Fukushima Unit 1 has been the site of significant new work on multiple fronts. We outline the new events and what they mean towards unraveling the aftermath.
Reactor Well Inspection
Start date: July 17, 2019
TEPCO has been conducting inspection work in the reactor well area of unit 1 in the latter half of July. Actual inspection work began July 17. The totality of the inspection effort has not been published so far. A few hints at what they have found were included in the road map documents released at the end of July.
The image below is from the robots inspecting beneath the reactor well concrete slabs. A pair of robots are being used to crawl between two layers of the dislodged slabs. The photo taken facing the spent fuel pool shows the fuel transfer gate in the background. Significant amounts of small debris are seen between the slabs. TEPCO provided no explanation for how this debris was created or became located within the slabs. The left photo is before debris removal, the right photo after. TEPCO did not explain the debris removal process. It is assumed the robot pair removed or moved the debris.
The diagram below shows the location of the robot storage box in red. The highlighted area with the number 3 in a circle shows the photo location and direction for the above photo where debris was moved. The inspection work in the reactor well is scheduled to continue into the end of August. This work includes taking 3D images, radiation readings and swipe samples to determine the types of contamination in the area.
Spent Fuel Pool Debris Removal
Start Date: March 2020
Status: in development
TEPCO has slowly progressed in removing debris from the refueling floor area. The debris around the spent fuel pool has been saved for last. Both the overhead crane and the fuel handling crane are stranded above the spent fuel pool. Work will need to take place to remove the layers of the roof, structural steel and various sizes of debris.
In conducting this work there is a risk of debris falling into the spent fuel pool. Larger pieces of debris could damage fuel assemblies in the pool. Smaller debris could cause more work later on to clean out the pool. To prevent debris from falling into the pool, TEPCO has created what can best be described as a giant air mattress.
The inflatable sheet will cover most of the spent fuel pool to catch debris that would otherwise fall in the pool. As the pool is currently covered with stranded equipment and building debris, deploying this sheet over the pool is not an easy task. A process explained in the diagrams below show how they plan to install this. It would be placed below the debris into the pool, while rolled up in a tube. Once over the pool it would be unrolled and inflated.
Diagram below shows how the floating sheet would cover the pool. The floating sheet appears in purple over the spent fuel racks.
A test run of the floating sheet concept was conducted. In an undisclosed location a version of the concept was installed and inflated on a shallow pool. TEPCO plans to install the working version of this concept at unit 1 in March of 2020.
Start date: currently undetermined
Status: preparatory work
Work to prepare a new access hatch into unit 1’s containment has been underway in recent months. The new hatch cuts through the personnel access doors, then places a contained tube and access port. The outer door has been cut through, the inner door cutting began in June. As the inner door was cut, an increase in radiation levels inside containment was detected. TEPCO cites the cutting time to be 6 minutes. The radiation levels eventually went back down to typical levels inside containment after work stopped. TEPCO didn’t record any increase on the monitoring equipment past the filtration system. TEPCO assumes this was caused by the water jet used to cut through the door striking a contaminated surface inside containment, or that the door itself harbored a concentration of contaminated residues.
Work to continue cutting the inner door is scheduled for September and beyond. No date for the actual inspection is provided in the newest roadmap. This all pushes the inspection far down the 2019 calendar.
Unit 1-2 Vent Tower Removal
Started: August 1, 2019
After multiple problems the removal of the unit 1-2 vent tower is underway. Improper calculations and a change in the crane used to do the work caused the cutting rig to sit too low to install at the top of the vent tower. A platform of fill dirt was added to enable the crane to sit high enough to begin the cutting work.
On July 17, TEPCO confirmed that the raised platform of dirt would allow the crane to lift the cutting rig high enough to clear the top of the vent tower.
TEPCO hopes to have the vent tower cut down by March 2020. Asahi Shimbun confirmed that cutting work began on August 1st.
This work is not without risk. The base of the vent tower was over 10 Sieverts/hr in 2011, how much over that is not known as the available testing equipment reached its maximum capability. By 2015 the base of the tower was still at 2 Sieverts/hr. Cutting and removing parts of this tower has the potential to release radioactive contamination to the environment. TEPCO has never clearly documented the types of contamination within the tower. Sump drain water from the tower was collected but detailed data on the water was not made public. The threat of alpha radiation or highly radioactive microparticles of fuel are always a concern. The cut pieces of the tower will be moved to the sea front concrete deck where they will be placed in shielded containers to then be moved to longer term storage.
As all of this work evolves we will have more updates.
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