Five killed by pipe rupture at Mihama

10 August 2004

Kansai Electric Power Company’s (Kepco’s) failure to conduct sufficient inspections has lead to an accident that has cost five workers their lives.

A carbon steel condensate pipe in the secondary circuit of Mihama’s third PWR unit suffered a 50cm rupture on 9 August, blasting eleven workers with boiling water and steam at about 150ºC.

The victims, from the Osaka-based Kiuchi Keisoku technical measurement firm, were on the second floor of the turbine building preparing for an inspection scheduled for 14 August. Four died at the scene, the fifth suffered 80% burns and died in hospital two weeks later.

It has emerged that the room, which has a seven metre ceiling, filled with steam so quickly that only workers who were standing up at the time of the rupture had a chance of escape. There was no radiological release and the 826MWe unit underwent a successful emergency shutdown.

The failed section was of a smaller diameter than the majority of the pipe, raising the possibility of turbulent flow within, and therefore an increased rate of erosion. Although the pipe was listed in maintenance guidelines as a ‘main inspection’ part it was not registered on the plant’s maintenance system and had not been ultrasonically tested for thickness since installation in 1976. This problem had been pointed out to Kepco by a subcontractor in April last year but, because similar parts at other plants had not worn thin, no action was taken ahead of the scheduled inspection.

However, the rate of erosion was greater than Kepco’s engineers had estimated and the pipe had been reduced from its initial thickness of 10mm to an ultimately catastrophic 1.4mm. The regulatory minimum is 4.7mm.

Quality control manager Koji Ebisuzaki reportedly said: “We are responsible” for slack management of plant inspection data while prime minister Junichiro Koizumi promised a thorough probe and Hiroshi Matsumura, managing director of Kepco, appeared on television to apologise. Kepco president, Yosaku Fuji, threw himself on the floor at the feet of bereaved relatives. “I will do everything I can to avoid this kind of accident happening again. I don’t know how to apologise to you” he said.

Trade minister Shoichi Nakagawa said: “We must not undermine trust in nuclear energy policy” before announcing that he will visit Mihama, 350km west of Tokyo on the Sea of Japan, to supervise investigations. But public support of nuclear in Japan has already been substantially undermined by incidents such as the September 1999 Tokai Mura criticality accident and, more recently, Tokyo Electric Power’s falsification of inspection data.

Kepco has begun a programme of pipe inspection at Mihama and their other plants, Ohii and Takahama, which necessitates the ordered shutdown of eight units in addition to three which are already offline. Pipe sections downstream of orifices have been identified as requiring detailed examination during the inspections which are expected to last about two weeks each. The programme is divided into three groups of units in order to maintain stability of supply.

A local government task force official asked Kepco executives why parts listed as main inspection parts were not regularly examined. The response from Kepco vice president Tetsuji Kishida was: “We just forgot them. From the beginning, they were not designated as such.”

Police in Fukui province are reportedly investigating the accident which could have been caused by ‘professional negligence’.


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