Primary explosive: Sensitive, used in very small quantities to initiate the main charge. Secondary explosive: Takes quite a bit of energy to initiate and ther

Primary explosive: Sensitive, used in very small quantities to initiate the main charge. Secondary explosive: Takes quite a bit of energy to initiate and therefore reasonably stable. High explosive: I forget the exact technical distinction from low explosive, but it refers to how it goes off, not how energetic it is. In the real world nobody but a terrorist uses any more of a primary explosive than is needed to set off the secondary, and they are kept apart for as long as possible. You put the charge in place then you insert the detonator cap. However, these are nukes. Obviously, we don't know details but there's another technology in use: exploding blast wires. A blast wire has no primary explosive, it is set off by pushing a huge current through a tiny wire. This is done both for timing reasons (you need great precision in imploding the plutonium) and because primary explosives cook off in fire. Secondary explosives often do not. While the heat causing a detonator to cook off isn't going to produce a nuclear yield it would still make an awful mess.