The Systems Nuclear Auxiliary POWER (SNAP) program was a program of experimental

radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), or radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the Decay heat, heat released by the decay of a suitable radioactive material i ...

s (RTGs) and space nuclear reactors flown during the 1960s by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

. The SNAP program developed as a result of Project Feedback, a Rand Corporation study of reconnaissance satellites completed in 1954. As some of the proposed satellites had high power demands, some as high as a few kilowatts, the U.S. Atomic Energy Commission (AEC) requested a series of nuclear power-plant studies from industry in 1951. Completed in 1952, these studies determined that nuclear power plants were technically feasible for use on satellites. In 1955, the AEC began two parallel SNAP nuclear power projects. One, contracted with The Martin Company, used radio-isotopic decay as the power source for its generators. These plants were given odd-numbered SNAP designations beginning with SNAP-1. The other project used nuclear reactors to generate energy, and was developed by the Atomics International Division of

North American Aviation North American Aviation (NAA) was a major American aerospace manufacturer that designed and built several notable aircraft and spacecraft. Its products included the T-6 Texan trainer, the P-51 Mustang fighter, the B-25 Mitchell bomber, the F- ...

. Their systems were given even-numbered SNAP designations, the first being SNAP-2. Most of the systems development and reactor testing was conducted at the

Santa Susana Field Laboratory The Santa Susana Field Laboratory (SSFL), formerly known as Rocketdyne, is a complex of industrial research and development facilities located on a portion of Southern California in an unincorporated area of Ventura County in the Simi Hills betw ...

, Ventura County, California using a number of specialized facilities.

Odd-numbered SNAPs: radioisotope thermoelectric generators

Radioisotope thermoelectric generators A radioisotope thermoelectric generator (RTG, RITEG), or radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioactive material into electr ...

use the heat of

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

to produce electricity.

SNAP-1

SNAP-1 was a test platform that was never deployed, using cerium-144 in a

Rankine cycle The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sour ...

with mercury as the heat transfer fluid. Operated successfully for 2500 hours.

SNAP-3

SNAP-3 was the first RTG used in a space mission (1961). Launched aboard U.S. Navy Transit 4A and 4B navigation satellites. The electrical output of this RTG was 2.5 watts.

SNAP-7

SNAP-7A, D and F was designed for marine applications such as lighthouses and buoys; at least six units were deployed in the mid-1960s, with names SNAP-7A through SNAP-7F. SNAP-7D produced thirty watts of electricity using (about four kilograms) of

strontium-90 Strontium-90 () is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.79 years. It undergoes β− decay into yttrium-90, with a decay energy of 0.546 MeV. Strontium-90 has applications in medicine a ...

as SrTiO₃. These were very large units, weighing between .

SNAP-9

After SNAP-3 on Transit 4A/B, SNAP-9A units served aboard many of the Transit satellite series. In April 1964 a SNAP-9A failed to achieve orbit and disintegrated, dispersing roughly of

plutonium-238 Plutonium-238 ( or Pu-238) is a radioactive isotope of plutonium that has a half-life of 87.7 years. Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitable for usage ...

over all continents. Most plutonium fell in the southern hemisphere. Estimated 630 TBq of radiation was released.

SNAP-11

SNAP-11 was an experimental RTG intended to power the

Surveyor Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. These points are usually on the ...

probes during the lunar night. The curium-242 RTGs would have produced 25 watts of electricity using 900 watts of thermal energy for 130 days. The hot junction temperature was , the cold junction temperature was . They had a liquid NaK thermal control system and a movable shutter to dump excess heat.SNAP-11 Surveyor Program, Third Quarterly Report
/ref>SNAP-11 Surveyor Program, Thirteenth Quarterly Report
/ref> They were not used on the Surveyor missions.

In general, the SNAP 11 fuel block is a cylindrical multi-material unit which occupies the internal volume of the generator. TZM (molybdenum alloy) fuel capsule, fueled with curium-242 (Cm₂O₃ in an iridium matrix) is located in the center of the fuel block. Capsule is surrounded by a platinum sphere, approximately inches in diameter, which provides shielding and acts as an energy absorber for impact considerations. This assembly is enclosed in graphite and beryllium sub-assemblies to provide the proper thermal distribution and ablative protection.

SNAP-19

SNAP-19(B) was developed for the Nimbus-B satellite by the Nuclear Division of the Martin-Marietta Company (now Teledyne Energy Systems). Fueled with plutonium-238, two parallel lead telluride thermocouple generators produced an initial maximum of approximately 30 watts of electricity. Nimbus 3 used a SNAP-19B with the recovered fuel from the Nimbus-B1 attempt. SNAP-19's powered the ''

Pioneer 10 ''Pioneer 10'' (originally designated Pioneer F) is a NASA space probe launched in 1972 that completed the first mission to the planet Jupiter. ''Pioneer 10'' became the first of five artificial objects to achieve the escape velocity needed ...

'' and ''

Pioneer 11 ''Pioneer 11'' (also known as ''Pioneer G'') is a NASA robotic space probe launched on April 5, 1973, to study the asteroid belt, the environment around Jupiter and Saturn, the solar wind, and cosmic rays. It was the first probe to Exploration ...

'' missions. They used n-type 2N-PbTe and p-type TAGS-85 thermoelectric elements. Modified SNAP-19B's were used for the '' Viking 1'' and '' Viking 2'' landers. A SNAP-19C was used to power a telemetry array at Nanda Devi in

Uttarakhand Uttarakhand (, ), also known as Uttaranchal ( ; List of renamed places in India, the official name until 2007), is a States and union territories of India, state in North India, northern India. The state is bordered by Himachal Pradesh to the n ...

for a CIA operation to track Chinese missile launches.

SNAP-21 & 23

SNAP-21 and SNAP-23 were designed for underwater use and used

as the radioactive source, encapsulated as either strontium oxide or strontium titanate. They produced about ten watts of electricity.

SNAP-27

Five SNAP-27 units provided electric power for the Apollo Lunar Surface Experiments Packages (ALSEP) left on the Moon by

Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

12, 14, 15, 16, and 17. The SNAP-27 power supply weighed about 20 kilograms, was 46 cm long and 40.6 cm in diameter. It consisted of a central fuel capsule surrounded by concentric rings of thermocouples. Outside of the thermocouples was a set of fins to provide for heat rejection from the cold side of the thermocouple. Each of the SNAP devices produced approximately 75 W of electrical power at 30 VDC. The energy source for each device was a rod of plutonium-238 providing a thermal power of approximately 1250 W. This fuel capsule, containing of

in oxide form (44,500 Ci or 1.65 PBq), was carried to the Moon in a separate fuel cask attached to the side of the

Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed s ...

. The fuel cask provided thermal insulation and added structural support to the fuel capsule. On the Moon, the Lunar Module pilot removed the fuel capsule from the cask and inserted it in the RTG. These stations transmitted information about moonquakes and meteor impacts, lunar magnetic and gravitational fields, the Moon's internal temperature, and the Moon's atmosphere for several years after the missions. After ten years, a SNAP-27 still produced more than 90% of its initial output of 75 watts. The fuel cask from the SNAP-27 unit carried by the

Apollo 13 Apollo 13 (April 1117, 1970) was the seventh crewed mission in the Apollo program, Apollo space program and would have been the third Moon landing. The craft was launched from Kennedy Space Center on April 11, 1970, but the landing was abort ...

mission currently lies in of water at the bottom of the

Tonga Trench The Tonga Trench is an oceanic trench located in the southwestern Pacific Ocean. It is the deepest trench in the Southern hemisphere and the second deepest on Earth after the Mariana Trench. The fastest plate-tectonic velocity on Earth is occurri ...

in the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

. This mission failed to land on the Moon, and the

lunar module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed s ...

carrying its generator burnt up during re-entry into the Earth's atmosphere, with the trajectory arranged so that the cask would land in the trench. The cask survived re-entry, as it was designed to do, and no release of plutonium has been detected. The corrosion resistant materials of the capsule are expected to contain it for 10 half-lives (870 years).

Even-numbered SNAPs: compact nuclear reactors

A series of compact nuclear reactors intended for space use, the even numbered SNAPs were developed for the U.S. government by the Atomics International division of

SNAP Experimental Reactor (SER)

The SNAP Experimental Reactor (SER) was the first reactor to be built by the specifications established for space satellite applications. The SER used uranium zirconium hydride as the fuel and eutectic sodium-potassium alloy ( NaK) as the coolant and operated at approximately 50 kW thermal. The system did not have a power conversion but used a secondary heat air blast system to dissipate the heat to the atmosphere. The SER used a similar reactor reflector moderator device as the SNAP-10A but with only one reflector. Criticality was achieved in September 1959 with final shutdown completed in December 1961. The project was considered a success. It gave continued confidence in the development of the SNAP Program and it also led to in depth research and component development.

SNAP-2

The SNAP-2 Developmental Reactor was the second SNAP reactor built. This device used Uranium-zirconium hydride fuel and had a design reactor power of 55 kW_t. It was the first model to use a flight control assembly and was tested from April 1961 to December 1962. The basic concept was that nuclear power would be a long term source of energy for crewed space capsules. However, the crew capsule had to be shielded from deadly radiation streaming from the nuclear reactor. Surrounding the reactor with a radiation shield was out of the question. It would be far too heavy to launch with the rockets available at that time. To protect the "crew" and "payload", the SNAP-2 system used a "shadow shield". The shield was a truncated cone containing

lithium hydride Lithium hydride is an inorganic compound with the formula Li H. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not solub ...

. The reactor was at the small end and the crew capsule/payload was in the shadow of the large end. Studies were performed on the reactor, individual components and the support system. Atomics International, a division of North American Aviation did the development and testing work. The SNAP-2 Shield Development unit was responsible for developing the radiation shield. Creating the shield meant melting lithium hydride and casting it into the form required. The form was a big truncated cone. Molten lithium hydride had to be poured into the casting mold a little at a time otherwise it would crack as it cooled and solidified. Cracks in the shield material would be fatal to any space crew or payload depending on it because it would allow radiation to stream through to the crew/payload compartment. As the material cooled, it would form kind of a hollowed vortex in the middle. The development engineers had to create ways to fill the vortex while maintaining the shield's integrity. And, in doing all this they had to keep in mind that they were working with a material that could be explosively unstable in a moist oxygen rich environment. Analysis also revealed that under thermal and radiation gradients, the lithium hydride could disassociate and hydrogen ions could migrate through the shield. This would produce variations of shielding efficacy and could subject the payloads to intense radiation. Efforts were made to mitigate these effects. The SNAP 2DR used a similar reactor reflector moderator device as the SNAP-10A but with two movable and internal fixed reflectors. The system was designed so that the reactor could be integrated with a mercury Rankine cycle to generate 3.5 kW of electricity.

SNAP-8

The SNAP-8 reactors were designed, constructed and operated by Atomics International under contract with the

National Aeronautics and Space Administration The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the United States's civil space program, aeronautics research and space research. Established in 1958, it su ...

. Two SNAP-8 reactors were produced: The SNAP 8 Experimental Reactor and the SNAP 8 Developmental Reactor. Both SNAP 8 reactors used the same highly enriched uranium zirconium hydride fuel as the SNAP 2 and SNAP 10A reactors. The SNAP 8 design included primary and secondary NaK loops to transfer heat to the mercury rankine power conversion system. The electrical generating system for the SNAP 8 reactors was supplied by Aerojet General. The SNAP 8 Experimental Reactor was a 600 kW_t reactor that was tested from 1963 to 1965. The SNAP 8 Developmental Reactor had a reactor core measuring , contained a total of of fuel, had a power rating of 1 MW_t. The reactor was tested in 1969 at the

SNAP-10A

The SNAP-10A was a space-qualified nuclear reactor power system launched into space in 1965 under the SNAPSHOT program.SNAPSHOT
NASA Glenn Research Center, March 20, 2007. Retrieved 3 April 2019.
Gunther's Space Page. Retrieved 3 April 2019. It was built as a research project for the Air Force, to demonstrate the capability to generate higher power than RTGs. The reactor employed two moveable beryllium reflectors for control, and generated 35 kW_t at beginning of life. The system generated electricity by circulating NaK around lead tellurium thermocouples. To mitigate launch hazards, the reactor was not started until it reached a safe orbit. SNAP-10A was launched into Earth orbit in April 1965, and used to power an Agena-D research satellite, built by Lockheed/Martin. The system produced 500W of electrical power during an abbreviated 43-day flight test. The reactor was prematurely shut down by a faulty command receiver. It is predicted to remain in orbit for 4,000 years.

Citations

General sources

"Nuclear Power in Space". U.S. Department of Energy, Office of Nuclear Energy, Science & Technology

External links

SNAP-8 Electrical Generating System Development Program, Final Report

SNAP-19, Phase 3. Quarterly Progress Report, 1 January – 31 March 1966

SNAP 19, Phase 3. Quarterly Progress Report, 1 Apr. – 30 Jun. 1966

Analysis of the need for Agena command destruct and/or generator eject systems on the Nimbus B/SNAP-19 mission

SNAP-19/Nimbus B integration experience

SNAP-27, Volume 1. Quarterly Report, 1 Jul. – 30 Sep. 1966

SNAP-27, Volume 2. Quarterly Report, 1 Jan. – 31 Mar. 1966

"Space Nuclear Power: Opening the Final Frontier"
by G. L. Bennett (2006)

{{Nuclear Technology Atomics International Electrical generators NASA programs North American Aviation Nuclear power in space Nuclear technology United States Atomic Energy Commission