Ah. Depleted uranium is just a heavy metal, spent fuel is a heavy metal with a higher radioactivity (not suitable to handle with a bare hands) but not enough

Ah. Depleted uranium is just a heavy metal, spent fuel is a heavy metal with a higher radioactivity (not suitable to handle with a bare hands) but not enough to go critical or even be close to go critical. You can pour any amount of water on it, at best you would get a slightly radioactive water. Just to be clear - you need something else, with a high yield, eg a chemical bomb with a high TNT equiv., to make a dirty bomb - still it wouldn't provide a self sufficient critical reaction.