@Azzurolix
Welcome. You can use C-4 to set plutonium off like so:


Just press 'download' and it will open when you press the 'open simulation' button it the Powder Toy window.

@Azzurolix
Cool. Good luck!
EDIT: It doesn't have to be C-4; it can be most explosives.

You do ~SAVEID. For example ~ 248959 without the space.

To find out your save ID, bring up your save on the website and find your ID from the adress bar...
So: https://powdertoy.co.uk/Browse/View.html?ID=165883 << That last bit is the save ID.<br />

Make a sphere of metal, empty it. At the center of the sphere, put another empty sphere at the center. Then, fill the center sphere with C4 or whatever you want. A detonator is easy to create: for example a little bit of gun powder, an electrode inside, wifi system.
Fill the external sphere with plutonium.

Prepare the trigger system in a bunker of wall. Put some brick walls and concrete wall to test the strength of your bomb.

From there, the design can vary. A fusion bomb is exactly the same design except there will be another external sphere with deuterium.

These are basic concept but you can look on wikipedia if you want to go more details. The most important is for you to understand what is the idea behind the engineering of it and not to copy it blindly and after add unrealistic cheat to make it work.

Mackrock:

in a bunker of wall

Lolwut?
Anyways, just wire a bit of metal to the C-4 then spark it to boom.

Not funny. I like my trigger to remain intact for another experiment. Actually, the design of most nuke in Powder Toy could not work in the reality. In reality, there must be an implosion to trigger the explosion. A pure explosion will fizzle the explosive fission of the plutonium. To simulate a real fission/fusion realistic weapon is pretty tough.