Fukushima Daiichi NPS Prompt Report 2013

This is follow-up information on the "water leak at a tank in the H4 area in Fukushima Daiichi Nuclear Power Station" found on August 19.

Following the water leakage from the drain valve at the H4 area tank dike, we have been conducting enhanced patrols of the tanks. During a patrol today (on August 31), we found 4 locations showing high dose equivalent rates (γ and β rays (70μm dose equivalent rate^*1)).
*1: 70μm dose equivalent rate = dose equivalent rate to the skin, etc.

We consider that there has been no leakage to the outside of the dike because the water levels of all of the relevant tanks have not decreased and also because the drainage valves have been closed.

· The floor surface of the connecting pipe section between Tank No.5 and Tank No.6 in Group IV in the H5 area: Approx. 230mSv/h (70μm dose equivalent rate)

· Near the bottom flange of Tank No.10 in Group A in the H3 area: Approx. 220mSv/h (70μm dose equivalent rate)
(This is the same location as the location that showed approx. 70mSv/h when the tanks were inspected on August 22.^*2)

· Near the bottom flange of Tank No.4 in Group B in the H3 area: Approx. 1,800mSv/h (70μm dose equivalent rate)
(This is the same location as the location that showed approx. 100mSv/h when the tanks were inspected on August 22.^*2)

· The bottom of Tank No.6 in Group II in the H4 area: Approx. 70mSv/h

For the positional relationship among the tanks, please refer to the following handout released on August 22:
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2013/images/handouts_130822_02-e.pdf

*2: These are locations where we measured the rates on August 22 and measured the rates again today (on August 31). The cause of the differences in values will be investigated.

With regards to the floor surface of the connecting pipe section between Tank No.5 and Tank No.6 in Group IV in the H5 area, the heat insulation material of the pipe placed above the floor surface was pressed after the high dose equivalent rate (not less than 100mSv/h (70μm dose equivalent rate)) was detected during a patrol. Then, one drop of water fell to the floor surface. The dose equivalent rate at a location on the floor surface to which the water fell was measured and confirmed to be approx. 230mSv/h (70μm dose equivalent rate).

Although water then stopped dropping from the connecting pipe, a discolored part (in a dry condition) was found on the floor surface under the pipe section. The size of this part was approx. 20cm×20cm. High dose equivalent rates were not detected in locations on the floor surface that are apart from the discolored part.

As emergency action concerning this location, we placed a drain pan at the discolored part on the floor surface, and wrapped the connection pipe with an absorption mat.

We will remove the heat insulation material of the pipe and investigate whether there has been contaminated water leakage, as soon as we are ready.

With regards to the other locations in the H3 and H4 areas, we confirmed that there has been no continuous water dropping, and will conduct detailed investigations as to whether there has been water leakage, etc. also in these locations.