On March 11 2011, turbines and reactors of Fukushima Daini Nuclear Power
Station Units 1 to 4 (Boiling Water Reactors, rated output 1,100
Megawatts) that we had been constantly operating at rated thermal output
were automatically shut down at 2:48 pm due to the Tohoku-Chihou-
Taiheiyou-Oki Earthquake.
                                       (previously announced on March 11)

At 7:15 am on March 15, the reactor of Unit 4 achieved cold shutdown. As
a result, all reactors of Units 1 to 4 at Fukushima Daini Nuclear Power
Station achieved cold shutdown.
                                       (previously announced on March 15)

As of 3:00 pm on October 24, the reactors of Units 1 to 4 are in cold
shutdown (please refer to the appendix). We will continue to make our
endeavors to further stabilize each plant of cold shutdown. 

<Updates are underlined> 

○ Unplanned radiation exposure of a partner company worker at Unit 4 
Today, we confirmed at 2:50 pm that a worker of partner company who was 
engaged in cleaning inside of Primary Containment Vessel of Unit 4 was 
exposed to an unplanned radiation amount more than 1 mSv*1, 1.58 mSv per
day.
The radiation exposure amount of the worker is not more than the exposure
limit*2 prescribed by law and it does not cause physical harm and no
radioactive materials were attached to the worker.
                                     (previously announced on October 17) 

<Result of the investigation> 
As a result of the investigation, the following matters were found out: 

· The place where the cleaning task was performed was in high radiation 
  dose area partitioned by shield mats. There is Clean Up Water System*3 
  Pipe Arrangement in the area. 
· Although a radiation administrator recognized high radiation dose by 
  prior measurement on September 7, another radiation administrator 
  missed high radiation dose (34.0mSv/h) on the surface of the pipe 
  arrangement in pre-operation measurement on October 14, as he did not 
  bring the data of September 7. Therefore, he set working times of 20 
  minutes without any respect to access to the pipe arrangement, taking 
  into account airborne radiation (2.0mSv/h) measured in advance of the 
  start of the cleaning work. 
· Although the radiation administrator had the cleaning workers know that 
  the work would be in high radiation dose area, he did not inform them 
  that it was especially high on the surface of the pipe arrangement. 
· There was no indication of high radiation dose area on the scene since 
  we restricted access to the persons in charge of the work to the inside 
  of the Primary Containment Vessel. 
· The cleaning worker did not find that his audible alarming personal 
  dosimeter*4 (hereinafter called, "APD") sounded due to noise, since he 
  had on his hood mask and was using a cleaner. 
· No consideration was made in advance for radiation dose lowering since 
  it was not a periodic inspection work. 

<Probable Cause> 
In the cleaning work, the radiation clerk missed the point of high 
radiation dose (Clean Up Water System Pipe Arrangement) confirmed by the 
prior measurement and set the working time based on the airborne 
radiation without any respect to access to the point. Furthermore, he was 
not aware of the sound of his APD and kept the work, since he had on his 
hood mask*5 and was using a cleaner. 
We supposed that the unplanned radiation exposure happened due to the 
abovementioned matters. 

<Countermeasures> 
· Radiation clerks will conduct pre-operation measurements using 
  preliminary measured data. 
· In works around high radiation dose area, radiation dose will be 
  measured in detail prior to the works taking into account work method, 
  and working time will be set based on the results of the measurements. 
  Moreover, workers will be informed of the results and then start their 
  works. Those will be thorough steps. 
· Results of measurements in high radiation dose area will be displayed 
  on the equipments and the shield mats and thus workers' attention will 
  be drawn. 
· Review meetings for radiation dose lowering will be held prior to works 
  in the area where radiation dose may exceed a certain amount at the 
  planning phase. 
· Cleaning work entering areas partitioned by shield mats will be 
  forbidden in principle. In the case that such entry is necessary, 
  workers must have on a whole face mask*6 so that they can confirm the 
  sound of their APD, or must use a APD with vibrator. 

*1: 1 mSv per day
It is prescribed by an official notice of Industrial Safety and Health
Act, "Reinforcement of measures for safety and health in radiation work
at nuclear power facilitiy", that work which may exceed 1 mSV per day
shall be notified to Labor Standards Office. As for this work, the
planned exposure was 0.9 mSv/day and it was not subject to notification.

*2: exposure limit
The exposure limit prescribed by law for workers who engage in radiation
work is 100 mSv/5years and 50 mSv/year.

*3: Clean Up Water System
Purification system to clean up impurities in the reactor water and to
maintain water quality. This is also used to drain off redundant water in
the reactor and to control reactor water level.

*4: audible alarming personal dosimeter
An instrument to measure radiation dose exposed by a worker himself and
entry time to controlled area. Alarm rings when radiation dose or entry
time reaches a certain level.

*5: hood mask
A protective equipment for aspiration with electric fan. This is used in
radiation controlled area to keep radioactive materials from being taken
to the body through mouth, nose and so on. One's whole head is covered by
this protective equipment.

*6: whole face mask
A protective equipment used in radiation controlled area to keep
radioactive materials from being taken to the body through mouth, nose
and so on. Only one's face is covered by this protective equipment. 

Appendix: TEPCO Plant Status of Fukushima Daini Nuclear Power Station 
                                                         (PDF 18.8KB)