BES-5

Spacecraft nuclear reactors are typically fast reactors for the following reasons. First, normal moderator materials (carbon, water) add bulk and mass which is not desirable in a spacecraft. Second, for reasons of nucleonics, the fuel must be highly enriched to have a lightweight critical mass (similar to small reactor designs on nuclear submarines). Note that some of the ²³⁸U (which is fertile and not fissile) will be converted to ²³⁹Pu during operation, and this is taken into consideration during the design and while estimating the power output and design life expectancy.

The design of the BES-5 FNR is such that a sub-critical assembly exists into which a rod of fissile material is inserted. Feedback and monitoring of the power level will keep the reactor delayed critical and not prompt critical, which can be done by a mechanical control system.

The fuel core of the reactor was 0.24 m in diameter, 0.67 m long and weighed, as an assembly, 53 kg, and contained 35-50 kg of enriched uranium. The entire reactor, including the radiation shielding, weighed 385 kg.

The uranium fuel was more than 90% enriched ²³⁵U and generated 3 kW of electrical power created by thermoelectric conversion of 100 kW of thermal output.

The BES-5 reactor was used in more than 31 satellite missions to power the radar units of the US-A surveillance satellites. The reactor was designed to be boosted to a high orbit at the end of its operational life, to prevent the radioactive fuel from re-entering Earth's atmosphere.

There were several mishaps related to failures in the ejection system, most notably Kosmos 954, which scattered radioactive debris over Canada. Kosmos 1402 also re-entered the atmosphere, but burned up over the Atlantic Ocean, away from populated areas. Kosmos 1900 failed to reach its disposal orbit and remains in low Earth orbit.