Press Releases 2013

On October 16, 2012, 2 spent fuel assembly water rods^*1 were found to be bent while inspecting the upper part (the clip) of fuel assembly channel box^*2 at Unit 5 at Kashiwazaki-Kariwa Nuclear Power Station under regular inspection.

In response to this matter, appearance inspection was performed on 65 fuel assemblies at Unit 5, and the water rods of 18 of them were found to be bent.

2 out of these 18 spent fuel assemblies on which major bent was found on the water rod were inspected further in detail utilizing a fiberscope. As a result, deformation was found near the water passage hole in the lower part of water rod and the fuel rods adjacent to the bent water rod were touching or close to one another. We received directive documents^*3 regarding this matter from the Nuclear Regulation Authority on October 19 and November 28, 2012. In accordance with the direction given, other units are being inspected for similar issue. As a result, water rods of 2 fuel assemblies at Unit 2 were also found to be partially bent.

(Previously announced on December 21, 2012)

In accordance with the directive documents received from the Nuclear Regulation Authority, data on the history of channel box installation, etc. was obtained and appearance inspection was performed on the current spent fuel pool and fuel assemblies stored in the reactor. An interim report including the progress status of cause investigation and the future investigation schedule has been submitted to the Nuclear Regulation Authority today.

As a result of appearance inspection so far performed on a total of 125 fuel assemblies (60 at Unit 2 and 65 at Unit 5) at Kashiwazaki-Kariwa Nuclear Power Station, water rods of 20 fuel assemblies (2 at Unit 2 and 18 at Unit 5) were found to be bent. The most significant bent found was approx. 2cm. These spent fuel assemblies were used during the period from 1994 to 2003, on which channel box was installed under water when they were still unused. These water rods are assumed to have been bent because of the excessive force applied when installing the channel box under water.

As a result of mockup test performed to investigate the bent water rods, it was confirmed that water rods may have been bent due to the excessive force applied when installing the channel box before the operation method was improved on 1998. When channel box was installed by following the current operation method (which was improved in 1998), water rods did not bend as the applied force was properly controlled.

Upon analyzing the impact of conditions such as the water rod being close to fuel rods and the water passage hole on the lower part of water rod being smaller due to deformation on the reactor core characteristics, it has been judged that there is no major impact considering that the area where fuel rods are touching one another is non-heat generating area without pellets.

We will continue to perform appearance inspection and mockup test in order to check the current conditions and identify the cause of the incident while evaluating the impact of this incident on safety analysis. The results will be summarized and submitted to the Nuclear Regulation Authority.

End

Attachment
- History of installing and removing channel box (Example: Unit 5 at Kashiwazaki-Kariwa Nuclear Power Station) (PDF 16.6KB)
- Category flow to determine the number of samples used for appearance inspection (PDF 47.9KB)
- The planned number of fuel assemblies to be inspected and the current progress status (PDF 117KB)
- Investigation schedule (PDF 67.8KB)

Reference (Report)
Progress of Investigation on the Bent Fuel Assembly Water Rods Found at our Nuclear Power Station (Interim Report) (PDF 44.0MB)

*1 Water rod
Hollow tube installed in the center of the fuel assembly in parallel with fuel rods. By passing water through the tube, the output inside the fuel assembly is optimized.

*2 Channel box
Square metal tube cover installed on the fuel assembly. By installing the channel box, the coolant flow path through the fuel assembly can be fixed. It also functions as a guide when the control rods operate and protects fuel assembly.

*3 Directive document (Received on October 19 and November 28, 2012)
Bent fuel assembly water rods found at Unit 5 of Kashiwazaki-Kariwa Nuclear Power Station (Direction)

The aforementioned documents are available only in Japanese. We apologize for the inconvenience this may cause.