The medusa weapon- possible??

IN the movie(and book), medusa was described as a thermonuclear bomb, of a 20MT yield. Now in the MOVIE, the bomb is shown to have a blast-kill radius of 50 miles,(but in the book, it is 25Mi, which would be more realistic), wherein everything, and everyone, in that range of the plane the bomb is in will be destroyed. Beyond the blast radius, however--will come the "medusa wave" a supposed "secondary wave" of EMP pulse which would be MUCH MUCH more powerful than the EMP pulse of a "normal" 20 megaton bomb--and one that would produce continent-wide results.

The movie (or book) does NOT go into HOW the secondary super-EMP pulse would be created, but one is led to believe that there is some type of "multiplier" device hooked to the secondary of the nuke, in which will somehow NOT increase the explosive yield of the bomb--BUT will somehow release all of its energy as an EMP pulse.

As someone who has LONG been interested in nuclear weapon technology, and who knows pretty much all there is to know that is "public' about the construction and operation of nukes, I just wonder--WHAT kind of device/material could be used to accomplish this in reality?? Keep in mind whatever it was HAD to be some type of "fissile/fusion" material or such--otherwise, it would NOT be possible to do anything--as it would be consumed in a fraction of a microsecond, when the secondary lights off. IF I had to make a guess--I would say it would be some combination of plutonium and some "exotic" material, like strontium or such, set up as so NOT to produce any real blast yield, but a VERY high level of ionizing radiation--which would THEN somehow be coupled to the ionosphere--where it would then be "distributed" around the country, or even world. . But then--I don't know THAT much about this stuff.

Anyone else care to guess how(or IF ) it could be made to work in reality??