At around 2:00 pm on October 23, 2011, we confirmed the presence of what we suspected was an oil puddle outside of the oil barrier at the temporary oil storage tank for the main transformer^*1 in the Wild Bird Forest within the grounds of the power station. On the following day, October 24, since oil film was detected on the surface of the water that had accumulated in the oil barrier and the oil puddle was confirmed at the track of water overspill from the oil barrier, we presumed that the oil discovered on October 23 had overflowed from the oil barrier due to the accumulation of rain water inside the oil barrier.

Since a tank investigation was not possible when the oil barrier was filled with water, we checked the status of the inside of the tank after decreasing the water level of the oil barrier via water transfer treatment and oil adsorption treatment. As a result, on June 25, 2012, we confirmed, that oil had leaked from the bottom of the oil gage of one of the 9 tanks and the oil levels had lowered at the other 4 tanks.  On the same day, we confirmed that the oil leakage stopped as the valve of the oil gage was closed.  Until June 25th, when we confirmed the decreased oil level, the estimated amount of oil leakage from the tank was approx. 39kl.  Other oil leakages are also being investigated.

(Previously announced on October 24, 2011 and June 30, 2012)

After another investigation of the storage amount of each tank, leakage from the other two tanks was confirmed, which led to the confirmation of existing leakage at 3 tanks and decreased oil levels was confirmed at 6 tanks in total, out of 9 tanks. The amount of oil leakage was re-evaluated at approx. 40kl.  For the other 3 tanks, the leakage point has not been identified via the site investigation and the leakage test. However, since the decreased oil level was observed, it is estimated that there was oil leakage due to some yet unconfirmed cause.
According to precipitation data from the Japanese Meteorological Agency, water mixed with oil that had accumulated in the oil barrier was believed to have leaked out of the oil barrier some time in the month of September, 2011. The total amount of oil leaking out of the oil barrier was estimated to be a maximum of 36kl, because until now we had already collected approx. 4kl of oil using adsorption mats etc.
There is some possibility that most of the 36kl of oil, which leaked out of the oil barrier, was soaked up by the soil around the oil barrier and the remaining parts that had leaked into a drain. However, the oil was believed to have not leaked into the ocean because the head of the drain near the oil barrier was blocked and there is no trace of any oil leakage around the soil.

The oil leaking from the storage tank is insulating oil taken from the main transformer of Unit 4, Fukushima Daiichi Nuclear Power Station due to the replacement of the transformer before the earthquake. Since the oil has a small amount of PCB (polychlorinated biphenyl) ^*2, we will deal with this matter while talking with related organizations.

Appendix "Leakage of transformer insulation oil from the outdoor hazardous material storage tank at the "Wild Bird Forest" in the Fukushima Daiichi Nuclear Power Station premise" (PDF 111KB)

*1 Main transformer
A transformer that boosts the voltage produced in a generator to transmission voltage levels.

*2 PCB
PCB stands for polychlorinated biphenyl that is insoluble, chemically stable, and it is an industrial compound that has a high boiling point. Although it was used as insulating oil, its production has been suspended since 1972, because of its toxicity to both the human body and the environment.