During the inspection of the upper part (clip part) of the fuel assembly channel box^*1 at Unit 5 of Kashiwazaki-Kariwa Nuclear Power Station (under regular inspection), a part of the water rod^*2 of 2 spent fuel assemblies being inspected was found to be bent.

Regarding this incident, we received a directive document^*3 from the Nuclear Regulation Authority requiring us to develop a plan to investigate the bent fuel assembly water rod found at Unit 5 of Kashiwazaki-Kariwa Nuclear Power Station and report the results. In accordance with the direction given, we have developed an investigation plan for Unit 5 of the power station and summarized the history of the 2 spent fuel assemblies which were found to be bent and their conditions in detail. Then, an appearance inspection was performed on a total of 34 spent fuel assemblies (33 which were scheduled to be inspected in detail and one out of 14^*4 for which an additional inspection was decided to be performed). The inspection results were put together as an interim report and were submitted to the Nuclear Regulation Authority. (Previously announced on November 6, 2012)

We are currently in the process of obtaining information on the current conditions of the bent fuel assemblies and investigating the root cause based on the developed investigation plan. Today, we received a directive document^*5 "Bent Fuel Assembly Water Rod at Unit 5 of Kashiwazaki-Kariwa Nuclear Power Station" from the Nuclear Regulation Authority.

In accordance with the direction given, we will promptly implement necessary measures and report the results to the Nuclear Regulation Authority.

*1 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 the fuel assembly.
*2 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.
*3 Directive document
"Bent fuel assembly water rod found at Unit 5 of TEPCO Kashiwazaki-Kariwa Nuclear Power Station (Direction)"
*4 14 fuel assemblies for which an additional inspection was decided to be performed
Though an appearance inspection is to be performed on a total of 30 fuel assemblies in order to identify the cause of the water rod bending, 16 out of the 30 are part of the 33 fuel assemblies for which an inspection of the upper part (clip) of the fuel assembly channel box is to be done. Therefore, 14 fuel assemblies are subject to an additional appearance inspection.
An appearance inspection will be performed as follows in order to identify the cause of the water rod bending.
- The number of fuel assemblies on which reused channel box was installed under water before loading in the reactor as unused fuels (similarly to the bent fuel assemblies): 10 (2 fuel assemblies of concern included)
- The number of fuel assemblies (which were manufactured during the same time as the bent fuel assemblies) without reused channel box installed under water: 10
- The number of fuel assemblies on which reused channel box was installed under water after the channel box installation method was changed so as to reduce the force used during installation: 10
*5 Directive document
"Bent fuel assembly water rod found at Unit 5 of TEPCO Kashiwazaki-Kariwa Nuclear Power Station (Direction)"

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