I took out this from this report (http://www.euronuclear.org/reflections/nuclear-facilities.ht...), which I think was also mentioned by jgrahamc:
> The studies showed that even if knowledgeable insiders could disable key safety equipment, redundant systems would provide backup protection. In scenarios where all engineered safety systems were ruined, there was still a period of hours to take corrective actions before onset of core damage.
Now, you may argue that what indeed went wrong in the case of the Fukushima plant was that the "redundant systems" didn't provide the "backup protection" the experts relied on. I see this report is confident this would not be the case were the terrorists to hit a nuclear plant, but based on what happened in the last few days I'd say the confidence level for that scenario is bellow 100%.
Anyway, thx for those downvotting me, and I hope I'll never be proven right.