With regard to the temperature of the lower part of the Reactor Pressure Vessel
of Unit 2 at Fukushima Daiichi Nuclear Power Station, since February 2, 2012,
it has shown a gradual increase at one point in the part and therefore we have
kept changing the water injection amount and monitoring the temperature. At
2:15 pm today, we confirmed that the temperature indicator has reached 82°C.
As the temperature of the lower part of the Reactor Pressure Vessel was not
below 80°C, at 2:20 pm today, we judged that it does not clear one of the
"Conditions for operation"*2, which are provided in the Reactor
Facility Safety Regulation*1.
When conditions for operation are not satisfied, as "Required countermeasures" *3,
it is required to take countermeasures against the temperature promptly, in
order to clear the conditions. We therefore injected water with boric acid as
countermeasure to prevent from reaching re-criticality from 11:38 am to 1:50 pm
today, and from 2:10 pm to 3:30 pm today we increased the water injection amount
through the core spray system from approx. 6.9m3/h to approx. 9.9m3/h.
We also adjusted the water injection amount through the reactor feed water system
from approx. 7.2m3/h to approx. 7.5m3/h. as it showed some fluctuation.
Although the temperature indicator for the lower part of the Reactor Pressure
Vessel of Unit 2 is increasing at the point, as there is only one point where the
temperature is rising and with respect to other areas the temperatures tend to
decrease due to the increase of water injection amount, temperature indicators
around the Reactor Pressure Vessel and in the Primary Containment Vessel show
decreasing and therefore the whole facility itself is presumed to be cooled, and
we could assume that there is some water around the point at stake and that it
cools down the point, judging from the relation between the entry pressure in the
Primary Loop Recirculation System and the amount of water injection in the
reactor feed water system, overall, we judge that cooling reactors works properly
now.
Considering this event, at 3:22 am today, we conducted a sampling of gas in the
Primary Containment Vessel of Unit 2 and have found that each of short half-life
nuclide Xe was below detection limit (9.5 x 10-2Bq/cm3) and the criterion
to judge re-criticality (1 Bq/cm3) and that there is no reaching criticality,
and we have confirmed that there is no increase of radiation (Cs-134 and 137).
There is no significant variation of the values of monitoring posts around the
site boundary of Fukushima Daiichi NPS and the consecutive dust monitors.
We will continue to the tendency of the temperatures, carefully.
*1 Reactor Facility Safety Regulation
Based on Clause 1, Article 37 of Act of the Regulation of Nuclear Source
Material, Nuclear Fuel Material and Reactors, it provides basic conditions
to be satisfied for operating nuclear power stations safely and keeping it
stable (operation management, fuel management, radiation management, emergency
treatment, and facility management based on "Mid-term safety securing" etc.).
It is approved by the national government.
*2 Conditions for operation
Reactor Facility Safety Regulation provides necessary conditions such as the
numbers of the permitted machines etc. or criteria of temperatures and
pressures for securing multiple safety function for operating reactors and
for keeping nuclear power stations stable and these are treated as conditions
for operation. When there happen some malfunctions of equipment provided in
the regulation and a nuclear power station can not clear the conditions
temporarily, operators have to take required countermeasures.
*3 Required countermeasures
Article 138 of Reactor Facility Safety Regulation provides that the temperature
of the reactor pressure vessels shall be below 80°C and that if it rises over
80°C, operators shall take countermeasures against the rising and clear the
temperature condition for operation.