but there was a burnt smell detected.
My educated guess says there was something burning but not on fire. No flames; no fire. The automatic system activated as it was supposed to and since this is halon, it was probably a room full of electronics.
The biggest hazard to halon exposure in this situation is oxygen displacement.
Yup. Electrical. Doesn't sound like the reactor scrammed. And doesn't sound like a containment issue or a pump failure.
Some kinda problem, but the system worked as designed.