Fission Power Systems

Fission Power Systems: High-Powered, Reliable Electricity in Space

Hello, and welcome back to the Beyond NERVA blog! Before we get started on today’s blog post, it has come to my attention that many people, even regulars to this blog, have been missing out on a lot of the content available here at BN. While the blog is a major focus of Beyond NERVA, it is far from the only thing that’s going on at beyondnerva.wordpress.com. There is a host of topical websites available, with a combination of content that’s been covered in this blog, as well as other areas that I didn’t have space for in the blog posts, didn’t fit anywhere, or that I discovered after I wrote the post on a subject. Each page is updated intermittently, depending on information received, time available, and other factors; but I will start posting on the homepage when each page is updated or added to make it easier to see what’s new, what’s updated, and what’s interesting. Two pages that have had major overhauls recently are the Fuel Elements page, and the Test Stands and Equipment page, so I’d encourage you to check them out. We also have the BN Facebook group, where I post page updates, interesting papers, and occasional 3D images (and now movie clips!) for the forthcoming YT channel (still a ways off yet, but work is continuing). Please check out the rest of the site, and come by to join the conversation on FB! With the community announcements out of the way, let’s get on to today’s blog post!

Fission-Based Electric Power systems

KRUSTY cutaway LANL

Kilopower cutaway diagram, image courtesy DoE

Today, we’re going to start looking at another way to use nuclear propulsion in space. One  not directly using the heat of the reactor like we’ve been examining for the last few months. Yes, today we’re beginning to look at nuclear electric propulsion. This will be an overview of the technology and options available, with a focus on the reactor and power conversion system options. This isn’t a new topic on this blog by any stretch of the imagination. Kilopower has been proposed as a spacecraft power supply to power electric thrusters; and, while this isn’t considered the most attractive option to many in NASA (they prefer to use the reactor to power surface installations, especially for crewed missions), this option is still being explored, researched, and utilized.

Most people that have looked into in-space nuclear power know that using a nuclear reactor for electrical power is nothing new. The US has flown one nuclear reactor (SNAP-10a); and the USSR and Russia have flown over thirty, using two different designs. All of these systems were designed to provide electrical power for a satellite. In the case of the US it was an experimental version of the Agena spacecraft; and for the USSR and Russia, it was used for RORSATs (Radar Ocean Reconnaissance SATellites). Radar is a notorious power hog, since resolution and power supply are directly proportional to each other. When these designs were first flown, photovoltaic panels were not nearly as efficient as they are today, so they weren’t considered a practical option.

We’ll look into these spacecraft more as this series continues on. For today, though, we’re going to take a broad overview of the mechanisms that a fission power supply needs; and a brief look at the different options available as far as the reactor itself, power conversion systems, and the reasons one would be used on a mission in the first place.

Electricity in On Earth

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San Onofre Nuclear Power Plant (now decommissioned), California, circa 2012. Image via Wikipedia

The vast majority of people are familiar with the way a nuclear power plant works on Earth: a reactor heats up water from the heat generated by fission in fuel elements (rods, bundles, or pellets, usually), which then goes through a steam generator. This steam then drives a set of steam turbines, which spin a generator and produce electricity. That water is then run through a cooling system (either cooling towers or a body of water), before being returned to the reactor core to start the process over again. Most reactors use what’s known as a two-loop system, meaning that the water that goes through the generator never enters the reactor core, instead the water from the core is run through a heat exchanger that transfers this heat to a second loop of water (the primary and secondary loops, respectively), to prevent the generator from becoming radioactive over time, while greatly simplifying the maintenance of the power plant. This is known as a two-loop pressurized water reactor (PWR), and is the most common type of nuclear reactor in the world. Another version actually produces the steam for the generators in the reactor vessel itself, and then runs this through the generator. This much simpler system is called a boiling water reactor (BWR), and has the advantage of mechanical simplicity; but adds the challenge of the turbine becoming radioactive over time, making maintenance more difficult. This design is most common in Japan. However, the systems are very similar: heat water, produce steam, run turbines, cool the water, return it to the core.

AGR Graphite Configuration

AGR graphite core configuration. The fuel cylinders are inserted into the wider, circular holes. Image via IAEA

One notable exception to this general trend for deployed reactors is in the UK, where gas cooled reactors are the most common type of reactor (in this case the AGR, or Advanced Gas Cooled Reactor). We’ve been looking at gas cooled reactors (sometimes known as HTGRs, or “High Temperature Gas Cooled Reactors”) a lot in the previous few months, so this concept should be familiar to regulars of Beyond NERVA. If you aren’t familiar, check out our Nuclear Thermal Rocket page for more information. The fuel elements have a different shape, and helium is used instead of hydrogen (as we’ve seen, if you can get away with NOT using hydrogen, that’s a good thing).

However, as many readers of this blog are familiar, nuclear power is going through a renaissance worldwide right now, with many reactor concepts that were proposed, but never deployed, being heavily investigated once again. Terms like “Liquid Metal Fast Breeder Reactor,” “Aqueous Homogeneous Reactor,” the ubiquitous “Molten Salt Reactor,” and others are being discussed, each with their own set of advantages and disadvantages over the current PWRs and BWRs that have been deployed worldwide. Even the HTGR is being talked about more, despite the fact that it’s been deployed in the UK fleet for decades, for many of the same reasons these other reactor designs are being investigated: it’s possible to make each of these designs “walk-away safe”, among other safety features inherent in their design, making nuclear accidents far more unlikely.

This doesn’t mean that these reactors are maintenance-free, however. Many of the components, especially the generators, require regular, sometimes quite difficult, maintenance. Some of the more advanced designs also have unique subsystems that will require maintenance or replacement (which is why many of the reactor designers are making modular designs, where the core itself, or peripheral components, can be disconnected, loaded onto a barge or truck, and delivered to a dedicated maintenance facility, while a replacement module is installed at the power plant) on a timeline that isn’t always clear – but will be once these reactors are being deployed for power generation. This brings us to the biggest driving factor in space nuclear power supply design, and leads the conversation to what we’re all here for: in-space nuclear power.

In-Space Fission Power Systems

STS 125 Grunsfeld and Feustel spacewalk

Spacewalk to repair Hubble, STS 125. Image courtesy NASA

With the notable exception of the Hubble Space Telescope; a few military and reconnaissance satellites retrieved by the now-long-retired Space Transport System (otherwise known as the Space Shuttle); and the various space stations launched by the US, the USSR/Russia, and now the Chinese; if something breaks after it’s launched into space, then it stays broken. Satellite operators have gotten very good over the decades at working around malfunctions, but the fact remains that on-orbit repair is by far the exception, not the rule. It’s difficult, if not impossible, to send a repair crew out to make repairs; and often those repairs would be impossible, even if it were possible to get to the satellite, due to the design considerations that went into the satellite in the first place.

 

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SNAP-10a Reactor Pre-flight testing, image courtesy DoE

This is just as true for an in-space nuclear reactor as any other system, and malfunctions have occurred on several missions involving fission power systems in space. Possibly the best known on-orbit failure is that of SNAP-10a, the only American nuclear reactor to ever fly in space. Shortly after achieving orbit, and after initial check-out of the spacecraft, the reactor was activated, and operated normally… for 14 minutes. Then, an electrical bus on the spacecraft (not part of the nuclear reactor) failed, and the entire mission was lost, including telemetry on reactor behavior. It still follows its original polar orbit, and will continue to do so for thousands of years, or until someone retrieves it at some future date (a challenging prospect for any satellite, and this was a larger-than-average one). Other failures have occurred as well, but we’ll look at these as we look at the individual systems that have been used in space so far, as well as proposed and tested systems.

There are two main concerns for space systems engineers: first, the satellite has to survive launch and deployment on orbit; and, second, everything on the spacecraft has to be as reliable as possible. The first consideration isn’t something that we are going to be examining in depth on this blog (at least not the near future). The second part, on the other hand, is something that affects most design decisions that are made in fission power supply design for space missions of all types. Maintenance is not an option in space.

The Parts of an In-Space Fission Power Supply

The same general parts for a terrestrial power plant are used for in-space fission power supply systems, with a reactor core using a working fluid (water or gas in the examples that we’ve seen so far), which then runs through a power conversion system (either a steam or gas turbine in terrestrial designs), and is then cooled before returning to the reactor core. However, water is heavy, and isn’t liquid over a very large range of temperatures unless it’s pressurized (which requires heavy pressure-tolerant pipes and reactor chambers), so it’s generally not used. Gas is sometimes used (often helium, argon, or xenon, all noble gases that won’t generally react chemically with the components of the spacecraft), but liquid metals (often sodium) are the most common option.

The Reactor Core

DU Core w clamps

KRUSTY/Kilopower reactor core and heat pipe configuration, image courtesy DoE

The geometry of the reactor core is often quite different from that seen in terrestrial reactor designs. The biggest difference is that it will be far smaller; often only a couple meters long and wide, or smaller; as opposed to the dozens or hundreds of meters that large terrestrial fission plants have. This is, of course, to save mass of the spacecraft, and to get more power out of less mass and volume. We’ll examine the different options for core geometry more as we go through this blog series, but the shape ranges from plugs of uranium oxide or carbide arranged in a row, surrounded by a power conversion system that we’ll look at later in this post; to cylindrical, square, or hexagonal fuel elements with coolant channels running through them (as we’ve seen with the Kilopower reactor, although that’s unique in that there’s just one fuel element, not several); to more exotic options, which all depend on a number of factors.

One big difference between these reactor cores and the ones that we’ve been examining in the nuclear thermal rocket posts is that they don’t run nearly as hot. While there are efficiency benefits to having a hotter reactor, as we’ve seen the thermal stresses, increased chemical reaction rates, and limitations on the materials that can be used, often mean that this is simply more trouble than its worth in most cases. An NTR has to be as hot as possible to maximize the specific impulse, or rocket efficiency, of the engine; whereas to produce electricity there are many options that work well enough at cooler temperatures, so engineers generally decide to run at cooler temperatures and save themselves a lot of the concerns and headaches that these high temperatures cause.

How hot is hot enough? Well, that depends on a number of factors, mostly to do with the power conversion system and the heat rejection system. The fact that these systems are linked together (in much the same way as the turbopumps, propellant being used, and other components of an NTR work together) is by now a familiar concept to regulars of this blog. These considerations, no matter what the details are, are called “balance of plant” issues; and if anything they’re the biggest concern for a reactor designer just beginning to design an in-space fission power system. Many concepts, like amount of power that needs to be provided, mass and volume of individual components, maximum and minimum working fluid outlet temperature, radiator requirements, and many other considerations, all work together to define a system; but perhaps the most important considerations are the first and third: power requirements and power conversion system. The power requirements, as well as mass requirements, will be defined by the mission that will use this reactor, so they can be taken as a given: if NASA wants a reactor to provide 100 megawatts of electricity (MWe) for a manned spacecraft using nuclear electric propulsion… well it’s the job of the reactor designer to do that, ideally within the mass budget allotted (or else other systems on the spacecraft have to get lighter, something that’s difficult or impossible to do).

Power Conversion Systems

A nuclear reactor doesn’t produce electricity directly. In most cases, what it does produce is heat (caused by the collision of the fission fragments and neutrons as they slow down inside the fuel element, or by hitting reactor components), which then has to be converted into electricity. There are a number of ways to do this, many more than are used on Earth in nuclear power plants. For the terrestrial reactors, it’s often a matter of using off-the-shelf technology for other power plant types – a nuclear reactor builder will likely buy their steam turbines from a company that makes steam turbines for coal-fired power plants, for instance (although for a single-loop system they have to be manufactured with much higher tolerances, because working on them will be much harder; so they have to break less, and require less maintenance).

In space, the field is much more wide open, partially because there’s very little off-the-shelf going on in space. This means that a reactor designer has a lot more options when it comes to their power conversion system. However, these designs tend to fall into three broad categories: heat engines, thermal-electric materials conversion, and charged particle conversion.

Heat Engines: The Workhorses of Modern Life

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Steam turbine, image via Wikipedia

The most familiar is the heat engine, where heat is turned into mechanical work, and this in turn spins a generator. The technical name for these types of devices is a heat engine, which was extensively studied by a 19th century military engineer, Nicholas Leonard Sadi Carnot, who worked out the theoretical limits of the efficiency of these types of engines in 1824. For this, he’s often called the “grandfather of thermodynamics.” These engines, no matter what specific type, have a hot side and a cold side, and use the temperature difference to create mechanical work. The bigger the difference, the more efficient; within certain limits. The most common types are the Rankine cycle (the working principle of  steam turbines), the Brayton cycle (the working principle of gas turbines), and the Stirling cycle (which we saw when we examined the Kilopower system). This temperature difference is why large cooling towers or ponds are used in most power plants: the amount of heat coming out is a fixed quantity, but the amount of cold available could be improved (down to ambient temperature at least) to maximize the efficiency of the system. In greatly simplified terms, the theoretical maximum efficiency of this type of engine is: [hot end temperature]-[cold end temperature])/[hot end temperature] (somewhere around 80%, but this is a matter of some debate). In practical terms, a free-piston Stirling engine can achieve 50% efficiency, while a supercritical-CO2 Brayton convertor (more on that in the power conversion blog post, coming up soon!) can achieve similar numbers – but with many more moving parts to break.

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Small gas turbine, image via Wikimedia

These power conversion systems will be covered in depth in a later post, but for right now let’s look at the thing that they all have in common: moving parts, often lots of moving parts. Moving parts wear out, they can get out of balance, they can grind against each other and create flecks of metal to get in the way or cause short circuits… so space fission power designers, as a general rule, don’t want to use them unless they have to.

 

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Free piston Stirling engine, image courtesy DoE

The Stirling engines used in Kilopower were tested on the ground for far longer than the life of any proposed mission; and not just for Kilopower. The Advanced Stirling Radioisotope Generator was the first program those exact same power conversion units were used in; and, indeed, KRUSTY reused the exact same hardware. These are incredibly mechanically simple systems, and that makes them attractive. Stirling engines are also able to achieve a higher theoretical efficiency (more on that in the power conversion systems blog post) than the other two common options, which makes them attractive as well; but it should be noted that theory and practice don’t always go hand in hand.

Regardless of how the motion is produced, it is then transferred to an electrical generator, which spins a magnet inside a coil of conductive wire, thereby producing an electric current. These generators, since they have moving parts, must be very carefully manufactured; and, again, are a potential source of mechanical or material failure over time.

Materials-Based Electricity Production

So, at this point, an interesting question rears its’ head: what if we could convert heat to electricity without moving parts? There is in fact a way, in fact a number of them, but they come with a big trade-off: single-digit power conversion efficiency. This tradeoff was still worth it to every nuclear electric power source designer that has had hardware fly, though. No moving parts, combined with rigorous quality control, means that whatever’s going to break is not going to be your power conversion system. There are two main options for directly converting heat to electricity, with similar names: the thermoelectric effect and the thermionic effect.

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Diagram of thermoelectric power conversion system, image via Wikipedia

The thermoelectric effect exploits how different metals hold onto electrons at different strengths, and also react to heat differently, by running heat along a join between two different metals, and along the seam having one side be heated and the other cooled. This creates an electrical current (how much depends on the temperature difference and the combination of metals used). The main downsides (other than low efficiency) to this process is that for any two given metals, the sweet spot for system efficiency is very small, so unless the reactor is at JUST the right temperature, and unless your radiators are working properly, no power is going to be produced. Additionally, these usually are relatively low-heat systems due to the materials that efficiently produce this effect in combination; although high-temperature thermoelectrics are gaining in efficiency. These systems are often used for energy capture in industrial facilities; wrapped around waste pipes, chimneys, and other areas.

Thermionic convertor IEEE

Diagram of thermionic conversion system, image courtesy IEEE

The next option is the thermionic effect. This is an incredibly ancient concept; known since the days of ancient Greece, and described in detail by Alexander Graham Bell, before the electron was ever discovered. Certain materials, when heated, emit electrons, which then build up a static charge on another material. The tendency for a coal on the end of a stick to attract ash was observed and described in ancient Greece, and this is that principle in action. Bell’s experiments were more efficient, since he used a vacuum: an incandescent bulb’s filament will emit electrons and become positively charged, depositing them on the glass of the bulb rather more efficiently than the coal, because there’s no air to capture the electrons and become mildly ionized. Once again, a static charge builds up on the glass, and this then can be drawn off to be used (assuming the positive side of the circuit connects back to the filament). In this case, the thermal difference being used is the filament (which gets quite hot) and the glass (which has a lot of surface area, and rejects heat by radiation of convection). This option is the one that has been used in space nuclear power sources, although with somewhat more efficient design of the power conversion system. These power conversion systems use an interesting property found in cesium (which we aren’t going to explore in this post), where it turns into a very unusual state of matter: Cesium-Ryberg vapor. This vapor collects the negative charge from the hot end of the power convertor, and then condenses on the cold end, transferring the electricity as it does so. Usually, a wick is then used to return the cooled cesium to the hot end of the convertor, and the process begins again.

TOPAZ Core configuration, Bennett

Diagram of TOPAZ reactor core and thermionic convertor system, image courtesy DoE

Theoretically, thermionics can achieve greater efficiency for a couple reasons: first, they can run much hotter; and, just as in all power conversion system options, the larger the thermal difference, the more efficient the system is. Secondly, the Rydberg matter state allows for greater conductivity than the metals of the thermoelectric effect, so less energy is lost in conduction (as heat, which makes the system less efficient the less conductive it is, in a couple different ways). These systems also have another property that makes them interesting to in-space fission power system designers: they can be installed in the core of the reactor itself. This was actually done in the TOPAZ series of nuclear reactors, which flew 30 times for the USSR and Russia, on the KOSMOS RORSAT program. Individual fuel elements were effectively wrapped in thermionic power conversion systems, in a design often called a “flashlight core,” since they look something like the batteries in an old-style flashlight handle.

However, these systems are still fundamentally limited, in that they are so low efficiency, usually in the single or low double digits. This means that if you’ve got a 1MWt nuclear reactor on your spacecraft, you’re going to have to get rid of that 1 MW of heat in order to keep your spacecraft running, but you’ll only get a few kilowatts of power. This is fine in many cases, and can be designed around (a fission reactor’s core doesn’t have to get much bigger to put out a lot more power, and fission and fusion scale up much better than they scale down), but that doesn’t mean that this is an ideal circumstance. Which leads to the next question: what other options are there?

Advanced Power Conversion Systems

As we saw from both of the previous concepts, there are theoretical fundamental limitations to both heat engines and material power conversion options; primarily, that the heat is needed to induce another phenomenon, and the difference between the hot and cold temperatures of the system or material define how efficient it can be. Is there a way to get around this thermal limitation? Are there other options available? The answer to the first question is, unfortunately, no, in most cases; but the answer to the second is a resounding yes!

MHD_generator_(En)

Image via Wikipedia

The first concept to look at is the magnetohydrodynamic generator (MHD). This is often discussed in spaceflight, but not as a power generator. This works in largely the same way that an electric generator and an electric motor are effectively the same thing, in reverse; it just matters if you’re putting in electricity to make the shaft spin, or spinning the shaft to make electricity. In this case, charged particles are sent through a series of magnetic coils, which slow down the charged particles, changing their kinetic energy into a charge in the coils, which is then used as a power source. This, obviously, requires a source of charged particles, though, which leads reactor designers that want to use this power conversion system to use some very different reactor core geometries, including one that we’ll look at more when we cover this subject in more depth: the vapor core reactor, where the fuel is a liquid that is sprayed in a very fine mist within a reaction chamber, allowing the particles to have less physical interaction while undergoing fission. These fission products are highly charged, and moving at very high speeds, so theoretically this can be a very efficient form of power production (and could make a good NTR as well, so we’ll be coming back to this a couple different ways). Work on a vapor core MHD propulsion system has been explored since the early 90s at the Institute for Nuclear Space Propulsion Innovation at the University of Florida, and shows promise; although only a handful of tests examining the criticality requirements of this reactor have been done, so it remains in advanced early stages of design (other experimentation has been done, and continues to be done, though).

AMTEC convertor

Image courtesy mpower.co.uk

The second option is an oddity among power conversion systems: AMTEC, or the Alkali Metal Thermal to Electric Convertor. Here, the capacity for alkali metals to carry high negative charge is exploited by running a liquid metal (usually sodium, but potassium has been used as well, at lower operating temperatures) in a closed loop, through convection between a high pressure hot end and a low pressure cold end. Due to the high energy potential difference, this process doesn’t require mechanical pumps, merely a difference in heat. The theoretical efficiency of this system approaches 40%. The fact that this approaches the efficiency of a heat engine, but without moving parts, makes this a very attractive option for space nuclear power designers, but the details get technical rather quickly, so we’ll leave them for the power conversion system post.

Getting Rid of Heat

As we all know, fission generates a lot of heat very efficiently, that’s the whole reason that it’s so attractive. Unfortunately, that heat all has to go somewhere, or two things will happen: the power conversion system stops working because the hot and cold ends approach the same temperature; and, your spacecraft gets baked until it’s destroyed. Heat always transfers from a higher temperature to a lower one, and how much heat is transferred depends on the temperature difference, the surface area of the thermal difference, and the thermal conduction of the materials involved. There are three mechanisms for heat to transfer: conduction (through physical contact with another material), convection (through gas heating up and then rising away from the heat source), and radiation (the infrared photons that actually make up the heat leave the substance in straight lines). Sadly, in the vacuum of space only radiation works, unless you’re throwing large amounts of gas overboard (as in an NTR, which operates on convection and radiation), which weighs a fair bit and takes up a lot of volume – and the container weighs a fair bit, too. This means that if we want to generate electricity, we have to get rid of heat through radiators, which heat a working fluid or other substance, and then use convection or conduction to an area with a large surface area to maximize the amount of surface area available for radiation. Radiators emit heat as a function of the temperature of the liquid (or gas) inside them, the surface area of the radiating surface, and several other factors, known as the Stefan-Boltzmann law (more on that in the heat rejection blog post coming up in a few posts). On the practical side of a radiator, if the substance used as a working fluid freezes at ambient temperatures, the designer needs to make sure it’s not going to freeze anywhere in the radiator and gum up the works.

STS128 EVA radiatorsOn the international space station, a pumped loop radiator using ammonia is used (and is responsible for a lot of the maintenance work done during EVAs, as was its predecessor on Mir). Higher temperature radiators can use sodium, lead-bismuth, or other liquids. The advantage to this system is that, to a degree, the more heat that needs to be rejected, the faster the pump can be run.

Hybrind screen-groove water heat pipe

Cutaway of Ti-H2O heat pipe for Kilopower, image courtesy NASA/ACT

The most popular type of radiator for modern spacecraft is the titanium-water heat pipe radiator. This system doesn’t require a pump, since it works through wicking, as all heat pipes do; which means one less set of moving parts is needed. Other heat pipes are possible, at higher temperatures, including the sodium heat pipes used in Kilopower; but in order to use these, and not lose power conversion efficiency, the reactor has to operate at a higher temperature. The advantage of these heat pipes is that, due to the Stefan-Boltzmann law, these radiators can get rid of much more heat with much less mass.

Belt radiator, NASA

Belt radiator concept, image courtesy NASA

More advanced options don’t use pipes to contain the liquid, but flexible membranes. The simplest uses a simple spray of liquid, which is sprayed in a controlled fashion into space toward a collector, which them pumps the liquid back into the reactor. This has a much higher surface area than a pipe-based heat radiator, since the droplets themselves have a huge surface area to volume ratio compared to the pipe surface area that’s facing space. A waterfall on the Moon may be a good method of rejecting heat, both due to the low gravity and the low average temperature (in the shade, at least).

 

Droplet Radiator NASA

Droplet radiator concept, image courtesy NASA

Of course, if the spacecraft maneuvers before all the spray is collected, a certain amount will be lost; so it can’t effectively be used during maneuvering… unless the drops are magnetic. This is the idea behind the Curie point liquid droplet radiator, which sprays liquid metal droplets that are then collected by electromagnets. The liquid itself will be sprayed at above the Curie point of the liquid, which means that it will become magnetic after it cools past its Curie point. This system is likely to be a heavy power hog, though, and the returning working mass is probably a solid, which has its own (not insurmountable) issues with return to the core to be recycled. One advantage to this is that the phase change from solid to liquid absorbs a fair bit of energy, and vice versa releases a fair bit, so it is more efficient at power transfer in that way.

RMMBLR Radiator Diagram

Spherical membrane radiator for Boeing RMMBLR, patent drawing

Another option, which would require less mass, is the membrane radiator, where the droplets are sprayed onto a membrane, which then collects them through centrifugal force. This has been proposed in a number of configurations, including spherical shapes, belts, and others. The most studied version was proposed by Boeing for a nuclear electric manned spacecraft, this reactor used liquid sodium that transferred the heat through a heat exchanger into water, and then had a distorted spherical membrane to catch the droplets. This system was called the Rotating Multi-Megawatt Boiling Liquid Reactor, and will be one of the systems we cover in the future.

However, these aren’t the only options for radiators. Anything that can efficiently conduct heat, and that has a high surface area, can be used as a radiator. Assuming carbon nanotubes are able to be made miles long, a collection of these would make a good radiator, due to their high thermal conductivity and large surface area. This is the principle behind “belt”, “wire”, or “spaghetti” radiator designs.

Ideas abound on how to maximize radiation abound, and we’ll cover them more in depth in a later post. I leave this topic with a chart comparing many different options for heat rejection systems, from Winchell Chung’s wonderful Atomic Rockets website, showing heat capacity, specific area mass, radiation area, and array mass (including supporting structures) for various proposed and historical systems, most of which we’ll cover at some later point.

Radiator Mass Chart Project Rho

Table courtesy Winchell Chung, Project Rho

Why Use Fission Power Systems, When Most Missions Use Less Power than My Microwave?

So what needs a lot of power in space? Not much, really. The ISS uses about the same amount of power as 14 normal American houses, but those are notorious for power loss, and this is to support multiple humans and dozens of experiments. Curiosity, NASA’s current flagship mission, is an SUV-sized, six wheeled rover with: two chemistry labs; two sets of cameras; a drill; a robotic arm; and a laser that vaporizes pinpricks of rock from dozens of feet away, and analyzes the resultant color of the vaporized rock to determine the composition. All of this is done with only 110 watts (admittedly, it only does one of these things at a time, and would be challenged to travel faster than a small tortoise, but still). Instrumentation as a general rule doesn’t need a lot of power, and NASA and other space agencies over the years have gotten very good at using less and less power to do more and more.

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Kilopower reactor powering conceptual Mars habitat, image courtesy NASA

There are really only four reasons for high power use: life support, in-situ resource utilization (ISRU), radar, and propulsion. Life support is a major focus of the Kilopower program, with a strong possibility that the first deployment of the system will be to the surface of the Moon, to support a crewed base (not a new idea, when we cover the BES-5, otherwise known as TOPAZ-II, we’ll look at a NASA mission concept that used a Soviet-built reactor to do the same thing). However, since while on-orbit in the inner solar system most life support systems can be supported by solar panels (unless there’s weather or an inconveniently long day-night cycle) those will likely be used.

ISRU Concept, NASA

Rocket propellant ISRU concept, image via NASA

In situ resource utilization is a very hot topic right now: most Mars sample return missions assume that either incredibly power-dense propellants will be used, or much of the propellant will be manufactured on Mars (often using some small portion of the raw materials brought from Earth, such as hydrogen feedstock, which is hard to get on Mars easily). Other uses include using either lunar or Martian regolith to construct habitats using 3D printing; extraction of valuable resources, either for their monetary or practical value; producing tools and other products for use on future crewed bases… the list is extensive. The other thing that’s extensive is the power requirements for this type of activity, not only for the process itself, but also for associated materials and tools needed. Many forms of chemical synthesis or extraction need precursor chemicals, solvents, reagents, and other substances that also have to be extracted, refined, and used. If a furnace is needed for metal refining, that furnace is likely going to have to be built in situ in order to be able to begin the actual end goal of refining metals, then the molds, forgings, or other associated metalworking tools to make a useful product need to be built as well. All of these things require large amounts of energy, and may require acres of solar panels (with intermittency issues, maintenance issues, and the cost of manufacturing THOSE in situ as well), or can be satisfied with a relatively small, power-dense fission power system.

RORSAT_by_Ronald_C._Wittmann,_1982

RORSAT, Smithsonian image by Ronald Whitman via Wikipedia

In Earth orbit, radar can now be provided by solar panels (unlike the earlier RORSATs, when photovoltaics were far less efficient than today). Within the main belt asteroids, unless a high degree of reliability is needed for crewed surface installations (the focus for Kilopower at the moment), solar power is, again, usually sufficient. Out of Earth orbit, other systems have been devised to gather similar data for far less power, but these systems have limitations as far as data gathering is concerned. Given a powerful, sufficiently power-dense , power source, we may see on-orbit ground penetrating radar used to, say, map the rocky core and liquid layer of Enceladus, or probe the depths of the ice giants; but, until Kilopower becomes operational, there’s not a power supply that can support that, and radar tends to be bulky – and, even worse, heavy.

In the outer solar system, the situation is quite different; but, even then, power requirements tend to be low. Kilopower’s 1 kWe system is more than was available to Cassini/Galileo, New Horizons, or any other outer planet flagship mission. All of these (except Juno, which used experimental, and very expensive, solar panels, with a collective surface area of a tennis court) were powered by radioisotope thermoelectric generators (another upcoming topic, if not for a blog post, then for the website), which are fundamentally limited by the decay of the isotope used. These missions usually run on just a few hundred watts of power, if not less, and bring back incredible science returns.

The last use is going to be the subject of our next blog post, since it’s another hugely diverse area in its’ own right: propulsion. Briefly, electric propulsion uses electric potential rather than chemical potential to accelerate a spacecraft. While it is often seen as a new thing, the concept has been around since the earliest days of spaceflight, although the details of how it’s used have changed greatly over time. The thrust and efficiency available in these systems is often dependent on having more electrical power, though, so fission power systems offer a uniquely attractive set of characteristics for this type of propulsion. More on that in the next post, though.

What’s Coming Up?

While this has been a long blog post, it’s also been a very brief summary of the basic needs of a fission power system, its limitations, and a couple of its uses, if not the one that most people think of. Each of these subsystems is a hugely important area, and each must balance with the others in terms of power output, mass, volume, and a host of other requirements, to make a cohesive whole, or a nuclear electric spacecraft would be impossible.

The next post will focus on the most commonly thought of reason for nuclear power in space: high powered electric propulsion. This will be another overview post, looking at the various options for electric propulsion, with some of the advantages and disadvantages of each.

In the following few posts in this series, we’ll focus on the basics of the power system itself. We’ll focus on the core of the reactor first, including different cooling systems for the reactor, and touch briefly on one sort of power conversion system: in-core thermionic power conversion systems. This may also be our introduction to the TOPAZ reactor, depending on how the blog post goes. After that, we’ll look at power conversion systems, a hugely diverse area… so that may become two posts as well, depending on how things go: heat engines (Rankine, Brayton, and Stirling engines) for one post, and materials and advanced power conversion options for the other. After that, we’ll move on to heat rejection technologies, an area with far more options than most people realize, and one that has some difficult-to-explain or unusual concepts to cover. Finally, we’ll have a set of blog posts for each type of electric thruster: electrostatic, electromagnetic, electrothermal, and photonic. Finally, we’ll bring it all together in a post examining the concept of balance-of-plant, and a few of the basics of the unique challenges of designing a nuclear electric spacecraft.

We will likely have interludes in here examining other subjects as well, possibly on different proposed reactor systems or spacecraft over the years… and maybe even a look at non-solid-core NTRs, depending on how things go. There will, of course, also be updates to the website on many subjects, as I come across more information on a subject, or to flesh out information from previous or forthcoming blog posts. Again, I’ll try and keep the webpage updated with information on which pages are new or updated!

This is going to be a long and grand adventure into the depths of nuclear electric spacecraft! Don’t worry if the blog posts start slow (not in frequency, though… many of these posts may come out more frequently than others have, due to their narrower focus): just like the systems we’re looking into, it may take time to get up to speed, but once everything’s had time to build up thrust the posts will seem to fly by!

 

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