Water leak within Fukushima Daiichi 2 reactor building

At 1.05pm on 9 August, a Tepco employee found that the water level of the unit 2 used fuel pool skimmer surge tank - used to confirm that the used fuel pool is full of water - had decreased. In order to investigate the cause of this decrease, at 4.36pm on the same day the primary pump for the used fuel pool cooling system was intentionally shut down.

Tepco said it had evaluated that with the cooling shut off, the temperature of the unit 2 used fuel pool water should initially rise about 0.06°C per hour and reach a maximum temperature of about 46°C, remaining below the limited condition of operation of 65°C.

The leaked water flowed into a drain on the floor connected to the water collection pit located in a room of the first basement floor.

A diagram of Fukushima Daiichi unit 2's used fuel pool system (Image: Tepco) 

At 6.52am on 10 August, Tepco confirmed that the level of stagnant water in the room had stopped rising and therefore determined that the leak had stopped. Furthermore, it confirmed that the level of stagnant water was lower than the subdrains in the vicinity of the building and that the leaked water has been contained within the reactor building.

"We will continue to monitor the unit 2 spent fuel pool (SFP) water level and its temperature and remain on standby so that we can implement SFP circulation cooling whenever necessary as we deliberate further investigations and countermeasures," Tepco said in a 13 August statement.

It added: "Based on field dose investigation results, we will formulate an investigation plan that utilises the remotely operated robot (SPOT). In particular, we shall inject filtered water into the skimmer surge tank and examine water leaking from the equipment in the FPC [fuel pool coolant cleansing system] pump room/heat exchanger room. Based on the results of the aforementioned investigations, we shall deliberate countermeasures for the future."

International Atomic Energy Agency Director General Rafael Mariano Grossi said on 15 August that Tepco had informed the IAEA about the leak. The agency said "it is important to highlight that this event is not related to the ALPS treated water discharge" adding that the IAEA  "was not informed about any breach of the radiation protection standards", and pledged to "continue to keep Member States informed of further developments, as appropriate".