Though water injection into the reactors of Unit 1-3 at Fukushima Daiichi Nuclear Power Station is continuously done by the regular reactor injection system, a decrease in the reactor injection water amounts at Unit 1-3 was found at 3:00 PM on August 30, 2012 at the regular data check. Since the technical specification for the nuclear reactor facility*1 stipulates an "operational requirement"*2 that the regular reactor injection system must maintain the necessary water injection amount, the shift supervisor judged that the "operational requirement" was not fulfilled at Unit 1 at 3:07 PM, Unit 2 at 3:00 PM and Unit 3 at 3:05 PM. (Previously mentioned on August 30, 2012)
Since this issue occurred on August 30, 2012, we have been monitoring (and adjusting when necessary) the reactor injection water amounts and investigating the cause of the issue. During the process, suspended materials were found in the buffer tank where the reactor injection water is stored in.
In response to this, we conducted buffer tank water purification utilizing an underwater pump to eliminate the suspended materials from September 8 to 10 and captured substances that are likely to clog the valves.
The decrease in the reactor injection water amounts is assumed to be due to the substances which flowed into the buffer tank and got stuck in the flow rate adjustment valves in the downstream side, narrowing the water flowing channel as a result. However, as no flow rate decrease was found since September 7 before the buffer tank purification was started, it is likely that some of the substances causing the valves to be clogged had already been eliminated by the time the purification was started (Some of the substances were caught in the strainer of the air-cooled chiller used for cooling the buffer tank water and others flowed towards the downstream side of the flow rate adjustment valves when the valves were adjusted).
Though we have continued to monitor the reactor injection water amounts until today, the shift supervisor has judged that the condition had recovered to the state which fulfills the operational requirement at 4:00 PM today, considering that the water injection amounts have been stable since the flow rates were adjusted on September 7 and that no substance which may cause a flow rate decrease was found in the strainer of the air-cooled chiller (used for buffer water cooling) this morning.
The reactor water injection amounts as of 4:00 PM on September 13 are as follows.
Unit 1: 4.8m3/h (Necessary water injection amount: 3.8m3/h)
Unit 2: 6.7m3/h (Necessary water injection amount: 5.4m3/h)
Unit 3: 6.8m3/h (Necessary water injection amount: 5.4m3/h)
We will continue our efforts in maintaining cold shut down condition and ensuring plant safety.
End
*1 Technical specification for the nuclear reactor facility
Government-approved fundamental rules for nuclear power station operators to follow in order to maintain safe operation and stable condition of nuclear power stations (including operation management, fuel management, radiation management, emergency measures and equipment/facility management based on the "Policy on the Mid-term Security"), which are stipulated by Article 37 Section 1 of the Act on the Regulations of Nuclear Source Material, Nuclear Fuel Material and Reactors.
*2 Operational requirement
In accordance with the technical specification for the nuclear reactor facility, operational requirements are stipulated to ensure safety and stability of power stations (such as the number of available equipments, temperatures and pressures). In the case that an issue arises with equipment subject to the technical specification and the operational requirements cannot be satisfied temporarily, appropriate measures must be implemented.
