Continuity mistake: When the purser's safe falls through the ceiling, we get to see the safe come to a rest, and we can see the door to the safe is still perfectly in position and still closed. The next shot looking down on the safe now shows the safe with the door now fully opened.
Revealing mistake: When the purser's safe falls through the ceiling, the ceiling metal is far too thin. Bearing in mind also, that the on the other side of the ceiling metal is supposed to be the floor of the next deck up, and that too is a thin flimsy metal, that doesn't even appear that it could support any kind of weight.
Answer: I just assumed that it was the plutonium on board. But I may be misremembering. It could have been uranium.
ChristmasJonesfan