From Wikipedia
NASA's Perseverance rover uses a nuclear power source to explore Mars:
The rover's nuclear power source is a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), which is essentially a nuclear battery. The MMRTG contains 10.6 pounds of plutonium dioxide, which decays to produce heat that powers the rover's instruments. The MMRTG generates 110 watts of power, which is about the same as a light bulb.
Who is lying, you or government?
Walt is correct. These are not reactors per se. A reactor would leverage the use of a critical mass of a fissionable radioisotope and the neutron flux that causes the reaction to "go". But a nuclear battery, as I understand it, simply generates heat from radioactive decay which is then converted into electrical power by a thermoelectric effect.
Apparently the Russians abandoned a truck ton of these things all over the Siberian arctic. Just a big ol' chunk of highly radioactive materials in a container rusting away in the wilds of Siberia.