Fukushima Daiichi NPS Prompt Report 2013

This is a follow-up report on the statuses of steam found wafting through the air near the central part of the fifth floor (equipment storage pool side) of Unit 3 on July 18.

As a result of the subsequent detailed investigation, we consider that the steam is likely to generate in the following mechanism. We will perform temperature and dose measurement after reducing dose by debris removal, and will evaluate the appropriateness of the assessment method.

Therefore, we will restart the debris removal work as soon as it is ready.

We are regularly performing assessment by measuring dose and dust in each point at the site including the upper part of Unit Reactor Building. Impact on the environment cased by this steam is small in the site overall.

If the generation of the steam is found along with the debris removal work, we will check the plant parameter and the monitoring post readings. In addition, we will inspect if there are no critical status or abnormality found in the plant. The results will be announced for the time being.

(The generation mechanism of the steam)
The steam is estimated to be the rainwater, which was streamed down the gap of the shield plug and was heated by the head of the PCV. Also, there was a difference between the nitrogen injection amount (16m³/h) and the extracted amount (13m³/h) of the RPV and the PCV. Therefore, the gas including this difference amount of water vapor (approx. 3m³/h) is likely to be leaked from the head of the PCV, etc. It is estimated that this water vapor was cooled since the surrounding air was relatively cold when it was emitted to the upper part of the fifth floor of the Reactor Building via the gap of the shield plug, and it was observed as the steam.