Nuclear Engineer Identifies Mechanism for Potential Catastrophic Drain Down of Fukushima Unit 4 Spent Fuel Pool
Lucas W. Hixson
June 14, 2012
During a review of events concerning the status of Fukushima Unit 4, nuclear engineer Chris Harris identified the “weakest link” which may initiate a spent fuel pool draindown event.
A major portion of the water in the Fukushima Unit 4 Spent Fuel Pool remains over the Fuel because of the, “Refueling Bulkhead and Bellows” Seal (see drawing above).
Although this component has received little attention, its integrity is vital to maintaining Spent Fuel Pool Level. This is because in the current configuration (Refueling Mode) of Unit 4 and the known problem of leaking Refueling Slide Gates, the Drain Down of the Spent Fuel Pool could occur via Failure of the “Refueling Bulkhead and Bellows”.
The Gate is a long rectangular “dam” in the side of the Fuel Pool which can be removed after the Reactor Refueling Cavity Well is filled so that Fuel can pass through the opening (Slot). The Fuel Handling Machine is then able to pass the fuel safely submerged. The Gate has seals so that the Fuel Pool doesn’t drain into the Cavity and dangerously expose Fuel Assemblies when not in Refueling Mode.
TEPCO noted that the refueling gate seal is maintained by adequate pool water levels, and may be lost in the event of a LOCA sequence.
The leakage through the Gate is caused by an apparent physical distortion which causes an unintended Flow Path between the Reactor Refueling Cavity Well and the Fuel Pool. Evidence of this condition includes a January incident where Fuel Pool water level had been maintained by water from the Reactor Refueling Cavity Well.
If the Gate were undamaged and installed correctly, then there should have been no direct communication between the Well and the Pool.
Because the damaged Gate provides a non isolable Flow Path, any loss of water from Reactor Refueling Cavity Well will Drain the Fuel Pool to the Bottom of the Gate.
Read the full report here
Published: June 15th, 2012 at 4:19 pm ET
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