First the spent fuel pits don’t need power to keep the rods cool. They are stored in casks, and basically kept underwater in a large pit. The water keeps things cool. The problem is many of these pits are now at or near capacity. The question is what do you do with the material long term. If a suicide bomber did get into the spent fuel pit and detonated himself, I doubt he would be do much more than make a mess by blowing himself up. Some of the old plants did keep the rods in the pool without any casks. These were a priority to get cleaned up. Most of those plants are shut down and are in a maintenance phase.
All power plants, not only nukes, are now required to harden themselves from EMP, cyberattack, and terrorist assault. Obviously this all isn’t going to happen overnight, but steps are being taken. Not sure about other industries, but power generators are pretty proactive on this issue.
Fukushima is a unique animal. Observing from a distance, it seems TEPCO is doing everything wrong. What they allowed to leak into the ocean is criminal. I personally would not eat any fish caught in the Pacfic. Their solution of building giant tanks to house all the water is crazy. Another quake in the area would split those tanks open like an egg.
Have heard conflicting stories about the meltdowns. Still have contacts in the industry and they are getting conflicting info. Always a sign that things might not be what they should be.
As far as plant mutations, from what I understand, it will take several generation of plants to see what the mutations are. Perhaps someone else can give further insight.