This is one section of our annual report, the full report can be found here.
We will be posting a section per day over the next week for ease of reading.
Spent fuel removal at unit 3 has been the main priority over the last year. After unit 4, unit 3’s spent fuel is considered one of the highest risk sources on site. Spent fuel removal began in April 2019 and is expected to be completed by the end of FY 2020 (March 2021).(2) Work was slowed by repeated malfunctions of the remote fuel removal equipment. This robotic equipment is used to remove spent fuel without direct human contact.
The equipment places spent fuel assemblies into a transit cask then prepares the cask for removal from the building. Failures of electrical components and robotic armatures have slowed this important work.
The remote equipment used to remove spent fuel from the unit 3 spent fuel pool suffered another failure in February. The left manipulator arm failed. TEPCO cites that they can continue to do fuel removal work with just the right manipulator arm. (28)
Damage to spent fuel assemblies in the pool was found as work progressed. A closer view of the assembly tops showed many had distortion or bent lifting handles. The condition of the assemblies down in the fuel racks is unknown. Earlier monitoring of the water in the pool didn’t indicate widespread fuel damage. This doesn’t mean assemblies are all in safe condition. Damage could make removing them from the fuel racks problematic. (29)
Currently, 13 deformed fuel assemblies are identified in unit 3’s spent fuel pool. The most recent group was found in January.
As of January 2020, 56 fuel assemblies had been removed from unit 3’s spent fuel pool. (30)
Of the 56 assemblies, 52 were unused fuel. The new assemblies have all been removed. All other fuel assemblies removed from the pool are spent fuel. A photo provided by TEPCO shows how much debris still remains in the spent fuel racks. (31)
(Top) Unit 3 spent fuel pool damage with fuel rack map.
(Bottom) Spent fuel assembly with damaged lifting handle.
A cask of spent fuel removed from unit 3 in February suffered a problem when workers tried to unload the fuel in the common pool. A fuel assembly became wedged in the square channel box of the fuel cask. (image below right)
Workers attempted to break the channel box free with a long pipe to no avail. The cask with the stuck fuel assembly will be given a location in the common pool to be stored there until they determine what to do with it. (32)
Unit 3 new questions from NRA
In January Japan’s nuclear regulator (NRA) announced their completed inspection inside unit 3. The inspection work managed to navigate part of the 3rd floor of the reactor building. This brief trip inside collected significant information towards understanding the failures at unit 3. (33)
New evidence of the directional nature of the reactor explosion was found. The extensive damage reinforced the concern that the blast during the meltdowns had a specific initiation point and was actually two explosions. Among this new evidence, the 4th floor area adjacent to the tool pit appears to be the initiation point or the area of the most severe explosive force outside of containment. We cover this new information more extensively than we could here, in our January report. (33)
Researchers in Japan and the UK developed a 3D imaging swimming robot capable of identifying fuel debris. In tests at the JAEA research facility the robot was able to successfully complete the task. This robot was made with some off the shelf parts and only cost $150 USD to make. No word yet if this would be deployed at unit 3.(34)
Work to further inspect inside unit 3’s containment
structure has been delayed indefinitely. The high water level in containment was cited as a complicating factor in any future fuel debris removal work at unit 3. The containment hatches that are the best candidates for introducing
larger equipment into unit 3 are all under water. Unit 3 has a higher retained water level than the other units. This unit is considered the least air tight containment structure of the three units. Radiation levels in the areas where fuel debris removal equipment would be installed remains high at unit 3 further complicating matters. (3)
Photo (right)showing the channel box stuck on the spent fuel assembly (photo TEPCO)
Unit 3’s fuel debris pile is larger than what can be observed at unit 2. Internal pedestal structures are more heavily damaged than what was found inside unit 2. This may be due to the massive explosion at unit 3 or a different melt through scenario that put more of the molten fuel into the pedestal in a short period of time.
Work to reduce the amount of contaminated stagnant water in building basements around the plant site discovered that the water in the unit 3 reactor building was highly contaminated. (3) Workers found highly radioactive water in the turbine building for unit 3 back in 2011. The specific cause for both has never been admitted by TEPCO.
Photo (below) of fuel debris in the unit 3 pedestal (photo TEPCO).
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