This page will be expanded soon, as we look at power conversion systems in the blog! Expect big changes to this page early in 2019!
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
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.
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.
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.
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.
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.
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!
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).
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.