see http://safetyfirst.nei.org/safety-and-security/fukushima-daini-model-of-a-safe-shutdown/
Fukushima Daini: Model of a Safe Shutdown
TEPCO Workers Lay Cables to Restore Power to Fukushima Daini
Struggling against earthquake aftershocks, devastating floodwaters and debris, employees at Tokyo Electric Power Co.’s (TEPCO)Fukushima Daini nuclear energy facility safely shut down all four of the facility’s reactors within days of the historic 9.0-magnitude earthquake and tsunami that struck on March 11. The facility is located only seven miles southwest of its sister plant, Fukushima Daiichi, and produces enough electricity to power roughly three million homes and businesses in Japan.
When the earthquake struck, the Fukushima Daini facility automatically shut down safely as designed. However, it went into a state of emergency following the tsunami when water damage disrupted heat removal systems in three of the four reactors.
TEPCO reactor operators were able to quickly bring reactor 3, which had retained its heat removal function, into stable condition in a matter of hours. Meanwhile, other employees worked feverishly around-the-clock to reestablish heat removal capability in the other three reactors and finished stabilizing them by March 15.
A key distinction between the post-disaster conditions at Fukushima Daini and Fukushima Daiichi was that off-site power was available at the Daini facility, whereas the Daiichi plant suffered a complete loss of electricity, including backup generators and, eventually, emergency batteries needed to power reactor cooling systems. Fukushima Daini workers were able to tap into electricity from a 500-kilovolt-transmission line – a key lifeline – to power a water injection system that helped cool the reactors as they shut down.
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