A Japan Atomic Industrial Forum paper has compared the earthquake and tsunami preparations at four nuclear power plants on the northeastern coast of Japan, Fukushima Daiichi, Daiini, Onagawa and Tokai Daini, in an attempt to explain why reactor meltdowns only occurred at one. It concludes that the key factor was that the other three stations were able to preserve some source of power to ensure reactor cooling.
First, it reports that the seismic resistance was about the same as the earthquake ground motion, so ensuring the safety of the reactor structures, but the earthquake heavily damaged offsite power systems, including all seven at Fukushima Daiichi, two of four at Daiini, four of five on Onagawa and all three at Tokai Daini, greatly hindering recovery efforts. It said, "reliability of power sources as a whole should be improved, including that of external sources."
Second, the actual tsunami height was more than double the assumed height at Fukushima Daiichi (13.1m); two-three metres higher than expected at Fukushima Daini (7-8m), half a metre less than expected at Onagawa (13m) and 1m less than expected at Tokai Daini (5.5m). (The report went on to particularly praise the design approach at Onagawa, which recognized that ground height is a major anti-tsunami measure, and set a station height of 15m above sea level).
At Fukushima Daiichi, the tsunami surge knocked out all emergency diesel generators (except one at unit 6), and DC power was lost too. With no offsite or onsite power at units 1-4, they sustained major damage. Unit 5 was able to tie into the unit 6 diesel generator and both 5&6 avoided severe damage.
At Fukushima Daiini, the tsunami destroyed emergency seawater cooling pumps, but power was able to be received from one offsite power line to cool all four reactors using reactor core isolation cooling (RCIC) systems.
At Onagawa, offsite power was maintained and the tsunami did not reach the main buildings on the site.
At Tokai Daini, although offsite power was lost, two emergency diesel generators charged batteries that powered RCICs.
At both Fukushima sites, seismic-isolated buildings (including a new one at Daini) were vital headquarters to manage disaster recovery efforts on site, the report said.