Ok here is how EMP works...in a simplistic viewpoint.
After detonation, the free neutrons decay to a beta particle and a proton. Half life for a free neutron is about 13 min.
The high energy beta particles (electrons) align themselves with the Earth’s magnetic field and actually orbit the lines of force, thus generating the EMP.
This is why an Atomic Bomb generates a minor EMP as the number of free neutrons are so much smaller than with a LARGE Thermonuclear warhead.
When I say large, I’m talking about tons of Lithium Deuteride and not just 100lbs which is what you would find in a modern MIRV warhead.
The Starfish Prime warhead which caused the first observable EMP was over 1MT yield and weighed well over 4,000lbs, flew straight up, and detonated over Johnston Island.
Delivering a 4,000lb warhead 8000 miles and detonating it 200 miles in the air is beyond NK abilities.
There are much easier and cheaper ways of crippling the USA than attempting an EMP attack.
Backing Democrats for office, for one!