Japan’s Chubu Electric Power Co has shut down its Hamaoka unit 5 reactor after excessive vibration in the low-pressure (LP) steam turbine. A visual inspection revealed that a number of blade vanes had failed and one had broken off.
The advanced boiling water reactor (ABWR), located in Shizuoka Prefecture, central Japan, was shut down on 15 June and subsequent investigation has revealed that one turbine vane had become detached while blades in all three LP turbines had cracked. The broken blade was on the 12th stage of the second LP turbine and was located at the bottom of the turbine casing.
So far, Chubu has found 662 of 840 blades on the three turbines to be damaged, of which about 50 are cracked or broken. It released a statement saying: “It has been confirmed that fractures or cracking had occurred in some of the connectors (forks) at the roots of the vanes in all three of the turbines. It has also been confirmed that cracking had occurred in some of the vane connectors on the shaft.”
The cause of the problem is currently under investigation by both Chubu and turbine manufacturers Hitachi, although those close to the industry suggest that the nature of the failures on stage 12 of all three turbines points to a design flaw. Weight was added to this argument when vanes were removed from the 13th stage on the generator side of the low-pressure turbine and the forks of these vanes were subjected to visual inspection and non-destructive testing. The results did not indicate that any cracking or other such faults were present. In addition, cycle fatigue is thought unlikely since the turbines are relatively new with the Hamaoka unit beginning commercial operations in 2005.
The Hitachi two-stage reheated TC6F-52 turbine, comprises one high-pressure turbine and three low-pressure turbines with 1.25 m blades. Steam turbines installed on Japanese reactors have previously used shorter 0.98m blades.
Chubu, meanwhile, has said that it expects the reactor to remain off line for the foreseeable future and certainly for the rest of the financial year which ends next March. In a recent statement the company said it has been able to maintain a supply margin of about 8% by running two units at its Taketoyo thermal station, which had been idle, and purchasing power from other companies. However, the additional expense associated with these operations has led the company to revise predictions for its results for 2006-07 with costs estimated at as much as ¥63 billion ($543 million).
The failure at Hamaoka 5 prompted the early July shut down of the Shika unit 2 and the subsequent discovery of similar cracks in 146 out of 840 blades on its three LP steam turbines. Shika has only been under commercial operation since March and these turbines were also supplied by Hitachi.
With the 1358 MWe reactor, located in Ishikawa prefecture, western Japan, expected to be shut down for at least nine months, owners Hokuriku Electric have also revised downward their financial performance for the year, cutting revenue outlook by ¥10 billion ($86 million) as it is expected to cut sales to other utilities in order to meet power demand from its own consumers. Hokuriku will also delay maintenance at its Fukui thermal plant to bolster short-term capacity.
The company has not yet decided to take legal action against Hitachi as the cause of the fault has so far not been determined, although a spokesman for Hitachi told NEI: “A design related problem is one of a number of possible causes being looked into.”
With either a manufacturing or design problem likely to be the root cause of damage at both plants, the fallout is expected to weigh heavily on Hitachi which not only faces the possibility of huge damages claims from Chubu and Hokuriku, but also acquiring a damning reputation in a highly competitive and reliability-led industry. Such an outcome is seen by many in the industry as all but unthinkable, with the turbines operating at the relatively low speed of 1800 rpm and at a modest 280oC steam temperature, considerably lower than many conventional thermal plants. With thorough testing from both the experienced manufacturers and the operators ahead of commercial operations, the failures have left many scratching their heads given Hitachi’s existing reputation for quality control and its experience in supplying 15 of Japan’s 53 operating reactors.
The development of a forging technique for making a large, solid low-pressure turbine spindle marked a significant development in BWR technology and Hitachi has continued using this technique as turbines have grown in size to improve efficiency. For instance, the Hamaoka 5 reactor has the same thermal output, at 3,926 MWth, as earlier units but boasts Japan’s highest electric power output at 1,380 MWe, according to Chubu.
Hitachi and Toshiba are the only two Japanese companies that develop BWR technology, which is expected to witness significant growth over the coming decades, notably in the USA. And NRG Energy has already awarded a consortium of Hitachi and General Electric a contract to build two ABWRs in Texas, although it is not clear if Hitachi is to supply the steam turbines. However, Hitachi shares have dropped by more than 5% since the reports.