Newsletter of EFN

Environmentalists For Nuclear Energy

11th August 2004


This document is archived on the internet and can be consulted at the following address: http://www.ecolo.org/archives/archives-nuc-en/


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 Causes of the industrial accident at Mihama in Japan

by Bruno Comby


Photo of the Mihama nuclear power plant seen from the nearby beach. The accident took place in the turbine hall of reactor number 3 which is the one on the right of this photo.

© Photo EFN - www.ecolo.org / Bruno Comby (April 2001)



Dear readers, dear friends,

Following the accident which occurred two days ago at the Japanese nuclear power station at Mihama, please find below some information about this event.

The cause of that accident was the suddden rupture of a pipe carrying pressurized water in the secondary circuit of the power station, after it had passed through the turbine and heat exchanger and before it returned to the steam generator (see photos and the schematic of the circuit below). In an instant, the pipe broke, and an enormous burst of steam was released into the turbine hall. 

Eleven persons who were in the turbine hall at that moment were seriously burned (scalded). Four of them died the very day of the accident. At least two of the other seven are in a critical state and their prognosis is not favorable.

The rupture occurred in a straight part of a black steel tube 56 centimeters (22 inches) in diameter carrying pressurized water. When the reactor was at full power, that steam would have been at a temperature of 195°C, under a pressure of 12.7 atmospheres (about 180 psi); we have been told that, at the moment of the accident, the temperature was 140°C and the pressure 9 atmospheres, but that information is yet to be confirmed.

When the power station was built, the wall of the tube was 10 mm thick (0.4 inch), and it was supposed to remain at least 4.7 mm thick to stay safe; but, as one can see from the photo below, it was almost entirely eroded and down to 1.4 to 1.5 mm thick in a large area around the rupture zone. There was a great deal of corrosion. This reflects a major fault, either at the stage of engineering design, where the phenomenon of corrosion was under-estimated or neglected (similar pipes in more recent nuclear power plants are now in stainless steel, not black steel); or in the inspection routine. Periodic inspections, correctly carried out using ultra-sound instruments currently in wide use, would have revealed the weakened state of this tube.

The Mihama-3 reactor, a Mitsubishi pressurized water reactor of 826/780 MW(e), diverged in January 1976 and was connected to the grid on 19th February 1976. It has thus operated for 28 years and should have been the subject of at least two major inspections. Major inspections on nuclear power plants are usually performed every 10 years. These inspections are particularly detailed. How could they not have detected this defect? According to today's news broadcast on the Japanese television, but yet to be confirmed, this pipe had not been verified even once since the reactor was put into operation.  It seems then that there have been one or several instances of severe incompetence.

During a press conference held at Mihama, 350 km west of Tokyo, Koji Ebisuzaki, director of quality control, said "We are responsible" Haruo Nakano, spokeman for KEPCO, the utility which owns and operates the plant, declared "We made visual inspections but never used ultra-sound." Ultra-sound inspections on this tube were not obligatory, but thaere is no other way to detect internal corrosion of such a tube. KEPCO had recently put off an in-depth technical evaluation of the very tube which failed. The inspection of this tube had been planned and should have been performed several months earlier, but KEPCO had decided to postpone this verification until the next inspection.

EFN reminds the world and insists upon the point that maintenance and safety in nuclear power stations are and must remain absolute priorities.

We note however that even though several mistakes were committed at different levels: the pipe was in black steel that corrodes easily, the corrosion was not checked, the Japanese safety authorities new that this pipe needed to be changed (several had already corroded and had been replaced by stainless steel pipes in other Japanese nuclear power plants), and above that the inspection which was planned (too late) was postponed, the basic principles of nuclear safety nevertheless still played their role effectively to prevent from a larger accident: the multi-level and redundant safety systems installed at different levels shut the reactor down automatically and immediately without any other incident, despite the extremely rare character of the event and the gravity of the situation.


Image - Mihama schematic of the primary and secondary circuit showing where the pipe has ruptured


Yesteday's news bulletin mentioned steam at 260 or 270°C. That would be the temperature at the exit of the steam generator, before passing through the turbine and condenser; but it was not there that the rupture occurred. At the site of the rupture, after the turbine and condenser, the temperature would be lower but still high enough to inflict serious burns on the unfortunate men working nearby in the turbine hall.

The nuclear reactor shut down spontaneously at the moment of the accident, but it is not clear at this moment whether it was the rupture of the tube which caused the emergency shutdown of the reactor (which is the most likely scenario), or whether some other event led to the automatic shut down of the reactor, causing an overpressure itself causing the corrosion-weakened tube to fail.

We wish to repeat that absolutely no radioactivity was released into the environment. The accident was not nuclear but industrial - it could have happened in any power station whether heated by nuclear energy or by burning fossil fuel.

With four victims (to date), this is a tragic event. In fact, it is the worst accident to have occurred in a nuclear power station since the Chernobyl accident of 1986. But we must view things in perspective: recent news items have shown us once again that natural gas is a killer, causing many fatal incidents and killing many more persons each year; not to speak of the thousands who die every year in coal mine accidents without getting the same attention from the media.

The flow in the secondary circuit of the power station, is about 1600 tonnes of water per hour, about 27 tonnes per minute, or, if you like, nearly a tonne every two seconds. And this under a pressure of about ten atmospheres. When the pipe ruptures like this, the pressure is reduced and the water (140°C) transforms into steam, is projected until it fills the entire machinery hall. Our brief calculation suggests that it would take only a couple of minutes to fill a hall of dimensions 100m x 50m x 20m, unless the leaking pipe could be isolated and the flow of steam stopped.

The ruptured circuit was isolated, and the steam projections stopped indeed after some time (how much? we don't know). According to information published by the Japanese media (asahi.com), there were 220 subcontractors (which is a lot) working inside the plant (how many of them in the turbine hall when the accident occurred? we don't know). The subcontractors were preparing a planned inspection which was about to start a few days later.

The 11 workers who were scalded seem to be those who were nearest to the pipe when it broke. Some of them had severe body burns because they received bursts of steam directly on their bodies, and others badly burned their lungs by breathing the hot steam as it spread throughout the turbine hall. Four have already died and seven of them are now struggling for life in the Fukui prefecture hospital. One of them is severely burnt on 80% of body surface, and will certainly not survive, and several others are in critical condition. Luckily, some workers managed to escape the turbine hall safely (how many? we don't know).

Hot pressurized water and steam are dangerous. In bygone days, industrial and domestic boiler and hot-water heater accidents were frequent, often involving equipment which was badly adjusted, badly maintained and without adequate safety systems. Many victims were scalded (burnt by steam) and many died of their burns. Sometimes it was faulty equipment, but often it was imprudence. For example, in steamboats navigating the Mississippi River a century ago, it was not uncommon to tie down the safety valves, especially in a boat race, or just to let the pressure rise higher, forcing the boat at a higher speed. In one case, on the Mississipi river in 1865, a steam explosion is said to have killed 1,547 people, many of whom were prisoners being trasferred, when the steam boiler powering the boat suddenly exploded. Many were scalded and died from the explosion itself, and others drowned. Nor were industrial boiler explosions rare. Since then, safety standards have been imposed and progressively tightened. In the United States, there is a long standing National Board of Boiler and Pressure Vessel Inspectors whose standards and rules are accepted world-wide. Chemical engineers are particularly alert to these questions. Nevertheless, steam burns and boiler explosions still kill frequently. Numerous references are obtained for example by searching for "boiler explosion" on the internet.

In any event, as the Mihama accident reminds us, there is no energy without risk, and hot pressurized water (whether produced in a nuclear reactor or not) is dangerous and should be handled with precaution. Accidents are always possible, and we must be extremely vigilant and prudent to create and strengthen a culture of safety and meticulously respect the rules. Although rarer than before, industrial accidents due to corrosion of pipes and steam explosions still occur, but they are hardly mentioned in the media, when they occur in places other than nuclear power plants.

What would be the impact of an accident as serious as Mihama, if it had occurred in any plant but nuclear? Perhaps an article in the local or regional paper, an item on the evening TV news, but it would certainly not receive the honors of notice in the international press as Mihama does these days. 

On the one hand, we should learn from this sort of experience and do everything to avoid the recurrence of accidents like Mihama. In case such an accident does occur, we must be able limit the damages by prevention.

On the other hand, we must emphasize the NON-NUCLEAR nature of this accident, which might have occurred in any other factory or in a power plant fired by gas or coal or oil. It was not a NUCLEAR accident. It was an INDUSTRIAL accident.

Greenpeace and other ecological movements around the world have already begun in their own shameful way to take advantage of this unfortunate accident, and to demand once again that the world abandon nuclear energy, in Japan and elsewhere. They are not impressed by the fact that THE ACCIDENT AT MIHAMA WAS NOT A NUCLEAR ACCIDENT. Recent gas explosions have had many more victims than Mihama, but we don't hear these associations calling to "stop gas" or for a "gas-free world" comparable to the "stop nuclear" or "nuclear-free world" shoutings which re-appeared today in reaction to this unfortunate INDUSTRIAL (NOT NUCLEAR) accident. Explosions are rather a speciality of gas, you know; they cause more frequent and bigger accidents, with many more victims than yesterday's accident in Japan. Is there any consistency in their behavior?

EFN extends its most sincere condolance to the families of the victims.

Sincerely yours,


Bruno Comby

President of EFN
Environmentalists For Nuclear Energy (civilian only, properly used, and with the greatest of precautions) 


Image - Outside photo of the reactor-3 building at Mihama, where the accident happened, showing the steam coming out of the turbine hallOn this photo taken on August 9th, 2004, one can see the steam rising from the turbine building of reactor number 3 at the Mihama Nuclear Power Plant in Fukui Prefecture. That steam comes from the inside of the turbine hall,.which is the rectangular building on the left hand side of the photo. The reactor itself containing the nuclear fuel is located inside the circular-shaped concrete confinement in the center of the photo. The nuclear reactor, the confinement building, and the primary circuit were not affected at all by the accident. Mihama was not a nuclear accident. The nuclear part of the installation was unaffected. No radiation leaked and the reactor itself always remained safe and under control.


Image - Photos of the ruptured pipe from different angles

Photos of the scene of the accident and ruptured pipe : source KEPCO - Kansai Electric Power Corporation (August 10th, 2004)

Bruno Comby, EFN founder and president (http://www.ecolo.org ), is the author of 10 books published in 15 languages on ecology and energy, including the bestseller "Environmentalists For Nuclear Energy" (published by TNR Editions). He is a graduate of the Ecole Polytechnique in France, and holds a postgraduate qualification as nuclear physicist from the Superior National University of Advanced Technology in Paris (ENSTA).


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