Atomic batteries are not at all powerful enough. Microwatts.
Running anything on batteries for days takes a lot of batteries. A Raspberry Pi for 2 days for example takes somewhere from 1.5 to 2 kg of li-ion batteries.
That sort of weight starts to be really problematic for a device which needs to survive a plane crash - since every bit of extra mass increases the force with which it hits.
They don't have to be microwatts. Galileo, for example, used 17 pounds of plutonium to generate 570W of power at launch, with a half life of about 88 years.
Not that you'd want to carry 17 pounds of plutonium around on an airliner all the time, but power output is not an insurmountable problem by itself.
Running anything on batteries for days takes a lot of batteries. A Raspberry Pi for 2 days for example takes somewhere from 1.5 to 2 kg of li-ion batteries.
That sort of weight starts to be really problematic for a device which needs to survive a plane crash - since every bit of extra mass increases the force with which it hits.