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SNAP-3 : In 1961, the first RTG used in a space mission was launched aboard a U.S. Navy Transit 4A and 4B navigation satellites. The electrical power output of this RTG, which was called (SNAP-3), was a mere 2.5 watts.
SNAP-7 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 electric power using 225 kilocuries (8.3 PBq) (about four kilograms) of strontium-90 as SrTiO3. These were very large units, weighing between 1,870 and 6,000 pounds (850 and 2,720 kg).
After SNAP-3 on Transit 4A/B, SNAP-9A units served aboard many of the Transit satellite series. On April 24, 1964 a SNAP-9A failed to achieve orbit and disintegrated, dispersing roughly 1 kilogram (2.2 lb) of plutonium-238 over all continents, and leading to NASA's increased development of solar photovoltaic energy technology.[better source needed]
SNAP-11, an experimental RTG intended to power the Surveyor probes during the lunar night. They were to be powered by curium-242 (900 watts thermal) and produce 25 watts of electricity for 130 days. Designed with 925 °F (496 °C; 769 K) hot junction and 350 °F (177 °C; 450 K) cold junction. They had a liquid NaK thermal control system and a movable shutter to dump excess heat. They were not used on the Surveyor missions.
"In general, the SNAP 11 fuel block is a cylindrically shaped multimaterial unit which occupies the internal volume of the generator. TZM (molybdenum alloy) fuel capsule, fueled with curium-242 (Cm203 in an iridium matrix) is located in the center of the fuel block. capsule is surrounded by a platinum sphere, approximately 2 - 1 / 4 inches in diameter, which provides shielding and acts as an energy absorber for impact considerations. This assembly is enclosed in graphite and beryllium subassemblies to provide the proper thermal distribution and ablative protection."
SNAP-19(B) was developed for the Nimbus-B satellite. "The SNAP 19 generators are fueled with plutonium 238 and employ lead telluride thermoelectric couples for energy conversion. Each of the  electrically paralleled generators produces approximately 30 watts at beginning of life. Each generator ... weighs less than 35 pounds and is ... 6-1/2 inches in diameter by 10-3/4 inches high. [extended by] six fins." Nimbus 3 used a SNAP-19B (with the recovered fuel from the Nimbus-B1 attempt.
SNAP-21 and SNAP-23 were designed for underwater use and used strontium-90 as the radioactive source, encapsulated as either strontium oxide or strontium titanate. They produced about ten watts.
Five SNAP-27 units provided electric power for the Apollo Lunar Surface Experiment Packages (ALSEP) left on the Moon by Apollo 12, 14, 15, 16, and 17. The fuel capsule, containing 3.8 kilograms (8.4 lb) of plutonium-238 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 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 70 watts.
The fuel cask from the SNAP-27 unit carried by the Apollo 13 mission currently lies in 20,000 feet (6,100 m) of water at the bottom of the Tonga Trench in the Pacific Ocean. This mission failed to land on the moon, and the lunar module 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).
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.
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 kWt. It was the first model to use a flight control assembly and was tested from April 1961 to December 1962. Studies were performed on the reactor, individual components and the support system. 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.
The SNAP-8 reactors were designed, constructed and operated by Atomics International under contract with the National Aeronautics and Space Administration. 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 kWt reactor that was tested from 1963 to 1965.
The SNAP 8 Developmental Reactor had a reactor core measuring 9.5 by 33 inches (24 by 84 cm), contained a total of 18 pounds (8.2 kg) of fuel, had a power rating of 1 MWt. The reactor was tested in 1969 at the Santa Susana Field Laboratory.
The SNAP-10A was a space-qualified nuclear reactor power system. 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 35KWT at beginning of life. The system generated electricity by circulating NaK around lead tellurium thermocouples. To mitigate launch hazards, the reactor was never started until it reached a safe orbit. It 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.