NASA's SNAP-10A satellite operated for 43 days before stopping.
Notes on verification
Multiple independent sources including DOE, Wikipedia, and Astronautix corroborate the 43-day operational period and the non-nuclear electrical cause of shutdown.
Sources
- Inside NASA's audacious plan to build a nuclear-powered ... (nationalgeographic.com)
- https://www.energy.gov/etec/articles/lasting-legacy-snap-program-laid-critical-groundwork-space-nuclear-power-systems (energy.gov)
- https://www.energy.gov/etec/system-nuclear-auxiliary-power-snap-overview (energy.gov)
- http://www.astronautix.com/s/snapshot.html (astronautix.com)