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2003 Northeast blackout<unk> Problem was that all but one of the reactors were tripped and allowed to poison out, preventing a early reconnection to the electricity grid.<unk> Problem was that all but one of the reactors were tripped and allowed to poison out, preventing a early reconnection to the electricity grid. Once shut down, all nuclear reactors take several days to return to service. Once shut down, all nuclear reactors take several days to return to service.