As reported by WNISR published on July 15, 2015:
"As a result of the Fukushima nuclear disaster, more pressing questions have been raised about the wisdom of operating older reactors. The Fukushima Daiichi Units (1 to 4) were connected to the grid between 1971 and 1974. The license for unit 1 had been extended for another 10 years in February 2011, a month before the catastrophe began. Four days after the accidents in Japan, the German government ordered the shutdown of seven reactors that had started up before 1981. These reactors, together with another unit that was closed at the time, never restarted. The sole selection criterion was operational age. Other countries did not adopt the same approach, but it is clear that the 3/11 events had an impact on previously assumed extended lifetimes in other countries as well, including in Belgium, Switzerland, and Taiwan." (p.38-39)
Thus, had the operating license at Fukushima Daiichi Unit 1 not been extended, just weeks earlier, Unit 1 would not have been operating on 3/11/11. Especially if its irradiated nuclear fuel had then been removed from the reactor core, a meltdown could not have occurred (by definition) -- as was the case at Fukushima Daiichi Units 4, 5, and 6 (which were not operating, and had cores off-loaded of nuclear fuel).
(Granted, off-loading a reactor core of its irradiated fuel, into the storage pool, simply transfers the risk another location. This was the dire situation at Fukushima Daiichi Unit 4, until the irradiated nuclear fuel was finally completely removed from the storage pool by Dec. 2014. Now, that irradiated nuclear fuel risk has been transferred to Fukushima Daiichi's ground level "common pool" -- not a risk-free location, but significantly less risky than the near-collapse Unit 4 reactor building, of which the storage pool is an integral part.)
It is also important to point out that some sources allege that the meltdown at Fukushima Daiichi Unit 1 was well under way even before the tsunami hit the site, about 50 minutes after the 9.0 earthquake had struck. That is, certain sources (citing the testimony of on-site workers' eye-witness experience) allege that the earthquake itself had so badly damaged Unit 1, that it was already in process of melting down, even before the tsunami struck the site (that is, tsunami or no tsunami, Unit 1 was likely doomed to melt down, due to earthquake damage).
This begs the question, how vulerable to earthquakes, or other shocks, are the oldest reactors still operating in the U.S., and around the world?