The Tokyo Electric Power Company, Inc. (TEPCO) has been cautiously continuing the
operation of Unit No.7 of the Kashiwazaki-Kariwa Nuclear Power Station by using
power suppression testing*1 in order to verify that radiation leakage from the
identified leaking fuel assembly is appropriately restrained and that the assembly's
state remains stable. During this operation, operational and monitoring data have
been collected and compiled for enhancing our knowledge.
Unit No.7 was also due for a shutdown for a partial replacement of fuel in late
September 2009 before its scheduled regular inspection. This was a decision made in
consideration of comprehensive issues, including that Unit No.7 was the first plant
that resumed operation after the Niigata-Chuetsu-Oki Earthquake.
The collection and compilation of operational and monitoring data for Unit No.7,
which continued until today, will end as we start the procedure to reduce the output
of the generator at 10:00 a.m. on September 25, 2009 and stop power generation at
around 11:00 p.m. Subsequently, the reactor will be shut down at around 1:00 a.m.
on September 26.
After the reactor shutdown, all the fuel assemblies loaded in the reactor will be
subjected to a sipping test*2 to detect the source of the radiation leak, and the
identified leaking assembly will be replaced. For the sake of providing better
reliability, the leaking assembly as well as 96 other assemblies that are not
equipped with a foreign matter filter will be replaced with a new type of fuel
assembly with a foreign matter filter*3.
Any fuel assemblies from which radiation leakage was detected during the sipping
test will be subjected to detailed inspection, including visual inspection, in
order to investigate the cause of the leak.
The results of the test will be summarized and reported to the national and local
governments.
TEPCO will step up efforts to make its power stations more resistant to disasters
and thereby provide better safety and peace of mind. At the same time, TEPCO will
appropriately perform inspections, evaluations and restoration work at its plants
and will responsibly explain the results to people in the local community and
society in general, just as it always has.
*1 Power suppression testing
When a radiation leak from fuel occurs during plant operation, where the leaking
assembly is loaded can be identified by operating control rods while restricting
the output of the plant, and then by taking high-sensitivity off-gas monitor
readings during that operation. This method also allows the plant to continue
stable operation at rated output, with the control rods around the identified
leaking assembly inserted for the restriction of output. Power suppression
testing is a proven technique that has been used in many cases for the purpose
of continuing plant operation.
*2 Sipping test
An inspection to identify a leaking fuel assembly by measuring the concentration
of the gaseous radioactive material released from the given assembly. For
measurement, the target fuel assembly, which is loaded in the reactor, is lifted
a few meters to reduce water pressure applied to it.
*3 New type of fuel assembly with a foreign matter filter
A fuel assembly equipped with the filtration function at its bottom in order to
further reduce the influence of foreign matter on the fuel.