Fukushima 3rd Anniversary; Technical Report Unit 4
Unit 4 saw the completion of the defueling building. Since early in 2011 TEPCO has considered the removal of fuel from the spent fuel pool to be an urgent matter. An emergency set of concrete supports were installed under the pool in 2011 to try to shore up the pool but concerns remained.
The cantilever design defueling building has been under construction for most of 2013. It was ready for use by September. The building interior gave the appearance of a shiny new unit 4 but beneath the new equipment resides the damaged hulk of the building. New cranes, an independent deck and a new elevator area were included in the defueling building. TEPCO has promoted this work as progress and success. In a way it has been, but the need for it has been a dire one.
As work began to remove the spent fuel from the pool TEPCO admitted there were a number of damaged fuel assemblies. Initially one damaged assembly was reviewed, finding cracks in the outer assembly cover. Later more extensive reviews found even more damage among assemblies. The damage included:
- significant signs of over heating near the top area of the assemblies and at the bottom
- damage to the lifting bails most likely from falling debris during the explosive damage to the buildings
- discoloration from potential excessive heat or oxide deposits
- varying depth scratches and dents along the length of the fuel assemblies
- horizontal crack identified in one of the fuel assemblies suggesting overheating of a spacer within the fuel assembly damage during fuel handling in the reactor core or spent fuel pool
- debris material at the lower ends which may be residue left over from heated debris formed on the fuel assembly accumulated explosion material that has settled into the bottom fuel rack area
Fuel removal began on November 18, 2013 but not without risks. We outlined here the inherent risks in the fuel transfer process. So far there have not been any major incidents in the fuel removal process but the risk remains until all of the assemblies are removed. The fewer assemblies in the pool, the lower the overall risk.
TEPCO has gone back and forth between wanting to have the press report on what they considered a success and an odd secrecy around the details of the casks being used. A press helicopter caught one of the first transfers on video, causing TEPCO to claim this violated security laws.
Two Yomiuri reporters were allowed into the containment structure for unit 4 and the torus area. They reported that the interior of unit 4’s containment structure was “heavily damaged” and that the torus room was flooded with contaminated water from unit 3.
By now about 418 fuel assemblies have been removed from unit 4’s spent fuel pool. Many more remain and there is no current plan for how they will remove the damaged assemblies. TEPCO hopes to have the spent fuel pool emptied within a year. They will need to continue emptying the common pool and moving older fuel to dry cask storage to make this happen. The latest report showed the common pool over 83% full and cask storage over 43% full.
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