Report On Fukushima Unit 1 Containment Inspection 2015
Hitachi’s shape changing robot (at least one of them, they have more) is abandoned and dead inside the containment of unit 1. The robot did manage to transmit quite a bit of information and was still working two days later when they cut the cord. This is a small bit of good news as it proved that the robot model could withstand fairly high radiation for a long period of time without the camera or electronics failing. The newest report considered that the robot may have gotten wedged as it tried to navigate between an HVH box and a pump. There was a piece of debris to one side so they set the robot to the far side between the two structures and that appears to be where it remains stuck. This point is one of the narrowest sections of the planned route. A photo of the mock up of this location shows a narrow passage and what looks like nearby pipes. So the failure may have been one of circumstance rather than equipment failure. This robot model does apparently have a problem freeing itself, one of the down sides of the small size.
TEPCO provided a limited number of images taken by the robot. The video clips provided had better quality video. They showed what appears to be remains of blistered yellow paint on the top side of the floor grating in places. The underside is mostly charred and black. Similar blistered paint was found inside unit 2’s containment in places, that paint was more of an orange color. Miscellaneous pieces of debris were found, some appeared to be random pieces of equipment. A couple of larger pieces could be concrete but the image quality is too poor to confirm. What appears to be a charred or blistered pipe can be made out along with a large valve wheel in one section of the video. TEPCO did confirm that the area in the cat walk that leads to the lower level in containment is open and they should be able to pass a robot down through the opening in the future. (images and diagrams below)
Temperatures found along the cat walk were stable and comparable to temperatures found in containment in 2011
NHK reported that they considered the cloudy wafts on the video to likely be steam coming from melted fuel under the standing water in the bottom of containment but didn’t explain where they got that information from or if it was their own conjecture. Outside temperatures on the day of the inspection was about 9c. So the general temperature on the catwalk of about 20c is certainly warmer than outside. It is also warmer than the temperature readings normally taken inside containment that average around 16c most of the time.
There was a brief peak to 24.9 Sv/hour at 14:16 on the video. This high reading appears to be somewhere between location B11 and B13. Otherwise the radiation levels ranged between 7 Sv/hour and about 13 Sv/hour. These may differ from TEPCO’s official readings if they stopped to take a sustained count at a location. Sensors put the radiation level in containment at around 5 Sv/hour back in 2012. Previous scope work found about 11.1 Sv/hour at the entry point.
Other inspection work of unit 1 found varying radiation levels.
- A scope inspection of the torus room in 2013 found over 900 mSv/hour near the torus tube.
- A 2012 scope inspection of the torus room in another location found readings as high as in containment and then went completely off the charts as they dropped the instrument lower into the water. TEPCO claimed the instrument must have malfunctioned but have done nothing to confirm that or reinspect the area.
- Readings around the venting pipe for the torus were found to be about 2.4 Sv/hour in 2014.
- Readings for the same pipe as it passes through the first floor were 5 Sv/hour
- A reading of 1.64 Sv/hour was found in the torus room near where they found a melted sandpocket drain pipe, the pipe appears to be plastic.
- The base of the vent stack outside was found to be 25 Sv/hr. This vent stack is shared with unit 2. At this point it isn’t completely clear what unit contributed to the high readings of the stack. Unit 2 could be the major contributor here.
Where the robot appears to have gotten stuck
Image capture from the video showing what appears to be a charred or blistered pipe and a valve wheel. The yellow on the upper side of the catwalk can be seen. (Please excuse the watermark on the video, we are working on producing unmarked captures after the software problem is sorted out)
This section showed considerable debris including a turnbuckle. TEPCO mentions that this is in front of the PLR pipe. They also included a diagram of a blanket cover over the PLR pipe that was installed before the disaster.
This mock up of the containment catwalk shows the tight location where the robot stuck itself.
The PLR pump box mentioned in the reports of the work would look something like this one below. The image is of a PLR pump at Kashiwazaki Kariwa. Those reactors are newer ABWR units so the design may vary but this gives a rough idea what the PLR box might look like
All of the related images for the inspection work can be found at the link below. We are continuing to analyze the data found in this inspection and may have follow up reports soon.
TEPCO Handout http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2015/images/handouts_150413_01-e.pdf
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