As part of preparation for collecting samples of control rod research materials^*1,
we conducted outlook inspection on the two spent hafnium flat tube control rods^*2 stored in Unit 7, Kashiwazaki Kariwa Nuclear Power Station on October 27, 2010.As a result, we observed a fine crack (with the maximum width of approx. 0.3mm) at the center of the tie rod^*3 on the circumference of one of the control rods.
In response to this event, we received an instruction document^*4 from NISA requesting to confirm the healthiness of the control rod on which the crack was found and the control rods whose specification is same as the cracked rod.
As a result of outlook investigations, we observed cracks on twenty-eight control rods out of the seventy-eight control rods (including the one identified on October 27, 2010) at Unit 7. However, those cracks are within the range of healthiness-evaluation which was conducted in response to the instruction and there is no safety problem that may threaten the healthiness of the structural safety and the function of insertion of the control rods.
We also conducted inspections on the twenty-nine control rods of the same type at Units 1, 2&4 of Fukushima Daiichi Nuclear Power Station, Unit 2 of Fukushima Daini Nuclear Power Station and Units 4&5 of Kashiwazaki Kariwa Nuclear Power Station and observed no crack on the tie rods.
(Announced by February 1, 2012)

We announce herewith that we reported to NISA about the cause of and countermeasures against the cracks which occurred on the tie rods of the hafnium flat
tube control rods today.

As a result of the investigation, we observed cracks either on the tie rods or the connecting points of sheathe and tie rods and concluded that those cracks were
caused by the following respective reasons.

・Cracks on tie rods
(1) There were firm layer which was formed in the machining manufacturing process on the tie rods which were cracked.
(2) Fine cracks appeared due to the stress corrosion cracking during the usage in the reactor.
(3) The cracks developed due to radiation inducement stress corrosion cracking, caused by neutron irradiation and weld residual stress.
 

・Cracks at the connecting points of sheath and tie rods
(1) Gaps were created due to the insufficient penetration welding.
(2) Fine cracks appeared due to corrosion during the usage in the reactors.
(3)The cracks developed due to radiation inducement stress corrosion cracking, caused by neutron irradiation and weld residual stress.

We will consider countermeasures in manufacturing the same type of control rods such as construction methods which do not cause gaps in the connecting points between the sheath and tie rods or improvement of manufacturing methods of tie rods.

As we found an error, which does not influence the result of the evaluation of healthiness, in the report to NISA on the healthiness evaluation dated December 8, 2010 and the report to NISA on the results of outlook investigation dated January 7, 2011, we submitted the revised report today.

<Attachment>
Exhibit 1: Results of investigation of the cracks on the spent hafnium flat tube control rods (Summary)(PDF 56.8KB)
Exhibit 2: Mechanism of crack occurrences(PDF 98.9KB)
Report: Report on cracks on the hafnium flat tube control rods in Unit 7, Kashiwazaki Kariwa Nuclear Power Station(PDF 82.7MB)
*The aforementioned attachments are only available in Japanese. We apologize for any inconvenience this may cause.

*1 Preparation for collecting samples of control rod research materials
The control rods, whose tie rods had fine cracks, were taken out from the reactor during the 4th periodical inspection (April, 2002 II August, 2002) and had been stored in the spent fuel pool. In January, 2006, in order to investigate the similar materials found in Unit 6 of Fukushima Daiichi  Nuclear Power Station, we conducted an utlook inspection in April 2006 and confirmed that there was no crack on the sheath that may affect their healthiness. Therefore, in view of enriching the knowledge of hafnium irradiation  characteristics, we planned to investigate the causes of cracks on the  sheaths of the control rods as part of the common research on electricity,"Research on healthiness evaluation of hafnium control rods".

*2 Hafnium flat tube control rods
Control rods containing hafnium, which has high neutron absorption capacity as neutron absorbent and made in the form of tubes.
   
*3 Tie rod
One of the structural members of the control rods, which connect the metal plates (sheath) and handles.

*4 Instruction document
"Action against cracks on control rods (Instruction)"
 (No.110, 22, NISA)

We have provided a Japanese press release version of the instruction document received from NISA. However, at this time we have reserved the right not to provide an English version due to potential misunderstandings that may arise from an inaccurate rendering of the original Japanese text. We may provide the English translation that NISA releases in our press releases. However, in principle we would advise you to visit the NISA website for timely and accurate information.