At the reactor building of Unit 1, Fukushima Daiichi Nuclear Power Station,
we need to install and calibrate a new reactor water level gauge and to 
establish a circulating cooling system to improve the status of the 
reactor to a more stable cooled condition. To do so, we have to improve 
the working environment inside the reactor building.
We will install an ambient air filtration system*, circulate air inside 
the reactor building for several days, remove radioactive substances with 
the filter and lower the density. In order for workers to enter the 
reactor building and conduct necessary works, we will open the airlock. 
Before opening the airlock, we will monitor the density of the radioactive 
substances within the reactor building and confirm that the influence to 
the outside is low enough.
We will announce beforehand in opening the airlock of the reactor building. 
At the same time, we will closely monitor and announce the figures at 
monitoring posts.
On May 2nd, we received the instruction from the Minister of Economy, 
Trade and Industry to report the work detail of "improvement of working 
environment inside the reactor building of Unit 1, Fukushima Daiichi 
Nuclear Power Station2, as well as the evaluation of the impact outside 
due to radioactive materials. On May 3rd, we organized the information 
based on this instruction and reported to the Minister of Economy, Trade 
and Industry. 
                           (previously announced on May 1st and 4th, 2011)

After the previous announcement, we have completed to install ambient air 
filtration system and ducts by May 5. Since May 5, we started the work to 
operate the system, circulate the air inside the reactor building, remove 
radioactive substances and lower the density of radioactive substances of 
the reactor building.

As a result, total density estimated as 4.8 Becquerel/cm3 based on the 
measured data on April 26th decreased to 1.97 x 10-2 Becquerel/ cm3, 
estimated based on the data taken at 3:15 on May 7th. Thus, sufficient 
decrease in density was confirmed and we will open the airlock from 8 pm 
today. (We will remove the ducts of the ambient filtration system set on 
the airlock in order to avoid sudden change of ambient air inside the 
reactor building.) We will remove the sheets used for installing the 
ambient air filtration system from 4 am tomorrow and workers will enter 
the reactor building to start measuring the radioactive density for the 
further construction work.
There are monitoring posts to monitor the radiation density of air on the 
site boundary of the power station. We evaluated the impact of opening the 
airlock on the figures of monitoring posts, which are approximately in the 
level of several tens of μSv per hour. We confirmed that that they would 
not affect the currently monitored level.

Attachement1:Impact evaluation of operating ambient air filtration system 
             at reactor building of Unit 1(PDF 10.1KB) 
Attachement2:Impact Assessment Results of Ventilating Reactor Building of 
             Unit 1(PDF 12.2KB) 
 
*Ambient air filtration system
This system vacuums up the ambient air ant the work area by a fan, filters 
impurities and clean the air.