On June 8th, 2011 at approximately 6:10 pm, during an inspection of the 
underground emergency tanks*1 of the High-voltage Start-up Transformer*2  
consisting of draining accumulated water from the tank's inside, a TEPCO 
employee verified the presence of oil that had leaked to the sea surface 
around the water discharge canal of Units 3 and 4. Presently, all water 
draining activities have been suspended. Furthermore, oil adsorption 
mattresses are being utilized to prevent the spread of oil and an oil 
fence barrier is being prepared.
                                    (Previously announced on June 8, 2011)

At 9:50 pm on June 8th, installation of the oil fences was completed. 
Afterwards, it was confirmed that an extremely thin sheet of oil had 
spread over the surface area of the sea enclosed by the oil fence.
Due to the March 11th earthquake, insulating oil from the high-voltage 
start-up transformer had leaked into the emergency underground tanks. 
On June 8th, during an inspection consisting of removing stored water 
from the tank, 32 ㎥ of water mixed with insulating oil had leaked to 
the sea surface.
The maximum estimated amount of the insulating oil was 0.5 ㎥.
                                    (Previously announced on June 9, 2011)

The following are the investigation results, the estimated causes, and 
the countermeasures taken regarding this event. 

[Investigation results] 
 We have found the following facts to be conclusive.
- Due to the March 11th earthquake, insulating oil from the inside of 
  the High-voltage transformer flowed into the water-proof underground 
  tank. Though we collected it, water containing remaining oil mixed 
  with rainwater was left in the tank. 
- A TEPCO supervisor verbally informed the workers in charge of the 
  commingling of insulating oil in the tank, but did not tell them to 
  pay attention to the risk of oil leakage from the tank.
- The TEPCO supervisor did not visit and inspect the site. He instructed 
  the workers to drain water minus a full understanding of site conditions.
- The workers from a partner company did not recognize that a mixture of 
  oil and water was left in the tank due to the earthquake or conduct any 
  surveys on oil content before beginning their duties. 

 In addition, we have found the following facts to be conclusive as a 
 result of investigating the leaked oil.
- We investigated the inside of approximately 940 meters of the drainage 
  canal from the tank to the water discharge canal and confirmed leaked 
  oil was accumulating in the drainage canal up to approximately 780 
  meters. 
- Based on the aforementioned conditions, we estimated that most 
  (approximately 0.49 ㎥) of the leaked oil from the tank (approximately 
  maximum 0.5 ㎥) had been accumulating inside the drainage and 
  approximately 0.01 ㎥ of oil (10 litters) had leaked to the sea surface. 
  We also found an extremely thin sheet of oil had spread over 
  approximately 250 ㎥ of the surface area of the sea enclosed by the oil 
  fence around the water discharge canal of Units 3 and 4. 

[Estimated causes]
 We have to conduct more inspections on the conditions of the sites, as 
 facility conditions may have changed due to the earthquake. However, we 
 ordered the draining of water minus a sufficient advance inspection and 
 did not clearly explain that there was oil remaining in the tank. 
 Therefore, we estimated that the event occurred as related employees were 
 unable to share that there was a risk of oil leakage into the sea due to 
 the drainage of water. 

[Countermeasures]
 We have completed collecting oil that has leaked up to the sea surface, 
 we will continue collecting oil that has accumulated inside the drainage 
 canal as well as implement the following countermeasures. 
- Based on the aforementioned conditions, we estimated that most 
  (approximately 0.49 ㎥) of the leaked oil from the tank (approximately 
  maximum 0.5 ㎥) was accumulating inside the drainage canal and 
  approximately 0.01 ㎥ of oil (10 litters) had leaked to the sea surface. 
  We also found an extremely thin sheet of oil had spread over 
  approximately 250 ㎥ of the surface area of the sea enclosed by the oil 
  fence around the water discharge canal of Units 3 and 4. 
- Regarding work at dangerous material storage facilities, a TEPCO 
  supervisor should be initially present at the scene of work to provide 
  work instructions. 
- Those who supervise the safety of dangerous materials should make others 
  aware of the conditions if they are not normal due to breakdowns and 
  tell workers to pay attention to them.

                                                                       END
*1 Emergency Underground Tank
   A tank used to store the leakage of insulating oil from the 
   High-voltage Start-up Transformer

*2 High-voltage Start-up Transformer
   A transformer supplying offsite power to the power station during the 
   plant's startup and shutdown. 

Appendix: Oil leakage conditions (PDF 72.1KB)