How to make a nuclear bomb in Minecraft?

You could install a mod like too much TNT, or install world edit and create a giant sphere of tnt. Or just place TNT down and light it if you don't want to install anything.

Without mods, the only way to get a large explosion is with TNT.

With mods, it depends on the mod. Several mods add nukes. ICBM, Volts (where Nuclear is a *medium* tier bomb), Too Much TNT, IC2, etc.

In vanilla minecraft put A LOT OF TNT in a clump and light it