Important Report from TEPCO

The Tokyo Electric Power Company, Incorporated, would like to make comments on subjects that are reported by the newspapers, TV or websites.

The structure of Unit 4 Spent Fuel Pool (SFP) doesn't easily cause water leak.

May 30, 2012

1. The SFP is made of reinforced concrete which has a thickness of approximately 140 - 185cm, lined with 6mm-thick stainless steel plate. The pool is supported by a steel and concrete structure, mostly isolated from the 4th and 5th floor walls of Unit 4 Reactor Building, which has been damaged by the explosion.

2. The pool water is circulated by injecting from the top of the pool and collecting the flooded water from the upper edge of the pool which is sent to the Skimmer Surge Tank*. There are no pipes or drainage holes that pass through the concrete structure of the pool, either on the side or at the bottom of the pool.

3. We continuously monitor the water level of the Skimmer Surge Tank*, adding the appropriate amount of water to the pool, the amount which evaporates from the surface of the pool. We could detect any water leakage as an abnormal water level fall at the Skimmer Surge Tank*, should the pipes get damaged. Furthermore, the pool itself is equipped with water leak detectors that lie inside the gap between the reinforced concrete and the stainless steel plate.

4. A backwater valve -which is not driven by motor, is installed in the pipe that injects water inside the pool. Should the pipe gets damaged, the backwater valve shuts down automatically, preventing the pool water from flowing out.

5. The pool depth is approximately 11m, about 7m above the top of the 4m-long spent fuels. If the pool loses its cooling function by the stop of water circulation, it takes about 3 weeks for 5m-level amount of water to evaporate. It is possible to add water into the pool either by restarting circulation by making necessary repairs or by directly pouring water from the concrete pump vehicles which stand by at the station.

* Skimmer Surge Tank
A tank which is placed to catch the flooded water from the SFP.

On June 12, we corrected the thickness of the stainless steel plate from 3mm to 6mm.

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