Assuming it works as intended.
The HEMP device or our grid?
If the HEMP device (which is not hard to make for a nuclear power) is only 10% effective, then a third of the US is fucked rather than the entire lower 48. If our grid is only 90% effective at dealing with it, that 1/3 of the country drags the entire country's grid down with it. Maybe in that scenario, the 2/3 that only got dragged down rather than being actively surged can be energized in a month or two, and they can help bring the actually attacked part of the grid up in another year.
I am in no way confident that we are that competent.
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