Press Releases 2013

On August 10, 2012, we received the directive document on the partial damage on the upper part of the fuel assembly channel box^*1 upper clip^*2 from NISA of the Ministry of Economy, Trade and Industry, regarding the damage on the channel box upper (clip) at Unit 3 at Onagawa NPS of Tohoku Electric Power, which was found on July 10, 2012. In response to this verbal instruction, we inspected the partial damage on the upper part of the fuel assembly channel box (clip), conducted investigation into it, evaluated and discussed the recurrence prevention countermeasures.

We completed the inspection of the damages on the upper clips, the investigation and evaluation and the discussion over recurrence prevention countermeasures. We have put these results together as a final report and submitted the report to NISA today.

As a result, we have found 162 channel boxes, whose connecting part of the upper clips were damaged, out of 18,586 channel boxes, which are loaded to assembly fuel inside the spent fuel pool. (Maximum 29 millimeter-long damage)

The damages were found only on the connecting parts of the upper clips, therefore, they had no influence on the function of the channel boxes. (Ensuring the flow-passage of the reactor cooling material and the control rod guidance function)

We conducted a visual check for the fuel assemblies inside these 162 channel boxes with partial damage, and found no abnormality such as injury or deformation. We have found a deformation (bend) in the water rods with regard to the rest 17 channel boxes, and that the bend was caused not by the damage on the channel box upper clip, but by too large a load when loading channel boxes.

We collected and analyzed the surrounding part of the damaged clips and found that the material, zirconium-base alloy corroded and that the material turned fragile fine powder. Therefore, we determined the damaged clips neither caused damage on the reactor core internals, nor had influence on the reactor facilities.

In conclusion, we found no influence on the channel box function, the fuel assembly, or the reactor facilities.

The causes of the damage vary among the channel-box manufacturers, however we assumed the followings could be the main causes. -When welding the clips on the upper part of the channel boxes, the usage of a stiffening of metal plates increased the heat input at the edge of the clips.

-Pre-heated stiffening of metal plates decelerated the cooling velocity after welding, and enlarged crystalline structure at the welded part.

-The chemical such as iron contained in the material inside the channel boxes decreased, and it degraded corrosion resistance.

As a recurrence prevention countermeasure, we will introduce a new welding facility in order to decrease the heat-input when welding, and will adopt a welding method using no stiffening of a metal plate.

We have confirmed that the channel boxes fully functioned, though they had partial damages on their upper clips. Therefore, we will continue to use the 68 channel boxes loaded on the reactor reload fuel (excluding 17 fuel assemblies whose water rods were bent).

We will monitor and check the upper clip of the channel boxes of all Units in a regular inspection, and will search for a new damage.

We will check the damage of the channel boxes at Fukushima Daiichi and Daini NPS, as needed.

[Circumstances so far]
· On August 10, 2012, we reported NISA on the details of the partial peel-off on the upper part of the fuel assembly channel box found in the past at Kashiwazaki-Kariwa Nuclear Power Station (overview of the matter investigation conducted and measures implemented) in response to the verbal instructions received. On the same day, we received the directive document^*3 regarding the partial damage on the upper clip from NISA.

(Announced on August 10, 2012)

· As a result of the investigation into the damages concerning the fuel assemblies inside the reactor and the spent fuel pool, we conducted the inspection for all the fuel assembly channel boxes in the spent fuel pool of Unit 1, 4, and 7 at Kashiwazaki-Kariwa Nuclear Power Station (which can be inspected at this point in time) by utilizing an under-water camera, the upper part of some of the channel boxes seemed to be whitened or damaged. (None at Unit 1, 10 assemblies at Unit 4, and 71 assemblies at Unit 7)

· We plan to continue the investigation into the clips in accordance with the direction given. The channel boxes which seem to be whitened or damaged will be lifted up by the fuel handling machine for a detailed inspection utilizing an underwater camera. The cause of this matter will be investigated in consideration of the investigations conducted by other operators of electric utilities that receive the same directive document, and the results will be reported to NISA.

(Announced before September 10, 2012)

· Appendix
The Outline of the Report on "Partial Damage on the Upper Part of the Fuel Assembly Channel Box (Clip) Found at TEPCO's Nuclear Power Station (Final Report)" (PDF 1.04MB)

· Reference
The Report on the Partial Damage on the Upper Part of the Fuel Assemble Channel Box (Clip) Found at TEPCO's Nuclear Power Station (Final Report) (PDF 67.4MB)

*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 Channel box upper clip
It serves as fixing the channel boxes to the fuel assembly, as well as installing jig tools, when loading or removing channel boxes to/from the fuel assembly.

*3 Directive document
"Partial damage on the upper part (clip) of the fuel assembly channel box (Direction)" (August 10, 2012 NISA No.2)

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