The big problem with older reactors is that they need active cooling for awhile after they are shut down. That's why Fukushima exploded. The diesel generators

The big problem with older reactors is that they need active cooling for awhile after they are shut down. That's why Fukushima exploded. The diesel generators flooded and couldn't run the cooling pumps. Chernobyl happened during an experiment shutting down the plant using the momentum in the generator to cool the plant. Newer reactor designs can shut down safely without active cooling.