Fukushima: Lessons Learned (Part Two)

Fukushima Nuclear Power Plant

The plant is located on the northeast coast of Japan about 120 miles from Tokyo.  It consists of six (6) GE-designed Boiling Water Reactors.  The steam energy from the boiling water is used to drive turbines which create electricity.  The steams cools and condenses back into water for re-use.  The 860-acre site is some 30 feet above sea level on a bluff overlooking the sea.

Construction planning for the site and reactors included consideration for protection against both earthquakes and tsunamis.  The tsunami from the 1960 Chilean earthquake that measured 10.2 feet when it came ashore nearby was used as a reference guide.

Breakwaters had been installed in the harbor to mitigate tsunami effects. (Many of the purpose built breakwaters were ineffective in the Tohoku quake and have even been implicated in worsening flooding in adjoining areas by re-directing the waves.) In 2002 the inundation protection level was increased to 18.7 feet when some back-up equipment was placed on higher ground. 

In addition, the plant was equipped with Emergency Diesel Generators (EDGs) in order to operate cooling systems for the reactors if AC power was lost. Control rooms for the reactors were created in paired groupings of 1 and 2, 3 and 4, 5 and 6.  They shared some common facilities and travel between the paired control rooms was easy.

The site also included storage for spent nuclear fuel utilizing dry cask storage and a separate area where fuel was stored in water-filled pools.

The severe shaking associated with the quake damaged the electrical infrastructure on-site and caused the collapse of electrical transmission towers.  It also exceeded the built-in vibration parameters and resulted in the three on-line reactors going into emergency mode with control rods fully inserted and effectively shutting down.  The diesel generators activated in order to allow for cooling to continue and to ensure that the fuel rods remained covered with adequate water. Any anomalies noticed in the reactor shutdowns were monitored and solved.  The heat produced in this mode was less than 10% of normal reactor operation but still enough to be of significant concern.

The quake struck at 1446 Japan time and the first of the tsunamis came ashore at 1527. These tsunamis, one over 45 feet high and well above any plant protection features, rolled over the harbor breakwaters and in and through the plant, inundating reactor buildings and flooding basements and other key areas.

The surging water knocked out all of the diesel generators except one, immersed electrical switch gear and panels causing shorting and grounding, and critically disabled the control rooms for the three operating reactors.

The ability to control the reactors began to degrade rapidly as AC and DC power faded and the control rooms, nerve centers for protecting the reactor cores, went completely dark.

Tomorrow:  Part Three, Flying Blind


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