"Broken arrow" does indeed refer to a nuclear accident or release of nuclear material.
Regarding the numbers stations, they're legit, and actually transmissions for and between clandestine stations. When I was in one of my previous lives, I was a Russian interpreter/translator and military intelligence analyst. I spent some time at a radio intercept site in Europe, and my last year at NSA headquarters outside Washington. We frequently intercepted these transmissions, which were recorded and/or copied by hand when in morse code, and turned over to the cryptanalysts. The specific type of five-character groups were known as Cheney/Hornblende.
The bad news is that now that you've read this I'm going to have to kill you.
