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Nuclear power and earthquakes

Nuclear Monitor Issue: 

(December 19, 1997) South Korea is having difficulties finding locations to build new nuclear power plants because more evidence has been found that proposed sites are in areas with dangerous seismic faults (see bottom). There is still a story that the disaster in Chernobyl (1986) occured after a earthquake took place. In the United States the Department of Energy is still willing to create a nuke waste repository at the Yucca Mountains, an area where quakes occur quite often. Enough reason to go into this issue more deeply. We took some news and backgrounds from Japan, probably the country wich is, looking at seismic faults, at most in danger.

(483/4.4802) Hiroo Komura / AMPO -There is this legend saying that a giant Catfish is buried deep in the sand beneath the Japanese islands who occasionaly shifts and changes position. When this happens the earth shakes and splits and fragile constructions of human beings collapse. It is also said that this ever present danger has contributed much to the Japanese sense of the ephemerality of all things. come they built nuclear power plants on the back of the catfish?

The nuclear industry itself gives the answer. The visitor's center at the Mihama Nuclear Power Complex has a earthquake safety demonstration display. If you ask "and what if there is an earthquake?" you can see the safety control rods drop smoothly into the reactor fuel and a voice says "see, the reactor can be safely stopped". There will be a day they will have to prove this; just under the Visititor's Center is an earthquake fault. The Japanese power company engineers keep saying "science and technology, properly employed, can overcome unpredictable accidents". When Chernobyl happened they where on TV saying that this could not happen in Japan; the NPP's are of a different type and have superior safety features. When the big earthquake hit San Francisco (USA, 1989) and all kinds of ferro-concrete buildings, highways and parts of the Bay Bridge came down engineers appeared on TV saying that the collapsed buildings and bridges where not built earthquake-proof: "In japan all large constructions are built to withstand any earthquake that might predictably occur". But than came the Kobe-disaster, January 1995, and every kind of construction came down. The experts where back on TV again, explaining: "The constructions where earthquake-proof, meaning they were built to withstand any quake that might predictably have come to Kobe. This earthquake was beyond prediction"... It occured on a number of different faults, some of which had not even been known to exist.

A government official, confronted with the question what would happen just IF such a earthquake would happen in an area with NPPs answered "that's like asking what would happen if the world blew up".

What exactly could happen is described below, kindly written by professor Komura Hiroo from the Shizuoka University, Hamamatsu, Japan.

Earthquake Would Cause Power Excursion on Nuclear Reactors
Japan is well-known as a country belonging to the unstable Pan-Pacific Earthquake Belt. Dangerous areas are specified and intensively monitored as the special zones, in which the big earthquakes did occur repeatedly in some historical span. Japan has 52 reactors in operation at present and most of these are located in or close to these dangerous zones. It is unbelievable that such dangerous zones were selected as the nuclear sites. Among them Hamaoka nuclear power station is most vulnerable, for it has been constructed at the center of the seismic source area of the anticipated Tokai Earthquake. There have been found many active faults on seabed, which are regarded as the source of earthquakes. Big earthquakes of magnitude 8 class in Richter Scale have been repeated every 120 years in this district and 140 years have passed since the last earthquake in 1854.

Pro-nuclear people are claiming that nuclear facilities are designed so as to even stand the biggest earthquake - not supposed to occur. The acceleration applied to important reactor components are determined for each facility and the largest value is 600 gals (a measuring standard for forces) for Hamaoka. But on Hanshin Earthquake some 800 or 900 gals of accelerations were actually observed. Immediately after Hanshin Earthquake, some of utilities announced their nuclear facilities could stand for such an earthquake and acceleration up to 1000 gals. But they did not give any evidence or data supporting their argument.

It is easy to suppose an earthquake causes damage on important reactor components or pipes, or reactor building walls. However, another possibility was recognized recently in Japan: Boiling Water Reactor (BWR), one of the typical light water reactor in Japan might suffer big damage from a power excursion - abrupt increase of output. This is an severe accident which might lead to a nuclear explosion or a steam explosion in the reactor. In 1993 an earthquake (M 5.9) happened in the northern part of Japan. Than Onagawa No. 1 reactor was being operated at the rated power level. It was automatically stopped (scrammed) by an earthquake. At that time actual rising of neutron flux was caused inside the reactor. In BWR's light water is used as the moderator, lowering the neutron velocity so that uranium can readily capture the neutron for a chain reaction. Usually, BWR is filled with an enormous amount of voids, that is to say, bubbles, and these bubbles do not act as moderator as liquid water does and the nuclear reaction is automatically suppressed by generation of bubbles. Quakes are likely to remove such bubbles stuck to fuel rods and push them up outside the reactor core. Then the nuclear reaction was increased and neutron flux level rose above 118 percent of rated value, at which the system is designed so as to generate the scram signal to avoid the serious condition.

This means the reactor almost went on power excursion. The scram signal was sent at 118% but this does not mean the actual value of neutron flux was restricted at that level. It only means the signal was generated. The actual neutron level can reach 400 % or even 1000 %. Fortunately, in every case the scram was successfully fulfilled so far. But this cannot be expected to be repeated all the time. Scram failures did occur in other situations and will continue to occur from now on.

Recently, cracks have been found on the core-shroud of many BWR's in United States, Japan and some European countries. The core-shroud is a huge cylinder surrounding a reactor core and regulating water flow in it. Those cracks, fairly long and deep, go along the circumferential direction and are located on the various parts of the shroud. National Regulatory Commission in the US and General Electric are very upset when cracks were found on the welded parts of the shrouds.

Most serious situation on this issue is that if the shroud would fall down to the bottom of a pressure vessel, lateral water flow will be generated and this flow disturbs the insertion of control rods at emergency situation. With the vibrating control rods and fuel rod assemblies, control rods would be blocked not to be inserted into the space gap among the fuel rods. In that case, we cannot expect the automatic shutdown - the last possibility of preventing a reactor from power excursion. And it should be noted that the falldown of a shroud is anticipated to be caused by the vibration of an earthquake. Utilities have a plan for repairing cracks on a shroud or replacing a shroud itself by new one within 5 years, but who knows when a huge earthquake will occur. From the technical point of view, nuclear reactors, especially BWR's, are extremely weak for earthquakes.

Power excursion mentioned above is likely to be caused even by a relatively small earthquake. A huge earthquake can destroy many components and pipes simultaneously. Coolant will flow out of a reactor, while water storage tanks (suppression pool etc.) attached to the reactor will be crashed. No water available for preventing loss of coolant accident. We cannot suppose what will actually happen when a nuclear reactor is exposed to a big earthquake.

Sources: AMPO magazine, Vol.26 No.3, 1995 and an article written by Hiroo KOMURA, Department of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Japan

Contact: Hiroo KOMURA, Department of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Japan
Tel & Fax : +81-53-478-1096
E-mail: [email protected]

"Not supposed to happen". The Republic Of Korea, already having hard times finding locations for new planned NPPs finally admits that seismic faults in the country causes extra dangers for reactors. According to safety officials "ongoing evaluations of seismic safety" will lead to stiffer standards. Three months ago a quake struck along the so-called Ulsan branch of the Yongsan fault, not far from the Kori PWR site in southeastern Korea. "This event was not supposed to happen", was the brave comment the Vice-President of the Korea Institute of Nuclear Safety gave. Evaluation of seismic safety could delay licensing of the Bonggil site, at the Sea of Japan coast, north to some known faultic area's.
(Nucleonics Week, October 9, 1997)