December 13, 2012
Bent rod found at TEPCO's Niigata reactor
Tokyo Electric Power Co. said Dec. 12 a bent water rod caused two fuel rods to come into contact inside a fuel rod assembly stored in a spent fuel storage pool for the No. 5 reactor of the Kashiwazaki-Kariwa nuclear power plant in Niigata Prefecture.
No damage to nuclear fuel or other anomalies have been reported, but the two fuel rods may have been in contact with each other when they were burning inside the reactor.
The situation had the potential to cause a serious fuel failure accident.
A fuel rod assembly is a bundle of 60 fuel rods. A water rod, or a passage for coolant water, runs through its center.
Water rods were found to be bending in 18 fuel rod assemblies in the storage pool. Closer studies found that in one fuel rod assembly, the bending water rod pushed one nearby fuel rod into contact with another.
At the Kashiwazaki-Kariwa plant, a number of unspent fuel rod assemblies had been exposed to excessive loads due to sloppy work procedures when they were encased in metal covers.
The two fuel rods in question may have come into contact during such work procedures, TEPCO officials said.
The fuel rod assembly was encased in a cover in 1994 and burned in the nuclear reactor from 1995 to 2000.
See : TEPCO press release site Dec.12:
Press Release (Dec 12,2012)
Bent Fuel Assembly Water Rod at Unit 5 of Kashiwazaki-Kariwa Nuclear Power Station (Follow-up Report)
We are currently investigating the bent found on part of spent fuel assembly water rod*1 while inspecting the upper part (clip) of fuel assembly channel box*2 at Unit 5 of Kashiwazaki-Kariwa Nuclear Power Station under regular inspection. As a result of appearance inspection, the water rods of 18 out of 65 spent fuel assemblies were found to be bent.
The 2 out of these 18 spent fuel assemblies on which major bent was found were inspected further utilizing a fiberscope. As a result of inspection, the following has been found.
- Deformation was found near the hole on the thin area in the lower part of water rod where strength is relatively low.
- Though some holes have become smaller due to deformation, no hole was completely closed.
- The fuel rods adjacent to the bent water rods were touching or close to one another.
As fuel rods were found to be touching one another due to the bent water rods, it has been judged by the Nuclear Regulatory Agency that the incident applies to Article 19-17, Paragraph 1, Item 3*3 of the Rule for the Installation, Operation, etc. of Commercial Nuclear Power Reactors (Rule for Commercial Nuclear Power Reactors).
Based on the inspection results, we will continue investigating the cause of the bent water rod and report the results to the Nuclear Regulation Authority.
"Fuel assembly inspection in response to the bent water rod found at Unit 5 of Kashiwazaki-Kariwa Nuclear Power Station"（PDF 208KB）
*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 Article 19-17, Paragraph 1, Item 3 of the Rule for the Installation, Operation, etc. of Commercial Nuclear Power Reactors (Rule for Commercial Nuclear Power Reactors)
As stipulated by Article 62-3 of the Act on the Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors, in the case of an incident which applies to the following, licensees of reactor operation (including former licensees of reactor operation, same as in the following Article and Article 24) must report the incident and countermeasures to the Nuclear Regulation Authority within 10 days from the incident occurrence.
When licensees inspect equipments critical for ensuring safety and the results do not comply to the standards stipulated in Article 9 or Article 9-2 of the Ordinance which specifies the technical standards of commercial nuclear power facilities or when the functions necessary to maintain safety of nuclear facilities are deemed inadequate.
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