Military planners understand the vulnerability of the grid to upset, but have been more concerned with man-made threats such as the electromagnetic pulse that would be generated by a high-altitude nucleardetonation.
They have long since outlived their design life and for at least 18 years none of them has been tested in an operationally realistic way (that is, with an underground nucleardetonation).
He was also aware of a more immediate risk in those cold-war days: a smaller visitor (such as the one that had exploded over Siberia in 1908, flattening 2, 000 square miles of forest) might be mistaken by either superpower for a nucleardetonation and trigger Armageddon.