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”.
TEPCO Analysis of Refueling Slide Gate
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.
Reactor Refueling Cavity Well Seal
The Reactor Refueling Cavity Well is formed by the Removal of the Drywell Dome, the Reactor Vessel Head, and the Installation of the Reactor Refueling Cavity Well Seal.
The “Refueling Bulkhead and Bellows” or Reactor Refueling Cavity Well Seal is a thin flexible Stainless Steel ring that keeps the water from dumping down into the Drywell when filling the Cavity. When the Cavity level is equalized with the Fuel Pool Level, then the Gate can be removed.
With the leaking Gate, the inventory of the Fuel Pool is relying on the Seal’s integrity. The Seal is not intended for long-term use. It is not able to be replaced if damaged while in use. It is not robust enough to withstand misalignment due to Reactor Building Structural damage or earthquakes. The Seal has been in service for greater than a year, presumably with no maintenance. Additionally, this Seal and all parts of the Primary Coolant System and Fuel Pool have been exposed to Salt Water, an environment for which its materials have not been analyzed.
If the Seal were to fail, then the Fuel Pool would Drain to dangerously low levels right through the damaged Gate. Further, because the Seal Failure would cause drainage directly to the Containment Drywell, there would be no way to Refill the Fuel Pool.
Such Seal failures are not uncommon. Below is a short Operating History of similar Seal failures. See: “Spent Fuel Draindown Events.”
“A Local Problem for Japan or a Global Mega Crisis?”
The following Consequences are identified for a Loss of Spent Fuel Pool Level:
* a nominal release of 10% of the SFP 4 inventory of cesium and strontium would represent 3-10 times the March 2011 release amounts
* Spent fuel pools are not protected in the same way as reactor cores, and the unit 4 building is seriously damaged
* the fuel rods currently produce about one megawatt (MVV) or more of waste energy in the form of heat (1,000,000 Watts!)
* If cooling water for the spent fuel pool is lost – either by collapse of the pool, formation of cracks in the pool, or other factors – a major release of radioactive material could result. Given the large amount of heat generated by the fuel rods, the temperature would rise quickly. These rods are surrounded by zirconium cladding, and at high temperatures, this cladding catalyzes hydrogen production, can generate additional heat, and even explode and burn (NRC, 2006).
* The water surrounding the fuel rods in the spent fuel pools serves two purposes: First, it conducts heat away from the fuel assemblies to avoid overheating. Second, it provides shielding from the extremely high radiation levels near the rods.
* loss of shielding by the cooling water could critically increase radiation levels in the entire Daiichi complex. High radiation is already a serious problem limiting worker and even robot access to the plant to perform repairs and mitigation, and to maintain cooling of the other spent fuel pools and reactors. Thus, a catastrophic failure of the unit 4 spent fuel pool could potentially cascade into additional releases from the other spent fuel pools and reactors.
The Fuel Pool Gates need to be repaired such that the Cavity Seal is not relied on solely for the maintenance of Fuel Pool level. Eventually, the Fuel must be removed from the Fuel Pool and moved to a safe location.
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