BWR reactors like Fukushima are all passive-safe in at least one way, since water moderates the neurons coming off the critical materials, slowing them enough to continue the reaction. If the water boils away, sure the thing will almost certainly melt but there won't be a runaway prompt-critical event like Chernobyl, and if the concrete "bathtub" around the core holds, there won't be much radioactive material released into the groundwater either.
From what I heard the concern at Fukushima was that the earthquake had compromised the integrity of the containment structure, so allowing the core to melt down was judged as too risky (I'm not sure if I agree with that decision, but of course my opinion in hindsight is not relevant). In fact the majority of radioactive material that was released in the incident might have leaked from the spent fuel storage, which was also damaged in the earthquake:
From what I heard the concern at Fukushima was that the earthquake had compromised the integrity of the containment structure, so allowing the core to melt down was judged as too risky (I'm not sure if I agree with that decision, but of course my opinion in hindsight is not relevant). In fact the majority of radioactive material that was released in the incident might have leaked from the spent fuel storage, which was also damaged in the earthquake:
http://web.mit.edu/nse/pdf/news/2011/Fukushima_Lessons_Learn...