Fukushima Unit 1 Reactor Well Inspection Results

The inspection of the unit 1 reactor well began in July, continuing into the end of August. A trio of remote controlled robots conducted the work. Radiation readings, images and dust swipes were collected as part of this work.
TEPCO’s newest report on this work here.

Our prior reporting on this inspection effort can be found here and here.

The robots doing the work:

Robot used for reactor well inspection with 3D camera and dosimeter to measure radiation

Robot with camera on a drop down cable, used to look down into the dislodged reactor well slabs.

Robot with smear collection device. This is used to collect dust and contamination smears for analysis.

Robots launch from the shielded box transported by crane to the refueling floor. Red checkered box in the below diagram is the shielded box location. This is on the tool pit side of the refueling floor (north), the opposite side of the building from the spent fuel pool.

diagram above shows camera view locations

Photo captions top left going clockwise:
Upper south plug and upper center
Through the gap of the rug, the roof is falling.
Top plug | Middle plug | Middle center plug | Middle east plug

A small pile of debris is identified in the red circle. TEPCO vaguely describes it as building debris.

Photo captions from top center going clockwise:
The middle plug floats up, It is in contact with the upper plug.
Middle plug | Middle center plug

The gap between the concrete slabs is large enough to be visible. Radiation levels in this location are high enough to cause interference on the camera. The random dots on the image indicate the interference.

 

Photo captions from top center going clockwise:
Middle east plug | Middle center plug | The east plug is inclined in the east direction.

Daylight can be seen in the distance. The far end of the upper slabs of the concrete reactor well cover were previously identified as being dislodged up out of the reactor well. Radiation levels are high enough to cause camera interference.

 

SUGV robot with a drop down camera:

This work dropped the camera on a cable down into the deeper portions of the reactor well.

Red dots = camera drop down locations
Yellow dashed lines = gaps in the concrete slabs
Center diagram = side view of image locations and north, middle and south slabs
Right diagram = camera locations from south side view. Slab locations and PCV containment cap in yellow.

Status of lower plug (image at the time of camera hanging)
1) from top center clockwise: Middle East Plug | Bottom Center Plug | Lower North Plug
2) top: Middle east plug | bottom: Lower south plug East end
3) top: Well Wall | Lower south plug

Images from this work show small amounts of radiation interference. SUGV’s drop down camera may be a different type of camera with higher radiation resistance, or the area is lower in radioactivity. All of these drop down inspection locations are around the outer areas of the reactor well.

Reactor Well Wall And PCV Cap:

SUGV was able to view the edge of the reactor well and down to the location where the yellow PCV containment cap bolts to the containment structure.

Image of the location of the shooting (from the perspective of the south side)
Captions from top center clockwise:
Upper middle east p | Survey Robots | Middle east plug | Middle center plug | Camera | PCV Head | Lower south plug | Middle plug

Location #4 is the edge of the reactor well. Location #5 is the bolt location of the PCV containment cap.

 

Captions unable to be translated.

This view of the reactor well wall shows what appears to be the exposed concrete of the well with the stainless steel liner peeled back. This is likely damage from the concrete reactor well cover slabs striking this area.


This image (above) shows work taking place at Fukushima DAINI. The reactors at Daini are newer designs than unit 1 at Fukushima DAIICHI. The reactor well areas are quite similar. In the photo along the top of the well, the stainless steel lined grooves for the concrete reactor well cover slabs can be seen. In the photo, the yellow PCV containment cap has already been removed. The RPV reactor vessel cap is in the process of being removed. In the lower area of the reactor well, a series of holes around the edge can be seen. These are the bolt holes for the PCV cap.

Inspection of the PCV containment cap base:

The drop down camera was used to look deep down into the reactor well. The area between the edge of the PCV containment cap and the wall of the reactor well was identified.

Title: Accumulated water and water crest
Top image: Lower South Plug
Diagram captions top right clockwise: Flange bolts | PCV cover | PCV | Drain | Pooled water range (Estimation) | Well Seal Bellows | Rib | Bellows top Plate surface | Well Wall | PCV Flange Surface  | Rib

A small amount of water was seen pooled in the bottom of the containment cap flange. No radiation readings for this area were provided.

The photo below shows a similar location at Fukushima DAINI taken during defueling work.

 

Radiation Measurements:

From the report:
At each measurement point, the dose is measured by switching the height and orientation of the dosimeter (up and down).  As a result of the measurement, the trend of high dose near the center of the central plug was confirmed.

Left diagram: Height of the middle plug floor 20mm, dosimeter downward
Where the dose rate is lowest (Approx. 640mSv/h)
Where the dose rate is highest (Approx. 1970mSv/h)

Right diagram: Height and dose recording of 240mm of middle plug floor
Where the dose rate is lowest (Approx. 630mSv/h)
Where the dose rate is highest (Approx. 1510mSv/h)

*The measured value is γ-ray dose rate.

 

left:
Height of the middle plug floor 250mm, dosimeter downward
Where the dose rate is lowest (Approx. 730mSv/h)
Where the dose rate is highest (Approx. 1560mSv/h)
right:
Height and dose recording of the middle plug floor 470mm
Where the dose rate is lowest (Approx. 690mSv/h)
Where the dose rate is highest (Approx. 1360mSv/h)

left:
Height and dosimeter downward of the middle plug floor 470mm
Where the dose rate is lowest (Approx. 600mSv/h)
Where the dose rate is highest (Approx. 940mSv/h)
right:
Height and dose recording of the middle plug floor 690mm
Where the dose rate is lowest (Approx. 460mSv/h)
Where the dose rate is highest (Approx. 820mSv/h)

*The measured value is γ-ray dose rate.
Date of measurement
July 25 and August 21, 2019
Note: The upper and middle plugs

Clearance determines measurement point

 

These depth related radiation readings indicate test site #2 as having the highest radiation and at the shallowest depth. Site #3 had the next highest reading but was taken at a deeper location. The north side of the reactor building is the side of the tool pit and the side that faces the administration building on site.

Based on this location for site #2, we looked at photos of refueling work at similar reactors.

This photo from Fukushima DAINI shows an access hatch on the PCV containment cap. The location of this hatch is near the tool pit divider wall. In this photo the thick removable divider wall can be seen off to the left. If the location of this hatch on the DAINI reactor is roughly the same as the location of the hatch on the PCV containment cap for unit 1 at Daiichi, this could explain the higher readings at this location. We do sometimes find deviations in the location of various features of the different builds of GE boiling water reactors, while some features remain the same across builds and models. It is a known problem during a reactor event, including meltdown type events, that the gaskets of access points into the containment structure are prone to failure. Heat and high pressure can cause gaskets to melt or otherwise fail. If this hatch location is accurate for unit 1, this may be the cause of this higher radiation reading.

The diagram above of a GE Mark 1 reactor similar to unit 1 shows the various layers of the reactor well. The reactor vessel cap has a vent line as shown in the top center of the cap. This is a known weakness and potential failure point in a reactor event.

The photos above of the unit 4 Daiichi reactor cap (RPV cap) shows two vent lines. The actual location of those when the cap is installed had not been identified from old photos. There are slight design differences between the model of unit 4 and unit 1. These vent lines are a potential route for failure and contamination.

The radiation readings found on this inspection are lower than those found for the reactor well area of unit 3. Unit 3’s readings so far have only been taken at the surface of the refueling floor and are considerably higher. Radiation readings at other locations inside unit 1 are higher than these refueling floor readings. The unit 1-2 vent tower, the torus room floor and a location near the TIPS room on the first floor of the reactor building are higher than these reactor well readings. We also noted in the past that unit 1’s reactor well did not release steam and smoke as units 2 and 3 did.

These new findings provide more evidence towards determining the failure scenario for unit 1. Upcoming containment inspections may provide more clues to combine with this information towards a better understanding of unit 1’s failure.

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