It is a easy calculation and you need a lot more than a meter for that kind of power. Even 1J on the outside is going to be a problem for passages on a 1+ hour flight, there are some quite high energy gammas (much higher than 1MeV). Its easy to deal with on the ground, not for flight. Gammas are a real PITA to shield against without some serious 1/r^2 “shielding” or in a pit etc. Water is not very good at dealing with gammas, dense materials do much better.
