I think the original premise of 99% of people instantly dying is a bit silly frankly. Something like a giant asteroid that could cause that to happen even within a day would pretty much render the world uninhabitable anyway. Realistically the only thing that could quickly eliminate all but a small percentage of humanity without taking out most of the buildings, plants and animals as well is an epidemic of some kind. Yet even something far more infectious and deadly than Covid would likely take months to achieve this. So I think you'd be looking at mass panic and anarchy long before there were only 1% of people left and a lot less intact resources left behind once there were.
As I recall from Fukushima the problem was that the earthquake caused the reactor to shut down and the tsunami took out the back up diesel generators. So there was no power for the pumps that circulated the coolant for the nuclear fuel and this risked the reactor going into meltdown. Presumably in this scenario with no human intervention safety cut outs in the nuclear reactors would shut them down, the diesel generators would cut in for a month or so until the fuel ran out and then all the reactors in the world would start cooking off. Not sure what would happen if we got hundreds of Chernobyls over the space of a month or two but it probably wouldn't be pretty for the Northern hemisphere. Then you'd have all humanity's other toxic messes from untended oil wells, chemical industry plants and so on to deal with. Overall I think you'd be better off dead.