Nuclear Power Plants Unable to Withstand Earthquakes

Japan is one of the world’s most earthquake-prone countries. Only a handful of regions on Earth experience frequent major earthquakes reaching seismic intensity levels of 6 or 7. Japan is one of them.

The Earth’s crust is divided into several tectonic plates. Earthquakes and volcanic activity are concentrated where these plates collide or where new land is formed.

Four of these plates converge in the waters off the coast of Japan. The Great East Japan Earthquake of March 11, 2011, occurred in the trench where the Pacific Plate and the North American Plate collide.

Its magnitude was 9.0. This ranks as the fourth-largest earthquake ever recorded on Earth.

Prior to the Great East Japan Earthquake, Japan had 54 nuclear power plants. After the disaster, some of them were decommissioned. Even so, there are still about 30 nuclear power plants, and three more are under construction.

There are nuclear power plants in countries all over the world. However, nuclear power plants worldwide are typically located away from areas prone to frequent earthquakes. Japan is the only country that builds nuclear power plants in such earthquake-prone areas.

So, how much seismic activity can Japan’s nuclear power plants withstand? They cannot withstand a seismic intensity of 6.

One unit used to measure seismic shaking is the gal (peak ground acceleration). The relationship between seismic intensity and gal is as follows:

Seismic Intensity Gal Intensity 7 1500 or higher Intensity 6 Upper 830–1500 Intensity 6 Lower 520–830 Intensity 5 Upper 250–520

The Great East Japan Earthquake recorded 2,933 gal. There have been earthquakes that recorded even higher gal values. The 2008 Iwate-Miyagi Inland Earthquake recorded 4,022 gal. Its magnitude was 7.2.

Furthermore, there have been 17 earthquakes exceeding 1,000 gal since the year 2000.

However, Japan’s nuclear power plants cannot withstand earthquakes of this magnitude. While seismic resistance varies by plant, there are no nuclear power plants in Japan capable of withstanding over 1,000 gal.

In the case of the Fukushima Daiichi Nuclear Power Plant, which suffered a nuclear accident during the Great East Japan Earthquake, there were no seismic standards in place at the time of its planning.

Following the example of the United States, where the reactor design manufacturer is based, it was designed to withstand 265 gal. This corresponds to seismic resistance for a seismic intensity of 5+ on the Japanese scale. The United States, with the exception of areas like California, is generally not prone to major earthquakes.

In 2009, safety review standards were established, requiring plants to withstand a magnitude 7.1 earthquake and 600 gal. This corresponds to seismic resistance for a seismic intensity of 6-lower.

As such, Japanese nuclear power plants cannot withstand a seismic intensity of 6.

Some readers may think that seismic resistance capable of withstanding 4,022 gal is excessive.

However, Mitsui Home’s houses are designed to withstand a seismic intensity of 7 on the Japanese scale 60 times. Converted to maximum gal, this amounts to 5,115 gal.

During the 2016 Kumamoto Earthquake, there were two earthquakes with a seismic intensity of 7, one day apart. As a result, buildings that withstood the first earthquake could not withstand the second and collapsed. This was because the buildings had been weakened by the first earthquake.

Consequently, some disaster victims chose to sleep in their cars rather than seek shelter in school gymnasiums.

A home capable of withstanding a magnitude 7 earthquake 60 times would provide peace of mind even in such situations.

In the June 2010 issue of “Shincho 45,” film director and comedian Beat Takeshi had the following exchange during a dialogue with Shunsuke Kondo, Chairman of the Nuclear Energy Commission:

“ It seems that if a nuclear power plant is damaged by an earthquake, it could lead to a major disaster. If it’s going to be that serious, they must have taken appropriate precautions. So, surprisingly, fleeing to a nuclear power plant might actually be the right choice during an earthquake. ” However, homes are safer than nuclear power plants.

Because the Great East Japan Earthquake originated 130 kilometers offshore, the seismic intensity along the Tohoku coast was 6.

At the Fukushima Daini Nuclear Power Plant, there were four external AC power lines (electricity from the general power grid), and two of them were shut down by the earthquake. One line was already offline for inspection, leaving only one line remaining. The tsunami caused the emergency diesel generator for Unit 1 to fail, putting the plant at risk of a total loss of power.

The Tohoku-Pacific Ocean Earthquake and the Subsequent Situation at the Fukushima Daini Nuclear Power Plant https://www.pref.fukushima.lg.jp/uploaded/attachment/188933.pdf

Additionally, at the Onagawa Nuclear Power Plant in Miyagi Prefecture, also in the Tohoku region, 650 locations — including minor issues — suffered damage due to the earthquake. While the emergency diesel generators did not fail, four out of five external AC power lines were damaged.

Damage to General Equipment: A Trigger for Emergency Generator Failure? Onagawa Nuclear Power Plant https://www.asahi.com/special/10005/TKY201105300344.html

Overview of Damage to the Onagawa Nuclear Power Plant Caused by the Great East Japan Earthquake https://www.jsme.or.jp/pes/Research/A-TS08-08/03/03tohoku.pdf

The concept of safety includes redundancy. This involves having multiple units of machinery that must not fail; if one fails, the system switches to another unit to keep the machinery running.

Some readers may interpret redundancy in the following way:

Imagine stacking five sheets of steel plate and firing a bullet vertically into them. Even if the bullet penetrates the first four sheets, if it is stopped by the fifth, the system is safe.

In this case, we can conclude that even if the same bullet is fired, it will be stopped by the fifth sheet.

Therefore, even if an earthquake of the same magnitude 6 strikes, in the case of the Onagawa Nuclear Power Plant, even if four of the four external AC power lines fail, the last one will stop the bullet.

In this way, applying this to the case of a nuclear power plant, even if multiple lines fail, as long as at least one remains, power can still be supplied, so safety should be assured.

However, the safety of redundancy is calculated based on probability. Suppose there is a system with redundant equipment. If it operates stably 90% of the time, the probability that both pieces of equipment will fail is 10% of 10%, or 1%. In this case, the probability that either one or both pieces of equipment will be operational and functioning normally is 99%.

Furthermore, unlike ordinary failures, an earthquake has the potential to damage all equipment at once.

An earthquake affects all external AC power lines. Just because one out of five lines remained functional during the Great East Japan Earthquake does not mean the same result will occur if a similar earthquake strikes again.

This is because, in the case of the Onagawa Nuclear Power Plant, simple probability theory suggests that any given line has an 80% chance of failing (the probability of 4 out of 5 lines being down). The probability of all five lines failing is approximately 33% (4/5 to the power of 5). Therefore, even with redundancy, it amounts to nothing more than leaving it up to chance.

In the case of the Onagawa Nuclear Power Plant, the emergency diesel generators might have survived, or they might not have. However, in the case of the Fukushima Daini Nuclear Power Plant, there was a probability of approximately 31% (3/4 to the fourth power) that a total loss of power would have occurred. A total loss of power would have meant that the reactors would have melted down, just as they did at the Fukushima Daiichi Nuclear Power Plant.

Furthermore, there is suspicion that the emergency diesel generators at the Fukushima Daiichi Nuclear Power Plant also failed due to the earthquake.

I refer to What Happened at the Fukushima Nuclear Power Plant: The Collapse of the Safety Myth (edited by Kotaro Kuroda and Hiromitsu Ino).

Two tsunamis were confirmed following the Great East Japan Earthquake. These were recorded at 3:27 p.m. and 3:35 p.m. by a wave gauge located 1.5 kilometers offshore from the Fukushima Daiichi Nuclear Power Plant.

It takes 70 to 80 seconds for the tsunami to travel from the wave gauge to the tip of the breakwater. Furthermore, analysis of digital photographs of the tsunami revealed that it took 56 seconds to travel from the tip of the breakwater to the plant’s shoreline.

The first wave could not overcome the 4-meter-high embankment. The second wave reached the plant site at 3:37 p.m., and it is estimated that it took 1 to 2 minutes to reach Unit 1.

However, although the exact time is unclear in the operation log, it states that the diesel generator for Unit 1 malfunctioned before 36 minutes had elapsed, and an operator also testified to this during a hearing by the Diet’s Accident Investigation Committee.

This was later revised to 37 minutes following an inquiry by TEPCO. However, under the rules of the Diet’s Accident Investigation Committee, changing testimony after the fact constitutes a violation of the rules.

Nevertheless, power companies’ disregard for earthquakes continues to this day.

Kansai Electric Power Company (KEPCO) claims, based on strong-motion prediction — a theory in seismology — that earthquakes exceeding 700 gal will not occur on the Oi Nuclear Power Plant site.

The reliable observational data used for this strong-motion prediction dates only from 2000 onward. It is a forecast of the future based on just over 20 years of data.

Under these conditions, KEPCO’s claim can only withstand a once-in-20-years earthquake, not a once-in-1,000-years event.

To put it another way, it’s as if aliens landed on this planet’s mid-latitude region, conducted a climate survey for a quarter of the planet’s orbital period, and concluded that this planet has a warm, comfortable climate. It’s like spring in the temperate zone.

It is premature to determine the future based on current strong-motion predictions.

As I wrote in the article “Electric Power Companies and Moral Hazard,” Kansai Electric’s President Yagi (at the time; he resigned following a scandal in which he accepted money and gifts from a former deputy mayor of a municipality hosting a Kansai Electric nuclear plant) tried to revive the myth of nuclear safety by claiming that larger plants are safer, so Kansai Electric cannot be trusted.

This is strictly taiga’s personal opinion.

That said, in earthquake-prone Japan, safety standards that deem a nuclear plant safe when it cannot withstand a seismic intensity of 6 are not credible.

Translated with DeepL.com and taiga.