Electrical Switching Equipment Design Flaw Doomed Nuke Plant

Japan Fukushima Daiichi Backup Generators Could Not Get Electricity

When electrcitiy from the grid was knock out by the tsumani, even if the swamped and inoperable backup generators would have been working, it would have done no good because the equipment used to switch the power from the diesel geneators to the reactors cooling systems was inoperable due to being flooded. Nuclear-power plants must continuously cool their hot, radioactive fuel. Those cooling systems run on electricity, which the plants ordinarily pulled from the nation's power grid. If the grid fails, on-site diesel generators kick on to keep the cooling systems running. If they don't, that plant is in danger of melting down.

The company used two different designs for safeguarding its 10 reactors at its two Fukushima sites. When the devastating quake struck on March 11, the five reactors at Fukushima Daini with the newer design withstood the resulting 45-foot tsunami without their vital cooling systems failing. Those reactors shut down safely. But the cooling systems failed at four reactors at Fukushima Daiichi with the older design. Backup diesel generators and electrical-switching equipment were swamped by seawater. As a result, fuel melted down at three reactors and there were explosions at four reactor buildings.

The placement of a single electric-switching station in a poorly protected outbuilding led to those switches being knocked out by the tsunami and rendering operating generators useless.    This design flaw was a relic of the original design. General Electric Company designed the older Fukushima reactors. All the Fukushima plants, including the newer ones, were based on GE designs. GE has expressed that any flaws at the Fukushima reactors weren't its fault because Tepco was in charge of design changes. The location of emergency diesel generators at the Fukushima Daiichi plant were reviewed and approved by Tepco and regulatory authorities.

The early reactors used GE's Mark 1 design. To keep the reactor compact and economical, the reactor buildings were made small.  Because Tepco's first reactor buildings were small, the generators had to go somewhere else. Engineers put them into neighboring structures that house turbines. The reactor buildings were fortress-like, with thick concrete walls and dual sets of sturdy doors. The turbine buildings were far less sturdy, especially their doors. Backup power generators are critical safety equipment, and it should've been a no-brainer to put them inside the reactor buildings.

Tepco the Mark II design in the No. 6 reactor building, which had enough space for the backup generators to go inside. By 1987, Tepco had opened its tenth and final reactor in Fukushima prefecture. Nos. 1 through 5 at Fukushima Daiichi had the old design. The other five at Daini had the newer design.

In 1998, to comply with new regulatory requirements, Tepco decided to give each reactor at Fukushima Daiichi at least two dedicated backup diesel generators, something that not all of them had. New backup generators for reactors Nos. 2 and 4 were placed in new buildings located higher on the mountainside next to the reactors. All six reactors were given access to generators housed outside of the vulnerable turbine buildings.

The switching stations for reactors Nos. 1 through 5 were in the poorly protected turbine buildings and that's where they stayed. (Because of its more-advanced original design, No. 6's switching station was already in the reactor building.)

Explosions at Nos. 1 and 3 severely damaged those reactor buildings. Hydrogen leaking from No. 3 is thought to have triggered a blast at the No. 4 reactor building, and No. 2 probably had an explosion, too. The multiple blasts released radiation into the outside air.

In contrast, reactors No. 5 and 6, and all four reactors at nearby Fukushima Daini, safely reached cold shutdown. At No. 6, the newest generator housed in a separate building kept working and supplying power through its undamaged switching station, secure inside the reactor building. Tepco was able to use that power to keep equipment at neighboring No. 5 running. (WSJ, 7/1/2011, photo and graphic courtesy WSJ)