The Downey Facility on Lakewood Blvd. in the City of Downey, was used by North American Aviation (NAA) to build airplanes during the 1940s and 1950s. It was later used by Rockwell International to build the Space Shuttle. In addition to these aerospace operations, a small portion of the Downey facility was also used for nuclear research from 1948 to 1955. Operations were conducted by the Atomic Energy Research Department (AERD), which later became the Atomics International (AI) Division when operations were moved to the Canoga Facility in late 1955.
AERD was engaged in research and development for controlled release of energy from the atom for the production of electric power. This research and development was then sponsored by the Atomic Energy Commission (AEC).
One of the projects was the design, construction and operation of the first nuclear reactor in California. A source of neutrons was needed for one of the reactor physics projects, so a small aqueous, homogeneous reactor called the Water Boiler Neutron Source (WBNS) was built and put into operation on April 21, 1952. The reactor was operated at power levels up to 4 watts and served as an excellent neutron source for a number of reactor physics programs. It didn't really boil water, as one might guess from the 4-watt power level, but a small amount of hydrogen and oxygen from decomposition of the water was released from the solution into a tank during operation - hence the designation "water boiler".
The following link briefly summarizes this initial nuclear research and documents a radiation survey of the facility that was conducted in 2000 prior to transferring ownership of the facility to the City of Downey.