Um, the root cause of the problems at Fukushima had nothing to do with the design of the reactors. The root cause was that their backup diesel generators, which were to supply power to the reactor control and safety systems in the event of a shutdown, along with their switchgear, were sited behind a seawall that got overcome by the tsunami. If the backup generators had been sited properly, this whole fiasco would never have gotten started; the reactors would have shut down safely, end of story. I'm actually amazed that the reactors have held up as well as they have in the face of having no cooling water over a significant part of the fuel rods for this long.