Fukushima 3rd Anniversary; Technical Report Unit 2
These events and new information were found on unit 2 within the last year. TEPCO announced in July that they think there may be leaks in the torus structures of units 1-3. At this point they didn’t identify a specific location on the unit 2 torus as the point of failure.
In April 2013 TEPCO sent in robots to inspect the MSIV room of unit 2. This room contains the large valves that open and close the steam lines that route from the reactor vessel to the turbine building. The room was unusually pristine with no visible damage or water leaks. The radiation levels were also quite low. By contrast the MSIV room of unit 3 had a damaged door, much higher radiation levels and contaminated water leaking from the room.
An investigation of the TIPS room at unit 2 discovered the probe tubes into the reactor vessel were blocked with melted debris. The substance appeared to be a mix of melted metals and melted fuel and read at 200 mS/h.
In late July it was admitted that the torus of unit 2 is leaking. TEPCO was unsure of the exact location.
Another attempt to scope the containment structure at unit 2 was done. This found new damage near the reactor pedestal. Dark charred areas and a broken control rod drive rail were found near the pedestal. Later reports showed radiation levels that were much lower for this work than previous attempts to look inside containment.
A later report showed charred debris that is similar to what melted fuel debris looks like on the edge of the control rod opening into the pedestal.
The containment pressure inside unit 2 began to drop in August. TEPCO made a vague excuse about an open valve that they later shut. The lower pressure trend continued and has been the norm for 2014 so far.
In early 2014 an ultrasound capable robot crawled the outside of the steel torus tube of unit 2. It discovered that the water level in the torus tube was 10-30mm lower than the water inside the torus room. TEPCO still does not have a specific mechanism for the water leakage out of unit 2’s containment.
The refueling floor of unit 2 has been too radioactive to allow workers to enter. An innovative method to insert a robot to the refueling floor and a set of periscope cameras will be installed to survey the refueling floor. Once more data is gathered TEPCO will decide if they are going to try to repair the equipment on the refueling floor or if they will remove the roof and install a new building over the top similar to what they are doing at unit 4.
TEPCO reported that unit 2 has a 3cm hole in the torus in January but did not provide further details.
Accurate temperature readings at unit 2 have been an ongoing problem. Workers short circuited a thermometer installed to help monitor the temperatures inside unit 2. This leaves even more uncertainty about the conditions inside unit 2.
These bits of data help understand the situation inside unit 2. None of this has given a solid identification of the melted fuel location. Previous data showed the likelihood of melted fuel in the pedestal but newer data shows major changes in the same areas. Much more inspection effort is needed at unit 2 to understand what has transpired. A scope inside the pedestal could reveal some badly needed answers.
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