On September 13, 2011, trace of iodine 131 (6 x 10-9 Bq/cm3, detection
limit 4.6 x 10-9 Bq/cm3) was detected during the weekly periodic sampling
in radiation monitor of main exhaust stack (*1).
Concentration of iodine detected this time was below measurement limit of
7 x 10-9 Bq/cm3 specified in the measurement guideline (*2) set by the
government.
There was no impact for the surrounding environment of the plant as no
change was observed in the monitor of main exhaust stack of unit 7 and in
the monitoring post located at the site boundary.
                   (previously released on September 14, 2011 in Japanese)

TEPCO has investigated the cause of the detection and compiled
countermeasures. As a result of the investigation, we have identified
following; 
1) During the periodic inspection, Unit 7 has placed countermeasures to
   control the release of iodine including the installation of local
   exhauster (*3), continuation of treatment in off gas system (*4) until
   the iodine concentration goes below sufficiently low level as fuel
   leak occurred during the operation in Unit 7.
2) The local exhausters located in the fuel pool cooling and filtering
   system (*5) and reactor building operating floor were stopped right
   after the leaked fuel assembly was moved to spent fuel pool for the
   investigation.
3) It is possible that the iodine moved into the air of reactor building
   operating floor as the iodine concentration in the spent fuel pool
   increased due to the stoppage of fuel pool cooling and filtering
   system while the leaked fuel assembly was stored inside the pool.

According to such investigation, TEPCO estimates that the iodine slightly
above the detection limit was detected in radiation monitor of main
exhaust stack of Unit 7 because the iodine concentration in the spent
fuel pool increased due to the stoppage of fuel pool cooling and
filtering system while the leaked fuel assembly was stored inside the
pool and such iodine moved into the air of reactor building operating
floor and reached exhaust stack through heating and ventilating air
conditioning and cooling system.

As a countermeasure to prevent the occurrence of the same incident, we
will build into the internal procedure that fuel pool cooling and
filtering system shall keep operation at least for a month after the
leaked fuel assembly is moved into the spent fuel pool.

We will continue implementing the sufficient release control in operating
the power plants.

*1: periodic sampling in radiation monitor of main exhaust stack
Periodic measurement conducted weekly basis in which the filter placed in
the sampling line of main exhaust stack is investigated for the existence
of radioactive materials in the form of gas. Iodine 131 detected this
time was from the filter placed during September 7 to 13, 2011.

*2: measurement guideline
Guideline by the nuclear safety commission of Japan setting the standard
to measure the amount of radioactive materials in the gas and liquid
waste released into the environment.

*3: local exhauster
Facility to absorb the iodine contained in the air by taking the air of
the operation area through fan and making such air go through the
activated carbon.

*4: off gas system
The system to release gas through exhaust stack after abstracting gas
which is not condensed in the condenser and reducing its radioactivity in
order to maintain vacuum inside the condenser. 

*5: fuel pool cooling and filtering system
Filtering system to maintain the water quality by removing the impurities
while cooling the water in the pool.