Wikipedia
Upravlyaemy Sputnik Aktivnyj , or US-A, also known in the west as Radar Ocean Reconnaissance Satellite or RORSAT, was a series of Soviet reconnaissance satellites. Launched between 1967 and 1988 to monitor NATO and merchant vessels using active radar, the satellites were powered by nuclear reactors.
Because a return signal from an ordinary target illuminated by a radar transmitter diminishes as the inverse of the fourth power of the distance, for the surveillance radar to work effectively, US-A satellites had to be placed in low Earth orbit. Had they used large solar panels for power, the orbit would have rapidly decayed due to drag through the upper atmosphere. Further, the satellite would have been useless in the shadow of Earth. Hence the majority of the satellites carried type BES-5 nuclear reactors fuelled by uranium-235. Normally the nuclear reactor cores were ejected into high orbit (a so-called "disposal orbit") at the end of the mission, but there were several failure incidents, some of which resulted in radioactive material re-entering the Earth's atmosphere.
The US-A programme was responsible for orbiting a total of 33 nuclear reactors, 31 of them BES-5 types with a capacity of providing about two kilowatts of power for the radar unit. In addition, in 1987 the Soviets launched two larger TOPAZ nuclear reactors (six kilowatts) in Kosmos satellites ( Kosmos 1818 and Kosmos 1867) which were each capable of 6 months of operation. The higher-orbiting TOPAZ-containing satellites were the major source of orbital contamination for satellites that sensed gamma-rays for astronomical and security purposes, as radioisotope thermoelectric generators (RTGs) do not generate significant gamma radiation as compared with unshielded satellite fission reactors, and all of the BES-5-containing spacecraft orbited too low to cause positron-pollution in the magnetosphere.
The last US-A satellite was launched 14 March 1988.