The Mihama Nuclear Power Plant Accident

On Monday, 9 August 2004 a fatal accident happened at the Hihama No. 3 nuclear power station in Fukui prefecture, Japan. The plant is owned and run by Kansai Electric Power corporation (KEPCO), the major power utility in Western Japan. Four workers were scalded to death by superheated steam, seven other workers were injured.

The accident happened when the reactor was about to undergo routine maintenance. The accident was caused by a bursting steam pipe in the non-radioactive part of the reactor. In 27 years of operation that 56 cm diameter pipe had not once been checked for corrosion, let alone replaced. By the time it burst, its walls had worn down from an initial 10 mm of carbon steel to a mere 1.4 mm. Regulations required the pipes to be replaced when the walls were eroded to a thickness of 4.7 mm. Nine months before the accident a subcontractor company had alerted the operators to the need for inspections, but the warning was ignored.

Between 1998 and 2003, KEPCO replaced carbon steel steam pipes in two other power stations, Takahama Nuclear Power Plant No. 3 and Oi Nuclear Power Plant No. 1, with stainless steel pipes. Inspections had revealed that the original pipes had worn so thin they would not have lasted another two years.

The Mihama No. 3 reactor, which started commercial operations in 1976, is of the pressurized water (PWR) design used by most Western countries (USA, Germany, France). High pressure water is used to carry heat from the reactor core to a heat exchanger, where its heat turns water of a secondary cooling cycle into steam, which then drives power turbines. The steam is condensated and fed back into the heat exchanger. Because of the heat exchanger separating the two cooling cycles, water in the secondary cycle and the steam turbines are not radioactively contaminated.

There are 15 nuclear power stations in Fukui prefectures, almost one third of the 47 in all of Japan. Plans to switch some of them to the use of mixed oxide (MOX) fuel have met political resistance, as there a questions about both the safety and economic viability of this type of fuel.

The accident raises renewed serious questions about the safety culture of Japanese Nuclear Power Plant operators and the legal framework that surrounds them. Existing regulations did not explicitly require the company to check the secondary cooling cycle steam pipes of nuclear power stations. By contrast, every car needs to undergo comprehensive safety checks every two years. In 1995 a serious accident forced a shutdown in the Monju Fast Breeder Reactor, also in Fukui prefecture. That accident was also caused by a broken cooling pipe. The operators then tried to conceal evidence to cover up the magnitude of the accident.

The MOX Myth: A report from the Netherlands that details the numerous problems with using plutonium for power generation.

Monju Problems: A thread that started before the 1995 Monju accident and outlined the problems in detail.

The Tokaimura Accident: About the 1999 accident in the JCO nuclear fuel factory.

TEPCO Scandal: Power company punished for faking critical safety check.

The Plutonium Economy: Why Fast Breeder Reactors and nuclear waste reprocessing are not the asnwer for Japan's energy problems.

Fukushima: Joe Wein blog posts about the nuclear accident.

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