Press Release Back Number(2006)

Causes of Damage to Vanes in the Low-pressure Turbines of Hamaoka Nuclear Power Station No.5 and Measures

October 27, 2006
Chubu Electric Power Co., Inc.

The causes of the damage that occurred to vanes in the 12th stages of the low-pressure turbines of Hamaoka Nuclear Power Station No. 5 (advanced boiling water reactor, rated electric output 1,380 MW) has been investigated, and measures have been adopted in response. The results of these investigations are presented here. A report has also been made to the government today.


1 Survey Results

Inspection of the low-pressure turbines showed that a vane located in the 12th stage of low-pressure turbine B had become detached. In addition to this, it was also confirmed that cracking occurred on the 12th stage of some of the connectors (forks) and at the roots of the vanes of low-pressure turbines A, B and C. No abnormalities were found on vanes of any other stages.

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

Because of this, it was estimated that the problem was limited to the 12th stages of the low-pressure turbines of the reactor No. 5.

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 during no load and low load operations 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 during load cutoff testing. We presume that these stresses affected the 12th stage, resulting in fatigue damage to the connections at the vane forks.

(This information was first announced on September 12, 2006.)


2 Causes

Random vibration and flashback vibration stress tests determined that the initial cracks in the forks of the vanes in the 12th stages occurred as a result of the simultaneous occurrence of random and flashback vibration in 20% load cutoff tests conducted during test operation of the reactor No. 5. The combined stresses generated repeatedly by random and flashback vibration resulted in high-cycle fatigue and the occurrence of cracking.

We estimate that the cracks later spread due to random and flashback vibration during test and commercial operation. The spread of cracking reduced the surface area of the fork of the single vane that became detached, making it unable to resist the pressure of centrifugal force, resulting in fracture.


3 Measures

In light of the incident, all of the vanes in the 12th stages of low-pressure turbines A, B and C (a total of 840) will be replaced with new vanes that have been designed and manufactured with consideration of random vibration and flashback vibration. Cracks also occurred at the roots of vanes, and new roots will also therefore be manufactures.

The design and manufacture of new vanes is expected to require a considerable amount of time, given the requirement for proving tests, etc.

In the interim, we will remove the vanes from the 12th stages of low-pressure turbines A, B and C, and replace them with pressure plates*4, the reliability of which has been proven in both nuclear and thermal power stations.

Examinations have been conducted to evaluate the effect on the rest of the facility of using the pressure plates, and to ensure safe and stable operation. The government will inspect the design of the plates and conduct further inspections before operation commences. No schedule has been established at this stage for the recommencement of operation.


Aspects of the design of the vanes employed when the turbines were increased in size were not examined by the manufacturer in sufficient detail, and changes will also therefore be made to the process of turbine design.


*1 High-cycle fatigue
This 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 vibration
This is an irregular fluid oscillation that occurs 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 Flashback phenomenon
The low-pressure turbines extract a portion of the steam to heat the water that is supplied into the nuclear reactor. This steam for heating 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 flashback. Load cut means an emergency stop of the generator as a result of a failure in transmission lines or for other causes.

*4 Pressure plate
Both moving and stationary vanes are employed in a turbine. Stationary vanes are positioned between moving vanes in order to adjust the flow of steam between the vanes. The flow of steam from the 11th stage moving vane is adjusted by the stationary vanes, and causes the moving vanes in the 12th stage to revolve. The next set of stationary vanes adjusts the flow again, and the steam flows to the 13th stage.
The moving vanes and the stationary vanes that are positioned in front of them in the 12th stages of the turbines will be removed, and the stationary vanes will be replaced with immovable pressure plates to adjust the flow of steam. We have conducted examinations to verify that the pressure plates will reduce pressure and adjust the flow of steam as effectively as the vanes that will be removed, and will not have any negative effect on the functioning of the other vanes.


Attachments 1:
PDFCauses of Damage to Vanes in the Low-pressure Turbines of Hamaoka Nuclear Power Station No.5 and Measures[PDF:220KB]

Attachments 2:
PDFOutline of Operation using Pressure Plates[PDF:320KB]

< 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 was conducted from August 30 till September 21. These inspections covered all stages of high pressure Turbines 1 through 7. Visual inspection and non-destructive testing were conducted without removing the vanes, and it was confirm that no abnormalities were found.

(These results were first announced on August 30 and September 22, 2006.)