My goodness but 2011 is shaping up to be an exciting one, and not necessarily in a good way, depending on where on the globe you're located. Definitely not if you're an inhabitant of Japan, living in proximity to the Fukushima Daiichi nuclear plant that is currently experiencing such difficulties in the wake of the massive earthquake and accompanying tsunami. Situations like this do highlight just how difficult it is to plan for worst-case scenarios.
Nuclear plants are expensive and complicated to build, complex to run, and challenging to maintain, and that's with everything working correctly. To give you an idea of the kinds of maintenance challenges that exist in nuclear power plants, I'd suggest reading the article, "Testing for Materials Aging in Nuclear Power Generators", published in May 2006 in Sensors. Engineers familiar with nuclear plants can build on decades of experience to make normal operation as safe and effective as possible. But how do you plan a plant that will safely survive a massive earthquake, one that's the largest to ever hit Japan, and the largest on record since 1900 when seismologists began keeping records? It's one thing to design a structure to survive smaller-magnitude earthquakes; there are a lot more of them and they're less destructive, so you can both acquire more real-world data and have lower stakes if your earthquake mitigation efforts fail. The truly massive earthquakes are rare and we're still learning about the different types of earthquakes and the differing sorts of damage they cause.
Any plant, whether it's for chemical production, petrochemical processing, or power generation involves design tradeoffs. The only completely safe plant is one that's not actually running and that doesn't contain any dangerous materials or people. For the rest of them, every major accident that has occurred ideally informs the design of the next generation of plants. We don't learn from success, success doesn't challenge us; we learn from failure. That's the reason that we now have such sophisticated safety and monitoring systems available and why they're getting more intelligent all the time. Heck, compare the radiation monitoring equipment available when Chernobyl happened to the more sensitive and capable instruments that are in use today, helping to keep tabs on the radiation exposure experienced by Fukushima personnel and first responders and keep them safe.