Development and Testing

SNAP-50: The Last of the SNAP Reactors

Hello, and welcome to Beyond NERVA, for our first blog post of the year! Today, we reach the end of the reactor portion of the SNAP program. A combination of the holidays and personal circumstances prevented me from finishing this post as early as I would have liked to, but it’s finally here! Check the end of the blog post for information on an upcoming blog format change. [Author’s note: somehow the references section didn’t attach to the original post, that issue is now corrected, and I apologize, references are everything in as technical a field as this.]

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. A fifth reactor, SNAP-4, also underwent some testing, but was meant for undersea applications for the Navy. 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.

The SNAP-50 would permanently alter the way that astronuclear reactors were designed, and would change the course of in-space reactor development for over 20 years. By the time of its cancellation in 1973, it had approached flight readiness to the point that funding and time allowed, but changes in launch vehicle configuration rang the death knell of the SNAP-50.

The Birth of the SNAP-50

Mockup of SNAP-50, image DOE

Up until now, the SNAP program had focused on a particular subset of nuclear reactor designs. They were all fueled with uranium-zirconium hydride fuel (within a small range of uranium content, all HEU), cooled with NaK-78, and fed either mercury Rankine generators or thermoelectric power conversion systems. This had a lot of advantages for the program: fuel element development improvements for one reactor could be implemented in all of them, challenges in one reactor system that weren’t present in another allowed for distinct data points to figure out what was going on, and the engineers and reactor developers were able to look at each others’ work for ideas on how to improve reliability, efficiency, and other design questions.

Tory IIA reactor inlet end, image DOE

However, there was another program that was going on at about the same time which had a very different purpose, but similar enough design constraints that it could be very useful for an in-space fission power plant: the Aircraft Nuclear Propulsion program (ANP), which was primarily run out of Oak Ridge National Laboratory. Perhaps the most famous part of the ANP program was the series of direct cycle ramjets for Project PLUTO: the TORY series. These ramjets were nuclear fission engines using the atmosphere itself as the working fluid. There were significant challenges to this approach, because the clad for the fuel elements must not fail, or else the fission products from the fuel elements would be released as what would be virtually identical to nuclear fallout, only different due to the method that it was generated. The fuel elements themselves would be heavily eroded by the hot air moving through the reactor (which turned out to be a much smaller problem than was initially anticipated). The advantage to this system, though, is that it was simple, and could be made to be relatively lightweight.

Another option was what was known as the semi-indirect cycle, where the reactor would heat a working fluid in a closed loop, which would then heat the air through a heat exchanger built into the engine pod. While this was marginally safer from a fission product release point of view, there were a number of issues with the design. The reactor would have to run at a higher temperature than the direct cycle, because there are always losses whenever you transfer heat from one working fluid to another, and the increased mass of the system also required greater thrust to maintain the desired flight characteristics. The primary coolant loop would become irradiated when going through the reactor, leading to potential irradiation of the air as it passed through the heat exchanger. Another concern was that the heat exchanger could fail, leading to the working fluid (usually a liquid metal) being exposed at high temperature to the superheated air, where it could easily explode. Finally, if a clad failure occurred in the fuel elements, fission products could migrate into the working fluid, making the primary loop even more radioactive, increasing the irradiation of the air as it passed through the engine – and releasing fission products into the atmosphere if the heat exchanger failed.

The alternative to these approaches was an indirect cycle, where the reactor heated a working fluid in a closed loop, transferred this to another working fluid, which then heated the air. The main difference between these systems is that, rather than having the possibly radioactive primary coolant come in close proximity with the air and therefore transferring ionizing radiation, there is an additional coolant loop to minimize this concern, at the cost of both mass and thermal efficiency. This setup allowed for far greater assurances that the air passing through the engine would not be irradiated, because the irradiation of the secondary coolant loop would be so low as to be functionally nonexistent. However, if the semi-indirect cycle was more massive, this indirect cycle would be the heaviest of all of the designs, meaning far higher power outputs and temperatures were needed in order to get the necessary thrust-to-weight ratios for the aircraft. Nevertheless, from the point of view of the people responsible for the ANP program, this was the most attractive design for a crewed aircraft.

Both SNAP and ANP needed many of the same things out of a nuclear reactor: it had to be compact, it had to be lightweight, it had to have a VERY high power density and it needed to be able to operate virtually maintenance-free in a variety of high-power conditions. These requirements are in stark contrast to terrestrial, stationary nuclear reactors which can afford heavy weight, voluminous construction and can thus benefit of low power density. As a general rule of thumb, an increase in power density, will also intensify the engineering, materials, and maintenance challenges. The fact that the ANP program needed high outlet temperatures to run a jet engine also bore the potential of having a large thermal gradient across a power conversion system – meaning that high-conversion-efficiency electrical generation was possible. That led SNAP program leaders to see about adapting an aircraft system into a spacecraft system.

Image DOE

The selected design was under development at the Connecticut Advanced Nuclear Engine Laboratory (CANEL) in Middletown, Connecticut. The prime contractor was Pratt and Whitney. Originally part of the indirect-cycle program, the challenges of heat exchanger design, adequate thrust, and a host of other problems continually set back the indirect cycle program, and when the ANP program was canceled in 1961, Pratt and Whitney no longer had a customer for their reactor, despite doing extensive testing and even fabricating novel alloys to deal with certain challenges that their reactor design presented. This led them to look for another customer for the reactor, and they discovered that both NASA and the US Air Force were both interested in high-power-density, high temperature reactors for in-space use. Both were interested in this high powered reactor, and the SNAP-50 was born.

PWAR-20 cross-section and elevation, image DOE

This reactor was an evolution of a series of test reactors, the PWAR series of test reactors. Three reactors (the PWAR-2, -4, and -8, for 2, 4, and 8 MW of thermal power per reactor core) had already been run for initial design of an aircraft reactor, focused on testing not only the critical geometry of the reactor, but the materials needed to contain its unique (at the time) coolant: liquid lithium. This is because lithium has an excellent specific heat capacity, or the amount of energy that can be contained as heat per unit mass at a given temperature: 3.558 J/kg-C, compared to the 1.124 J/kg-C of NaK78, the coolant of the other SNAP reactors. This means that less coolant would be needed to transport the energy away from the reactor and into the engine in the ANP program, and for SNAP this meant that less working fluid mass would be needed transferring from the reactor to the power conversion system. The facts that Li is much less massive than NaK, and that less of it would be needed, makes lithium a highly coveted option for an astronuclear reactor design. However, this design decision also led to needing novel concepts for how to contain liquid lithium. Even compared to NaK, lithium is highly toxic, highly corrosive in most materials and led, during the ANP program, to Pratt and Whitney investigating novel elemental compositions for their containment structures. We’ll look at just what they did later.

SNAP-50: Designing the Reactor Core

This reactor ended up using a form of fuel element that we have yet to look at in this blog: uranium nitride, UN. While both UC (you can read more about carbide fuels here) and UN were considered at the beginning of the program, the reactor designers ended up settling on UN because of a unique capacity that this fuel form offers: it has the highest fissile fuel density of any type of fuel element. This is offset by the fact that UN isn’t the most heat tolerant of fuel elements, requiring a lower core operating temperature. Other options were considered as well, including CERMET fuels using oxides, carbides, and nitrides suspended in a tungsten metal matrix to increase thermal conductivity and reduce the temperature of the fissile fuel itself. The decision between UN, with its higher mass efficiency (due to its higher fissile density), and uranium carbide (UC), with the highest operating temperature of any solid fuel element, was a difficult decision, and a lot of fuel element testing occurred at CANEL before a decision was reached. After a lot of study, it was determined that UN in a tungsten CERMET fuel was the best balance of high fissile fuel density, high thermal conductivity, and the ability to manage low fuel burnup over the course of the reactor’s life.

From SNAP-50/SPUR Design Summary

Perhaps the most important design consideration for the fuel elements after the type of fuel was how dense the fuel would be, and how to increase the density if this was desired in the final design. While higher density fuel is generally speaking a better idea when it comes to specific power, it was discovered that the higher density the fuel was, the lower the amount of burnup would be possible before the fuel would fail due to fission product gas buildup within the fuel itself. Initial calculations showed that there was an effectively unlimited fuel burnup potential of UN at 80% of its theoretical density since a lot of the gasses could diffuse out of the fuel element. However, once the fuel reached 95% density, this was limited to 1% fuel burnup. Additional work was done to determine that this low burnup was in fact not a project killer for a 10,000 hour reactor lifetime, as was specified by NASA, and the program moved ahead.

These fuel pellets needed a cladding material, as most fuel does, and this led to some additional unique materials challenges. With the decision to use lithium coolant, and the need for both elasticity and strength in the fuel element cladding (to deal with both structural loads and fuel swelling), it was necessary to do extensive experimentation on the metal that would be used for the clad. Eventually, a columbium-zirconium alloy with a small amount of carbon (CB-1ZR-0.6C) was decided on as a barrier between the Cb-Zr alloy of the clad (which resisted the high-temperature lithium erosion on the pressure vessel side of the clad) and the UN-W CERMET fuel (which would react strongly without the carburized layer).

This decisions led to an interesting reactor design, but not necessarily one that is unique from a non-materials point of view. The fuel would be formed into high-density pellets, which would then be loaded into a clad, with a spring to keep the fuel to the bottom (spacecraft end) of the reactor. The gap between the top of the fuel elements and the top of the clad was for the release of fission product gasses produced during operation of the reactor. These rods would be loaded in a hexagonal prism pattern into a larger collection of fuel elements, called a can. Seven of these cans, placed side by side (one regular hexagon, surrounded by six slightly truncated hexagons), would form the fueled portion of the reactor core. Shims of beryllium would shape the core into a cylinder, which was surrounded by a pressure vessel and lateral reflectors. Six poison-backed control drums mounted within the reflector would rotate to provide reactor control. Should the reactor need to be scrammed, a spring mechanism would return all the drums to a position with the neutron poison facing the reactor, stopping fission from occurring.

SNAP-50 flow diagram, image DOE

The lithium, after being heated to a temperature of 2000°F (1093°C), would feed into a potassium boiler, before being returned to the core at an inlet temperature of 1900 F (1037°C). From the boiler, the potassium vapor, which is 1850°F (1010°C), would enter a Rankine turbine which would produce electricity. The potassium vapor would cool down to 1118°F (603°C) in the process and return – condensed to its liquid form – to the boiler, thus closing the circulation. Several secondary coolant loops were used in this reactor: the main one was for the neutron reflectors, shield actuators, control drums, and other radiation hardened equipment, and used NaK as a coolant; this coolant was also used as a lubricant for the condensate pump in the potassium system. Another, lower temperature organic coolant was used for other systems that weren’t in as high a radiation flux. The radiators that were used to reject heat also used NaK as a working fluid, and were split into a primary and secondary radiator array. The primary array pulled heat from the condenser, and reduced it from 1246°F (674°C) to 1096°F (591°C), while the secondary array took the lower-temperature coolant from 730°F (388°C) to 490°F (254°C). This design was designed to operate in both single and dual loop situations, with the second (identical) loop used for high powered operation and to increase redundancy in the power plant.

These design decisions led to a flexible reactor core size, and the ability to adapt to changing requirements from either NASA or the USAF, both of which were continuing to show interest in the SNAP-50 for powering the new, larger space stations that were becoming a major focus of both organizations.

The Power Plant: Getting the Juice Flowing

By 1973, the SNAP 2/10A program had ended, and the SNAP-8/ZrHR program was winding down. These systems simply didn’t provide enough power for the new, larger space station designs that were being envisaged by NASA, and the smaller reactor sizes (the 10B advanced designs that we looked at a couple blog posts back, and the 5 kWe Thermoelectric Reactor) didn’t provide capabilities that were needed at the time. This left the SNAP-50 as the sole reactor design that was practical to take on a range of mission types… but there was a need to have different reactor power outputs, so the program ended up developing two reactor sizes. The first was a 35 kWe reactor design, meant for smaller space stations and lunar bases, although this particular part of the 35 kWe design seems to have never been fully fleshed out. A larger, 300 kWe type was designed for NASA’s proposed modular space station, a project which would eventually evolve into the actual ISS.

Unlike in the SNAP-2 and SNAP-8 programs, the SNAP-50 kept its Rankine turbine design, which had potassium vapor as its working fluid. This meant that the power plant was able to meet its electrical power output requirements far more easily than the lower efficiency demanded by thermoelectric conversion systems. The CRU system meant for the SNAP-2 ended up reaching its design requirements for reliability and life by this time, but sadly the overall program had been canceled, so there was no reactor to pair to this ingenious design (sadly, it’s so highly toxic that testing would be nearly impossible on Earth). The boiler, pumps, and radiators for the secondary loop were tested past the 10,000 hour design lifetime of the power plant, and all major complications discovered during the testing process were addressed, proving that the power conversion system was ready for the next stage of testing in a flight configuration.

One concern that was studied in depth was the secondary coolant loop’s tendency to become irradiated in the neutron flux coming off the reactor. Potassium has a propensity for absorbing neutrons, and in particular 41K (6% of unrefined K) can capture a neutron and become 42K. This is a problem, because 42K goes through gamma decay, so anywhere that the secondary coolant goes needs to have gamma radiation shielding to prevent the radiation from reaching the crew. This limited where the power conversion system could be mounted, to keep it inside the gamma shielding of the temporary, reactor-mounted shield, however the compact nature of both the reactor core and the power conversion system meant that this was a reasonably small concern, but one worthy of in-depth examination by the design team.

The power conversion system and auxiliary equipment, including the actuators for the control drums, power conditioning equipment, and other necessary equipment was cooled by a third coolant loop, which used an organic coolant (basically the oil needed for the moving parts to be lubricated), which ran through its own set of pumps and radiators. This tertiary loop was kept isolated from the vast majority of the radiation flux coming off the reactor, and as such wasn’t a major concern for irradiation damage of the coolant/lubricant.

Some Will Stay, Some Will Go: Mounting SNAP-50 To A Space Station

SNAP50 mounted to early NASA modular space station concept, image DOE

Each design used a 4-pi (a fully enclosing) shield with a secondary shadow shield pointing to the space station in order to reduce radiation exposure for crews of spacecraft rendezvousing or undocking from the space station. This primary shield was made out of a layer of beryllium to reflect neutrons back into the core, and boron carbide (B4C, enriched in boron-10) to absorb the neutrons that weren’t reflected back into the core. These structures needed to be cooled to ensure that the shield wouldn’t degrade, so a NaK shield coolant system (using technology adapted from the SNAP-8 program) was used to keep the shield at an acceptable temperature.

The shadow shield was built in two parts: the entire structure would be launched at the same time for the initial reactor installation for the space station, and then when the reactor needed to be replaced only a portion of the shield would be jettisoned with the reactor. The remainder, as well as the radiators for the reactor’s various coolant systems, would be kept mounted to the space station in order to reduce the amount of mass that needed to be launched for the station resupply. The shadow shield was made out of layers of tungsten and LiH, for gamma and neutron shielding respectively.

Image DOE

When it came time to replace the core of the reactor at the end of its 10,000 hour design life (which was a serious constraint on the UN fuels that they were working with due to fuel burnup issues), everything from the separation plane back would be jettisoned. This could theoretically have been dragged to a graveyard orbit by an automated mission, but the more likely scenario at the time would have been to leave it in a slowly degrading orbit to give the majority of the short-lived isotopes time to decay, and then design it to burn up in the atmosphere at a high enough altitude that diffusion would dilute the impact of any radioisotopes from the reactor. This was, of course, before the problems that the USSR ran into with their US-A program [insert link], which eliminated this lower cost decommissioning option.

Image DOE

After the old reactor core was discarded, the new core, together with the small forward shield and power conversion system, could be put in place using a combination of off-the-shelf hardware, which at the time was expected to be common enough: either Titan-III or Saturn 1B rockets, with appropriate upper stages to handle the docking procedure with the space station. The reactor would then be attached to the radiator, the docking would be completed, and within 8 hours the reactor would reach steady-state operations for another 10,000 hours of normal use. The longest that the station would be running on backup power would be four days. Unfortunately, information on the exact docking mechanism used is thin, so the details on how they planned this stage are still somewhat hazy, but there’s nothing preventing this from being done.

A number of secondary systems, including accumulators, pumps, and other equipment are mounted along with the radiator in the permanent section of the power supply installation. Many other systems, especially anything that has been exposed to a large radiation flux or high temperatures during operation (LiH, the primary shielding material, loses hydrogen through outgassing at a known rate depending on temperature, and can almost be said to have a half-life), will be separated with the core, but everything that was practicable to leave in place was kept.

This basic design principle for reloadable (which in astronuclear often just means “replaceable core”) reactors will be revisited time and again for orbital installations. Variations on the concept abound, although surface power units seem to favor “abandon in place” far more. In the case of large future installations, it’s not unreasonable to suspect that refueling of a reactor core would be possible, but at this point in astronuclear mission utilization, even having this level of reusability was an impressive feat.

35 kWe SNAP-50: The Starter Model

In the 1960s, having 35 kWe of power for a space station was considered significant enough to supply the vast majority of mission needs. Because of this, a smaller version of the SNAP-50 was designed to fit this mission design niche. While the initial power plant would require the use of a Saturn 1B to launch it into orbit, the replacement reactors could be launched on either an Atlas-Centaur or Titan IIIA-Centaur launch vehicle. This was billed as a low cost option, as a proof of concept for the far larger – and at this point, far less fully tested – 300 kWe version to come.

NASA was still thinking of very large space stations at this time. The baseline crew requirements alone were incredible: 24-36 crew, with rotations lasting from 3 months to a year, and a station life of five years. While 35 kWe wouldn’t be sufficient for the full station, it would be an attractive option. Other programs had looked at nuclear power plants for space stations as well, like we saw with the Manned Orbiting Laboratory and the Orbital Workshop (later Skylab), and facilities of that size would be good candidates for the 35 kWe system.

The core itself measured 8.3 inches (0.211 m) across, 11.2 inches (0.284 m) long, and used 236 fuel elements arranged into seven fuel element cans within the pressure vessel of the core. Six poison-backed control drums were used for primary reactor control. The core would produce up to 400 kW of thermal power. The pressure vessel, control drums, and all other control and reflective materials together measured just 19.6 inches (4.98 m) by 27.9 inches (7.09 m), and the replaceable portion of the reactor was between four and five feet (1.2 m and 1.5 m) tall, and five and six feet (1.5 m and 1.8 m) across – including shielding.

SNAP-50 powered probe concept, image DOE

This reactor could also have been a good prototype reactor for a nuclear electric probe, a concept that will be revisited later, although there’s little evidence that this path was ever seriously explored. Like many smaller reactor designs, this one did not get the amount of attention that its larger brother offered, but at the time this was considered a good, solid space station power supply.

300 kWe SNAP-50: The Most Powerful Space Reactor to Date

While there were sketches for more powerful reactors than the 300 kWe SNAP-50 variant, they never really developed the reactors to any great extent, and certainly not to the point of experimental verification that SNAP-50 had achieved. This was considered to be a good starting point for possibly a crewed nuclear electric spacecraft, as well as being able to power a truly huge space station.

The 300 kWe variant of the reactor was slightly different in more than size when compared to its smaller brother. Despite using the same fuel, clad, and coolant as the 35 kWe system, the 300 kWe system could achieve over four times the fuel burnup of the smaller reactor (0.32% vs 1.3%), and had a higher maximum fuel power density as well, both of which have a huge impact on core lifetimes and dynamics. This was partially achieved by making the fuel elements almost half as narrow, and increasing the number of fuel elements to 1093, held in 19 cans within the core. This led to a core that was 10.2 inches (0.259 m) wide, and 14.28 inches (0.363 m) long (keeping the same 1:1.4 gore geometry between the reactors), and a pressure vessel that was 12” (0.305 m) in diameter by 43” (1.092 m) in length. It also increased the thermal output of the reactor to 2200 kWt. The number of control drums was increased from six to eight longer control drums to fit the longer core, and some rearrangement of lithium pumps and other equipment for the power conversion system occurred within the larger 4 pi shield structure. The entire reactor assembly that would undergo replacement was five to six feet high, and six to seven feet in diameter (1.5 m; 1.8 m; 2.1 m).

Lander-based SNAP-50 concept, image DOE

Sadly, even the ambitious NASA space station wasn’t big enough to need even the smaller 35 kWe version of the reactor, much less the 300 kWe variants. Plans had been made for a fleet of nuclear electric tugs that would ferry equipment back and forth to a permanent Moon base, but cancellation of that program occurred at the same time as the death of the moon base itself.

Mass Tradeoffs: Why Nuclear Instead of Solar?

By the middle of the 1960s, photovoltaic solar panels had become efficient and reliable enough for use in spacecraft on a regular basis. Because of this, it was a genuine question for the first time ever whether to go with solar panels or a nuclear reactor, whereas in the 1950s and early 60s nuclear was pretty much the only option. However, solar panels have a downside: drag. Even in orbit, there is a very thin atmosphere, and so for lower orbits a satellite has to regularly raise itself up or it will burn up in the atmosphere. Another down side comes from MM/OD: micro meteorites and orbital debris. Since solar panels are large, flat, and all pointing at the sun all the time, there’s a greater chance that something will strike one of those panels, damaging or possibly even destroying it. Managing these two issues is the primary concern of using solar panels as a power supply in terms of orbital behavior, and determines the majority of the refueling mass needed for a solar powered space station.

Image DOE, from SNAP-50 Design Summary

On the nuclear side, by 1965, there were two power plant options on the table: the SNAP-8 (pre-ZrHR redesign) and the SNAP-50, and solar photovoltaics had developed to the point that they could be deployed in space. Because of this, a comparison was done by Pratt and Whitney of the three systems to determine the mass efficiency of each system, not only in initial deployment but also in yearly fueling and tankage requirements. Each of the systems was compared at a 35 kWe power level to the space station in order to allow for a level playing field.

One thing that stands out about the solar system (based on a pair of Lockheed and General Electric studies) is that it’s marginally the lightest of all the systems at launch, but within a year the total system maintenance mass required far outstrips the mass of the nuclear power plants, especially the SNAP-50. This is because the solar panels have a large sail area, which catches the very thin atmosphere at the station’s orbital altitude and drags the station down into the thicker atmosphere, so thrust is needed to re-boost the space station. This is something that has to be done on a regular basis for the ISS. The mass of the fuel, tankage, and structure to allow for this reboost is extensive. Even back in 1965 there were discussions on using electric propulsion for the reboosting of the space station, in order to significantly reduce the mass needed for this procedure. That discussion is still happening casually with the ISS, and Ad Astra still hopes to use VASIMR for this purpose – a concept that’s been floated for the last ten or so years.

Overall, the mass difference between the SNAP-50 and the optimistic Lockheed proposal of the time was significant: the original deployment was only about 70 lbs (31.75 kg) different, but the yearly maintenance mass requirements would be 5,280 lbs (2395 kg) different – quite a large amount of mass.

Because the SNAP-50 and SNAP-8 don’t have these large sail areas, and the radiators needed can be made aerodynamically enough to greatly reduce the drag on the station, the reboost requirements are significantly lower than for the solar panels. The SNAP-50 weighs significantly less than the SNAP-8, and has significantly less surface area, because the reactor operates at a far higher temperature, and therefore needs a smaller radiator. Another difference between the reactors is volume: the SNAP-50 is physically smaller than the SNAP-8 because of that same higher temperature, and also due to the fact that the UN fuel is far more dense than its U-ZrH fueled counterpart.

These reactors were designed to be replaced once a year, with the initial launch being significantly more massive than the follow-up launches, benefitting of the sectioned architecture with a separation plane just at the small of the shadow shield as described above. Only the smaller section of shield remained with the reactor when it was separated. The larger, heavier section, on the other hand, would remain with the space station, as well as the radiators, and serve as the mounting point for the new reactor core and power conversion system, which would be sent via an automated refueling launch to the space station.

Solar panels, on the other hand, require both reboost to compensate for drag as well as equipment to repair or replace the panels, batteries, and associated components as they wear out. This in turn requires a somewhat robust repair capability for ongoing maintenance – a requirement for any large, long term space station, but the more area you have to get hit by space debris, which means more time and mass spent on repairs rather than doing science.

Of course, today solar panels are far lighter, and electric thrusters are also far more mature than they were at that time. This, in addition to widespread radiophobia, make solar the most widespread occurrence in most satellites, and all space stations, to date. However, the savings available in overall lifetime mass and a sail area that is both smaller and more physically robust, remain key advantages for a nuclear powered space station in the future

The End of an Era: Changing Priorities, Changing Funding

The SNAP-50, even the small 35 kWe version, offered more power, more efficiency, and less mass and volume than the most advanced of SNAP-8’s children: the A-ZrHR [Link]. This was the end of the zirconium hydride fueled reactor era for the Atomic Energy Commission, and while this type of fuel continues to be used in reactors all over the world in TRIGA research and training reactors (a common type of small reactor for colleges and research organizations), its time as the preferred fuel for astronuclear designs was over.

In fact, by the end of the study period, the SNAP-50 was extended to 1.5 MWe in some designs, the most powerful design to be proposed until the 1980s, and one of the most powerful ever proposed… but this ended up going nowhere, as did much of the mission planning surrounding the SNAP program.

At the same time as these higher-powered reactor designs were coming to maturity, funding for both civilian and military space programs virtually disappeared. National priorities, and perceptions of nuclear power, were shifting. Technological advances eliminated many future military crewed missions in favor of uncrewed ones with longer lifetimes, less mass, less cost – and far smaller power requirements. NASA funding began falling under the axe even as we were landing on the Moon for the first time, and from then on funding became very scarce on the ground.

The transition from the Atomic Energy Commission to the Department of Energy wasn’t without its hiccups, or reductions in funding, either, and where once every single AEC lab seemed to have its own family of reactor designs, the field narrowed greatly. As we’ll see, even at the start of Star Wars the reactor design was not too different from the SNAP-50.

Finally, the changes in launch system had their impact as well. NASA was heavily investing in the Space Transport System (the Space Shuttle), which was assumed to be the way that most or all payloads would be launched, so the nuclear reactor had to be able to be flown up – and in some cases returned – by the Shuttle. This placed a whole different set of constraints on the reactor, requiring a large rewrite of the basic design. The follow-on design, the SP-100, used the same UN fuel and Li coolant as the SNAP-50, but was designed to be launched and retrieved by the Shuttle. The fact that the STS never lived up to its promise in launch frequency or cost (and that other launchers were available continuously) means that this was ultimately a diversion, but at the time it was a serious consideration.

All of this spelled the death of the SNAP-50 program, as well as the end of dedicated research into a single reactor design until 1983, with the SP-100 nuclear reactor system, a reactor we’ll look at another time.

While I would love to go into many of the reactors that were developed up to this time, including heat pipe cooled reactors (SABRE at Los Alamos), thermionic power conversion systems (5 kWe Thermionic Reactor), and other ideas, there simply isn’t time to go into them here. As we look at different reactor components they’ll come up, and we’ll mention them there. Sadly, while some labs were able to continue funding some limited research with the help of NASA and sometimes the Department of Defense or the Defense Nuclear Safety Agency. The days of big astronuclear programs, though, were fading into a thing of the past. Both space and nuclear power would refocus, and then fade in the rankings of budgetary requirements over the years. We will be looking at these reactors more as time goes on, in our new “Forgotten Reactors” column (more on that below).

The Blog is Changing!

With the new year, I’ve been thinking a lot about the format of both the website and the blog, and where I hope to go in the next year. I’ve had several organizational projects on the back burner, and some of them are going to be started here soon. The biggest part is going to be the relationship between the blog and the website, and what I write more about where.

Expect another blog post shortly (it’s already written, just not edited yet) about our plans for the next year!

I’ve got big plans for Beyond NERVA this year, and there are a LOT of things that are slowly getting started in the background which will greatly improve the quality of the blog and the website, and this is just the start!


SNAP-50/SPUR Program Summary, Pratt and Whitney staff, 1964

35 and 300 kWe SNAP-50/SPUR Power Plants for the Manned Orbiting Space Station Application, Pratt and Whitney staff, 1965

Uranium Nitride Fuel Development SNAP-50, Pratt and Whitney staff, 1965

SNAP Program Summary Report, Voss 1984

2 replies »

  1. This is really great work! Thanks!
    Sadly, when I add up all the numbers, it looks like even the SNAP 50 in its 300KWe configuration only manages about 45 W/kg, even if we remove shielding and structure to leave the bare minimum components.

    This does not compare favourably to modern solar panels with 100 to 1000 W/kg or more…

    Perhaps a modern version of a nuclear-electric reactor could become competitive again!


    • Certainly, there are advantages and disadvantages to this type of system. The biggest limitation that this concept ran into is that the fuel was significantly thermally limited… the only advantage it really had was a high theoretical fissile fuel density, but in order to achieve that they sacrificed both upper temp and fissile burnup.

      The discussion of the drag tradeoff with solar panels, as well as the increased surface area being more susceptible to MM/OD, was an interesting one that I only briefly summarized, but is well worth both a read (in the Summary Report linked at the bottom of the post) and an update with modern systems.

      A final point I’ll throw out there: K Rankine systems (or any metal vapor Rankine) have their own limitations, which can in mot cases be more severe than either a Brayton or Stirling conversion system… but high-efficiency, low-mass Braytons are still sci-fi. Sandia NL spent over a decade on a supercritical CO2 Brayton system that I’ll be covering in the future, but the corrosion issues were (are) still significant enough when mass ISN’T a concern that I’m not holding my breath. IIRC it’s what killed Prometheus as well.


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