Press Releases 2013

On October 16, 2012, during inspection of the upper parts (clip parts) of the fuel assembly channel boxes^*1 at Kashiwazaki-Kariwa Nuclear Power Station Unit 5 (under regular inspection), parts of the water rods^*2 of 2 spent fuel assemblies under appearance inspection were found bent.

Regarding this incident, we received directive documents^*3 from the Nuclear Regulation Authority on October 19 and November 28, 2012. In accordance with the documents, we are currently conducting appearance inspection of fuel assemblies at Units 1-7 of Kashiwazaki-Kiriwa Nuclear Power Station and at Unit 4 of Fukushima Daini Nuclear Power Station.

By the time of the previous announcement, we had found bent water rods in a total of 26 spent fuel assemblies at Units 1, 2 and 5 of Kashiwazaki-Kiriwa Nuclear Power Station. Each of these assemblies is one on which a channel box was installed under water before the assembly was loaded in the reactor as fresh fuel. We presumed that the water rod was bent due to application of an excess load when the channel box was installed.

We announced that we were to make progress in mock-up tests simulating a fuel assembly as well as in evaluation of impacts on the safety analyses while continuing the appearance inspection in order to further grasp the situation and investigate the cause.

(Previously announced on March 19, 2013)

In accordance with the directive documents from the Nuclear Regulation Authority, we compiled an interim report on results of the cause investigation on bent water rods, the evaluation of impacts on the safety analyses, and the current progress status of the appearance inspection. Today, we have submitted the interim report to the Nuclear Regulation Authority.

In order to find out the cause of bent water rods, we conducted a mock-up test simulating a fuel assembly over its entire length and mock-up tests simulating parts (the upper and lower parts) of a water rod. As a result, we confirmed that our earlier presumption about the cause was correct, that is, the bend of the water rods was caused by application of excess loads to the water rods when channel boxes were installed on fuel in the spent fuel pool.

With regards to installation work of a channel box, we have made revisions to the work procedure since 1998. In the fuel assembly inspection conducted so far, none of the water rods of fuel assemblies that have channel boxes installed according to the revised work procedure were found bent. Further, we reconfirmed the validity of the current work procedure through the mock-up tests.

Impacts of a bent water rod on the safety analyses were evaluated on the basis of impacts on local nuclear characteristics of the fuel assembly and reactor core characteristics. As a result, the impacts on an analysis regarding abnormal transient changes during operation and on the other analyses such as the accident analysis were evaluated as insignificant, whereby we confirmed that the safety analyses are not affected.

With regards to the appearance inspection of fuel assemblies, approximately 70% (a total of 419 assemblies, which are broken down into 74, 89, 99, 50 and 72 assemblies at Kashiwazaki-Kariwa Nuclear Power Station Units 1, 2, 5, 6, and 7, respectively, and 35 assemblies at Fukushima Daini Nuclear Power Station Unit 4) has been completed. Bent water rods were found in a total of 26 spent fuel assemblies (6, 2 and 18 assemblies in Kashiwazaki-Kariwa Nuclear Power Station Units 1, 2, and 5, respectively), that is, no additional bent water rod has been found since the announcement on March 19.

We will conduct the appearance inspection on the remaining 202 assemblies. Out of these 202 assemblies, 5 assemblies are fuel where channel boxes were attached and detached under water when these assemblies were low irradiated fuel after being used in a short time period in a reactor during the experimental operation of Kashiwazaki-Kariwa Nuclear Power Station Unit 6. These 5 assemblies are considered to have characteristics similar to those of the new fuel where bent water rods were found. Therefore, the inspection will be conducted on these assemblies with the aim of acquiring broader knowledge.

We will continue conducting the appearance inspection of fuel assemblies, compile a report on the results of the appearance inspection and other related issues, and submit the report to the Nuclear Regulation Authority.

Attachment
"Results of Cause Investigation on Bent Fuel Assembly Water Rods at Our Nuclear Power Stations (Interim Report) (Summary)" (PDF 1.05MB)

Reference
"Results of Cause Investigation on Bent Fuel Assembly Water Rods at Our Nuclear Power Stations (Interim Report)"(PDF 115MB)

*1 Channel box
Square metal tubular 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 documents
"Bent fuel assembly water rod found at Unit 5 of TEPCO Kashiwazaki-Kariwa Nuclear Power Station (Direction)" (received on October 19, 2012)
"Bent fuel assembly water rod found at Unit 5 of TEPCO Kashiwazaki-Kariwa Nuclear Power Station (Direction)" (received on November 28, 2012)

*4 Characteristic similar to new fuel
There is a known phenomenon (irradiation hardening) where a zirconium alloy component of the fuel assembly hardens by receiving irradiation of neutrons when a fuel assembly has been used in a reactor. A water rod of irradiated fuel suffers from this irradiation hardening and becomes harder and inflexible. On the other hand, irradiation hardening does not fully progress in a water rod of irradiated fuel used only for a short time period, and such a water rod is similar to a water rod of new fuel in terms of hardness characteristic.

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