Results of Inspection, Causes, and Countermeasures for Cracks, Etc. in Mounting of Low-Pressure Turbine Moving Vanes in Unit 3 and Unit 4 of the Hamaoka Nuclear Power Station
June 21, 2013
Chubu Electric Power Co.,Inc.
Chubu Electric Power has confirmed that cracking occurred in the moving vane mounting assembly (on the shaft side) in the low-pressure turbine of Hamaoka Nuclear Power Station Unit 4 (rated electric output 1,137 MW, undergoing its 13th regular inspection), and has obtained inspection results that also indicate the presence of cracking in the same parts in Unit 3 (rated electric output 1,100 MW, undergoing its 17th regular inspection).
It was also confirmed that cracking and fractures have occurred in the moving vane mounting assembly (on the moving vane side) in the low-pressure turbine of Unit 4. Inspection results were also obtained indicating the presence of cracking in the moving vane mounting assembly (on the moving vane side) in Unit 3.
Detailed investigation has been conducted of low-pressure turbine moving vane mounting assemblies (on the shaft side and the moving vane side) in which the presence of cracking and fractures has been confirmed to date. An organized summary has been made of the inspection results together with causes and countermeasures, and this is to report the findings regarding the shaft and the moving vanes.
Shaft Inspection Results with Causes and Countermeasures
1. Inspection Results
Areas where significant indicative waveforms were observed in ultrasonic testing (Note 1) were subjected to magnetic particle testing (Note 2), and this confirmed cracking in portions of the low-pressure turbine moving vane mounting assemblies (on the shaft side).
The depth of the cracking was investigated by cutting away the affected portion until the cracking had been removed. Those results were used to perform strength evaluation, the results of which confirmed the existence of some places where restoration of the moving vane would be problematic.
Samples were taken and subjected to surface examination by microscope. The results confirmed that the cracking followed along grain boundaries in the metallographic structure.
The surface examination of cracking by microscope and the conditions of materials, operating environment, and stress generated in the low-pressure turbine moving vane mounting assemblies (on the shaft side) suggested that the cracking was a result of stress corrosion cracking (SCC).
(1) Inspection and Evaluation
The shaft in its current condition was subjected to restoration as described in the following paragraph (2). In that state, the shaft was evaluated to determine whether it could be operated in future on the assumption that stress corrosion cracking would occur again and that it would progress. The results confirmed that such operation would be possible for a minimum of approximately four more years (three cycles).
Inspections of the areas in question will be conducted in future, and the soundness of those areas will be evaluated.
(2) Restoration Method
Restoration will be performed by the below method in those areas where the cracking was deeper and restoration of the moving vanes was problematic in terms of their strength.
 Installation of Shims
Those moving vanes will be replaced by shims that would be less influenced by the centrifugal force that was applied to moving vane mounting assemblies (on the shaft side).
 Installation of Pressure Plates
At turbine stages where the installation of shims is difficult, all of the moving vanes will be removed from that stage and a pressure plate will be installed that reduces the pressure by a comparable amount to the moving vanes, as well as simultaneously adjusting the flow.
 Replacement of Shafts
Since it proved difficult to restore the low-pressure turbine C of Unit 4 using shims and pressure plates, we will manufacture a new shaft to replace the old one. In the course of shaft replacement, we will plan countermeasures to limit the occurrence of stress corrosion cracking in the moving vane mounting assemblies (on the shaft side).
Restoration of the shaft is scheduled to take place by the end of fiscal year 2014, which is also the target for completion of tsunami countermeasures and severe accident countermeasures for filter vent equipment, etc. Evaluation of the pressure plates indicates that installation will reduce generator output during operation by approximately 7% of its rated value.
4. Replacement of Shafts
Plans will also be made to replace shafts in low-pressure turbines A to C of Unit 3 and low-pressure turbines A and B of Unit 4. In the course of shaft replacement, we will plan countermeasures to limit the occurrence of stress corrosion cracking in the moving vane mounting assemblies (on the shaft side), including also the shaft of low-pressure turbine C in Unit 4.
Moving Vane Inspection Results with Causes and Countermeasures
1. Inspection Results
(1) Unit 3
Moving vane mounting assemblies in low-pressure turbines (on the moving vane side) in which significant indicative waveforms were found through ultrasonic testing were subjected to visual inspection and magnetic particle testing. The results confirmed that the significant indicative waveforms obtained by ultrasonic testing were from the detection of scratches that had no influence on the function of the moving vanes. No cracking or fractures were found.
(2) Unit 4
The implementation of visual inspection, ultrasonic testing, and magnetic particle testing confirmed the presence of cracking and fractures in a portion of the moving vane mounting assemblies (on the moving vane side) in low-pressure turbines. The areas where cracking and fractures were found were subjected to surface examination using a microscope. The results confirmed the presence of fine line patterns that are characteristic of high-cycle fatigue (Note 3) in fracture surfaces.
In addition, a portion of the cracking had rust and other such corrosion extending to the very ends of the cracks. No fresh metal fracture surfaces were found, suggesting that the cracks had occurred in the past and had not advanced into surrounding areas since that time.
The results from evaluation of the occurrence of high-cycle fatigue fractures confirmed that the natural frequency (the characteristic frequency of an oscillating body that is allowed to oscillate freely) of the moving vanes was close to the frequency that was 9 or 11 times the rotating frequency of the turbine (30 rotations per second, or 30 Hz). This resulted in the generation of resonance and an increase in vibrational stress, which is thought to have caused high-cycle fatigue fractures.
When the natural frequency of the moving vanes and the frequency that is an integral multiple of the rotating frequency of the turbine (hereafter the exciting frequency) are close, there is a possibility that resonance will occur. For that reason, the natural frequency of the moving vanes is checked during the design phase, but the exciting frequencies at or above the 8th order (eight times the rotating frequency of the turbine) that were found here had not been checked because past operating performance suggested that they would have little influence.
Those moving vanes on which cracking had been confirmed not to have advanced into surrounding areas after its occurrence had been subjected to a structural modification in the past for the purpose of reducing vibrational stress due to resonance. It is thought that for this reason the high-cycle fatigue fractures that occurred before the structural modification stopped progressing after its completion.
A design will be adopted that prevents high-cycle fatigue fractures by also taking 8th order and higher exciting frequencies into account during the mounting of moving vanes. In conjunction with this measure, all the moving vanes in which cracking or fractures were found will be replaced with new vanes and restoration is scheduled to take place by the end of fiscal year 2014.
In addition, after the moving vanes have been replaced, the natural frequency of the moving vanes will be confirmed and the influence of increased vibrational stress due to resonance will be confirmed.
(Note 1) Ultrasonic testing
This is a form of testing in which ultrasound waves are sent into the test object and the interior of the object is examined by the reflection of the ultrasound waves. It is capable of testing the shaft with the moving vanes attached.
(Note 2) Magnetic particle testing
This is a form of testing that uses the patterns of magnetic particles formed on the test object when exposed to a magnetic field to investigate the surface of the object (including the interior close to the surface).
(Note 3) High-cycle fatigue
The phenomenon whereby metallic material repeatedly subjected to force above a certain level (for 10,000 to 100,000 times or more) develops cracking that progresses to the point of damage.