topic in this post, due to the sheer variety of possible designs and extreme engineering requirements. We’ll be discussing the different designs in depth in upcoming blog posts, but it’s worth doing an overview of the different designs, their strengths and weaknesses, here.

Closed Cycle

One half of the lightbulb configuration, McLafferty et al 1968

The simplest design to describe is the closed cycle gas core, which in many ways resembles a vapor core NTR. In most iterations, a sealed cylinder with a piston at one end (similar in many ways to the piston in an automobile engine), is filled with UF6 gas. This is compressed in order to reach critical geometry, and fission occurs in the cylinder. The walls of the cylinder are generally made out of quartz, which is transparent to the majority of the radiation coming off the fissioning uranium, and is able to resist the fluorination from the gas (other options include silicon dioxide, magnesium oxide, and aluminum oxide). Additionally, while the quartz will darken under the heat, the radiation actually “anneals” the quartz to keep it transparent, and coolant is run through the cylinder to maintain the material within thermal limits; a vortex is induced during fission which, when properly managed, also keeps the majority of the uranium (now in a charged state) from coming in contact with the walls of the chamber as well, reducing thermal load on the material. Some designs have used pressure waves in place of the piston to induce fission, but the fluid-mechanical result is very similar. This results in a lightbulb-like structure, hence the common nickname “nuclear lightbulb.” One variation mentioned in Russian literature also uses a closed uranium loop, circulating the fissile fuel to minimize the fission product buildup and maintain the fissile density of the reactor.

The main advantage to these types of designs is that all fission products and particle radiation are contained within the bulb structure, meaning that fission product and radiation release into the environment is eliminated, with only gamma and x-ray radiation during operation being a concern. However, due to the fact that there’s a solid structure between the fuel element and the propellant, this engine is thermally limited more than any other gas core design, and its performance in both thrust and specific impulse suffers as a result.

Open Cycle

The next very broad category is an open cycle gas core. Here, there is usually no solid structure between the fissioning uranium and the propellant, meaning that core temperatures can reach astoundingly high temperatures (sometimes limited only by the melting temperature of the materials surrounding the active reactor zone, such as reflectors and pressure vessel). Sadly, this also means that actually containing the fuel is the single largest challenge in this type of reactor, and the exhaust tends to be incredibly radioactive as a result, On the plus side, this sort of rocket can achieve isp in the tens of thousands of seconds (similar to or better than electric propulsion), and also achieve high thrust.

Perhaps the easiest way to make a pure open cycle gas core NTR is to allow the fuel and the propellant to fully mix, similarly to how the droplet core NTR was done, and either ensure all (or most) of the fissile fuel is burned before leaving the rocket nozzle. Insanely radioactive, sure, but with a complete mixing of the fissioning atoms and the propellant the theoretically most efficient transfer of energy is possible. However, the challenge of fully fissioning the fuel in such a short period of time is significant, and I can’t find any evidence of significant research into this type of gas core reactor.

Due to the challenges of burning the fissile fuel completely enough during a single pass through the reactor, though, it is generally considered required to maintain a more stable fissile structure within the reactor’s active region. Maintaining this sort of structure is a challenge, but is generally done through gasdynamic effects: the propellant injected into the reactor is used to push the fuel back into the center of the reactor. This involves the use of a porous outer wall of the reactor, where the hydrogen propellant is inserted at a high enough pressure and evenly enough spaced intervals to counterbalance both the tendency of the plasma to expand until it’s not able to undergo fission and the tendency of the fuel to leave the nozzle before being burned.

Soviet-type Vortex Stabilized open cycle, image Koroteev et al 2007

The next way is to create a low pressure stagnant area in the center of the core, which will contain the fissile fuel. In order to maintain this type of pressure differential, a solid structure is usually needed, generally made out of a high temperature refractory metal. In a way this is a hybrid closed/open cycle gas core (even though the plasma isn’t in direct contact with the structure of the reactor itself), because the structure itself is key to generating this low pressure zone necessary for maintaining this plasma body fuel element. This type of NTR has been the focus of Russian gas core research since the 1970s, and will be covered more in the future.

Spherical gas core diagram, image NASA

As I’m sure most of you have guessed, fuel containment is a very complex and difficult problem, and one that’s had many solutions over the years (which we’ll cover in a future post). Most recent gas core NTR designs in the US are based on the spherical gas core. Here, the plasma is held in the center of the active zone using jets of propellant from all sides. This is generally called a porous wall gas core NTR, and while it takes advantage of any vortex stabilization that may occur in the fuel, it does not rely on it; in many ways, it’s a lot like an indoor skydiving arena with air jets blowing from all sides. This design, first proposed in the 1970s, uses high pressure propellant to contain the fuel in the reactor, and in many designs the flow can be adjusted to deal with the engine being under thrust, pushing the fuel toward the nozzle in traditional design configurations. Most designs suffer from massive erosion of the fuel by shear forces from the propellant eroding the fuel from the outside edge, but in some conceptual sketches this can be gotten around using non-traditional nozzle configurations which have a solid structure along the main thrust axis of the rocket. (More on that in a future post. I’m still trying to track down the sources to fully explain that pseudo-aerospike concept).

Hybrid gas core diagram, Beveridge 2017

The most promising designs as far as fuel loss rates minimize the amount of plasma required to maintain the reaction. This is what’s known as a hybrid solid-gas NTR, first proposed by Hyland in the 1970s, and also one of the designs which has been most recently investigated by Lucas Beveridge. Here, the fissile fuel is split between two components: the high-temperature plasma fuel is used for final heating of the propellant, but isn’t able to sustain fission independently. Instead, a sphere of solid fuel encases the outside of the active zone of the reactor. This minimizes the amount of fuel that can be easily eroded while ensuring that a critical mass of fissile material is contained in the active region of the reactor. This really is less complicated than it sounds, but is difficult to summarize briefly without delving into the details of critical geometry, so I’ll try to explain it this way: the interior of the reactor is viewed by the neutrons in the reactor as a high-density low temperature fuel area, surrounding a low density high temperature fuel area, with the coolant/moderator passing through the high density area and flowing around the low density area, making a complete reactor between these parts while minimizing how much of the low density fuel is needed and therefore minimizing the fuel loss. I wish I was able to make this more clear in less than a couple pages, but sadly I’m not that good at summarizing in non-technical terms. I’ll try and do better on the hybrid core post coming in the future.

All of these designs suffer from massive fuel loss, leading to highly radioactive exhaust and incredibly inefficient engines which are absurdly expensive to operate due to the amount of highly enriched fissile fuel needed. (Because everything going into the reactor needs to fission as quickly as possible, every component of the fuel itself needs to undergo fission as easily as possible.) This is the major Achilles heel of this NTR type: despite the massive potential promise, the fuel loss, and radioactive plume coming off these reactors, make them unusable with current engineering.

There’s going to be a lot more that I’m going to write about this type of NTR, and I skipped a lot of ideas, and variations on these ideas, so expect a lot more in the coming year on this subject.

Cooling the Reactor/Heating the Propellant

Finally there’s cooling, which usually comes in one of two varieties:

  1. cooling using the propellant, as in most NTR designs that we’ve seen, to reject all the heat from the reactor
  2. cooling in a closed loop, as is done in an NEP system
Hybrid gas core with secondary cooling diagram, Beveridge 2017

While the ideal situation is to reject all the heat into the propellant, which maximizes the thrust and minimizes the dry mass of the system, this is the exception in many of these systems, rather than the norm. There’s a couple reasons for this: containing a fluid with fast-moving (or high pressure) hydrogen is challenging because the gas wants to strip away the mass that it comes in contact with (far easier in a fluid than a solid), H2 is insanely difficult to contain at almost any temperature, and these reactors are designed to achieve incredibly high temperatures which can outstrip the available heat rejection area that the reactor designs allow.

Complicating the issue further, hydrogen is mostly transparent to the radiation that a nuclear reactor puts off (mostly in the hard UV/X/gamma spectrum), meaning that it takes a lot of hydrogen to reject the heat produced in the reactor (a common complaint in any gas-cooled reactor, to be fair), and that hydrogen doesn’t get heated that much on an atom-by-atom basis, all things considered.

There’s a way around this, though, which many designs, from LARS on the liquid side to basically every gas core design I’ve ever seen use: microparticle or vapor seeding. This is a form of hybrid propellant, which I mention in my NTR propellants page. Basically, a metal is ground incredibly fine (or is vaporized), and then included in the propellant feed. This captures the high-wavelength photons (due to its higher atomic mass, and greater opacity to those wavelengths as a result), which are re-emitted at a lower frequency which is more easily absorbed by the propellant. While the US prefers to use tungsten microparticles in their designs, the USSR and Russia have also examined two other types of metals: lithium and NaK vapor. These have the advantage of being lower mass, impacting the overall propellant mass less, and also far easier to control fluid insertion rates (although microparticles can act as fluidized materials due to their small size, and maintain suspension in the H2 propellant well). This is a subject that I’ll cover in more depth in the future in the gas core NTR post.

(Side note: I’ve NEVER seen data on non-hydrogen propellant in a liquid-fueled NTR, but this problem would be somewhat ameliorated by using a higher atomic mass fuel, but which one is used will determine both how much more radiation would be directly absorbed, and what kind of loss in specific impulse would accompany this substitution. Also, using other elements/molecules would significantly change the neutronic structure and hydrodynamic behavior of the reactor, a subject I’ve never seen covered in any paper.)

Sadly, in many designs there simply isn’t the heat capacity to remove all of the reactor’s thermal energy through the propellant stream. Early gas core NTRs were especially notorious for this, with some only able to reject about 3% of the reactor’s thermal energy into the propellant. In order to prevent the reactor and pressure vessel from melting, external radiators were used – hence the large, arrowhead-shaped radiators on many gas core NTR designs.

This is unfortunate, since it directly affects the dry mass of the system, making it not only heavier but less power efficient overall. Fortunately, due to the high temperatures which need to be rejected, advanced high temperature radiators can be used (such as liquid droplet radiators, membrane radiators, or high temperature liquid metal radiators) which can reject more energy in less mass and surface area.

Another example, one which I’ve never seen discussed before (with one exception) is the use of a bimodal system. If significant amounts of heat are coming off the reactor, then it may be worth it to use a power conversion system to convert some of the heat into electricity for an electric propulsion system to back up the pure thermal system. This is something that would have to be carefully considered, for a number of reasons:

  1. It increases the complexity of the system: power conversion system, power conditioning system, thrusters, and support subsystems for each must be added, and each needs extensive reliability testing.
  2. It will significantly increase the mass of the system, so either the thrust needs to be significantly increased or the overall thrust efficiency needs to offset the additional dry mass (depending on the desire for thrust or efficiency in the system).
    1. Knock on mass increases will be extensive, with likely additions being: an additional primary heat loop, larger radiators for heat rejection, main truss restructuring and brackets, additional radiation shielding for certain radiation sensitive components, possible backup power conditioning and storage systems, and many other subsystem support structures.
  3. This concept has not been extensively studied; the only example that I’ve seen is the RD-600, which used a low power mode with an MHD that the plasma passed directly through in a closed loop system (more on this system in the future); this is obviously not the same type of system being discussed here. The only other similar parallel is with the Werka-type dusty plasma fission fragment rocket, which uses a helium-xenon Brayton turbine to provide about 100 kWe for housekeeping and system electrical power. However, this system only rejected less than 1% of the total FFRE waste heat.
    1. The proper power conversion system needs to be selected, thruster selection is in a similar position, and other systems would go through similar selection and optimization processes would need to be done. This is made more complex due to the necessity to match the PCS and thermal management of the system to the reactor, which has not been finalized and is currently very inefficient in terms of fissile material. If a heat engine is used, the quality of the heat reduces, meaning larger (and heavier) radiators are needed, as well.

Fluid Fuels: Promises of Advanced Rockets, but Many Challenges to Overcome

As we’ve seen in this brief overview of fluid fueled NTRs, the diversity in advanced NTR designs is broad, with an incredible amount of research having been done over the decades on many aspects of this incredibly promising, but challenging, propulsion technology. From the chemically challenging liquid fuel NTR, with several materials and propellant feed challenges and options, to the reliable vapor core, to the challenging but incredibly promising gas core NTR, the future of nuclear thermal propulsion is far more promising than the already-impressive solid core designs we’ve examined in the past.

Coming up on Beyond NERVA, we will examine each of these types in detail in a series of blog posts, and the information both in this post and future posts will be adapted into more-easily referenced web pages. Interspersed with this, I will be working on filling in details on the Rover series of engines and tests on the webpage, and we may also cover some additional solid core concepts that haven’t been covered yet, especially the pebble-bed designs, such as Timberwind and MITEE (the pebble-bed concept is also sometimes called a fluidized bed, since the fuel is able to move in relation to the other pellets in the fueled section of the reactor in many designs, so can be considered a hybrid system in some ways).

With the holiday season, life events, and concluding the project which has kept me from working as much as I would have liked on here in the coming months, I can’t predict when the next post (the first of three on liquid fueled NTRs) will be published, but I’ve already got 7 pages written on that post, six on the next (bubblers), and 6 on the final in that trilogy (radiator LNTR) with another 4 on vapor cores, and about 10 pages on the basic physics principles of gas core reactor physics (which is insanely complex), so hopefully these will be coming in the near future!

As ever, I look forward to your feedback, and follow me on Twitter, or join the Beyond NERVA Facebook page, for more content!

References

This is just going to be a short list of references, rather than the more extensive typical one, since I’m covering all this more in depth later… but here’s a short list of references:

Liquid fuels

“Analysis of Vaporization of Liquid Uranium, Metal, and Carbon Systems at 9000 and 10000 R,” Kaufman et al 1966 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660025363.pdf

“A Technical Report on Conceptual Design Study of a Liquid Core Nuclear Rocket,” Nelson et al 1963 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19650026954.pdf

“Performance Potential of a Radiant Heat Transfer Liquid Core Nuclear Rocket Engine,” Ragsdale 1967 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670030774.pdf

Vapor and Droplet Core

“Droplet Core Nuclear Reactor (DCNR),” Anghaie 1992 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920001887.pdf

“Vapor Core Propulsion Reactors,” Diaz 1992 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920001891.pdf

Gas Core

“Analytical Design and Performance Studies of the Nuclear Light Bulb Engine,” Rogers et al 1973 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730003969.pdf

“Open Cycle Gas Core Nuclear Rockets,” Ragsdale 1992 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920001890.pdf

“A Study of the Potential Feasibility of a Hybrid-Fuel Open Cycle Gas Core Nuclear Thermal Rocket,” Beveridge 2017 https://etd.iri.isu.edu/ViewSpecimen.aspx?ID=439

2 replies on “Fluid Fueled NTRs: A Brief Introduction”

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