In the last couple of versions for TPT, implosion nukes haven't been working right. In the next version this should be fixed so we can use implosion nukes like we could in version 42.3. Type "implosion nuk" in the "find and open a simulation" thing and open one and you will notice the plutonium falls to the bottom of the nuke when you initiate it.
there isn't really a source or material that will create a pressure that is designated inward, things explode because the energy created inside flows outward.
Any type of explosion makes pressure flowing outward with no regard. There isn't much you can do about this. Likewise there isn't a bomb or destructible material that will create negative pressure without balancing itself with positive pressure.
I agree with Phlegmkitty. Implosion nukes used to be a lot of fun. It has nothing to do with creating negative pressure but rather compressing the Plutonium while fission occurs.
Right now the Plut sinks to the bottom and kinda ruins the whole reaction. Run a search on implosion nukes to see what we mean. Check it against v42