Though water injection into Units 1-3 reactors at Fukushima Daiichi Nuclear Power Station is continuously done by the regular use reactor injection system, a TEPCO employee found that Unit 3 reactor injection water amount had increased from 5.8m3/h (as of 10:00 AM) to 7.0m3/h at 11:00 AM today during the regular data check.
The technical specification for the nuclear reactor facility*1 stipulates that an increase amount of reactor injection water from the regular use reactor injection system must be 1.0m3/h or less during any given 24 hours as an "operational requirement"*2.
Since the increase of Unit 3 reactor injection water exceeded 1.0m3/h, the shift supervisor on duty judged that the condition did not fulfill the "operational requirement" stipulated by the technical specification for the nuclear reactor facility at 11:00 AM.
As the "required measure to be taken" for this incident is to start implementing measures to reduce the increase amount of reactor injection water down to the allowed range, the reactor injection water amount at Unit 3 was set back to 6.0m3/h at 11:10 AM.
Unit 3 PCV gas control system indicates no change in xenon 135 density which is below the detection limit (3.4×10-1 [Bq/cm3]) and subcritical.
There has been no significant change in the monitoring post data and no leakage around the injection line between the regular use reactor injection system and the building entrance. The reactor injection water amounts at Units 1 and 2 remain stable.
We will investigate the cause of the incident and implement necessary measures.
*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 requirements
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.