Development and Testing

History of US Astronuclear Reactors part 1: SNAP-2 and 10A

Hello, and welcome to Beyond NERVA! Today we’re going to look at the program that birthed the first astronuclear reactor to go into orbit, although the extent of the program far exceeds the flight record of a single launch.

Before we get into that, I have a minor administrative announcement that will develop into major changes for Beyond NERVA in the near-to-mid future! As you may have noticed, we have moved from to For the moment, there isn’t much different, but in the background a major webpage update is brewing! Not only will the home page be updated to make it easier to navigate the site (and see all the content that’s already available!), but the number of pages on the site is going to be increasing significantly. A large part of this is going to be integrating information that I’ve written about in the blog into a more topical, focused format – with easier access to both basic concepts and technical details being a priority. However, there will also be expansions on concepts, pages for technical concepts that don’t really fit anywhere in the blog posts, and more! As these updates become live, I’ll mention them in future blog posts. Also, I’ll post them on both the Facebook group and the new Twitter feed (currently not super active, largely because I haven’t found my “tweet voice yet,” but I hope to expand this soon!). If you are on either platform, you should definitely check them out!

The Systems for Nuclear Auxiliary Propulsion, or SNAP program, was a major focus for a wide range of organizations in the US for many decades. 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.

Due to the sheer scope of the SNAP program, even eliminating systems that aren’t fission-based, this is going to be a two post subject. This post will cover the US Air Force’s portion of the SNAP reactor program: the SNAP-2 and SNAP-10A reactors; their development programs; the SNAPSHOT mission; and a look at the missions that these reactors were designed to support, including satellites, space stations, and other crewed and uncrewed installations. The next post will cover the NASA side of things: SNAP-8 and its successor designs as well as SNAP-50/SPUR. The one after that will cover the SP-100, SABRE, and other designs from the late 1970s through to the early 1990s, and will conclude with looking at a system that we mentioned briefly in the last post: the ENISY/TOPAZ II reactor, the only astronuclear design to be flight qualified by the space agencies and nuclear regulatory bodies of two different nations.

SNAP capabilities 1964

SNAP Reactor Capabilities and Status as of 1973, image DOE

The Beginnings of the US Astronuclear Program: SNAP’s Early Years

Core Cutaway Artist

Early SNAP-2 Concept Art, image courtesy DOE

Beginning in the earliest days of both the nuclear age and the space age, nuclear power had a lot of appeal for the space program: high power density, high power output, and mechanically simple systems were in high demand for space agencies worldwide. The earliest mention of a program to develop nuclear electric power systems for spacecraft was the Pied Piper program, begun in 1954. This led to the development of the Systems for Nuclear Auxiliary Power program, or SNAP, the following year (1955), which was eventually canceled in 1973, as were so many other space-focused programs.

s2 Toroidal Station

SNAP-2 powered space station concept image via DOE

Once space became a realistic place to send not only scientific payloads but personnel, the need to provide them with significant amounts of power became evident. Not only were most systems of the day far from the electricity efficient designs that both NASA and Roscosmos would develop in the coming decades; but, at the time, the vision for a semi-permanent space station wasn’t 3-6 people orbiting in a (completely epic, scientifically revolutionary, collaboratively brilliant, and invaluable) zero-gee conglomeration of tin cans like the ISS, but larger space stations that provided centrifugal gravity, staffed ‘round the clock by dozens of individuals. These weren’t just space stations for NASA, which was an infant organization at the time, but the USAF, and possibly other institutions in the US government as well. In addition, what would provide a livable habitation for a group of astronauts would also be able to power a remote, uncrewed radar station in the Arctic, or in other extreme environments. Even if crew were there, the fact that the power plant wouldn’t have to be maintained was a significant military advantage.

Responsible for both radioisotope thermoelectric generators (which run on the natural radioactive decay of a radioisotope, selected according to its energy density and half-life) as well as fission power plants, SNAP programs were numbered with an even-odd system: even numbers were fission reactors, odd numbers were RTGs. These designs were never solely meant for in-space application, but the increased mission requirements and complexities of being able to safely launch a nuclear power system into space made this aspect of their use the most stringent, and therefore the logical one to design around. Additionally, while the benefits of a power-dense electrical supply are obvious for any branch of the military, the need for this capability in space far surpassed the needs of those on the ground or at sea.

Originally jointly run by the AEC’s Department of Reactor Development (who funded the reactor itself) and the USAF’s AF Wright Air Development Center (who funded the power conversion system), full control was handed over to the AEC in 1957. Atomics International Research was the prime contractor for the program.

There are a number of similarities in almost all the SNAP designs, probably for a number of reasons. First, all of the reactors that we’ll be looking at (as well as some other designs we’ll look at in the next post) used the same type of fissile fuel, even though the form, and the cladding, varied reasonably widely between the different concepts. Uranium-zirconium hydride (U-ZrH) was a very popular fuel choice at the time. Assuming hydrogen loss could be controlled (this was a major part of the testing regime in all the reactors that we’ll look at), it provided a self-moderating, moderate-to-high-temperature fuel form, which was a very attractive feature. This type of fuel is still used today, for the TRIGA reactor – which, between it and its direct descendants is the most common form of research and test reactor worldwide. The high-powered reactors (SNAP 2 and 8) both initially used variations on the same power conversion system: a boiling mercury Rankine power conversion cycle, which was determined by the end of the testing regime to be possible to execute, however to my knowledge has never been proposed again (we’ll look at this briefly in the post on heat engines as power conversion systems, and a more in-depth look will be available in the future), although a mercury-based MHD conversion system is being offered as a power conversion system for an accelerator-driven molten salt reactor.

SNAP-2: The First American Built-For-Space Nuclear Reactor Design

S2 Artist Cutaway Core

SNAP-2 Reactor Cutaway, image DOE

The idea for the SNAP-2 reactor originally came from a 1951 Rand Corporation study, looking at the feasibility of having a nuclear powered satellite. By 1955, the possibilities that a fission power supply offered in terms of mass and reliability had captured the attention of many people in the USAF, which was (at the time) the organization that was most interested and involved (outside the Army Ballistic Missile Agency at the Redstone Arsenal, which would later become the Goddard Spaceflight Center) in the exploitation of space for military purposes.

The original request for the SNAP program, which ended up becoming known as SNAP 2, occurred in 1955, from the AEC’s Defense Reactor Development Division and the USAF Wright Air Development Center. It was 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 epithermal neutron spectrum would remain popular throughout much of the US in-space reactor program, both for electrical power and thermal propulsion designs. This design would later be adapted to the SNAP-10A reactor, with some modifications, as well.


S2 Critical Assembly

SNAP Critical Assembly core, image DOE

SNAP-2’s 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.

S2ER Cross Section

SNAP-2 Experimental Reactor core cros section diagram, image DOE

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. Dry (Sept 15) and wet (Oct 20) critical testing was completed the same year, and 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. Following transient and other testing, 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. Between May 19 and June 15, 1961, the reactor was disassembled and decommissioned. Testing on various reactor materials, especially the fuel elements, was conducted, and these test results refined the design for the Development Reactor.

S2ER Schedule and Timeline

S2DR Core Xsec

SNAP 2 Development Reactor core cross section, image DOE

The SNAP-2 Development Reactor (S2DR or SDR, also called the SNAP-2 Development System, S2DS) was installed in a new facility at the Atomics International Santa Susana research facility to better manage the increased testing requirements for the more advanced reactor design. While this wasn’t going to be a flight-type system, it was designed to inform the flight system on many of the details that the S2ER wasn’t able to. This, interestingly, is much harder to find information on than the S2ER. This reactor incorporated many changes from the S2ER, and went through several iterations to tweak the design for a flight reactor. Zero power testing occurred over the summer of 1961, and testing at power began shortly after (although at SNAP-10 power and temperature levels. Testing continued until December of 1962, and further refined the SNAP-2 and -10A reactors.

S2DR Development Timeline

A third set of critical assembly reactors, known as the SNAP Development Assembly series, was constructed at about the same time, meant to provide fuel element testing, criticality benchmarks, reflector and control system worth, and other core dynamic behaviors. These were also built at the Santa Susana facility, and would provide key test capabilities throughout the SNAP program. This water-and-beryllium reflected core assembly allowed for a wide range of testing environments, and would continue to serve the SNAP program through to its cancellation. Going through three iterations, the designs were used more to test fuel element characteristics than the core geometries of individual core concepts. This informed all three major SNAP designs in fuel element material and, to a lesser extent, heat transfer (the SNAP-8 used thinner fuel elements) design.

Extensive testing was carried out on all aspects of the core geometry, fuel element geometry and materials, and other behaviors of the reactor; but by May 1960 there was enough confidence in the reactor design for the USAF and AEC to plan on a launch program for the reactor (and the SNAP-10A), called SNAPSHOT (more on that below). Testing using the SNAP-2 Experimental Reactor occurred in 1960-1961, and the follow-on test program, including the Snap 2 Development reactor occurred in 1962-63. These programs, as well as the SNAP Critical Assembly 3 series of tests (used for SNAP 2 and 10A), allowed for a mostly finalized reactor design to be completed.

S2 PCS Cutaway Drawing

CRU mercury Rankine power conversion system cutaway diagram, image DOE

The power conversion system (PCS), a Rankine (steam) turbine using mercury, were carried out starting in 1958, with the development of a mercury boiler to test the components in a non-nuclear environment. The turbine had many technical challenges, including bearing lubrication and wear issues, turbine blade pitting and erosion, fluid dynamics challenges, and other technical difficulties. As is often the case with advanced reactor designs, the reactor core itself wasn’t the main challenge, nor the control mechanisms for the reactor, but the non-nuclear portions of the power unit. This is a common theme in astronuclear engineering. More recently, JIMO experienced similar problems when the final system design called for a theoretical but not yet experimental supercritical CO2 Brayton turbine (as we’ll see in a future post). However, without a power conversion system of useable efficiency and low enough mass, an astronuclear power system doesn’t have a means of delivering the electricity that it’s called upon to deliver.

Reactor shielding, in the form of a metal honeycomb impregnated with a high-hydrogen material (in this case a form of paraffin), was common to all SNAP reactor designs. The high hydrogen content allowed for the best hydrogen density of the available materials, and therefore the greatest shielding per unit mass of the available options.

s10 FSM Reactor

SNAP-2/10A FSM reflector and drum mechanism pre-test, image DOE

Testing on the SNAP 2 reactor system continued until 1963, when the reactor core itself was re-purposed into the redesigned SNAP-10, which became the SNAP-10A. At this point the SNAP-2 reactor program was folded into the SNAP-10A program. SNAP-2 specific design work was more or less halted from a reactor point of view, due to a number of factors, including the slower development of the CRU power conversion system, the large number of moving parts in the Rankine turbine, and the advances made in the more powerful SNAP-8 family of reactors (which we’ll cover in the next post). However, testing on the power conversion system continued until 1967, due to its application to other programs. This didn’t mean that the reactor was useless for other missions; in fact, it was far more useful, due to its far more efficient power conversion system for crewed space operations (as we’ll see later in this post), especially for space stations. However, even this role would be surpassed by a derivative of the SNAP-8, the Advanced ZrH Reactor, and the SNAP-2 would end up being deprived of any useful mission.

The SNAP Reactor Improvement Program, in 1963-64, continued to optimize and advance the design without nuclear testing, through computer modeling, flow analysis, and other means; but the program ended without flight hardware being either built or used. We’ll look more at the missions that this reactor was designed for later in this blog post, after looking at its smaller sibling, the first reactor (and only US reactor) to ever achieve orbit: the SNAP-10A.

S2 Program History Table

SNAP-10: The Father of the First Reactor in Space

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.

In the beginning, this design was superficially quite similar to the Romashka reactor that we’ll examine in the USSR part of this blog post, with plates of U-ZrH fuel, separated by beryllium plates for heat conduction, and surrounded by radial and axial beryllium reflectors. Purely conductively cooled internally, and radiatively cooled externally, this was later changed to a NaK forced convection cooling system for better thermal management (see below). The resulting design was later adapted to the SNAP-4 reactor, which was designed to be used for underwater military installations, rather than spaceflight. Outside these radial reflectors were thermoelectric power conversion systems, with a finned radiating casing being the only major component that was visible. The design looked, superficially at least, remarkably like the RTGs that would be used for the next several decades. However, the advantages to using even the low power conversion efficiency thermoelectric conversion system made this a far more powerful source of electricity than the RTGs that were available at the time (or even today) for space missions.

Reactor and Shield Cutaway

SNAP-10A Reactor sketch, image DOE

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. This also led to the NaK cooling system being implemented on the SNAP-10A. Many of the test reactors for the SNAP-2 system were also used to develop the SNAP-10A. This is because the final design, while lower powered, by 20 kWe of electrical output, was largely different in the power conversion system, not the reactor structure. This reactor design was tested extensively, with the S2ER, S2DR, and SCA test series (4A, 4B, and 4C) reactors, as well as the SNAP-10 Ground Test Reactor (S10FS-1). The new design used a similar, but slightly smaller, conical radiator using NaK as the working fluid for the radiator.

This was a far lower power design than the SNAP-2, coming in at 30 kWt, but with the 1.6% power conversion ratio of the thermoelectric systems, its electrical power output was only 500 We. It also ran almost 100°C cooler (200 F), allowing for longer fuel element life, but less thermal gradient to work with, and therefore less theoretical maximum efficiency. This tradeoff was the best on offer, though, and the power conversion system’s lack of moving parts, and ease of being tested in a non-nuclear environment without extensive support equipment, made it more robust from an engineering point of view. The overall design life of the reactor, though, remained short: only about 1 year, and less than 1% fissile fuel burnup. It’s possible, and maybe even likely, that (barring spacecraft-associated failure) the reactor could have provided power for longer durations; however, the longer the reactor operates, the more the fuel swells, due to fission product buildup, and at some point this would cause the clad of the fuel to fail. Other challenges to reactor design, such as fission poison buildup, clad erosion, mechanical wear, and others would end the reactor’s operational life at some point, even if the fuel elements could still provide more power.

SNAP Meteorological Satellite

SNAP-10A satellite concept, image DOE

The SNAP-10A was not meant to power crewed facilities, since the power output was so low that multiple installations would be needed. This meant that, while all SNAP reactors were meant to be largely or wholly unmaintained by crew personnel, this reactor had no possibility of being maintained. The reliability requirements for the system were higher because of this, and the lack of moving parts in the power conversion system aided in this design requirement. The design was also designed to only have a brief (72 hour) time period where active reactivity control would be used, to mitigate any startup transients, and to establish steady-state operations, before the active control systems would be left in their final configuration, leaving the reactor entirely self-regulating. This placed additional burden on the reactor designers to have a very strong understanding of the behavior of the reactor, its long-term stability, and any effects that would occur during the year-long lifetime of the system.

Reflector Ejection Test

Reflector ejection test in progress, image DOE

At the end of the reactor’s life, it was designed to stay in orbit until the short-lived and radiotoxic portions of the reactor had gone through at least five product half-lives, reducing the radioactivity of the system to a very low level. At the end of this process, the reactor would re-enter the atmosphere, the reflectors and end reflector would be ejected, and the entire thing would burn up in the upper atmosphere. From there, winds would dilute any residual radioactivity to less than what was released by a single small nuclear test (which were still being conducted in Nevada at the time). While there’s nothing wrong with this approach from a health physics point of view, as we saw in the last post on the BES-5 reactors the Soviet Union was flying, there are major international political problems with this concept. The SNAPSHOT reactor continues to orbit the Earth (currently at an altitude of roughly 1300 km), and will do so for more than 2000 years, according to recent orbital models, so the only system of concern is not in danger of re-entry any time soon; but, at some point, the reactor will need to be moved into a graveyard orbit or collected and returned to Earth – a problem which currently has no solution.

The Runup to Flight: Vehicle Verification and Integration

1960 brought big plans for orbital testing of both the SNAP-2 and SNAP-10 reactors, under the program SNAPSHOT: Two SNAP-10 launches, and two SNAP-2 launches would be made. Lockheed Missiles System Division was chosen as the launch vehicle, systems integration, and launch operations contractor for the program; while Atomics International, working under the AEC, was responsible for the power plant.

The SNAP-10A reactor design was meant to be decommissioned by orbiting for long enough that the fission product inventory (the waste portion of the burned fuel elements, and the source of the vast majority of the radiation from the reactor post-fission) would naturally decay away, and then the reactor would be de-orbited, and burn up in the atmosphere. This was planned before the KOSMOS-954 accident, when the possibility of allowing a nuclear reactor to burn up in the atmosphere was not as anathema as it is today. This plan wouldn’t increase the amount of radioactivity that the public would receive to any appreciable degree; and, at the time, open-air testing of nuclear weapons was the norm, sending up thousands of kilograms of radioactive fallout per year. However, it was important that the fuel rods themselves would burn up high in the atmosphere, in order to dilute the fuel elements as much as possible, and this is something that needed to be tested.


RFD-1 Experimental Payload

Enter the SNAP Reactor Flight Demonstration Number 1 mission, or RFD-1. The concept of this test was to demonstrate that the planned disassembly and burnup process would occur as expected, and to inform the further design of the reactor if there were any unexpected effects of re-entry. Sandia National Labs took the lead on this part of the SNAPSHOT program. After looking at the budget available, the launch vehicles available, and the payloads, the team realized that orbiting a nuclear reactor mockup would be too expensive, and another solution needed to be found. This led to the mission design of RFD-1: a sounding rocket would be used, and the core geometry would be changed to account for the short flight time, compared to a real reentry, in order to get the data needed for the de-orbiting testing of the actual SNAP-10A reactor that would be flown.

So what does this mean? Ideally, the development of a mockup of the SNAP-10A reactor, with the only difference being that there wouldn’t be any highly enriched uranium in the fuel elements, as normally configured; instead depleted uranium would be used. It would be launched on the same launch vehicle that the SNAPSHOT mission would use (an Atlas-Agena D), be placed in the same orbit, and then be deorbited at the same angle and the same place as the actual reactor would be; maybe even in a slightly less favorable reentry angle to know how accurate the calculations were, and what the margin of error would be. However, an Atlas-Agena rocket isn’t a cheap piece of hardware, either to purchase, or to launch, and the project managers knew that they wouldn’t be able to afford that, so they went hunting for a more economical alternative.

RFD1 Flight Path

RFD-1 Mission Profile, image DOE

This led the team to decide on a NASA Scout sounding rocket as the launch vehicle, launched from Wallops Island launch site (which still launches sounding rockets, as well as the Antares rocket, to this day, and is expanding to launch Vector Space and RocketLab orbital rockets as well in the coming years). Sounding rockets don’t reach orbital altitudes or velocities, but they get close, and so can be used effectively to test orbital components for systems that would eventually fly in orbit, but for much less money. The downside is that they’re far smaller, with less payload and less velocity than their larger, orbital cousins. This led to needing to compromise on the design of the dummy reactor in significant ways – but those ways couldn’t compromise the usefulness of the test.

Sandia Corporation (which runs Sandia National Laboratories to this day, although who runs Sandia Corp changes… it’s complicated) and Atomics International engineers got together to figure out what could be done with the Scout rocket and a dummy reactor to provide as useful an engineering validation as possible, while sticking within the payload requirements and flight profile of the relatively small, suborbital rocket that they could afford. Because the dummy reactor wouldn’t be going nearly as fast as it would during true re-entry, a steeper angle of attack when the test was returning to Earth was necessary to get the velocity high enough to get meaningful data.

Scout rocket

Scout sounding rocket, image DOE

The Scout rocket that was being used had much less payload capability than the Atlas rocket, so if there was a system that could be eliminated, it was removed to save weight. No NaK was flown on RFD-1, the power conversion system was left off, the NaK pump was simulated by an empty stainless steel box, and the reflector assembly was made out of aluminum instead of beryllium, both for weight and toxicity reasons (BeO is not something that you want to breathe!). The reactor core didn’t contain any dummy fuel elements, just a set of six stainless steel spacers to keep the grid plates at the appropriate separation. Because the angle of attack was steeper, the test would be shorter, meaning that there wouldn’t be time for the reactor’s reflectors to degrade enough to release the fuel elements. The fuel elements were the most important part of the test, however, since it needed to be demonstrated that they would completely burn up upon re-entry, so a compromise was found.

The fuel elements would be clustered on the outside of the dummy reactor core, and ejected early in the burnup test period. While the short time and high angle of attack meant that there wouldn’t be enough time to observe full burnup, the beginning of the process would be able to provide enough data to allow for accurate simulations of the process to be made. How to ensure that this data, which was the most important part of the test, would be able to be collected was another challenge, though, which forced even more compromises for RFT-1’s design. Testing equipment had to be mounted in such a way as to not change the aerodynamic profile of the dummy reactor core. Other minor changes were needed as well, but despite all of the differences between the RFD-1 and the actual SNAP-10A the thermodynamics and aerodynamics of the system were different in only very minor ways.

Testing support came from Wallops Island and NASA’s Bermuda tracking station, as well as three ships and five aircraft stationed near the impact site for radar observation. The ground stations would provide both radar and optical support for the RFD-1 mission, verifying reactor burnup, fuel element burnup, and other test objective data, while the aircraft and ships were primarily tasked with collecting telemetry data from on-board instruments, as well as providing additional radar data; although one NASA aircraft carried a spectrometer in order to analyze the visible radiation coming off the reentry vehicle as it disintegrated.

RFD-1 Burnup Splice

Film splice of RV burnup during RFD-1, image DOE

The test went largely as expected. Due to the steeper angle of attack, full fuel element burnup wasn’t possible, even with the early ejection of the simulated fuel rods, but the amount that they did disintegrate during the mission showed that the reactor’s fuel would be sufficiently distributed at a high enough altitude to prevent any radiological risk. The dummy core behaved mostly as expected, although there were some disagreements between the predicted behavior and the flight data, due to the fact that the re-entry vehicle was on such a steep angle of attack. However, the test was considered a success, and paved the way for SNAPSHOT to go forward.

The next task was to mount the SNAP-10A to the Agena spacecraft. Because the reactor was a very different power supply than was used at the time, special power conditioning units were needed to transfer power from the reactor to the spacecraft. This subsystem was mounted on the Agena itself, along with tracking and command functionality, control systems, and voltage regulation. While Atomics International worked to ensure the reactor would be as self-contained as possible, the reactor and spacecraft were fully integrated as a single system. Besides the reactor itself, the spacecraft carried a number of other experiments, including a suite of micrometeorite detectors and an experimental cesium contact thruster, which would operate from a battery system that would be recharged by electricity produced by the reactor.

S10FSM Vac Chamber

FSEM-3 in vacuum chamber for environmental and vibration tests, image DOE

In order to ensure the reactor would be able to be integrated to the spacecraft, a series of Flight System Prototypes (FSM-1, and -4; FSEM-2 and -3 were used for electrical system integration) were built. These were full scale, non-nuclear mockups that contained a heating unit to simulate the reactor core. Simulations were run using FSM-1 from launch to startup on orbit, with all testing occurring in a vacuum chamber. The final one of the series, FSM-4, was the only one that used NaK coolant in the system, which was used to verify that the thermal performance of the NaK system met with flight system requirements. FSEM-2 did not have a power system mockup, instead it used a mass mockup of the reactor, power conversion system, radiator, and other associated components. Testing with FSEM-2 showed that there were problems with the original electrical design of the spacecraft, which required a rebuild of the test-bed, and a modification of the flight system itself. Once complete, the renamed FSEM-2A underwent a series of shock, vibration, acceleration, temperature, and other tests (known as the “Shake and Bake” environmental tests), which it subsequently passed. The final mockup, FSEM-3, underwent extensive electrical systems testing at Lockheed’s Sunnyvale facility, using simulated mission events to test the compatibility of the spacecraft and the reactor. Additional electrical systems changes were implemented before the program proceeded, but by the middle of 1965, the electrical system and spacecraft integration tests were complete and the necessary changes were implemented into the flight vehicle design.


SNAP-10A F-3 (flight unit for SNAPSHOT) undergoing final checks before spacecraft integration. S10F-4 was identical.

The last round of pre-flight testing was a test of a flight-configured SNAP-10A reactor under fission power. This nuclear ground test, S10F-3, was identical to the system that would fly on SNAPSHOT, save some small ground safety modifications, and was tested from January 22 1965 to March 15, 1966. It operated uninterrupted for over 10,000 hours, with the first 390 days being at a power output of 35 kWt, and (following AEC approval) an additional 25 days of testing at 44 kWt. This testing showed that, after one year of operation, the continuing problem of hydrogen redistribution caused the reactor’s outlet temperature to drop more than expected, and additional, relatively minor, uncertainties about reactor dynamics were seen as well. However, overall, the test was a success, and paved the way for the launch of the SNAPSHOT spacecraft in April 1965; and the continued testing of S10F-3 during the SNAPSHOT mission was able to verify that the thermal behavior of astronuclear power systems during ground test is essentially identical to orbiting systems, proving the ground test strategy that had been employed for the SNAP program.

SNAPSHOT: The First Nuclear Reactor in Space

In 1963 there was a change in the way the USAF was funding these programs. While they were solely under the direction of the AEC, the USAF still funded research into the power conversion systems, since they were still operationally useful; but that changed in 1963, with the removal of the 0.3 kWe to 1 kWe portion of the program. Budget cuts killed the Zr-H moderated core of the SNAP-2 reactor, although funding continued for the Hg vapor Rankine conversion system (which was being developed by TRW) until 1966. The SNAP-4 reactor, which had not even been run through criticality testing, was canceled, as was the planned flight test of the SNAP-10A, which had been funded under the USAF, because they no longer had an operational need for the power system with the cancellation of the 0.3-1 kWe power system program. The associated USAF program that would have used the power supply was well behind schedule and over budget, and was canceled at the same time.

The USAF attempted to get more funding, but was denied. All parties involved had a series of meetings to figure out what to do to save the program, but the needed funds weren’t forthcoming. All partners in the program worked together to try and have a reduced SNAPSHOT program go through, but funding shortfalls in the AEC (who received only $8.6 million of the $15 million they requested), as well as severe restrictions on the Air Force (who continued to fund Lockheed for the development and systems integration work through bureaucratic creativity), kept the program from moving forward. At the same time, it was realized that being able to deliver kilowatts or megawatts of electrical power, rather than the watts currently able to be produced, would make the reactor a much more attractive program for a potential customer (either the USAF or NASA).

Finally, in February of 1964 the Joint Congressional Committee on Atomic Energy was able to fund the AEC to the tune of $14.6 million to complete the SNAP-10A orbital test. This reactor design had already been extensively tested and modeled, and unlike the SNAP-2 and -8 designs, no complex, highly experimental, mechanical-failure-prone power conversion system was needed.

On Orbit Artist

SNAP-10A reactor, artist’s rendering (artist unknown), image DOE

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.

While the threshold for mission success was set at 90 days, for Atomics International wanted to prove 1 year of capability for the system; so, in those 90 days, the goal was that the entire reactor system would be demonstrated to be capable of one year of operation (the SNAP-2 requirements). Atomics International imposed additional, more stringent, guidelines for the mission as well, specifying a number of design requirements, including self-containment of the power system outside the structure of the Agena, as much as possible; more stringent mass and center-of-gravity requirements for the system than specified by the US Air Force; meeting the military specifications for EM radiation exposure to the Agena; and others.


SNAPSHOT launch, image USAF via Gunter’s Space Page

The flight was formally approved in March, and 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. One of the impedance probes failed before launch, and a part of the micrometeorite detector system failed before returning data. A number of other minor faults were detected as well, 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.

The reactor was given the startup order 3.5 hours into the flight, when the two gross adjustment control drums were fully inserted, and the two fine control drums began a stepwise reactivity insertion into the reactor. Within 6 hours, the reactor achieved on-orbit criticality, and the active control portion of the reactor test program began. For the next 154 hours, the control drums were operated with ground commands, to test reactor behavior. Due to the problems with the ion engine, the failure sensing and malfunction sensing systems were also switched off, because these could have been corrupted by the errant thruster. Following the first 200 hours of reactor operations, the reactor was set to autonomous operation at full power. Between 600 and 700 hours later, the voltage output of the reactor, as well as its temperature, began to drop; an effect that the S10-F3 test reactor had also demonstrated, due to hydrogen migration in the core.

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.

What caused the failure? There are many possibilities, but when the telemetry from the spacecraft was read, it was obvious that something badly wrong had occurred. The only thing that can be said with complete confidence is that the error came from the Agena spacecraft rather than from the reactor. No indications had been received before the blackout that the reactor was about to scram itself (the reflector ejection was the emergency scram mechanism), and the problem wasn’t one that should have been able to occur without ground commands. However, with the telemetry data gained from the dwindling battery after the shutdown, some suppositions could be made. The most likely immediate cause of the reactor’s shutdown was traced to a possible spurious command from the high voltage command decoder, part of the Agena’s power conditioning and distribution system. This in turn was likely caused by one of two possible scenarios: either a piece of the voltage regulator failed, or it became overstressed because of either the unusual low-power vehicle loads or commanding the reactor to increase power output. Sadly, the cause of this system failure cascade was never directly determined, but all of the data received pointed to a high-voltage failure of some sort, rather than a low-voltage error (which could have also resulted in a reactor scram). Other possible causes of instrumentation or reactor failure, such as thermal or radiation environment, collision with another object, onboard explosion of the chemical propellants used on the Agena’s main engines, and previously noted flight anomalies – including the arcing and EM interference from the ion engine – were all eliminated as the cause of the error as well.

Despite the spacecraft’s mysterious early demise, SNAPSHOT provided many valuable lessons in space reactor design, qualification, ground handling, launch challenges, and many other aspects of handling an astronuclear power source for potential future missions: Suggestions for improved instrumentation design and performance characteristics; provision for a sunshade for the main radiator to eliminate the sun/shade efficiency difference that was observed during the mission; the use of a SNAP-2 type radiation shield to allow for off-the-shelf, non-radiation-hardened electronic components in order to save both money and weight on the spacecraft itself; and other minor changes were all suggested after the conclusion of the mission. Finally, the safety program developed for SNAPSHOT, including the SCA4 submersion criticality tests, the RFT-1 test, and the good agreement in reactor behavior between the on-orbit and ground test versions of the SNAP-10A showed that both the AEC and the customer of the SNAP-10A (be it the US Air Force or NASA) could have confidence that the program was ready to be used for whatever mission it was needed for.

Sadly, at the time of SNAPSHOT there simply wasn’t a mission that needed this system. 500 We isn’t much power, even though it was more power than was needed for many systems that were being used at the time. While improvements in the thermoelectric generators continued to come in (and would do so all the way to the present day, where thermoelectric systems are used for everything from RTGs on space missions to waste heat recapture in industrial facilities), the simple truth of the matter was that there was no mission that needed the SNAP-10A, so the program was largely shelved. Some follow-on paper studies would be conducted, but the lowest powered of the US astronuclear designs, and the first reactor to operate in Earth orbit, would be retired almost immediately after the SNAPSHOT mission.

Post-SNAPSHOT SNAP: the SNAP Improvement Program

The SNAP fission-powered program didn’t end with SNAPSHOT, far from it. While the SNAP reactors only ever flew once, their design was mature, well-tested, and in most particulars ready to fly in a short time – and the problems associated with those particulars had been well-addressed on the nuclear side of things. The Rankine power conversion system for the SNAP-2, which went through five iterations, reached technological maturity as well, having operated in a non-nuclear environment for close to 5,000 hours and remained in excellent condition, meaning that the 10,000 hour requirement for the PCS would be able to be met without any significant challenges. The thermoelectric power conversion system also continued to be developed, focusing on an advanced silicon-germanium thermoelectric convertor, which was highly sensitive to fabrication and manufacturing processes – however, we’ll look more at thermoelectrics in the power conversion systems series of blog posts, just keep in mind that the power conversion systems continued to improve throughout this time, not just the reactor core design.

LiH FE Cracking

Fuel element post-irradiation. Notice the cracks where the FE would rest in the endcap reflector

On the reactor side of things, the biggest challenge was definitely hydrogen migration within the fuel elements. As the hydrogen migrates away from the ZrH fuel, many problems occur; from unpredictable reactivity within the fuel elements, to temperature changes (dehydrogenated fuel elements developed hotspots – which in turn drove more hydrogen out of the fuel element), to changes in ductility of the fuel, causing major headaches for end-of-life behavior of the reactors and severely limiting the fuel element temperature that could be achieved. However, the necessary testing for the improvement of those systems could easily be conducted with less-expensive reactor tests, including the SCA4 test-bed, and didn’t require flight architecture testing to continue to be improved.

The maturity of these two reactors led to a short-lived program in the 1960s to improve them, the SNAP Reactor Improvement Program. The SNAP-2 and -10 reactors went through many different design changes, some large and some small – and some leading to new reactor designs based on the shared reactor core architecture.

By this time, the SNAP-2 had mostly faded into obscurity. However, the fact that it shared a reactor core with the SNAP-10A, and that the power conversion system was continuing to improve, warranted some small studies to improve its capabilities. The two of note that are independent of the core (all of the design changes for the -10 that will be discussed can be applied to the -2 core as well, since at this point they were identical) are the change from a single mercury boiler to three, to allow more power throughput and to reduce loads on one of the more challenging components, and combining multiple cores into a single power unit. These were proposed together for a space station design (which we’ll look at later) to allow an 11 kWe power supply for a crewed station.

The vast majority of this work was done on the -10A. Any further reactors of this type would have had an additional three sets of 1/8” beryllium shims on the external reflector, increasing the initial reactivity by about 50 cents (1 dollar of reactivity is exactly break even, all other things being equal; reactivity potential is often somewhere around $2-$3, however, to account for fission product buildup); this means that additional burnable poisons (elements which absorb neutrons, then decay into something that is mostly neutron transparent, to even out the reactivity of the reactor over its lifetime) could be inserted in the core at construction, mitigating the problems of reactivity loss that were experienced during earlier operation of the reactor. With this, and a number of other minor tweaks to reflector geometry and lowering the core outlet temperature slightly, the life of the SNAP-10A was able to be extended from the initial design goal of one year to five years of operation. The end-of-life power level of the improved -10A was 39.5 kWt, with an outlet temperature of 980 F (527°C) and a power density of 0.14 kWt/lb (0.31 kWt/kg).


Interim 10A 2

Interim SNAP 10A/2, image DOE

e design modifications led to another iteration of the SNAP-10A, the Interim SNAP-10A/2 (I-10A/2). This reactor’s core was identical, but the reflector was further enhanced, and the outlet temperature and reactor power were both increased. In addition, even more burnable poisons were added to the core to account for the higher power output of the reactor. Perhaps the biggest design change with the Interim -10A/2 was the method of reactor control: rather than the passive control of the reactor, as was done on the -10A, the entire period of operation for the I-10A/2 was actively controlled, using the control drums to manage reactivity and power output of the reactor. As with the improved -10A design, this reactor would be able to have an operational lifetime of five years. These improvements led the I-10A/2 to have an end of life power rating of 100 kWt, an outlet temperature of 1200 F (648°C), and an improved power density of 0.33 kWt/lb (0.73 kWt/kg).

10A2 Table

Interim SNAP 10A/2 Design References, image DOE

This design, in turn, led to the Upgraded SNAP-10A/2 (U-10A/2). The biggest in-core difference between the I-10A/2 and the U-10A/2 was the hydrogen barrier used in the fuel elements: rather than using the initial design that was common to the -2, -10A, and I-10A/2, this reactor used the hydrogen barrier from the SNAP-8 reactor, which we’ll look at in the next blog post. This is significant, because the degradation of the hydrogen barrier over time, and the resulting loss of hydrogen from the fuel elements, was the major lifetime limiting factor of the SNAP-10 variants up until this point. This reactor also went back to static control, rather than the active control used in the I-10A/2. As with the other -10A variants, the U-10A/2 had a possible core lifetime of five years, and other than an improvement of 100 F in outlet temperature (to 1300 F), and a marginal drop in power density to 0.31 kWt/lb, it shared many of the characteristics that the I-10A/2 had.

SNAP-10B: The Upgrade that Could Have Been

10B Cutaway System

SNAP=10B Cutaway Diagram, image DOE

One consistent mass penalty in the SNAP-10A variants that we’ve looked at so far is the control drums: relatively large reactivity insertions were possible with a minimum of movement due to the wide profile of the control drums, but this also meant that they extended well away from the reflector, especially early in the mission. This meant that, in order to prevent neutron backscatter from hitting the rest of the spacecraft, the shield had to be relatively wide compared the the size of the core – and the shield was not exactly a lightweight system component.

The SNAP-10B reactor was designed to address this problem. It used a similar core to the U-10A/2, with the upgraded hydrogen barrier from the -8, but the reflector was tapered to better fit the profile of the shadow shield, and axially sliding control cylinders would be moved in and out to provide control instead of the rotating drums of the -10A variants. A number of minor reactor changes were needed, and some of the reactor physics parameters changed due to this new control system; but, overall, very few modifications were needed.

The first -10B reactor, the -10B Basic (B-10B), was a very simple and direct evolution of the U-10A/2, with nothing but the reflector and control structures changed to the -10B configuration. Other than a slight drop in power density (to 0.30 kWt/lb), the rest of the performance characteristics of the B-10B were identical to the U-10A/2. This design would have been a simple evolution of the -10A/2, with a slimmer profile to help with payload integration challenges.

10B Basic Table

Image DOE

The next iteration of the SNAP-10B, the Advanced -10B (A-10B), had options for significant changes to the reactor core and the fuel elements themselves. One thing to keep in mind about these reactors is that they were being designed above and beyond any specific mission needs; and, on top of that, a production schedule hadn’t been laid out for them. This means that many of the design characteristics of these reactors were never “frozen,” which is the point in the design process when the production team of engineers need to have a basic configuration that won’t change in order to proceed with the program, although obviously many minor changes (and possibly some major ones) would continue to be made up until the system was flight qualified.


Up until now, every SNAP-10 design used a 37 fuel element core, with the only difference in the design occurring in the Upgraded -10A/2 and Basic -10B reactors (which changed the hydrogen barrier ceramic enamel inside the fuel element clad). However, with the A-10B there were three core size options: the first kept the 37 fuel element core, a medium-sized 55-element core, and a large 85-element core. There were other questions about the final design, as well, looking at two other major core changes (as well as a lot of open minor questions). The first option was to add a “getter,” a sheath of hydrophilic (highly hydrogen-absorbing) metal to the clad outside the steel casing, but still within the active region of the core. While this isn’t as ideal as containing the hydrogen within the U-ZrH itself, the neutron moderation provided by the hydrogen would be lost at a far lower rate. The second option was to change the core geometry itself, as the temperature of the core changed, with devices called “Thermal Coefficient Augmenters” (TCA). There were two options that were suggested: first, there was a bellows system that was driven by NaK core temperature (using ruthenium vapor), which moves a portion of the radial reflector to change the core’s reactivity coefficient; second, the securing grids for the fuel elements themselves would expand as the NaK increased in temperature, and contract as the coolant dropped in temperature.

Between the options available, with core size, fuel element design, and variable core and fuel element configuration all up in the air, the Advanced SNAP-10B was a wide range of reactors, rather than just one. Many of the characteristics of the reactors remained identical, including the fissile fuel itself, the overall core size, maximum outlet temperature, and others. However, the number of fuel elements in the core alone resulted in a wide range of different power outputs; and, which core modification the designers ultimately decided upon (Getter vs TCA, I haven’t seen any indication that the two were combined) would change what the capabilities of the reactor core would actually be. However, both for simplicity’s sake, and due to the very limited documentation available on the SNAP-10B program, other than a general comparison table from the SNAP Systems Capability Study from 1966, we’ll focus on the 85 fuel element core of the two options: the Getter core and the TCA core.

A final note, which isn’t clear from these tables: each of these reactor cores was nominally optimized to a 100 kWt power output, the additional fuel elements reduced the power density required at any time from the core in order to maximize fuel lifetime. Even with the improved hydrogen barriers, and the variable core geometry, while these systems CAN offer higher power, it comes at the cost of a shorter – but still minimum one year – life on the reactor system. Because of this, all reported estimates assumed a 100 kWt power level unless otherwise stated.

Yt Getter FE

Fuel element with yttrium getter, image DOE

The idea of a hydrogen “getter” was not a new one at the time that it was proposed, but it was one that hadn’t been investigated thoroughly at that point (and is a very niche requirement in terrestrial nuclear engineering). The basic concept is to get the second-best option when it comes to hydrogen migration: if you can’t keep the hydrogen in your fuel element itself, then the next best option is keeping it in the active region of the core (where fission is occurring, and neutron moderation is the most directly useful for power production). While this isn’t as good as increasing the chance of neutron capture within the fuel element itself, it’s still far better than hydrogen either dissolving into your coolant, or worse yet, migrating outside your reactor and into space, where it’s completely useless in terms of reactor dynamics. Of course, there’s a trade-off: because of the interplay between the various aspects of reactor physics and design, it wasn’t practical to change the external geometry of the fuel elements themselves – which means that the only way to add a hydrogen “getter” was to displace the fissile fuel itself. There’s definitely an optimization question to be considered; after all, the overall reactivity of the reactor will have to be reduced because the fuel is worth more in terms of reactivity than the hydrogen that would be lost, but the hydrogen containment in the core at end of life means that the system itself would be more predictable and reliable. Especially for a static control system like the A-10B, this increase in behavioral predictability can be worth far more than the reactivity that the additional fuel would offer. Of the materials options that were tested for the “getter” system, yttrium metal was found to be the most effective at the reactor temperatures and radiation flux that would be present in the A-10B core. However, while improvements had been made in the fuel element design to the point that the “getter” program continued until the cancellation of the SNAP-2/10 core experiments, there were many uncertainties left as to whether the concept was worth employing in a flight system.

The second option was to vary the core geometry with temperature, the Thermal Coefficient Augmentation (TCA) variant of the A-10B. This would change the reactivity of the reactor mechanically, but not require active commands from any systems outside the core itself. There were two options investigated: a bellows arrangement, and a design for an expanding grid holding the fuel elements themselves.

A-10B Bellows Diagram

Ruthenium vapor bellows design for TCA, image DOE

The first variant used a bellows to move a portion of the reflector out as the temperature increased. This was done using a ruthenium reservoir within the core itself. As the NaK increased in temperature, the ruthenium would boil, pushing a bellows which would move some of the beryllium shims away from the reactor vessel, reducing the overall worth of the radial reflector. While this sounds simple in theory, gas diffusion from a number of different sources (from fission products migrating through the clad to offgassing of various components) meant that the gas in the bellows would not just be ruthenium vapor. While this could have been accounted for, a lot of study would have needed to have been done with a flight-type system to properly model the behavior.

A-10B Expandable BaseplateThe second option would change the distance between the fuel elements themselves, using base plate with concentric accordion folds for each ring of fuel elements called the “convoluted baseplate.” As the NaK heated beyond optimized design temperature, the base plates would expand radially, separating the fuel elements and reducing the reactivity in the core. This involved a different set of materials tradeoffs, with just getting the device constructed causing major headaches. The design used both 316 stainless steel and Hastelloy C in its construction, and was cold annealed. The alternative, hot annealing, resulted in random cracks, and while explosive manufacture was explored it wasn’t practical to map the shockwave propagation through such a complex structure to ensure reliable construction at the time.

A-10B Expandable Baseplate 2While this is certainly a concept that has caused me to think a lot about the concept of a variable reactor geometry of this nature, there are many problems with this approach (which could have possibly been solved, or proven insurmountable). Major lifetime concerns would include ductility and elasticity changes through the wide range of temperatures that the baseplate would be exposed to; work hardening of the metal, thermal stresses, and neutron bombardment considerations of the base plates would also be a major concern in this concept.

These design options were briefly tested, but most of these ended up not being developed fully. Because the reactor’s design was never frozen, many engineering challenges remained in every option that had been presented. Also, while I know that a report was written on the SNAP-10B reactor’s design (R. J . Gimera, “SNAP 1OB Reactor Conceptual Design,” NAA-SR-10422), I can’t find it… yet. This makes writing about the design difficult, to say the least.

Because of this, and the extreme paucity of documentation on this later design, it’s time to turn to what these innovative designs could have offered when it comes to actual missions.

The Path Not Taken: Missions for SNAP-2, -10A

Every space system has to have a mission, or it will never fly. Both SNAP-2 and SNAP-10 offered a lot for the space program, both for crewed and uncrewed missions; and what they offered only grew with time. However, due to priorities at the time, and the fact that many records from these programs appear to never have been digitized, it’s difficult to point to specific mission proposals for these reactors in a lot of cases, and the missions have to be guessed at from scattered data, status reports, and other piecemeal sources.

SNAP-10 was always a lower-powered system, even with its growth to a kWe-class power supply. Because of this, it was always seen as a power supply for unmanned probes, mostly in low Earth orbit, but certainly it would also have been useful in interplanetary studies as well, which at this point were just appearing on the horizon as practical. Had the SNAPSHOT system worked as planned, the cesium thruster that had been on board the Agena spacecraft would have been an excellent propulsion source for an interplanetary mission. However, due to the long mission times and relatively fragile fuel of the original SNAP-10A, it is unlikely that these missions would have been initially successful, while the SNAP-10A/2 and SNAP-B systems, with their higher power output and lifetimes, would have been ideal for many interplanetary missions.

As we saw in the US-A program in our last post (which can be found here), one of the major advantages that a nuclear reactor offers over photovoltaic cells – which were just starting to be a practical technology at the time – is that they offer very little surface area, and therefore the atmospheric drag that all satellites experience due to the thin atmosphere in lower orbits is less of a concern. There are many cases where this lower altitude offers clear benefits, but the vast majority of them deal with image resolution: the lower you are, the more clear your imagery can be with the same sensors. For the Russians, the ability to get better imagery of US Navy movements in all weather conditions was of strategic importance, leading to the US-A program. For Americans, who had other means of surveillance (and an opponent’s far less capable blue-water navy to track), radar surveillance was not a major focus – although it should be noted that 500 We isn’t going to give you much, if any resolution, no matter what your altitude.

SNAP Meteorological Satellite

SNAP-10 powered meteorological satellite, image DOE

One area that SNAP-10A was considered for was for meteorological satellites. With a growing understanding of how weather could be monitored, and what types of data were available through orbital systems, the ability to take and transmit pictures from on-orbit using the first generations of digital cameras (which were just coming into existence, and not nearly good enough to interest the intelligence organizations at the time), along with transmitting the data back to Earth, would have allowed for the best weather tracking capability in the world at the time. By using a low orbit, these satellites would be able to make the most of the primitive equipment available at the time, and possibly (speculation on my part) have been able to gather rudimentary moisture content data as well.

However, while SNAP-10A was worked on for about a decade, for the entire program there was always the question of “what do you do with 500-1000 We?” Sure, it’s not an insignificant amount of power, even then, but… communications and propulsion, the two things that are the most immediately interesting for satellites with reliable power, both have a linear relationship between power level and capability: the more power, the more bandwidth, or delta-vee, you have available. Also, the -10A was only ever rated for one year of operations, although it was always suspected it could be limped along for longer, which precluded many other missions.

The later SNAP-10A/2 and -10B satellites, with their multi-kilowatt range and years-long lifespans, offered far more flexibility, but by this point many in the AEC, the US Air Force, NASA, and others were no longer very interested in the program; with newer, more capable, reactor designs being available (we’ll look at some of those in the next post). While the SNAP-10A was the only flight-qualified and -tested reactor design (and the errors on the mission were shown to not be the fault of the reactor, but the Agena spacecraft), it was destined to fade into obscurity.

SNAP-10A was always the smallest of the reactors, and also the least powerful. What about the SNAP-2, the 3-6 kWe reactor system?

Initial planning for the SNAP-2 offered many options, with communications satellites being mentioned as an option early on – especially if the reactor lifetime could be extended. While not designed specifically for electric propulsion, it could have utilized that capability either on orbit around the Earth or for interplanetary missions. Other options were also proposed, but one was seized on early: a space station.

S2 Cylinder Station

Cylindrical space station, image DOE

At the time, most space station designs were nuclear powered, and there were many different configurations. However, there were two that were the most common: first was the simple cylinder, launched as a single piece (although there were multiple module designs proposed which kept the basic cylinder shape) which would be finally realized with the Skylab mission; second was a torus-shaped space station, which was proposed almost a half a century before by Tsiolkovsky, and popularized at the time by Werner von Braun. SNAP-2 was adapted to both of these types of stations. Sadly, while I can find one paper on the use of the SNAP-2 on a station, it focuses exclusively on the reactor system, and doesn’t use a particular space station design, instead laying out the ground limits of the use of the reactor on each type of station, and especially the shielding requirements for each station’s geometry. It was also noted that the reactors could be clustered, providing up to 11 kWe of power for a space station, without significant change to the radiation shield geometry. We’ll look at radiation shielding in a couple posts, and look at the particulars of these designs there.

s2 Toroidal Station

Hexagonal/Toroid space station. Note the wide radiation shield. Image DOE

Since space stations were something that NASA didn’t have the budget for at the time, most designs remained vaguely defined, without much funding or impetus within the structure of either NASA or the US Air Force (although SNAP-2 would have definitely been an option for the Manned Orbiting Laboratory program of the USAF). By the time NASA was seriously looking at space stations as a major funding focus, the SNAP-8 derived Advanced Zirconium Hydride reactor, and later the SNAP-50 (which we’ll look at in the next post) offered more capability than the more powerful SNAP-2. Once again, the lack of a mission spelled the doom of the SNAP-2 reactor.

Hg Rankine Cutaway Drawing

Power conversion system, SNAP-2

The SNAP-2 reactor met its piecemeal fate even earlier than the SNAP-10A, but oddly enough both the reactor and the power conversion system lasted just as long as the SNAP-10A did. The reactor core for the SNAP-2 became the SNAP-10A/2 core, and the CRU power conversion system continued under development until after the reactor cores had been canceled. However, mention of the SNAP-2 as a system disappears in the literature around 1966, while the -2/10A core and CRU power conversion system continued until the late 1960s and late 1970s, respectively.

The Legacy of The Early SNAP Reactors

The SNAP program was canceled in 1971 (with one ongoing exception), after flying a single reactor which was operational for 43 days, and conducting over five years of fission powered testing on the ground. The death of the program was slow and drawn out, with the US Air Force canceling the program requirement for the SNAP-10A in 1963 (before the SNAPSHOT mission even launched), the SNAP-2 reactor development being canceled in 1967, all SNAP reactors (including the SNAP-8, which we’ll look at next week) being canceled by 1974, and the CRU power conversion system being continued until 1979 as a separate internal, NASA-supported but not fully funded, project by Rockwell International.

The promise of SNAP was not enough to save the program from the massive cuts to space programs, both for NASA and the US Air Force, that fell even as humanity stepped onto the Moon for the first time. This is an all-too-common fate, both in advanced nuclear reactor engineering and design as well as aerospace engineering. As one of the engineers who worked on the Molten Salt Reactor Experiment noted in a recent documentary on that technology, “everything I ever worked on got canceled.”

However, this does not mean that the SNAP-2/10A programs were useless, or that nothing except a permanently shut down reactor in orbit was achieved. In fact, the SNAP program has left a lasting mark on the astronuclear engineering world, and one that is still felt today. The design of the SNAP-2/10A core, and the challenges that were faced with both this reactor core and the SNAP-8 core informed hydride fuel element development, including the thermal limits of this fuel form, hydrogen migration mitigation strategies, and materials and modeling for multiple burnable poison options for many different fuel types. The thermoelectric conversion system (germanium-silicon) became a common one for high-temperature thermoelectric power conversion, both for power conversion and for thermal testing equipment. Many other materials and systems that were used in this reactor system continued to be developed through other programs.

Possibly the most broad and enduring legacy of this program is in the realm of launch safety, flight safety, and operational paradigms for crewed astronuclear power systems. The foundation of the launch and operational safety guidelines that are used today, for both fission power systems and radioisotope thermoelectric generators, were laid out, refined, or strongly informed by the SNAPSHOT and Space Reactor Safety program – a subject for a future web page, or possibly a blog post. From the ground handling of a nuclear reactor being integrated to a spacecraft, to launch safety and abort behavior, to characterizing nuclear reactor behavior if it falls into the ocean, to operating crewed space stations with on-board nuclear power plants, the SNAP-2/10A program literally wrote the book on how to operate a nuclear power supply for a spacecraft.

While the reactors themselves never flew again, nor did their direct descendants in design, the SNAP reactors formed the foundation for astronuclear engineering of fission power plants for decades. When we start to launch nuclear power systems in the future, these studies, and the carefully studied lessons of the program, will continue to offer lessons for future mission planners.

More Coming Soon!

The SNAP program extended well beyond the SNAP-2/10A program. The SNAP-8 reactor, started in 1959, was the first astronuclear design specifically developed for a nuclear electric propulsion spacecraft. It evolved into several different reactors, notably the Advanced ZrH reactor, which remained the preferred power option for NASA’s nascent modular space station through the mid-to-late 1970s, due to its ability to be effectively shielded from all angles. Its eventual replacement, the SNAP-50 reactor, offered megawatts of power using technology from the Aircraft Nuclear Propulsion program. Many other designs were proposed in this time period, including the SP-100 reactor, the ancestor of Kilopower (the SABRE heat pipe cooled reactor concept), as well as the first American in-core thermionic power system, advances in fuel element designs, and many other innovations.

Originally, these concepts were included in this blog post, but this post quickly expanded to the point that there simply wasn’t room for them. While some of the upcoming post has already been written, and a lot of the research has been done, this next post is going to be a long one as well. Because of this, I don’t know exactly when the post will end up being completed.

After we look at the reactor programs from the 1950s to the late 1980s, we’ll look at NASA and Rosatom’s collaboration on the TOPAZ-II reactor program, and the more recent history of astronuclear designs, from SDI through the Fission Surface Power program. We’ll finish up the series by looking at the most recent power systems from around the world, from JIMO to Kilopower to the new Russian on-orbit nuclear electric tug.

After this, we’ll look at shielding for astronuclear power plants, and possibly ground handling, launch safety, and launch abort considerations, then move on to power conversion systems, which will be a long series of posts due to the sheer number of options available.

These next posts are more research-intensive than usual, even for this blog, so while I’ll be hard at work on the next posts, it may be a bit more time than usual before these posts come out.




SNAP Reactor Overview, Voss 1984


Preliminary Results of the SNAP-2 Experimental Reactor, Hulin et al 1961

Application of the SNAP 2 to Manned Orbiting Stations, Rosenberg et al 1962

The ORNL-SNAP Shielding Program, Mynatt et al 1971



SNAP-10A Nuclear Analysis, Dayes et al 1965

SNAP 10 FS-3 Reactor Performance Hawley et al 1966


SNAPSHOT and the Flight Safety Program

SNAP-10A SNAPSHOTProgram Development, Atomics International 1962

Reliability Improvement Program Planning Report for the SNAP-10A Reactor, Coombs et al 1961

Aerospace Safety Reentry Analytical and Experimental Program SNAP 2 and 10A Interim Report, Elliot 1963

SNAPSHOT orbit, Heavens Above

SNAP Improvement Program

Static Control of SNAP Reactors, Birney et al 1966

SNAP Systems Capabilities Vol 2, Study Introduction, Reactors, Shielding, Atomics International 1965

Progress Report, SNAP Reactor Improvement Program, April-June 1965

Progress Report for SNAP General Supporting Technology May-July 1964


2 replies »

  1. This is a very impressive report, needless to say. Congratulations.

    BTW, we incorporated samarium as the burnable poison to partially accommodate fuel burn-up and hydrogen loss. Our prediction of power loss over the 43 days of operation was almost exact with the actual data. Is that graph available in any of the literature?

    Liked by 1 person

    • I believe it is, in the FSM-4 report IIRC. I didn’t include it in the links because I didn’t source any data out of it, but I believe I have it on my home computer.

      There was also a lot of work in the 2/10A RIP, as you know, but I don’t know if I’ve seen any burnup or irradiation tests out of that program. Were there any in core tests done on the advanced fuel elements?


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s