The reason that the disaster hasn’t been worse was that the Japanese system had many, many safety precautions in place. Japanese reactors are required to be designed and built to survive substantial earthquake accelerations. They are required to have backup power systems in place. And so on and so on. Everyone in the nuclear industry is trained to think about safety from day one.
Security, by contrast, is something most people in the nuclear industry might get a half-hour briefing on once a year. If you have intelligent adversaries, it wouldn’t be a surprise to have the main power for the cooling and the backup power both fail – the adversaries would plan to make sure that happened. This transforms all the probabilities we rely on for safety. The global nuclear safety regime needs strengthening – but it is far stronger than the global nuclear security regime. And that applies to theft of nuclear material as well as sabotage of facilities.
On the other hand, only the power of Mother Nature would likely be able to cause the scale of damage we’ve seen at several reactors at once.