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
North American Aviation.
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 kWt. 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. 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, with a target of delivering 600 kW of power for greater than 10,000 hr. 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 reactor that was tested from 1963 to 1965. Early testing demonstrated SCRAM shutdown under planned and unplanned conditions, followed by 60 days delivering 450 kW. The reactor passed all the planned tests. At the end of the run, of the 211 fuel elements in the core, 44 were found to be intact and 167 had cracked cladding. These cracks allowed hydrogen and fission products to escape into the NaK coolant and for the coolant to react with the fuel. The SNAP 8 Developmental Reactor had a reactor core measuring , contained a total of of fuel, had a power rating of 600 kW with 1300 °F outlet temperature, with a maximum 1 MWt power out put at 1100 °F outlet temp. The reactor power testing began in January 1969 at the
Santa Susana Field Laboratory. Because of the observed fuel element cracking in SNAP 8 Experimental Reactor, the Development Reactor was more closely monitored for radiation in the NaK coolant. Fission products (Xe-133, Xe-135 and Cs-137) appeared in the circulating coolant in May, demonstrating fuel cladding failure. The reactor was continued to be run until December, demonstrating 7,023 hours of 600 W output. Upon disassembly, 72 of the 211 fuel elements were judged to have cracked.
SNAP-10A The SNAP-10A was a space-qualified nuclear reactor power system launched into space in 1965 under the
SNAPSHOT program. 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 kWt 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. Although much of the fission products will have decayed in that time, the vast majority of uranium will remain, eventually returning to earth as fallout from burn up of the satellite. ==See also==