US-A

Incidents

• Launch failure, 25 April 1973. Launch failed and the reactor fell into the Pacific Ocean north of Japan. Radiation was detected by US air sampling airplanes.
• Kosmos 367, 3 October 1970, failed 110 hours after launch, moved to higher orbit.
• Kosmos 954. The satellite failed to boost into a nuclear-safe storage orbit as planned. Nuclear materials re-entered the Earth's atmosphere on 24 January 1978 and left a trail of radioactive pollution over an estimated 124,000 square kilometres of Canada's Northwest Territories.
• Kosmos 1402. Failed to boost into storage orbit in late 1982. The reactor core was separated from the remainder of the spacecraft and was the last piece of the satellite to return to Earth, landing in the South Atlantic Ocean on 7 February 1983.
• Kosmos 1900. The primary system failed to eject the reactor core into storage orbit, but the backup managed to push it into an orbit below its intended altitude.

Other concerns

• During 16 reactor core ejections, approximately 128 kg of NaK-78 escaped from the primary coolant systems of the BES-5 reactors. The smaller droplets have already decayed/reentered, but larger droplets were still in orbit as of 2012. Since the metal coolant was exposed to neutron radiation, it contains some radioactive argon-39, with a half-life of 269 years. There is no risk of surface contamination, as the droplets will burn up completely in the upper atmosphere on re-entry and the argon, a chemically inert gas, will dissipate. The major risk of the now solid objects is impact with operational satellites.
• An additional mechanism for radioactive material release is through the impact of space debris hitting intact contained coolant loops. A number of these old satellites are punctured by orbiting space debris—calculated to be 8 percent over any 50-year period—and release their remaining NaK coolant into space. The coolant self-forms into frozen droplets of solid sodium-potassium of up to approximately several centimeters in size, and these solid objects then become a significant source of space debris themselves.

List of US-A satellites