Press Release Back Number(2006)

Status of Inspection of Low-Pressure Turbine at Hamaoka Nuclear Power Station No. 5 (Follow-up)

September 12, 2006
Chubu Electric Power Co., Inc.

Chubu Electric Power Co., Inc. announced today the results of inspections of Low-Pressure Turbine at Hamaoka Nuclear Power Station No. 5 and of off-site testing and analyses that have been conducted to date, and other related matters.


1.Open Inspection of Low-Pressure Turbines

The casings of turbines were opened and in addition to turbine B, which experienced a detached vane, low pressure turbines A and C were visually inspected and non-destructive testing conducted. The inspections confirmed that fractures or cracking occurred on the 12th stage, which experienced the detached vane, of some of the connectors (forks) and at the roots of the vanes in all three of the turbines. No abnormalities were found on vanes of any other stages.

Based on these results, we assume that the problem is common to low-pressure turbines A through C, limited to the 12th stage.


2.Testing and Analyses Results

The results of testing and analyses performed off site are as follows.


(1) Results of the Fractographic Study

Observation of the fractures to the connections at the vane roots confirmed the presence of signs of high-cycle fatigue.*1


(2) Investigation of the Cause of the Fractures

The factors causing the high-cycle fatigue was assumed as the result of stress caused by random vibrations*2 arising from turbulence in steam flows within the turbines and of vibration stress caused by a flashback phenomenon*3 arising from high-speed reverse flows of steam in the turbine from water supply heater.

We presume that these stresses affected the 12th stage, possibly resulting in fatigue damage to the connections at the vane roots.

We are confirming any correlation between signs of fracture and turbine operating histories, including test operations.


3. Future Measures

We will continue to investigate the precise causes of this problem and will examine countermeasures to return the turbines to operation.



*1 High-cycle fatigue is a phenomenon in which pressure is applied to metal 10,000 to 100,000 times or more, resulting in cracks that develop into fractures.

*2 Random vibrations are irregular fluid oscillations that occur in vanes as a result of turbulence (large-scale reverse flows and eddies) in the steam within the turbine. Random vibrations occur when the turbine is under zero and low loads.

*3 The low-pressure turbines extract a portion of the steam to heat the water that is supplied into the nuclear reactor. This steam is called bleed air. When the load is cut or the turbine stops automatically, the steam supplied to the turbine is reduced rapidly, resulting in a drop in the pressure inside the turbines connected to the vacuum condensers, and a high-speed reverse flow of the bleed air into the turbine. This phenomenon is called flash-back.
Load cut means an emergency stop of the generator as a result of a failure in transmission lines or for other causes.


Attachment 1:
Hamaoka Nuclear Power Station Reactor No. 5 Low-Pressure Open Turbine Inspection Status and Survey Results

Attachment 2:
Overview of Random Vibration and Flashback Phenomenon

PDFReference:
Hamaoka Reactor No. 5 Operations History and Fractographic Study Results (Beachmarks)[PDF:182KB]



< Content of Previous Releases >


Hamaoka Nuclear Power Station No. 5 (advanced boiling water reactor; rated electric output 1.38 million kW) was in operation at constant thermal output when an alarm was generated for excessive turbine vibration at 8:39 a.m. JST on June 15, 2006. When the turbine was stopped, the nuclear reactor also shut down automatically.

(This incident was first announced on June 15, 2006.)



Work began on June 19 to open the casing of the low-pressure turbine. After the casing was opened, it was found that a vane on one of the vane wheels, which are located in the12th stage (third stage from the outside) of low-pressure turbine B, had become detached from the shaft and had fallen into the lower part of the turbine. It has been also confirmed that some portions of the forks were damaged and the pins used to hold the vane in place on the shaft were also partially broken. All of the remaining vanes on the stage from which the vane fell (139 in total) were removed and checked visually. It was confirmed that 46 of the vanes had fractures or cracks in some portions of their forks.
It has also been confirmed that there was some scoring damage and denting on other vanes and parts in the surrounding area.

(These results were first announced on June 23 and June 30, 2006.)



Visual Inspections and non-destructive tests were conducted on the other two units; turbines A and C, in addition to turbine B, where the vane drop was confirmed. As a result, fractures and cracks were confirmed in some portions of 185 vanes in turbine A, 247 vanes in turbine B and 230 vanes in turbine C. As for the vane wheels on the 13th and 14th stages on the side where the vane in turbine B dropped, no abnormalities were observed.

(These results were first announced on July 6 and July 11, 2006.)



Recovery of the damaged components inside low-pressure Turbine B is almost complete. Recovery of components that are thought to have traveled outside the turbine began on August 24 and is expected to be completed by October.

(This information was first announced on August 23, 2006.)



In light of the vane damage that occurred in low-pressure Turbine B, inspections of the high-pressure turbines began on August 30. These inspections will cover all stages of high pressure Turbines 1 through 7. Visual inspection and non-destructive testing will be conducted without removing the vanes to confirm the safety of the turbines.

(This information was first announced on August 30, 2006.)