The theory of a nuclear reator at earth's core has problems, the chief one being the random reversal of the magnetic poles. A reactor would be expotentially declining but during magnetic reversal events we see, in the fossil record, a weakening field(it's declined some 10% in the last 150 years or so)seems to indicate fluid dynamo currents in the outer core as the B dot origin. It may continue to weaken then suddenly REVERSE(N=S, S=N), or, as the fossil record shows, return to the initial orientation. The offset fields of Uranus and Neptune also indicate a fluid flow origin for planetary magnetic fields, not a central core origin. Also, from diamond cell/ high pressure studies, it would seem that the inner core is made of hexagonal iron crystals and does not generally participate in the earth's overall magnetic field.
It seems that the magnetic field source remains elusive. That natural nuclear reactor that was found in Gabon (it was extinct) is in the crust, so I wonder how much sense it really made for Herndon to extrapolate in the first place.
Neither does the shaft of a motor. The windings rotate through the field on a shaft(core).