Fukushima Daiichi NPS Prompt Report 2013

At around 1:31 AM on October 20, 2013, a TEPCO employee on a patrol found that water was dropping from the crack of the heat insulate wrapped around the piping valve at H9 (west) area tank (H9WA1).
We analyzed the dropping water, and found that the surface dose equivalent rate was 0.005mSV/h (γ- and β ray, 70μm dose equivalent rate), and that it was equal to the natural radiation background. In addition, we removed the heat insulate from the valve, and kept watching the point where the dropping was found for more than 10 minutes, but no water leak was found.
The measurement results of the dropping water sampled at the same time were as follows. These results are fully lower than the average all-β concentration (100,000Bq/L) of RO (reverse osmosis membrane type) treated water stored inside the tank.

<Measurement result of the dropping water>
Cesium-134: 430Bq/L
Cesium-137: 580Bq/L
All-β: 1,100Bq/L

Taking all these above into account, we determined that the dropping water was rainwater.