Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[rhody]

I would simply prefer an explanation of why the I-134 activity is incorrect, if it is, rather than just a statement to the effect that "the data for iodine-134 announced earlier in the day was actually for another substance that has a longer half-life." My response - "What other substance?!"

From Washington Post, today...

Perhaps this http://www.chron.com/disp/story.mpl/ap/top/all/7493328.html" explains it:

Tepco said it miscalculated the radioactivity measurement in the unit 2 turbine building because it mistook the data for iodine-134 for the data of cobalt-56, which has a longer half-life

Does this make more sense Astronuc, AntonL ?

Rhody...

 

[M. Bachmeier]

From Washington Post, today...

Perhaps this http://www.chron.com/disp/story.mpl/ap/top/all/7493328.html" explains it:


Does this make more sense Astronuc ?

Rhody...

According to what was said at the press conference (NHK) http://www3.nhk.or.jp/daily/english/27_24.html?play

They were not making a certain claim that iodine-134 was not what was detected, but would take another sample. Note: unless translation was incorrect.

 

[PietKuip]

extracted from attached report for sea-water analysis - basement water would have have been done very similar. Any clues in this statement regarding their near-site measuring capabilities?

There is a huge difference in activity and dilution of course.

One would bring such dilute samples off site. It should be difficult to measure seawater with the backgrounds at the accident site.

So they had a sample of half a litre and they measured it for 1000 seconds. Probably in a Marinelli beaker with a germanium detector.

On-site they seem to have a car with monitoring instruments. They should have portable spectrometers with them too.

 

[jlduh]

Actually, the Tepco study designed the highest possible tsunami at + 5,7m above "OP" (don't know what "O.P." means), confirmed by this study, and the platform on which the plant is built is 10 to 13m above "OP". So it was presented almost as a 2X safety factor in comparison with the "highest possible designed tsunami"!

I didn't even mention, in addition to "earthquake tsunamis", megatsunamis which can have various origins but one of the main being big landslides for example due to explosions on a volcano...

http://en.wikipedia.org/wiki/Megatsunami

But some may say that with waves bigger than 100 meters, anyway the catastroph would be terrible even without any nuclear plant...

 

[rhody]

According to what was said at the press conference (NHK) http://www3.nhk.or.jp/daily/english/27_24.html?play

They were not making a certain claim that iodine-134 was not what was detected, but would take another sample. Note: unless translation was incorrect.

M Bachmeier,

If I understand what you said, if it is cobalt-56, then it needs to be confirmed with a new set of measurements, no ? Another problem here is you are reporting on information from a press conference (real time I assume) whereas the WP article must be older, and as such the information is older as well. When I reported it, google news said it was about 10 minutes old. If that is correct it may have been reported in the press conference, which I did not watch, and am hoping you can confirm or refute. I don't know how long the WP takes to review and post breaking news, but can't imagine it could take too long.

Rhody...

 

[AntonL]

I would simply prefer an explanation of why the I-134 activity is incorrect, if it is, rather than just a statement to the effect that "the data for iodine-134 announced earlier in the day was actually for another substance that has a longer half-life." My response - "What other substance?!"

Here is the explanation, basically they extrapolate the measurement back to the time of sampling, and because of short life time of I-134 the value sky-rockets. Apparently they mistook Cobolt-56 but there seems to be confusion about that as well.

http://www.asahi.com/national/update/0327/TKY201103270213.html][/URL]
TEPCO, 27, announced that after 10 million times the radiation detected in normal water reactor turbine building entrapment in Fukushima Daiichi Unit 2 nuclear power plant, and at night, "mistaken for a different matter, " an interview with , re-corrected the early morning of July 28 further correction. Substantially revised and the number measured in the turbine building where even the 26th. Not share information, office workers exposure to high radiation-related locations (nuked) the accident happened. The lack of communication, has spurred the confusion.

Entrapment when the water is examined in Unit 2, calculated back to the time taken for the radioactivity of a substance called iodine-134 decreased very quickly, better, 1cc were around 2.9 billion becquerels. 1 million times the normal. Once found in large quantities so, could even have occurred in a fission reactor. TEPCO Nuclear Safety Commission will be asked to re-evaluate.

TEPCO and measured again, but should decrease rapidly if the radioactive iodine-134 was not much less. TEPCO 27 night, "could be mistaken for those slower decrease of cobalt 56, " said. However, in the early morning of July 28 meeting was "instead of cesium 134 Cobalt 56" was corrected again. The morning briefing "was detected" two different substances was announced, in fact, had not come out.

Mutou Sakai, vice president is "(the analysis) There was not enough in the process of examining the contents, " he said.

TEPCO spokesman said, "despite the possibility that measurement uncertainty, and make public, was hidden after [and] criticism, " to worry about. Nuclear and Industrial Safety Agency, METI also have to give priority to public for the same reason.

 

[M. Bachmeier]

M Bachmeier,

If I understand what you said, if it is cobalt-56, then it needs to be confirmed with a new set of measurements, no ? Another problem here is you are reporting on information from a press conference (real time I assume) whereas the WP article must be older, and as such the information is older as well. When I reported it, google news said it was about 10 minutes old. If that is correct it may have been reported in the press conference, which I did not watch, and am hoping you can confirm or refute. I don't know how long the WP takes to review and post breaking news, but can't imagine it could take too long.

Rhody...

My intention was only to clarify, not to be critical of any effort to increase the level of information. Accurate information is hard to come by and is often distorted as it passes through communication channels, so please don't take offense.

Please note link... and am relying on translation as reported...

 

[jlduh]

Again...

http://www.chron.com/disp/story.mpl/ap/top/all/7493328.html

Officials acknowledged there was radioactive water in all four of the Fukushima Dai-ichi complex's most troubled reactors, and that airborne radiation in Unit 2 measured 1,000 millisieverts per hour, four times the limit deemed safe by the government.

An other example of "mistake" in the way it is presented.

1,000 millisieverts per hour, Four times the ANNUAL limit deemed safe by the government FOR WORKERS IN THE NUCLEAR INDUSTRY IN CASE OF NECESSITY (which is currently the case!).

Mistake of the press or in the declaration of the autorities, how hell can the normal citizen who reads this can have a clear view of what is safe or unsafe?

 

[PietKuip]

Here is the explanation, basically they extrapolate the measurement back to the time of sampling, and because of short life time of I-134 the value sky-rockets.

That is no good reason. Just take a spectrum of the sample every half hour. It will show what the half-life of the peaks are.

 

[Astronuc]

From Washington Post, today...

Perhaps this http://www.chron.com/disp/story.mpl/ap/top/all/7493328.html" explains it:


Does this make more sense Astronuc, AntonL ?

Rhody...

Yes - that's plausible to a degree.

The decay of Co-56 does produce a gamma ray of 846.771 keV, with a relative intensity of 99.94. It could be mistaken for the 847 and 885 keV gammas from I-134. But those two gammas have relatively low intensity to lower energy gammas or X-rays.

Normally, one would not expect a high rate of Co-56 (from neutron activation of Fe-56) to be present in a sample. On the other hand, this is not a normal situation, and there is probably a lot of activated corrosion products in the wate with the fission products. Normally, the corrosion products would be captured on filters, while iodine would remain in the cooling water.

 

[Astronuc]

That is no good reason. Just take a spectrum of the sample every half hour. It will show what the half-life of the peaks are.

Yes - they would need to take a second count on the same sample and allow for decay of different isotopes. However, I would imagine that the lab procedure would allow for one sample - because it assumes normal conditions, not abnormal conditions. Normally, they would be sampling reactor coolant from the primary system - not seawater or samples from the torus, or wherever.

I hope they improve their sampling techniques.

 

[M. Bachmeier]

Yes - that's plausible to a degree.

The decay of Co-56 does produce a gamma ray of 846.771 keV, with a relative intensity of 99.94. It could be mistaken for the 847 and 885 keV gammas from I-134. But those two gammas have relatively low intensity to lower energy gammas or X-rays.

Normally, one would not expect a high rate of Co-56 (from neutron activation of Fe-56) to be present in a sample. On the other hand, this is not a normal situation, and there is probably a lot of activated corrosion products in the wate with the fission products. Normally, the corrosion products would be captured on filters, while iodine would remain in the cooling water.

Does this have any relation to the Co-56 question?

"Upper limits for yields of certain isotopes of argon from fission of 235U by thermal neutrons were determined radiochemically as follows: 37Ar, ≤1×10-7%; 39Ar, ≤4×10-7%; 41Ar, ≤3×10-9%; 42Ar, ≤3×10-11%. The upper limit for the fission yield of 56Co (daughter of 56Ni) was determined as ≤8×10-8%. The yields of these isotopes are three to seven orders of magnitude lower than those expected from thermal-neutron-induced ternary fission of 235U as described by Muga."

From: http://prola.aps.org/abstract/PR/v142/i3/p716_1

 

[rhody]

please don't take offense.

M Bachmeier,

None taken...

AntonL,

After your last post, thus the reason for new measurements. I believe that clarifies the issue.

Edit: Astronuc

I hope they improve their sampling techniques.

Rhody...

 

[PietKuip]

Yes - they would need to take a second count on the same sample and allow for decay of different isotopes. However, I would imagine that the lab procedure would allow for one sample - because it assumes normal conditions, not abnormal conditions. Normally, they would be sampling reactor coolant from the primary system - not seawater or samples from the torus, or wherever.

I hope they improve their sampling techniques.

Indeed. It is irresponsible to send back some person into that basement just for reasons of lab protocol.

 

[Astronuc]

Actually, the Tepco study designed the highest possible tsunami at + 5,7m above "OP" (don't know what "O.P." means), confirmed by this study, and the platform on which the plant is built is 10 to 13m above "OP". So it was presented almost as a 2X safety factor in comparison with the "highest possible designed tsunami"!

I didn't even mention, in addition to "earthquake tsunamis", megatsunamis which can have various origins but one of the main being big landslides for example due to explosions on a volcano...

http://en.wikipedia.org/wiki/Megatsunami

But some may say that with waves bigger than 100 meters, anyway the catastroph would be terrible even without any nuclear plant...

The 535 eruption of Krakatoa (Krakatau) apparently blew up many cubic miles of earth!
http://en.wikipedia.org/wiki/Krakatoa

While it was an island for many centuries ~535 to 1883, it has been posited that there was continuous land between what is now Sumatra and Java.
http://en.wikipedia.org/wiki/Krakatoa#535_AD_event

Yet - we have not experienced such events since.

 

[PietKuip]

http://www.ustream.tv/channel/cnic-news is a technical seminar about the accident with English translation. Live, I think.

Sorry, that was yesterday.

 

[ivars]

So its Co-56 that has activity around 10^6 Bq/cm^3? Is that better news? Where did it came from? Its not in the table...But I read it can be created from Fe-56 by neutron activation. Where do neutrons come from? Why it is in the water?

 

[RealWing]

Indeed. It is irresponsible to send back some person into that basement just for reasons of lab protocol.

As a former NPP manager, I've been closely following this event from the beginning, but just found this forum yesterday. I've been trying to put myself in their shoes to better understand what they are dealing with.
I would fully expect that they are doing their best to limit doses to workers ALARA - even in these very stressful circumstances. They would not be irresponsibly simply ordering someone to go and get a sample. They would do their best to use long handled sampling, different routes etc and HP briefings to minimize dose.

 

[TCups]

Why are they retracting the data in such a public way.
a) Is it because of high dosage 2.9 x 10^9 , or
b) Because of the short 1/2-life of 53 minutes and thus should not be present 16 or 17 days later

if b) then how will they explain Tc-99p 6hr 1/2-life, that should also not be present

Reference: http://www.bnl.gov/bnlweb/history/tc-99m.asp

I do not know if it is pertinent to nuclear reactors, but in the hot labs of radiology departments everywhere, technetium-99m is generated from elution of molybdenum-99, which has a half-life of 66 hours, allowing it to be transported over fairly long distances. Could the presence of a longer half-lived molybdenum-99 be the source of the technetium?

 

[Astronuc]

Yes - that's plausible to a degree.

The decay of Co-56 does produce a gamma ray of 846.771 keV, with a relative intensity of 99.94. It could be mistaken for the 847 and 885 keV gammas from I-134. But those two gammas have relatively low intensity to lower energy gammas or X-rays.

Normally, one would not expect a high rate of Co-56 (from neutron activation of Fe-56) to be present in a sample. On the other hand, this is not a normal situation, and there is probably a lot of activated corrosion products in the wate with the fission products. Normally, the corrosion products would be captured on filters, while iodine would remain in the cooling water.

I have to correct myself on this. Co-56 undergoes electron capture to Fe-56. Co-56 would come from electron capture by Ni-56.

 

[83729780]

2.9 trillion becquerels I-134 per liter! => assuming the fission stopped on the 14th that would be some thing in the range of 2.9*10^9= N *(0.5)^((27-14)*24))
2,419*10^103 atom of I-134 just for this liter (some one clever should be able to calculate the weight of that)

well that's 4.14*10^81 kg

which is about 13 orders of magnitude heavier than the entire universe

perhaps you miscalculated?

 

[AntonL]

The 535 eruption of Krakatoa (Krakatau) apparently blew up many cubic miles of earth!
http://en.wikipedia.org/wiki/Krakatoa

While it was an island for many centuries ~535 to 1883, it has been posited that there was continuous land between what is now Sumatra and Java.
http://en.wikipedia.org/wiki/Krakatoa#535_AD_event

Yet - we have not experienced such events since.

Bit off topic - you forgetting http://en.wikipedia.org/wiki/Mount_St._Helens" 0.7 cubic miles got shifted in 1980.

and what about a repeat of the http://en.wikipedia.org/wiki/Tunguska_event"

 

[Gilles]

extracted from attached report for sea-water analysis - basement water would have have been done very similar. Any clues in this statement regarding their near-site measuring capabilities?

seen on this report :
Te-132 (3 days) : 2.2E-01 Bq/cm^3
I-232 (2hours) : 3.3 +00 Bq/cm^3

correct me if I'm wrong, but if I-232 is a https://www.physicsforums.com/showpost.php?p=3212764&postcount=1491", with a much shorter lifetime, shouldn't the activities be equal ? (because each Te-132 decay is quickly followed by the corresponding I-232 decay ?)

that's all so messy ...

 

[PietKuip]

http://www.tepco.co.jp/cc/press/betu11_j/images/110327o.pdf

Tepco's new numbers. On a different forum, TH1960 writes: "The first column is "ayamari" - mistaken. "

 

[KateB]

Are they solely relying on gamma spectroscopy? Co-56 decays through positron emissions, while I-134 decays through traditional beta decay... It seems it would hard to confuse the two, though I am no physicist.

 

[Astronuc]

Bit off topic - you forgetting http://en.wikipedia.org/wiki/Mount_St._Helens" 0.7 cubic miles got shifted in 1980.

and what about a repeat of the http://en.wikipedia.org/wiki/Tunguska_event"

Mt. St. Helens is inland, to there would be no consequence of tsunami. The Tunguska event was also overland, but it probably would have produced some king of tsunami if it had happened over the ocean.

The point is such catastrophic events that would cause massive destruction or mega-tsunami are exceedingly rare. However, we do need to have a realistic assessment of likely events - and the recent earthquake and tsunami - are more likely than Krakatoa and Tunguska. Most nuclear power plants are not sited where they might be affected by volcanoes or large tsunamis, or perhaps large earthquakes.

 

[Astronuc]

Are they solely relying on gamma spectroscopy? Co-56 decays through positron emissions, while I-134 decays through traditional beta decay... It seems it would hard to confuse the two, though I am no physicist.

The chart of nuclides indicates that electron capture is the predominant mode of decay for Co-56. Nevertheless, I expect they are looking for characteristic gammas (gamma spectroscopy) with which to identify the radionuclides.

Decay gammas are generally discrete, as opposed to beta decay which represents a continuum of beta (and antineutrino) energies.

 

[rmattila]

seen on this report :
Te-132 (3 days) : 2.2E-01 Bq/cm^3
I-232 (2hours) : 3.3 +00 Bq/cm^3

correct me if I'm wrong, but if I-232 is a https://www.physicsforums.com/showpost.php?p=3212764&postcount=1491", with a much shorter lifetime, shouldn't the activities be equal ? (because each Te-132 decay is quickly followed by the corresponding I-232 decay ?)

Only if you assume that both isotopes get transported from the fuel to the location of measurement in an identical manner.

 

[Astronuc]

Reference: http://www.bnl.gov/bnlweb/history/tc-99m.asp

I do not know if it is pertinent to nuclear reactors, but in the hot labs of radiology departments everywhere, technetium-99m is generated from elution of molybdenum-99, which has a half-life of 66 hours, allowing it to be transported over fairly long distances. Could the presence of a longer half-lived molybdenum-99 be the source of the technetium?

Mo-98 (n,γ) Mo-99 (ß) Tc-99

Mo-99 is a low yield fission product, and Tc-99 is even lower probability.

 

[turbo]

Most nuclear power plants are not sited where they might be affected by volcanoes or large tsunamis, or perhaps large earthquakes.

I understand that Diablo Canyon is due for a review next month re: a 20-year extension, but that's now on hold.

 

[jlduh]

Most nuclear power plants are not sited where they might be affected by volcanoes or large tsunamis, or perhaps large earthquakes.

Here is a simulation of the Sanriku tsunami earthquake of 1896 i was talking about (height 36m), the wave is simulated 10 minutes after the earthquake:

http://www.netimago.com/image_183025.html

source
http://drh.edm.bosai.go.jp/database/item/26ae4c7ae7dfe46a0527e0dfb3cc3fcd4abf7a9f

And this is a map of the same area with current nuke plants in red:
http://www.netimago.com/image_183027.html

The two bottom ones are Fukushima Daichi and Daini (6 reactor and 4 reactors)

The one at the North is Onagawa Power Plant (3 reactors)

A nice picture a Onagawa NPP, hummm looks quite close from the sea level from this point of view...

http://www.netimago.com/image_183029.html

Wonder if the highest possible tsunami here has also been designed with similar hypothesis than at Fukushima, no?

For information, it can be of interest because of the proximity of the two plants to read what is on the wiki for the Daini plant situation hit also by the 14m tsunami (the plant is 12 miles south from Daïchi):

http://en.wikipedia.org/wiki/Fukushima_II_Nuclear_Power_Plant

The Fukushima II Nuclear Power Plant (福島第二原子力発電所 Fukushima Dai-N

The March 11, 2011 Tōhoku earthquake resulted in ground accelerations of 0.19 g (1.86 m/s2) to 0.28 (2.77 m/s2) at the plant site, which is well below the design basis. [5] All four units were automatically shut down immediately after the earthquake, according to Nuclear Engineering International,[2] and the diesel engines were started to power the reactor cooling.[7] TEPCO estimated that the tsunami that followed the earthquake and inundated the plant was 14 meters high which is more than twice the designed height.[5] This flooded the pump rooms used for heat transfer to the sea, the ultimate heat sink of the reactors.[7] While the cooling system for unit 3 was undamaged, the other reactors were affected. The cooling systems remained operational, but heated up due to the lack of a heat sink. The high pressure coolant injection (HPCI) system (powered by reactor steam) was used as additional cooling.[7] On March 12, the cooling system for three reactors (numbers 1, 2 and 4) at the torus had topped 100 °C between 05:30 and 06:10 JST,[8][9][10] rendering all cooling systems (depending on temperature difference between the torus and the reactor) ineffective.[7] The coolant systems in the pump room were repaired and activated in Units 1, 2 and 4 in the days following the emergency shutdown after cooling could recommence[8] Coolant temperatures below 100 °C (cold shutdown) were reached in reactor 2 about 34 hours after the emergency shut down (SCRAM).[8] Reactors 1 and 3 followed at 1:24 and 3:52 on March 14 and Reactor 4 at 7:00 on March 15.[11] The loss of cooling water at reactors 1, 2 and 4 was classified a level 3 on the International Nuclear Event Scale (serious incident) by Japanese authorities as of March 18.[12][13][14]

Officials made preparations for release of pressure from the plant on March 12.[15][16] As of March 20, however, no pressure release had been reported.[8][17]
An evacuation order was issued to people living within 3 kilometres (1.9 mi) of the plant,[18] subsequently expanded to 10 km (6.2 mi).[19] Air traffic was restricted in a 10 km (6.2 mi) radius around the plant, according to a NOTAM.[20] These zone were superseded by the 20 km evacuation and 30 km no-fly zones around Fukushima I on March 12 and 15, respectively.

TEPCO announced that a worker who had been seriously injured by the earthquake, and trapped in the crane operating console of the exhaust stack was transported to the ground at 5:13 p.m. and confirmed dead at 5:17 p.m.[19][21][22][23][24]

I wonder also where the generators were on Daini plant, and what made the significant differences between the two plants concerning the consequences of the tsunami on the nuclear process.

 

[TCups]

Mt. St. Helens is inland, to there would be no consequence of tsunami. The Tunguska event was also overland, but it probably would have produced some king of tsunami if it had happened over the ocean.

The point is such catastrophic events that would cause massive destruction or mega-tsunami are exceedingly rare. However, we do need to have a realistic assessment of likely events - and the recent earthquake and tsunami - are more likely than Krakatoa and Tunguska. Most nuclear power plants are not sited where they might be affected by volcanoes or large tsunamis, or perhaps large earthquakes.

It seems to me significant that the diesel generators were located in the turbine buildings. In looking at a retroactive correction to a fundamental design flaw made 40 years ago, it might be understandable that the reasoning was that should a tsunami hit that is big enough to wipe out the generator, then it will be big enough to cause enough more collateral damage that the loss of the generator probably would not matter anyway -- who knows? Idle speculation.

By their nature, though natural disasters are unpredictable. Generators relocated up the back slopes of the site after a 9.0 quake might have just tumbled down the hill like Jack and Jill, for all our ability to predict the "what if's".

Anecdotally, I do have a friend who makes his living servicing large generators and bringing in temporary generators in emergency situations. I haven't spoken to him recently, but I believe I remember him complaining about all the different interface types that were involved in "hooking up" an emergency generator if power were needed quickly. Not always an easy task.

One prudent design criteria or retrofit for power plants everywhere that might be worth considering: A helicopter pad big enough to accommodate a military style, heavy lift helicopter and a standardized connection interface such that an emergency generator could be flown in, hooked up and made operational in a matter of hours, and of course, available back up generators with that interface which could be airlifted to a disaster location quickly, if needed.

Forgive my rambling, please.

 

[Astronuc]

I understand that Diablo Canyon is due for a review next month re: a 20-year extension, but that's now on hold.

It's seismic capabilities are up for review. That's one of the sites that happens to be fairly close to a potentially active fault.

http://diablocanyonpge.com/pdf/24_SFZ_Appendix_J_splay_fault_analysis.pdf

However, one would have to determine whether or not an earthquake related that that or nearby faults would result in a seismic event in excess of that for which the plant was designed.

The plant has been operated fairly conservatively. The chose not to uprate the plant, for instance.

 

[divmstr95]

I take that to mean they were reporting activity for another radionuclide, possibly one with a gamma-ray (or beta) of similar energy. The precursor to I-134 (t_1/2 = 52.5 min) is Te-134 (t_1/2 = 41.8 min).

Now I'm curious as to what they think they were measuring.

Actually their retraction never states that they did not detect I-134. It merely states the levels stated were for a different isotope, Co-56.

 

[divmstr95]

Concerning the tsunami assessment problem relative to nuclear installations AND their safety devices like EDG, I would like to know more about how was designed the tsunami model made by TEPCO "according to JSCE method published in 2002"? They are saying they are modelling the "highest possible tsunami" but this doesn't sounds easily understandable taking account some basic facts...

http://www.netimago.com/image_182963.html
http://www.jnes.go.jp/seismic-symposium10/presentationdata/3_sessionB/B-11.pdf

In particular, are they taking into account the fact that a specific type of tsunamis, called "Tsunamis earthquakes", can happen and create huge waves even if the magnitude of the source earthquake is not that big, because of some specific conditions (with slow rupture at the fault and many other complex parameters)?

http://www.scidev.net/fr/latin-america-and-caribbean/news/un-mod-le-simple-pourrait-pr-voir-les-rares-s-ismes-provoquant-des-tsunamis-.html
http://www.eri.u-tokyo.ac.jp/seno/Papers/2002GL014868.pdf

The record 36m high tsunami at Sanriku (1896) is believed to be a tsunami of that type (tsunami earthquake), and Sanriku have been a place of huge tsunamis even if earthquakes were not so big than the current one:

1896 (magnitude 7,2 / wave height 36m): http://en.wikipedia.org/wiki/1896_Meiji-Sanriku_earthquake



1933 (magnitude 8,4 / wave height 28m): http://en.wikipedia.org/wiki/1933_Sanriku_earthquake

If some people on the forum are knowledgeable on these subjects and methods please don't hesitate to bring some infos. Putting the EDG at a certain height which is safe seems good sense but what has to be this certain height? How all this stuff is really elaborated? Should be reviewed quickly as i feel many nuke plants are not so far from the ocean... and not so far from Sanriku either!

The methodology used to predict Tsunami considers the geology of segments along a specific seismic zone as well as several other factors (hydrographic profiles, etc.). It also considers paleotsunami information for a given area based upon previous earthquakes. It may be that the tsunami model for that particular area did not generate a run up and inundation model that high.

Tsunami prediction is not perfect but when you're planning for development of coastal areas it is the best you've got to go on.