If it is true that NK does have a super EMP weapon the nuclear explosion would not have to be very big
Apparently, one of the signatures of a Super-EMP weapon is that it has a very low explosive yield, just several kilotons because the weapon is converting the energy of the nuclear warhead into gamma rays.
In 2004, the Russian generals told the EMP Commission that North Korea was getting help developing a Super-EMP nuclear weapon from contractors from Russia, China, Pakistan and elsewhere, and could probably test such a weapon ?in a few years.? A few years later, in 2006, North Korea tested its mysterious ?nuclear device? that produced an explosive yield of only several kilotons, and so was derided by the Western press as a failure?but hailed as a success by North Korea.
In 2012, a military commentator for the People?s Republic of China told a Hong Kong journal that North Korea has Super-EMP nuclear warheads.
One nuclear EMP warhead attack could collapse the national electric grid and other critical infrastructures?communications, transportation, banking and finance, food and water?that sustain modern civilization and the lives of millions.
A Super-EMP attack on the U.S. would cause much more damage than a primitive nuclear weapon, and will likely result in catastrophic consequences that will be irreversible. A Super-EMP attack would inflict maximum damage and virtually ensure a world without America.
Russia has something called the Fractional Orbital Bombardment System (FOBS), that would use an ICBM like a Space Launch Vehicle to put a nuclear warhead into orbit, like a satellite. They have apparently shared this with North Korea. Instead of using the ICBM to lob a warhead on a more accurate arcing ballistic trajectory, the FOBS would loft the warhead into a partial orbit. FOBS could reach any nation or threaten any target anywhere on earth.
The missile test in December 2012, was actually a FOBS test.
Thus, North Korea already possesses an ICBM and poses a mortal nuclear threat to the United States, and to all nations on Earth?right now.
North Korea, during the successful test of its ICBM on December 12, 2012, orbited a satellite weighing 100 kilograms (about 200 pounds). One design of a Super-EMP warhead would be a modified neutron bomb, an Enhanced Radiation Warhead (ERW) because it produces not only many neutrons but also many gamma rays. Gamma rays cause the EMP effect. The EMP field is so large that detonating anywhere over the U.S. would have catastrophic consequences.
North Korea orbited its satellite around the Earth at an altitude of about 500 kilometers. The trajectory of North Korea?s satellite is no accident?they deliberately aimed for and achieved this orbit and altitude, as announced before their launch.
An ideal altitude for making an EMP attack that places the field over the entire lower 48 United States is? wait for it? 500km.
The primary purpose of FOBS is a surprise attack. The first warning of the attack would be ?lights out?. North Korea and Iran have made clear in their threats that they wish to take down the United States. They perceive themselves to be at war with the U.S. and would like nothing more than to destroy the largest and most successful society on Earth.
There is more reading here as well
http://www.washingtontimes.com/news/...-now/?page=all