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).

The design of the BES-5 FNR is such that a sub-critical assembly exists into which a rod of fissile material is inserted.

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.

[citation needed] There were several mishaps related to failures in the ejection system, most notably Kosmos 954, which scattered debris over Canada.