Since this seems to be a topic of interest as of late, I’ve decided to come up with my own basic concept of an improvised nuke that is much more plausible than most other methods — using the TNT equivalent instead of actual fissile material.
So, let’s say you want to make a Tsar bomb. How much TNT would you need? 50 megatons. However, the next problem would be actually obtaining enough of the materials neededto make TNT. In most first world nations, the chemicals needed to create explosives are heavily regulated (like nitric acid). So instead, my solution to that problem would be to use gunpowder, which isn’t made from all those harsh laboratory chemicals that would be needed to make TNT or RDX. All you would need is Potassium Nitrate, Charcoal and Sulfur. And you could even make gunpowder from your own urine. You would just need enough of it for it to be the equivalent of 50 megatons of TNT.
But the next problem is how exactly you’re going to detonate this device. Since obtaining the materials and engineering necessary to make an ICBM would be universally implausible, I’ll just say to use the Hamas method, in that you would make sugar rockets, provided that you have enough of it to launch the warhead to the desired destination (i.e. from the United States to Israel, for example), and for the guidance, you could use a homemade RC Servo mechanism that’s big enough to weigh down and direct the rocket to anywhere you want it to go. As for the detonation, you could just use a nail and a bullet, with the explosive charge tightly packed in the explosive shell to hold it in place and for it to burn faster upon detonation. You could even add some radioactive materials as a finishing touch; say radioactive cobalt or any other radioactive material that you can find.
Also, my design is of a rocket, not of a device that is dropped from an aircraft or fired from a submarine.
But would this actually be plausible? It sounds kinda stupid, since you would need sufficient amounts of each material, but at least on paper it’s possible.