Physics simulator game: Nuclear Design Bureau (Steam) Kerbal Space Program for nuclear kaboom, if much, much less cute. Design your own nuclear weapon.

Draw the cross section of a nuclear weapon of your own design. Brick of U-235 goes here, explosives go there, etc. Hit the detonate button and get a realistic physics simulation (oops, K less than 1, subcritical, try again). Don't worry, the game's tutorial will walk you through the details of the science. Absolutely not the cuteness of Kerbal. Lots of charts and graphs. Interesting charts and graphs if you like this sort of thing, or want to learn by doing. $6.50 CDN on Steam. I'm sure you are doing this only with peaceful purposes in mind, like a nuclear bomb powered spacecraft or a nuclear bomb civil engineering project (Project Orion and Project Plowshares)