Following further analysis of the failure of the low pressure steam turbines at unit 5 of Chubu Electric Power Co’s Hamaoka nuclear plant in Shizuoka Prefecture, central Japan, the company has revealed the cause of the failure as turbulence in steam flows within the turbines and vibration stress caused by a flashback phenomenon.
The problem first emerged in mid-June, when the turbine was shut down due to “excessive vibration” in the low-pressure sections of its three steam turbines. A number of blade vanes were discovered to have failed and one had broken off. All of the failures occurred in the blade roots at the twelfth stage and Chubu subsequently found 662 of 840 12th stage blades on the three turbines to be damaged, specifically 185 vanes in turbine A, 247 vanes in turbine B and 230 vanes in turbine C.
Portions of the forks were damaged and the pins used to hold the vanes in place on the shaft were also partially broken with 46 of the vanes having fractures or cracks in some portions of their forks. There was also some scoring damage and denting on other vanes and parts in the surrounding area.
Further analysis of the Hitachi two-stage reheated TC6F-52 turbine, which comprises one high-pressure section and three low-pressure sections with 1.25 m blades, confirmed that fractures occurred at the vane roots on the 12th stage only of all three LP turbines and showed signs of high-cycle fatigue. The cracked portions of the forks underwent fractographic study confirming signs of high-cycle fatigue ‘beachmarks’ from which it is possible to determine the direction in which cracks are spreading and the history of their development. Materials, manufacture, assembly, operation and maintenance were subsequently ruled out as a cause of the failure.
The fatigue fractures are believed to result from stress caused by random vibrations arising from turbulence in steam flows within the turbines occurring under zero and low loads and of vibration stress caused by a flashback phenomenon arising from unintended high-speed reverse flows of steam into the turbine from the water supply heater.
Under normal operations a portion of the steam entering the LP turbines is extracted to heat the reactor supply water - known as bleed air. However, at low loads the steam supplied to the turbine is reduced rapidly and the consequent drop in pressure inside the turbines, which are connected to high vacuum condensers, results in hot water in the water supply heater boiling at low pressure. This causes a high-speed flashback, or reverse flow, of the bleed air into the turbine. This reverse flow steam hits the turbine’s vanes causing vibration. In Hamaoka unit 5 the bleed air pipe emerges just before the 12th stage.
In addition, it is also known that random vibration occurs in the LP turbine, especially in the final 14th stage and possibly into the 13th stage. However, the analysis indicates that as the turbines increase in size and the vanes get longer - previous LP turbines used blades of around 1 m - this phenomenon can extend to the 12th stage.
Recovery of the damaged components inside LP turbine B is almost complete with recovery of components that are thought to have “travelled outside the turbine” expected to be completed by October. The company also intends to continue to investigate the precise causes of the problem and examine countermeasures to return the turbines to operation. Nonetheless, Chubu expects the 1380 MWe advanced boiling water reactor (ABWR) to remain off line for at least the rest of the financial year, which ends in March, while Hokuriku Electric’s Shika unit 2, located in the Ishikawa prefecture of western Japan - which has a similar turbine and has been revealed to have similar blade failures - is expected to be shut down for at least nine months.
The failures prompted Chubu to revise its earnings predictions for 2006-07 with costs estimated at as much as ¥63 billion ($538 million) while Hokuriku have also revised downward their financial performance for the year, cutting revenue outlook by ¥10 billion ($85 million).
Hitachi has subsequently announced a downward revision to its consolidated earnings for the fiscal year 2006-07 totalling ¥80 billion ($683 million). While a precise breakdown of the costs associated with the failures has not been revealed, Hitachi’s Power & Industrial Systems segment will book repair costs related to “turbine damage at certain nuclear power plants” and is now expected to record a result of some ¥75 billion ($641 million) below previous expectations. However, while this figure just about covers the expected losses from Chubu and Hokuriku, with a design problem now firmly implicated the issue of potential compensation remains a thorny one for Hitachi shareholders.
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