what helicopter do you expect EXCEPT a navy helicopter to be flying through the fallout from a nuclear explosion?

especially with all the nuclear tests they were doing for decades?

Ambulance and rescue helicopters, if there are any left, and the crew didn't just take the helicopter and run as far as possible...

Which will almost certainly be military, because civilian helicopters and staff aren’t equipped or trained for that kind of environment except for perhaps a every couple of years hazmat/mass casualty training.