Seismic effect on Japan nuclear power plants

Seismic effect on Japan nuclear power plants

While global nuclear power plants are designed to withstand earthquakes, in the event of earthquakes and in the movement of major earth movement, to shut down safely.

In 1995 the closest  nuclear power plants, some 100 km north Kobe, were un affected by severe Kobe-Osaka earthquake, but 2004,2005,2007,2009 and 2011 Japanese reactors shut down automatically due to ground acceleration exceeding their trip settings.

In 1979 three nuclear reactors shut down automatically during divesting Taiwan  earth quake, and were  restarted two days later.

In March  2011 eleven operating nuclear power plant shut down automatically during the major earthquakes. Three of these subsequently caused an INES level 5 accident due to loss of power leading to loss of cooling

Nuclear facilities are designed so that earth quakes and other external events will not jeopardize the safety of plant. In France for instance nuclear power plants are designed to withstand an earthquake twice as strong as 1000 year calculated each site. It is estimated that world wide 20 per cent nuclear reactors are operating in the area of 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 frequency and magnitude of earthquake in Japan, particular attention is paid to Seismic issues in the siting,design and construction of nuclear power plants.


Japanese nuclear power plants are designed to withstand specified earthquake intensities evident in ground motion. These used to be specified as S1and S2 but S's in Gal units. These plants are fitted with seismic detectors.  The logarithmic Richer  Magnitude scale or more precisely the moment Magnitude scale generally used to day) measures the overall energy released in the earthquakes and there is not always a good correlation between the intensity that and intensity (ground motion) in particular place.

Japan revised regulatory guide for reviewing seismic design of nuclear power reactors facilities in September  2006 increased the S's figure to be equivalent to an earthquake 6.7 on the Richter or moment magnitude scale directly under reactor- factor 1.5 (up from magnitude 6.5). PGA or design basic 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 largest earth quake which can reasonably to expected to occur at the site of nuclear power plant, based on the known seism city     of the area and local active faults. A power reactor could continue to operate safely during an SI level earthquake, though in practice they are set to trip at lower levels. If it did shut down ,reactor would be expected to restart soon after an SI event. The revised seismic regulation released in May 2007 increased the S1 figure  to be equivalent to 6.7 on the logarithmic Richer scale- a factor 1.5 (up from 6.5). PGA is measured in Galileo units Gal (cm/ sec2) or g -the force of gravity, one g being 980  Gal the non S1 unit used here.

Larger earth quakes ground motions in the region, considering the tectonic structure's  and other factors must also be taken  into account, although their probability is very low. The largest conceivable such ground motion was upper limit design basis extreme earthquake ground motion (PGA) S2 generally assuming a magnitude 6.5 earthquake directly under the reactor. The plant safety systems would be effective during an S2 level earthquake to ensure safe shut down without release of radio activity though extensive inspection would be required before re-start. 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 facility in Japan reviewed along with design guide lines for their construction . The Japanese nuclear safety commission (NSC) then approved new standards were also thoroughly reviewed  at this time.  After recalculating the seismic design criteria required for nuclear power plant to survive near the epicenter of large earthquake the NSC concluded that under current guide lines such plant could survive quake of magnitude 7.75. The Kobe earthquake was 7.2.

PGA has long been considered un satisfactory indicator of damage to structures and some seismologists are proposing to replace it with cumulative average velocity (CAV) as more useful measures since it brings in displacement and duration.

Japan’ s  Rokkasha  reprocessing plant and associated facilities are built on stable rock and are designed to withstand on earthquake of magnitude 8.25 there.

Following a magnitude 7.3 earthquake in 2000 in then area where no geological fault was known, Japan’s NSC ordered full view of country’s seismic guide lines which had been adopted by NSC in 1981 and particularly revised in 2001 ) in 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 reactor facilities was published in September 2006 and resulted in NSC and industrial safety agency (NISA) calling for reactor owners with NISA to under take plant specific reviews of seismic safety, to be completed in2008 .

The main result of this review was that the SI-S2 system was formerly replaced by NSC in September 2006 with  a single design basis earth quake ground motion (DBGM S's) still measured in Gal. the guide states that main reactor facilities “ shall maintain safety functions under seismic force caused byDBGM S's . “ they and ancillary facilities should also with stand the “ seismic force loading of those caused by elastically dynamic design earthquake ground  motion Sd(ED-GM Sd) calculated from stress analysis and being at least half the Ss figure.

In March 2008 TEPCO up graded it estimates of likely design basis earthquake ground motion S's for Fakushima in 600 Gal other operators have adopted the same figure (magnitude 9.0 Tohoku-Taiheiyou-ok earthquake in March did not exceed this at Fukushima). In October 2008 TEPCO accepted 1000 Gal (1.02g)DB GM as new design basis for  ,Kashiwazaki , Kariwa,  following the July 2007 earthquake.

Japanese nuclear power plants such as Hamaoka near Tokai are in regions where earthquake of magnitude 8.5 may be expected. In fact the Tokai region has been racked by every major earthquake about every 150 years and it is 155 years since last big one Chub u -Harmaoka reactors were designed to withstand  such 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 S's standard established in September 2007 required 800 Gal. Since these units 3-5 have been upgraded to the new S's standard 1000 Gal. in August 2009 magnitude 6.5 earthquake nearby automatically shut down Harnaoka 4 &5 with ground motion of 426 Gal being recorded at unit 5. Some ancillary equipment was damaged and reactor 3&4 were restarted after rechecking. Restart unit 5 was repeatedly differed as company analyzed  why such seismic acceleration was recorded on it, coupled with some planned maintenance being  under taken during shut down. It restarted in January 2011.

Harnaoka units 1&2 had been shut down since 2001 and 2004 respectively pending seismic upgrading- they have originally designed to with stand only 450 Gal. in December 2008 the company decided to write off and a new reactor to replace them. Modifying the 1970’s units to new seismic standards would have cost about U.S. dollars 3.3 billion and has been  un economic so Chub u opted for U.S. dollar 1.7 billion written down instead.

Early in 2010 Japan’s METI confirmed that seismic safety of Manju fast reactor was adequate under new standards requiring S's Gal PG A. Assessments were carried out in conjunction with Kansas Mahima plant and JAPC s Tsuruga plants both nearby.

South Korea new APR-1400 reactor is designed to with stand 300 Gal seismic acceleration. The older is designed for 200 Gal but being up graded to at least 300 Gal so as to offered to Turkey and Jordan.

In U.S.A the Diablo Canyon plant is designed for 735 Gal peak ground acceleration and San Onopre  plant is designed for a 0.657 Gal peak ground acceleration.

Japan March 2011 Tohoku-Taiheiyou -Oki


The magnitude 9.0 Tohoku-Taiheiyou-Oki earth quake at 2.46 P.M. on 11th March did considerable damage and 14 meter tsunami it created caused even more. It appears to have been double quake giving a severe duration of about 3 minutes and was centered 130 km off-shore of the city Sendai in Miyagi prefecture on the eastern coast of Honshu Island. It moved Honstru  4 meters east and apparently subsided the nearby coast line by half meter. Eleven reactors at four nuclear power plants in region were operating at that time all shut down automatically when quake hit. Power  was available to run cooling pumps at most of the units and they achieved cold shut down in few days. However at TEPCO  Fukhushima  Daaichi  plant where three reactors were shut down by earthquake  the emergency diesel generators started as expected as expected but then shut down an hour  later when submerged by tsunami . Other  systems proved  inadequate and led authorities to order, and subsequently extend an evacuation while  engineers worked to re start power. About nine hours later mobile power supply units reached the plant and were being connected. Meanwhile units 1-3 had only battery power in sufficient to drive the cooling pumps.

The operating units which shut down were TEPCO ‘s  Fukushima Daiichi 1,2,3 Fakushima Daini 1,2,3,4 Tohoku’ s Onagawa 1,2,3 and Japcos  Tokai Onagawa 1 briefly suffered  a fire in non-turbine building, but the main problem centered on Fukushima Daiichi units 1-3 . First pressure inside the containment structures increased steadily and led to this being vented to the  atmosphere on going basis. Vented gases and vapor including hydrogen, produced by  exothermic interaction of fuels very hot Zirconium cladding with water. Later on 12th there was hydrogen explosion in the building above unit 1 reactor containment, and another  one  two days later in unit 3, from venting as hydrogen mixed with air. Then on 15th unit 2 ruptured its pressure suppression chamber under actual reactor  releasing some radio activity. Inside water level had dropped, exposing fuel, and this was addressed by pumping sea water into reactor pressure vessels.

Then separate set of problems arose as spent fuel ponds in upper part of the reactors structures were found to be depleted in water. In unit 4  the fuel there got hot enough to form hydrogen and other explosion destroyed the top of building and damaged unit’s  3 super structure further .The  focus since has been on replenishing the water pond’s  of unit 3&4 with some success , though the gaps in the roof and cladding . Unit 4 was undergoing maintenance and all its 548 fuel assemblies were  in that pond, along with new and used fuel, total 1535 assemblies giving it heat load about 3 MW thermal, according to France  ISRN. Unit 3 & pool contained fuel assemblies.

Japan’s nuclear & industrial  safety agency eventually declared the accident as level 5 on INES scale- an accident with wider consequences, the same level as three Mile Island 1979. The design basis acceleration for both Fakhushima  plants had been up graded in 2008 and now quoted horizontal 441- 489 Gal for Daiichi and 415-434 Gal for Daini.  The interim recorded data for both plants shows that 550 Gal was maximum if Daiichi in foundation of unit 2 ( other figures 281-548 Gal) and 254 Gal maximum for Daini. Unit 2,3and 5 exceed the maximum response acceleration design basis in E-W direction by about 20 % . Recording was 130-150 seconds. ( ground acceleration was around 2000 Gal few kilometers north on sediments.

In view of this it is being noticed that nuclear safety commission( NSC) of Japan had taken several steps to reduce impact earthquakes on nuclear power plants. But March 2011 earthquake (magnitude 9.0) combined with tsunami affected  Fakushima Daiichi plant which seems to be a rare case. However it is advised  to take  necessary safety steps to reduce impact of earthquake magnitude 9 and above for safe shut down of reactors automatically in event of earth quake associated with tsunami  . Nuclear scientists and engineers to design reactors that can with stand especially taking consideration of tsunami  without spreading any radiation in the event of core melt .