Fukushima Daiichi NPS Prompt Report 2013

At around 1:00 AM today (on August 24), in Unit 2, we conducted on-site confirmation of the primary containment vessel (PCV) gas control system, etc. after noticing that the pressure inside the PCV had dropped and that the flow rate of exhaust gas from the PCV gas control system was on a downward trend.

As a result of the confirmation, we found airflow through a U-shaped sealing section in a drainage line installed at the entrance side of the line of this system. In response, we closed a valve located upstream of the U-shaped sealing section at 10:40 AM, and have been monitoring the state.

After the valve closure, we confirmed that both of the pressure inside the PCV and the flow rate of exhaust gas from the PCV gas control system were on an upward trend as at 3:00 PM. We will continue to monitor the parameters.

No change was found in the monitoring post readings after this incident occurred. In addition, for the conformation purpose, we conducted dust measurement on the Turbine Building 1st floor which corresponds to the exit part of the drainage line's U-shaped sealing section. The dust measurement resulted in values below the detection limit values, which we consider to indicate no abnormality.

<Unit 2 PCV pressure change over time>
･ At 11:00 PM on August 23: 13.19 Kpa g
･ At 11:40 AM on August 24 (after the valve closure): 6.62 Kpa g
･ At 3:00 PM on August 24: 7.18 Kpa g

<Flow rate change in Unit 2 PCV gas control system over time>
･ At 11:00 PM on August 23: 16.03Nm³/h
･ At 11:40 AM on August 24 (after the valve closure): 12.66Nm³/h
･ At 3:00 PM on August 24: 12.66Nm³/h