This wouldn’t be the development of a full-scale multi-stage thermonuclear weapon; it’s a slight modification to an implosion fission device (which the NORKs already have) basically putting some fusionable material in the middle of the imploding Pu or HEU to generate more neutrons and improve the fission explosion.
OK. I’m not into nuclear science, don’t have the brains for it. So, what would be the range of this missile, as far as we know, and what would the impact of the detonation be? As far as we know.