Fission Electrical Power Plant Reactor Cores

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).

Russian/Soviet Reactor Cores


Romashka (Russian for the flower chamomile) was the first Sovet astronuclear design. Designed to use a thermionic in-core power conversion system, it was ground tested but never flown. A series of disc-shaped uranium carbide (UC2) fuel elements were used in this reactor (90% 235U), with holes drilled through the center, and roughly halfway from the central hole of the disc to the edge of the fuel element. Both of these holes were used to thread the thermionic power conversion system through the core of the reactor. Spacing of the fuel elements was provided by a mixture of beryllium oxide and graphite, which was also used to slightly moderate the neutron spectrum – but the neutron spectrum in the reactor remained in the fast spectrum. Surrounding the reactor core itself, both radially and at the ends of the core, were beryllium reflectors. Boron and boron nitride control rods placed in the radial reflector and base axial reflector were used to maintain reactor control through the use of a hydraulic system, however a large negative thermal reactivity coefficient in the reactor core was also meant to largely control the reactor in the case of normal operations. Finally, the reactor was surrounded by a finned steel casing that provided all heat rejection through passive radiation – no pumps required! The nominal operating temperature of the reactor was meant to be between 1200 C and 1800 C at the center of the core, and about 800 C at the edges of the core at the ends of the cylinder.

BES-5 Bouk

The Bouk (“Beech”) reactor, also known as the “Buk,” or BES-5 reactor, is arguably the most successful astronuclear design in history. Begun in 1960 by the Krasnya Zvesda Scientific and Propulsion Association, this reactor promised greater power output than the Romashka, at the cost of additional complexity, and requiring coolant to operate. From 1963 to 1969, testing of the fuel elements and reactor core was carried out without using the thermoelectric fuel elements (TFE), which were still under development. From 1968 to 1970, three reactor cores with full TFEs were tested at Baikal; and, with successful testing completed, the reactor design was prepared for launch, integrated into the Upravlenniye Sputnik Aktivny (US-A; in the West, RORSAT, for Radar Ocean Reconnaisance SATellite) spacecraft, designed to use radar for naval surveillance.

Rather than having stacked discs of UC2, the BES-5 used 79 fuel rods made out of uranium (90% enriched, total U mass 30 kg) molybdenum alloy metal, encased in high-temperature steel. NaK was used as a coolant for the reactor, pumped using the energy from 19 of the fuel assemblies to run an electromagnetic fuel pump. Producing over 100 kW of thermal energy, after electric conversion using in-core germanium-silicon thermoelectric power conversion elements (which use the difference in charge potential between two different metals along a boundary to create an electrical charge when a temperature gradient is applied across the join; again, more in a later post), a maximum of 5 kW of electrical energy was available for the spacecraft’s instrumentation. The fact that this core used thermoelectric conversion rather than thermionic is a good indicator that the common use of the term, TOPAZ, for this reactor is incorrect. Reactor control was provided by six beryllium reflector drums that would be slowly lowered through holes in the radial reflector over the reactor’s life to increase the local neutron flux to account for the buildup of neutron poisons.


The TOPOL program was begun in the 1960s, under the Russian acronym for Thermionic Experimental Converter in the Active Zone, which translates directly into Topaz in English, but ground testing didn’t begin until 1970. This was a multi-cell thermionic fuel element design similar in basic concept to Romashka, however it was a far more complex design. Instead of a single stack of disc-shaped fuel elements, a “garland” of fuel elements were formed into a thermionic fuel element. The fissile fuel element was surrounded by a thimble of tungsten or molybdenum, which formed the cathode of the thermionic converter, while the anode of the converter was a thin niobium tube; as with most thermionic converters the gap between cathode and anode was filled with cesium vapor. The anode was cooled with pumped NaK, although some sources indicate that lithium was also considered as a coolant for higher-powered versions of the reactor.

The differences between the BES-5 and TEU-5 were far more than the power conversion system. Instead of being a fast reactor, the Topaz was designed for the thermal neutron spectrum, and as such used zirconium hydride for in-core moderation (also creating a thermal limitation for the materials in the core; however, hydrogen loss mitigation measures were taken throughout the development process). Rather than using the metal fuels that its predecessor had, or the carbides of the Romashka, the Topol used a far more familiar material to nuclear power plant operators: uranium oxide (UO2), enriched to 90% 235U. This, along with reactor core geometry changes, allowed the amount of uranium needed for the core to drop from 30 kg in the BES-5 to 11.5 kg. NaK remained the coolant, due to its low melting temperature, good thermal conductivity, and neutronic transparency. The cathode temperature in the TEU-5 was in the range of 1500-1800C, which resulted in an electrical power output of up to 10 kW.

Yenesiy/Topaz II

The third and final Soviet astronuclear power supply for use on small-to-medium spacecraft, the Yenesiy (sometimes called Enisy or Topaz-II in American literature) was a higher-power evolution of the TEU-5, incorporating both materials and structural advances in its core and power conversion system. 

In its final iteration, it was designed to have a thermal output of 115 kWt (at the beginning of life), with a mission requirement of at least 6 kWe at the electrical outlet terminals for at least three years. Additional requirements included a ten year shelf life after construction (without fissile fuel, coolant, or other volatiles loaded), a maximum mass of 1061 kg, and prevention of criticality before achieving orbit (which was complicated from an American point of view, more on that below). The coolant for the reactor remained NaK-78, a common coolant in most reactors we’ve looked at so far. Cesium was stored in a reservoir at the “bottom” (away from the spacecraft) end of the reactor vessel, to ensure the proper partial pressure between the cathode and anode of the fuel elements, which would leak out over time (about 0.5 g/day during operation). 

Even before the fall of the Soviet Union, the need for foreign capital and additional funding for the program saw quiet discussions lead to a lend/lease program with the United States, called Topaz International. This program would see the US lease one test reactor, one flight-capable reactor, and take out options on another reactor as well. Plans were drawn up for a nuclear electric propulsion testbed, as well as the possibility of using one of the reactors on the Lunar surface as part of the Strategic Exploration Initiative.

Due to political complications, increased costs associated with the international nature of the research, and lack of funding, the program was cancelled on the US side in 1993. Additional funding on the Russian side never materialized, killing the program.

Recent and Current Russian Fission Power Systems


This reactor, the power supply for the Transport and Energy Module spacecraft currently under development in Russia,  is a gas cooled, fast neutron spectrum, oxide fueled reactor, designed with an electrical output requirement rather than a thermal output requirement, oddly enough of 1 Mwe. This requires a thermal output of at least 4 MWt, although depending on power conversion efficiency it may be higher. Currently, though, the 4 MWt figure seems to be the baseline for the design. It is meant to have a ten year reactor lifetime. While initially Brayton power conversion was discussed, in early 2020 it was announced that the system would move to a more typical thermionic power conversion system, resulting in a loss of power conversion efficiency.

Additionally, details of a smaller version of the TEM, called the Nucleon, have come to light in late 2020. It is unclear if the reactor will be the same, just operating the less efficient thermionic systems, or if this is a new design.

American Astronuclear Cores

Systems for Nuclear Auxiliary Power (SNAP)

SNAP was the main focus of a number of organizations in the US looking to use nuclear power in space and in remote regions on Earth where maintenance of a fission power system wasn’t feasible.

The program extended everywhere from the bottom of the seas (SNAP-4, which we won’t be covering in this post) to deep space travel with electric propulsion. SNAP was divided up into an odd/even numbering scheme, with the odd model numbers (starting with the SNAP-3) being radioisotope thermoelectric generators, and the even numbers (beginning with SNAP-2) being fission reactor electrical power systems.

The first three astronuclear systems (SNAP-2, SNAP-10, and SNAP-8) are all very similar systems: self-moderating LiH fuel, NaK primary coolant, and the use of open-backed control drums were common to all three systems. While the lower-powered SNAP-10 (and later versions of the SNAP-8) had thermoelectric conversion system, a mercury boiler turbine was developed for SNAP-2 and -8 (although it was never completed).

SNAP-50 developed from a different research program, using uranium nitride fuel, potassium coolant, and (eventually) a thermionic power conversion system, although a Rankine system was developed for it as well. 

SNAP Experimental and Developmental Reactors

The SNAP program used a number of experimental reactors to test materials, critical geometry, and the like. While some of them were specific to a particular reactor, many were used as general testbeds for the similar SNAP-2, SNAP-10, and SNAP-8 reactors, which all used the same fuel type and coolant, and were of similar power levels.

SNAP Critical Assembly

The first critical assembly test was in October of 1957, shortly after Sputnik-1’s successful launch. With 93% enriched 235U making up 8% of the weight of the U-ZrH fuel elements, a 1” beryllium inner reflector, and an outer graphite reflector (which could be varied in thickness), separated into two rough hemispheres to control the construction of a critical assembly; this device was able to test many of the reactivity conditions needed for materials testing on a small economic scale, as well as test the behavior of the fuel itself. The primary concerns with testing on this machine were reactivity, activation, and intrinsic steady state behavior of the fuel that would be used for SNAP-2. A number of materials were also tested for reflection and neutron absorbency, both for main core components as well as out-of-core mechanisms. This was followed by the SNAP-2 Experimental Reactor in 1959-1960 and the SNAP 2 Development Reactor in 1961-1962.

SNAP Critical Assembly 4

More commonly known as the SNAP Critical Assembly - 4B (due to upgrades to the fuel and assembly structure), this was one of the true workhorse reactors in the SNAP program. Used to test everything from thermal compatibility in fuel, loss of coolant accidents, and failure-to-orbit scenarios where the reactor could fall into water, or even worse mud, this reactor was completed in the middle 1950s and operated through much of the 1960s (sadly, the original documentation of this program is not available online that I've seen). While it informed many design decisions for SNAP-2 and -10B (as well as 8), it has been largely forgotten. However, a detailed analysis of the neutronic and thermal behavior was carried out in 2005 by Krass and Goluoglu, which is available below.

SNAP [2] Experimental Reactor

The SNAP-2 Experimental Reactor (S2ER or SER) was built to verify the core geometry and basic reactivity controls of the SNAP-2 reactor design, as well as to test the basics of the primary cooling system, materials, and other basic design questions, but was not meant to be a good representation of the eventual flight system. Construction started in June 1958, with construction completed by March 1959. Power operations started on Nov 5, 1959. Four days later, the reactor reached design power and temperature operations, and by April 23 of 1960, 1000 hours of continuous testing at design conditions were completed. The reactor was shut down for the last time on November 19, 1960, just over one year after it had first achieved full power operations. Testing on various reactor materials, especially the fuel elements, was conducted, and these test results refined the design for the Development Reactor.

SNAP-2 Development Reactor

The SNAP-2 Development Reactor (S2DR) was the next stage in testing for the SNAP-2 program, Sometimes called the SNAP-2 Development System (S2DS or SDS), it was designed to refine the design of particular components of the SNAP-2 flight reactor design, test materials issues, and provide better calibrations and design optimization for the system that was meant to fly as part of the SNAPSHOT program. By the end of the S2DR’s operational history in December 1962, it operated for 11,290 hours with a total energy release of 272,900 kWh. 2060 hours of this were at SNAP-2 operating conditions (55 kW, 1200F), 1544 hours at 30.5 kW and 1100F, and 1150 hours at SNAP-10 operating conditions (30.5 kW, 950F). The reactor was disassembled, and the fuel elements examined post-irradiation, in 1963.

SNAP-8 Experimental Reactor

The SNAP 8 Experimental Reactor (S8ER), went critical in May 1963, and operated until 1965. It operated for 2522 hours at above 600 kWt, and over 8000 hours at lower power levels. The fuel elements for the reactor were 14 inches in length, and 0.532 inches in diameter, with uranium-zirconium hydride enriched to 93.15% 235U, with 6 X 10^22 atoms of hydrogen per cubic centimeter.

SNAP-8 Development Reactor

The final reactor in the SNAP-8 testing and development series, the SNAP-8 Development Reactor was a shorter-lived reactor, in part because many of the questions that needed to be answered about the geometry had been answered by the S8ER, and partly because the unanswered materials questions were able to be answered with the SCA4 reactor. This reactor underwent dry critical testing in June 1968, and power testing began at the beginning of the next year. From January 1969 to December 1969, when the reactor was shut down for the final time, the reactor operated at nominal (600 kWt) power for 668 hours, and operated at 1000 kWt for 429 hours.

SNAP Mission Reactors

While only a single US astronuclear reactor has ever undergone launch or been tested in space, many highly advanced (in terms of research and experimentation) systems were developed. Three of these were from the original family of SNAP reactors, two were from similar designs based on an adapted aircraft nuclear reactor, and two more were unique designs. While many other designs have been proposed over the years, very few have undergone nuclear testing, and only one, the SNAP-10B, ever flew. 


The original request for the SNAP program, which ended up becoming known as SNAP 2, occurred in 1955, for possible power sources in the 1 to 10 kWe range that would be able to autonomously operate for one year, and the original proposal was for a zirconium hydride moderated sodium-potassium (NaK) metal cooled reactor with a boiling mercury Rankine power conversion system (similar to a steam turbine in operational principles, but we’ll look at the power conversion systems more in a later post), which is now known as SNAP-2. The design was refined into a 55 kWt, 5 kWe reactor operating at about 650°C outlet temperature, massing about 100 kg unshielded, and was tested for over 10,000 hours. This design would later be adapted to the SNAP-10A reactor, with some modifications, as well, creating the SNAP-2/10A program.


At about the same time as the SNAP 2 Development Reactor tests (1958), the USAF requested a study on a thermoelectric power conversion system, targeting a 0.3 kWe-1kWe power regime. This was the birth of what would eventually become the SNAP-10 reactor. This reactor would evolve in time to become the SNAP-10A reactor, the first nuclear reactor to go into orbit.

Originally only internally conductively cooled, this was the simplest of the designs for an astronuclear reactor in the family. Within a short period, however, the design was changed dramatically, resulting in a design very similar to the core for the SNAP-2 reactor that was under development at the same time. Modifications were made to the SNAP-2 baseline, resulting in the reactor cores themselves becoming identical.


SNAPSHOT consisted of a SNAP-10A fission power system mounted to a modified Agena-D spacecraft, which by this time was an off-the-shelf, highly adaptable spacecraft used by the US Air Force for a variety of missions. An experimental cesium contact ion thruster (read more about these thrusters on the Gridded Ion Engine page) was installed on the spacecraft for in-flight testing. The mission was to validate the SNAP-10A architecture with on-orbit experience, proving the capability to operate for 9 days without active control, while providing 500 W (28.5 V DC) of electrical power. Additional requirements included the use of a SNAP-2 reactor core with minimal modification (to allow for the higher-output SNAP-2 system with its mercury vapor Rankine power conversion system to be validated as well, when the need for it arose), eliminating the need (while offering the option) for active control of the reactor once startup was achieved for one year (to prove autonomous operation capability); facilitating safe ground handling during spacecraft integration and launch; and, accommodating future growth potential in both available power and power-to-weight ratio.

The launch occurred on April 3, 1965 on an Atlas-Agena D rocket from Vandenberg Air Force Base. The launch went perfectly, and placed the SNAPSHOT spacecraft in a polar orbit, as planned. Sadly, the mission was not one that could be considered either routine or simple. Many minor but irritating faults were discovered both during and after launch, but perhaps the most troubling was that there were shorts and voltage irregularities coming from the ion thruster, due to high voltage failure modes, as well as excessive electromagnetic interference from the system, which reduced the telemetry data to an unintelligible mess. This was shut off until later in the flight, in order to focus on testing the reactor itself.

On May 16, just over one month after being launched into orbit, contact was lost with the spacecraft for about 40 hours. Some time during this blackout, the reactor’s reflectors ejected from the core (although they remained attached to their actuator cables), shutting down the core. This spelled the end of reactor operations for the spacecraft, and when the emergency batteries died five days later all communication with the spacecraft was lost forever. Only 45 days had passed since the spacecraft’s launch, and information was received from the spacecraft for only 616 orbits.


SNAP 8 was the first reactor designed with these space stations in mind in mind. While SNAP-10A was a low-power system (at 500 watts when flown, later upgraded to 1 kW), and SNAP-2 was significantly larger (3 kW), there was a potential need for far more power. Crewed space stations take a lot of power (the ISS uses close to 100 kWe, as an example), and neither the SNAP-10 or the SNAP-2 were capable of powering the space stations that NASA was in the beginning stages of planning.

Initially designed to be far higher powered, with 30-60 kilowatts of electrical power, this was an electric supply that could power a truly impressive outpost for humanity in orbit. While the reactor was far higher powered than the SNAP 2, both used the same fuel (with minor exceptions) and coolant, and initially both used similar control drum structures, mercury Rankine cycle power conversion systems, and perhaps most attractively both were able to evolve with lessons learned from the other part of the program. Over the course of testing, though, the SNAP-8 would evolve into a family of reactors in its own right.

 In 1973 head of the AEC’s Space Nuclear Systems Division said that, given the lower funding levels that NASA was forced to work within, “…the missions which were likely to require large amounts of energy, now appear to be postponed until around 1990 or later.” This led to the cancellation of all nuclear reactor systems, and a shift in focus to radioisotope thermoelectric generators, which gave enough power for NASA and the DoD’s current mission priorities in a far simpler form.


The SNAP-50 was the last, and most powerful, of the SNAP series of reactors, and had a very different start when compared to the other three reactors that we’ve looked at. The SNAP-50 reactor started life in the Aircraft Nuclear Propulsion program for the US Air Force, and ended its life with NASA, as a power plant for the future modular space station that NASA was planning before the budget cuts of the mid to late 1970s took hold.

Because it came from a different program originally, it also uses different technology than the reactors we’ve looked at on the blog so far: uranium nitride fuel, and higher-temperature, lithium coolant made this reactor a very different beast than the other reactors in SNAP. However, these changes also allowed for a more powerful reactor, and a less massive power plant overall, thanks to the advantages of the higher-temperature design. It was also the first major project to move the space reactor development process away from SNAP-2/10A legacy designs.

Strategic Defense Initiative and early 21st Century US Reactors


The SP-100 reactor began in 1983 as a joint program between the Department of Defense, Department of Energy, and NASA. The first major program to design an astronuclear heat pipe reactor, it evolved from primarily a military and beamed power reactor into a more Nuclear Electric Propulsion focused system as NASA took a larger role in the program following the demise of the Soviet Union.

Uranium nitride fuel and lithium heatpipes (very similar to the basic components of the SNAP-50) were used in the reactor, which was designed for 2.5 MW of thermal power, mounted to (what would eventually be narrowed down to) a thermoelectric power conversion system.

Sadly, a combination of reorganizations in the program, cost overruns, scheduling delays, and loss of political appetite for the program saw its cancellation in 1995. However, the design of the SP-100 influenced many reactors after it, and many within the US astronuclear community today started their careers working on this system.


A joint program between NASA and the Department of the Navy, Project Prometheus was the power supply project within the Jupiter Icy Moons Orbiter (JIMO) program, meant to use a high-powered nuclear electric system to map the Jovian moons using a suite of sensors, including high-powered radar. While some design work preceeded this program, it officially began in 2003.

The design called for 200 kW of electrical power for at leat 10 years (up to 20), necessitating the use of a Brayton power conversion system and a 1 MWt power plant. This would be fueled with uranium oxide fuel, and cooled with a mix of helium and xenon gas. 

While the program showed a lot of promise, the amount of funding at NASA was low enough to necessitate cutbacks in astronuclear funding, and Prometheus ended up losing out to the Fission Surface Power and Nuclear Thermal Propulsion programs. Despite completing Phase A (technology feasibility) studies successfully, it was cancelled in 2005.

Recent and Current US Fission Power Supply Systems

Fission Surface Power

Fission Surface Power reflected the increased focus on returning to the lunar surface, and was the first reactor specifically designed for surface deployments (rather than being adaptible to them) out of all the major astronuclear programs in the US. Begun in 2006, it focused on a different set of nuclear technologies than either Prometheus or SP-100 had.

A uranium oxide fueled, NaK cooled (via pumped loop) reactor, it was designed for 185 kWt, feeding into a set of Stirling engines to produce a maximum of 48 kWe. A complex, multi-stage radiator was designed to both fold into a fairing for delivery to the Lunar surface as well as maximize heat rejection.

Further budget cuts occurred before electrically heated system testing could occur, resulting in only part of the heat rejection system being tested in 2016. It remains a potential reactor architecture for higher-powered missions, but due to the greater simplicity of the Kilopower system likely only in higher-power configurations than those that were initially tested.


The current focus of US astronuclear power supply development, Kilopower is the result of work on many different systems since the SDIO programs in the early 1980s. Designed as a simple, robust system, Kilopower began as a design study for an internal Los Alamos National Laboratory program, and despite minimal funding succeeded in being the first astronuclear reactor to go through fission powered testing since the SNAP-50 in the early 1970s.

A set of four uranium-molybdenum metal fuel elements (which take up the entire diameter of the core individually) are surrounded by – or in the case of higher power levels have embedded in them – sodium heat pipes, used as the primary coolant for the reactor. These feed Stirling engines, which then reject heat through a water heat pipe radiator. 

Design power ranges from 1 kWe to 100 kWe, depending on the size of the reactor and the needs of the mission.

While designed for both spacecraft and surface operation, focus for the design has been placed on Lunar outpost support, likely as part of the Artemis Program, as well as potential support of a future Martian mission.