Why would salt water ingress cause a nuclear reaction in Little Man? Also it seems like this was the scariest of the two 'gadgets' to fly with; those emergency

Why would salt water ingress cause a nuclear reaction in Little Man? Also it seems like this was the scariest of the two 'gadgets' to fly with; those emergency procedures (removing gunpowder!) seem like they probably wouldn't have much chance of success in a real flight emergency.