Earthquakes and Seismic Protection for Japanese Nuclear Power Plants

Context of the nuclear accident

The tsunami inundated about 560 sq km and resulted in a human death toll of about 19,000, and much damage to coastal ports and towns, with over a million buildings destroyed or partly collapsed. 
Some damage was caused by the earthquake, but most was by the tsunami.

A March 2012 Japan National Police Agency report confirmed 15,858 deaths (mostly by drowning), 3021 missing, and 6080 injured across twenty prefectures. In addition there were 129,855 buildings totally collapsed, 257,739 buildings 'half collapsed', and another 705,773 dwellings and 58,695 other buildings partially damaged. The earthquake and tsunami also caused extensive and severe damage to roads and railways as well as fires in many areas, and the Fujinuma dam collapse. The Japanese Prime Minister Kan said: "In the 65 years after the end of World War II, this is the toughest and the most difficult crisis for Japan.” Around 4.4 million households in north-eastern Japan were left without electricity and 1.5 million were without water for some weeks.

Fifteen ports were located in the disaster zone, and Hachinohe, Sendai, Ishinomaki and Onahama were destroyed. Sendai airport was flooded and severely damaged. An oil refinery near Tokyo owned by Cosmo Oil burned out of control for 10 days and killed six workers. Another oil refinery owned by JX Nippon Oil & Energy burned longer, but with no fatalities. An LNG plant at Sendai was damaged and out of action for a month. The environmental impact of each of these, both from the toxic chemicals from the fire and the groundwater contamination, is worse than the Fukushima accident. The long term health effects will inevitably be worse.

The economic cost estimated in April 2011 by the World Bank was US$235 billion, making it the most expensive natural disaster in world history.

The Environment Ministry said removal and disposal of 17 million tonnes of tsunami debris in Miyagi and Iwate prefectures was almost complete early in 2014, but for Fukushima prefecture only two-thirds of the debris was removed. The three prefectures were the ones that bore the brunt of the tsunami damage. In addition 11 million tonnes of marine sediment was washed ashore and deposited, of which 89% had been cleared early in 2014. (Jiji 21/2/14)

Earthquakes and seismic protection

Nuclear facilities are designed so that earthquakes and other external events will not jeopardise the safety of the plant. In France for instance, nuclear plants are designed to withstand an earthquake twice as strong as the 1000-year event calculated for each site. It is estimated that, worldwide, 20% of nuclear reactors are operating in areas of significant seismic activity. The International Atomic Energy Agency (IAEA) has a Safety Guide on Seismic Risks for Nuclear Power Plants. Various systems are used in planning, including Probabilistic Seismic Hazard Assessment (PSHA), which is recommended by IAEA and widely accepted.

Because of the frequency and magnitude of earthquakes in Japan, particular attention is paid to seismic issues in the siting, design and construction of nuclear power plants. The seismic design of such plants is based on criteria far more stringent than those applying to non-nuclear facilities. Power reactors are also built on hard rock foundations (not sediments) to minimise seismic shaking.

Japanese nuclear power plants are designed to withstand specified earthquake intensities evident in ground motion. These used to be specified as S1 and S2, but now simply Ss, in Gal units. The plants are fitted with seismic detectors. If these register ground motions of a set level (formerly 90% of S1, but at Fukushima only 135 Gal), systems will be activated to automatically bring the plant to an immediate safe shutdown. The logarithmic Richter magnitude scale (or more precisely the Moment Magnitude Scale more generally used today*) measures the overall energy released in an earthquake, and there is not always a good correlation between that and intensity (ground motion) in a particular place. Japan has a seismic intensity scale in shindo units 0 to 7, with weak/strong divisions at levels 5 & 6, hence ten levels. This describes the surface intensity at particular places, rather than the magnitude of the earthquake itself.

Originally, seismologists measured the magnitude of short-period seismic waves to indicate earthquake magnitude, and in the 1960s it became possible to measure longer-period seismic waves, which more accurately indicate the size of large earthquakes. They then started quantifying earthquakes according to seismic moment, using these longer-period wave measurements. To connect this scale to the old magnitude one a moment-magnitude scale was proposed. Up to magnitude 8 this gives the same result as the old scale, but for larger quakes such as Sumatra in 2004 and Tohoku 2011 it reflects the true size. Instead of 9.3 and 9.0 respectively these would have registered 8.6 and 8.2 respectively on the old scale.

Japan's revised Regulatory Guide for Reviewing Seismic Design of Nuclear Power Reactor Facilities in September 2006 increased the Ss figure to be equivalent to an earthquake of 6.7 on the Richter or Moment Magnitude scale directly under the reactor – a factor of 1.5 (up from magnitude 6.5). PGA or design basis earthquake ground motion is measured in Galileo units – Gal (cm/sec2) or g – the force of gravity, one g being 980 Gal.

The former design basis earthquake ground motion or peak ground acceleration (PGA) level S1 was defined as the largest earthquake which can reasonably be expected to occur at the site of a nuclear power plant, based on the known seismicity of the area and local active faults. A power reactor could continue to operate safely during an S1 level earthquake, though in practice they are set to trip at lower levels. If it did shut down, a reactor would be expected to restart soon after an S1 event.

Larger earthquake ground motions in the region, considering the tectonic structures and other factors, must also be taken into account, although their probability is very low. The largest conceivable such ground motion was the upper limit design basis extreme earthquake ground motion (PGA) S2, generally assuming a magnitude 6.5 earthquake directly under the reactor. The plant's safety systems would be effective during an S2 level earthquake to ensure safe shutdown without release of radioactivity, though extensive inspection would be required before restart. In particular, reactor pressure vessel, control rods and drive system and reactor containment should suffer no damage at all.

After the magnitude 7.2 Kobe earthquake in 1995 the safety of nuclear facilities in Japan was reviewed along with the design guidelines for their construction. The Japanese Nuclear Safety Commission (NSC) then approved new standards. Building and road construction standards were also thoroughly reviewed at this time. After recalculating the seismic design criteria required for a nuclear power plant to survive near the epicentre of a large earthquake the NSC concluded that under current guidelines such a plant could survive a quake of magnitude 7.75. The Kobe earthquake was 7.2.

PGA has long been considered an unsatisfactory indicator of damage to structures, and some seismologists are proposing to replace it with Cumulative Average Velocity (CAV) as a more useful measure since it brings in displacement and duration.

Following a magnitude 7.3 earthquake in 2000 in an area where no geological fault was known, Japan's NSC ordered a full review of the country's seismic guidelines (which had been adopted by the NSC in 1981 and partially revised in 2001) in the light of newly accumulated knowledge on seismology and earthquake engineering and advanced technologies of seismic design. The new Regulatory Guide for Reviewing Seismic Design of Nuclear Power Reactor Facilities was published in September 2006 and resulted in NSC and the Nuclear & Industrial Safety Agency (NISA) calling for reactor owners with NISA to undertake plant-specific reviews of seismic safety, to be completed in 2008.

The main result of this review was that the S1 - S2 system was formally replaced by NSC in September 2006 with a single design basis earthquake ground motion (DBGM Ss), still measured in Gal. The Guide states that the main reactor facilities "shall maintain their safety functions under the seismic force caused by DBGM Ss." They and ancillary facilities should also withstand the "seismic force loading of those caused by Elastically Dynamic Design Earthquake Ground Motion Sd (EDGM Sd)" calculated from stress analysis and being at least half the Ss figure.

In March 2008 Tepco upgraded its estimates of likely design basis earthquake ground motion Ss for Fukushima to 600 Gal, and other operators have adopted the same figure. (The magnitude 9.0 Tohoku earthquake in March 2011 did not exceed this at Fukushima.) In October 2008 Tepco accepted 1000 Gal (1.02g) DBGM as the new Ss design basis for Kashiwazaki Kariwa, following the July 2007 earthquake there, and Chubu accepted the same for Hamaoka. Tohoku has also accepted it for Onagawa, though the maximum recorded there in March 2011 was 568 Gal. In 2014 Hokuriku accepted the 1000 Gal level for Shika 2 ABWR, and apparently also for Shika 1, raising it from 600 Gal. Kyushu raised the DBGM for Sendai from 540 to 620 Gal, and Kansai raised that for Takahama 3&4 from 550 to 700 Gal in 2014, with Nuclear Regulation Authority (NRA) approval. The new NRA is now responsible for determining DBGM levels required.

Japanese nuclear plants such as Hamaoka near Tokai are in regions where earthquakes of up to magnitude 8.5 may be expected. In fact the Tokai region has been racked by very major earthquakes about every 150 years, and it is 155 years since the last big one. Chubu's Hamaoka reactors were designed to withstand such an anticipated Tokai earthquake and had design basis S1 of 450 Gal and S2 of 600 Gal. Units 3&4 were originally designed for 600 Gal, but the Ss standard established in September 2007 required 800 Gal. Since then units 3-5 have been upgraded to the new Ss standard of 1000 Gal.

But the focus has been on earthquakes, and not tsunamis, and further information on the whole question is in the Nuclear Power Plants & Earthquakes information paper, which also gives detail of major Japanese earthquakes relevant to nuclear plants.

In the 1960s the original design basis tsunami height was 3.1 m for Daiichi based on assessment of the 1960 Chile tsunami and so the plant had been built about 10 metres above sea level with the seawater pumps 4 m above sea level. The Daini plant was built 13 metres above sea level. In 2002 the design basis was revised to 5.7 metres above, and the seawater pumps were sealed. During the life of the Daiichi site, Tepco personnel reevaluated design-basis assumptions for tsunami height at least five times; and actions were taken on two occasions to prepare for increasingly large tsunamis.

In 2008 Tepco had investigated the potential for large tsunamis on the Fukushima coast. But because there were no previously recorded earthquakes off that coast to use as a model, engineers referred to the 1896 Meiji-Sanriku-oki earthquake off the coast of Iwate prefecture which had resulted in a 38-metre tsunami, and killed more than 27,000 people. Calculations based on this suggested a 15.7-metre tsunami at the Daiichi site. Senior Tepco managers directed that the validity of this possible scenario be checked with tsunami experts at the Japan Society of Civil Engineers, but the review was incomplete in March 2011.

Source: INPO 11-005 Addendum, Aug 2012.