At around 11:07 AM on September 22, during debris removal from the upper part of Unit 3 Reactor Building, a steel beam (approx. 300mmx200mmx7m, approx. 470kg) which had been on the side of the spent fuel pool (SFP) and found to be unstable slipped and fell into the pool when a worker was trying to grab it using an oil-pressure fork attached to the head of a crane. It was confirmed that there was no problem with the operation status of the spent fuel pool alternative cooling system and the skimmer surge water level. No significant change was found before and after the incident (by comparing the results obtained after the incident and the most recent results before the incident) as a result of the radioactivity density analysis of the spent fuel pool water*1. In addition, no significant change was found with the monitoring post data, the atmosphere dose rate measured near the spent fuel pool and the spent fuel pool water level.
From September 24, the conditions of the inside of the spent fuel pool (location where the steel beam fell into, conditions of the spent fuel and storage rack, soundness of the pool, etc.) have been investigated using an underwater camera. As a result, no problem was found with the upper part of the spent fuel and storage rack as well as the pool wall in the area captured by the underwater camera. During the investigation conducted on September 26, the location of the steel beam which had fallen into the pool was identified.
On September 26, we received a directive document*2 "A steel beam falling into Unit 3 spent fuel pool at Fukushima Daiichi Nuclear Power Station (Direction)" from the Nuclear Regulatory Authority. (Previously announced on September 26, 2012)
In accordance with direction given, we evaluated the impact of this incident on the soundness of the spent fuel, the spent fuel storage rack and the spent fuel pool while investigating the cause of this matter and discussing recurrence prevention measures. Today, we have submitted a report to the Nuclear Regulatory Authority.
"Report on the Steel Beam Falling into Unit 3 Spent Fuel Pool at Fukushima Daiichi Nuclear Power Station to the Nuclear Regulatory Authority"(PDF 1.45MB)
*1 Radioactivity density analysis results
As a result of radioactivity density analysis of Unit 3 spent fuel pool water, no significant change was found comparing before and after the incident.
*2 Directive document
"A steel beam falling into Unit 3 spent fuel pool at Fukushima Daiichi Nuclear Power Station (Direction)"
The aforementioned documents are available only in Japanese. We apologize for the inconvenience this may cause.