Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[alpi]

... Also, it is not exactly known where the melted nuclear fuel fell. If it has fallen down from the reactor pressure vessel, as many experts point out, even if that container's temperature is less than 100 degrees, it is a far cry from saying it is safe.

If the melted fuel (corium) has fallen down from the reactor pressure vessel I think it would be easy/easier to get it covered with water since water naturally flows downwards.

What's the importance of a temperature of less than 100 degrees? Also boiling water has good cooling capacity. Or does the steam escaping from the reactors still contain considerable amounts of volatile substances being radioactive? Has anyone seen any data on the radioactivity of the steams?

 

[joewein]

What's the importance of a temperature of less than 100 degrees? Also boiling water has good cooling capacity.

Cold shutdown is a precondition for being able to open the RPV to remove fuel, which you can't do while there's steam pressure.

This aspect loses some of its significance once the fuel has melted, especially if it has left the accessible RPV, as it becomes quite difficult to remove anyway.

Or does the steam escaping from the reactors still contain considerable amounts of volatile substances being radioactive? Has anyone seen any data on the radioactivity of the steams?

Until the discovery of the 5 Sv/h and 10 Sv/h hotspots in the hardened vent path, the most radioactive spot outside the containments had been a steamy place in unit 1 discovered by a robot, with 4 Sv/h. The steam there seemed to come up from the suppression chamber.

http://search.japantimes.co.jp/cgi-bin/nn20110604x1.html

I think this is one of the reasons behind building reactor covers.

 

[tsutsuji]

http://www.47news.jp/CN/201108/CN2011082201001124.html After 7:00 AM on 22 August, a very high radiation of 3 Sv/hour was found during maintenance work at SARRY, causing to suspend that maintenance work, consisting in replacing parts. The radiation was found during flushing. Then the radiation decreased and the facility was restarted at around 8:15 PM - a 6 hour delay from schedule - without replacing the parts.

http://mainichi.jp/select/wadai/news/20110823k0000e040061000c.html Tepco announced that there is a possibility that during vessel replacement, a valve actuated by the vessel pressure called "float" was dislodged, causing contaminated water to leak into a pipe. It is believed that a cesium concretion was sticking to the pipe. Junichi Matsumoto said that they don't understand why the concretion, believed to be several grams heavy, sticked to the pipe instead of being adsorbed. The alternative injection method using the reactor spray line will be started on 26 August at unit 3 reactor.

http://www.fnn-news.com/news/headlines/articles/CONN00206003.html Tepco said that they will take measures such as closing an exhaust gas valve, in order to prevent the SARRY problem from occurring again.

http://www.bloomberg.co.jp/apps/news?pid=90920019&sid=aJHTUKQD6I50 With 3.47 mSv, no one among the 23 workers exceeded the regulatory 5 mSv limit as a result of the exposure to the 3 Sv/hour source at SARRY. SARRY was restarted at 3:00 PM on 23 August.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110823_02-e.pdf SARRY: "Outline of the location where High Radiation Dose occurred"

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110823_03-e.pdf SARRY float valve diagram explaining the 3 Sv/hour problem and countermeasure.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110823_01-e.pdf Unit 3 alternative cooling method using core spray system.

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_110823_04-j.pdf (not yet translated into English) (page 2) At around 12:30 PM 23 August, a tiny leak was found in a SFP4 cooling system primary circuit hose. The cooling system is kept running.

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110823_04-e.pdf "Unit 4 Spent Fuel Pool Circulating Cooling System Oozing Water from Flexible Hose"

 

[rowmag]

That suggests the neutron flux is not measuring the number escaping the pools, as that is a pretty meaningless quantity which has no relevance to the sulfur production recorded. So are they talking about neutron flux per square meter of core area?

It would seem so. From the text of their paper:

The attenuation length of neutrons in water at room temperature is 2.8 cm and increases at higher temperatures (21). Because of the high absorption cross-section of ³⁵Cl, seawater has more attenuation. The value of the attenuation length of the neutrons in seawater at temperatures higher than 1,000 °C is not known. For simplicity, the attenuation length was taken to be 2.8 cm. The concentration of ³⁵SO4²⁻ at the source (reactor core) was assumed to be 10 times higher than the model-calculated ³⁵SO4²⁻ concentration in the marine boundary layer. Considering all the possible reactions of neutrons with seawater (3), we estimate that a total of 4 × 10¹¹ neutrons per m² were released before March 20 in which a fraction of 2 × 10⁸ neutrons per m² reacted with ³⁵Cl to make ³⁵S.

They had to make some pretty rough assumptions and approximations throughout, so I would suspect their number could be off by orders of magnitude even if their mechanism is correct.

 

[zapperzero]

http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_110823_01-e.pdf Unit 3 alternative cooling method using core spray system.

On page 5, this document has a schematic which shows, and I quote "Cooling from bottom of fuel by FDW" which is to be supplemented by "cooling from top of fuel by CS", with a nice pool of water in the bottom head of the RPV! I thought the fuel had melted down?

 

[rmattila]

Well TEPCO's claim is true - and disingenuous, I believe. Normally, venting is done via the suppression pool. In major emergencies, iirc, venting is performed directly to the atmosphere via something called a hardened vent.

In the Swedish/Finnish system, it is possible to hard vent either from the wet or the dry side. The automatic rupture discs are on the dry side (so that they are certainly operable even if the wet well becomes flooded), but the manual ventings are preferably done from the wet side in order to take additional advantage of the scrubbing capacity of the containment (both blowdown through wetwell and containment spraying, if it happens to be available).

The design basis for the scrubbing systems in the Finnish BWR:s was that a complete core meltdown should not lead to more than 100 TBq release of Cs -137, and the danger caused by other isotopes should not be higher than that from Cs-137, and that there would be no electricity for 24 hrs to cool the containment. This translated into requirements for being able to remove 99 % of aerosols and elemental iodine, and the required iodine filtering capacity means that the water in the system can not be pure water but needs certain amounts of NaOH and Na2S2O3. NaOH is added to the containment water as well in a severe accident situation, but it can not be done timely enough to prevent too large iodine release to exceed the requlatory 100 TBq limit for severe accidents.

Regarding the FILTRA gravel filter at Barsebäck, I am not familiar with its design basis.

 

[berkeman]

(Moderator's Note -- Discussion of the Virginia US earthquake and nuclear power plant has been broken off into its own thread.)

 

[LabratSR]

The latest by Dr. Michio Ishikawa - Passage to Core Solidification

http://www.gengikyo.jp/english/shokai/Tohoku_Jishin/article_20110808.html

 

[etudiant]

The latest by Dr. Michio Ishikawa - Passage to Core Solidification

http://www.gengikyo.jp/english/shokai/Tohoku_Jishin/article_20110808.html

The only reason mentioned for a push to get the cores solidified is that it is embarrassing to have an ongoing need to cool the cores for years to come, which serves to remind everyone that the accident is not yet over.

There is plenty of cleanup to be done at that site, enough to keep people very busy even after the cores cool enough to solidify. The cores appear to be in a relatively stable state and on a path to gradually cool down. The cost benefit of any effort to speed up that process does not seem compelling.

 

[rowmag]

The only reason mentioned for a push to get the cores solidified is that it is embarrassing to have an ongoing need to cool the cores for years to come, which serves to remind everyone that the accident is not yet over.

There is plenty of cleanup to be done at that site, enough to keep people very busy even after the cores cool enough to solidify. The cores appear to be in a relatively stable state and on a path to gradually cool down. The cost benefit of any effort to speed up that process does not seem compelling.

Not sure I see this. As long as the cores are liquid, there remains the chance of something happening to cause a new release of radioactive contaminants to the environment. Once they harden, that danger drops significantly. Then you can take your time cleaning up the site.

 

[etudiant]

Not sure I see this. As long as the cores are liquid, there remains the chance of something happening to cause a new release of radioactive contaminants to the environment. Once they harden, that danger drops significantly. Then you can take your time cleaning up the site.

That is certainly true.
However, it is difficult to envisage a method that might cool the core enough to allow it to solidify which does not pose even greater risk. One way to solidify the core is to extract it and spread it, so the heat can dissipate away from the core material. That seems nightmarish difficult at best. Certainly it has never been done before.
Alternatively, it may be possible to try to dilute the core with enough inert material to get a similar effect, but as the cores are still producing some megawatts of heat, cooling will still be needed. It may simply create a bigger lump of near molten stuff. That too has not been done before.
In this case, the best is very much the enemy of the good. If the cores have not left the RPVs yet, which is afaik the implication of the continuing temperature readings from the bottom of the RPVs, the disaster is still somewhat contained. Sustained cooling seems a safer bet than trying some pioneering fix that may make things worse.

 

[alpi]

Ishikawa says here http://www.gengikyo.jp/english/shokai/Tohoku_Jishin/article_20110413.htm that the melted cores likely have crusts of "around 20-30 centimeters" which seems believable. I think there shouldn't be much difference between a completely solid core and a core with a thick crust.

 

[tsutsuji]

http://www3.nhk.or.jp/news/genpatsu-fukushima/20110823/index.html The investigation panel made a press conference, saying they have auditioned 126 people for 300 hours, including plant manager Yoshida who was auditioned 4 times for a total of 19 hours and that they all answered with accuracy. The panel has its third meeting on 27 September and plans to issue an interim report within this year.

 

[zapperzero]

Ishikawa says here http://www.gengikyo.jp/english/shokai/Tohoku_Jishin/article_20110413.htm that the melted cores likely have crusts of "around 20-30 centimeters" which seems believable. I think there shouldn't be much difference between a completely solid core and a core with a thick crust.

The cited article is dated April 11. In view of later announcements of possible melt-throughs, it is of limited relevance.

EDIT: Upon further reading, I found this gem:

It may be time to consider allowing evacuees, who are living a life of inconvenience and hardship, to go home.

That's about it for this person's credibility, as far as I am concerned.

 

[marsupio]

@etudiant

hi and agree with your philosophy of troubleshoot. But first point, I (we ?) thunk that tragedy as Cernobyl or Fukushima will never come again.
Second point, Cernobyl learned us to have the better as possible information to act in the best way.

All this to exprim a point of view : test pionneer issue on a very little part of meltdown. For the future.

My advice. I let the professinnal of the topic discuss about boxite properties as unmagnetic.

My best regards

 

[zapperzero]

The latest by Dr. Michio Ishikawa - Passage to Core Solidification

http://www.gengikyo.jp/english/shokai/Tohoku_Jishin/article_20110808.html

I hadn't read this. I have now, due to alpi's post. Here's an excerpt:

The other option is air-cooling.This involves suspending the current cooling operation to let the core melt further and drop on the bottom of the containment vessel so as to increase the surface area before resuming cooling. Implementing this option requires the examination of several technical challenges, including the prevention of steam explosion.It is possible to safely implement this approach, but the implementation requires courage and meticulous care.

Umm... wait, what? Is this guy for real?

 

[LabratSR]

The cited article is dated April 11. In view of later announcements of possible melt-throughs, it is of limited relevance.

EDIT: Upon further reading, I found this gem:

That's about it for this person's credibility, as far as I am concerned.

http://www.gengikyo.jp/english/shokai/Tohoku_Jishin/article_20110620.html

 

[rmattila]

A report from 1988 concerning filtered venting capabilities of US plants:

http://www.osti.gov/energycitations/servlets/purl/6945722-maXGrD/6945722.pdf

Within the United States, the only commercial reactors approved to vent during severe accidents are boiling water reactors having water suppression pools. The pool serves to scrub and retain radionuclides. The degree of effectiveness has generated some debate within the technical comnunity. The decontaminatlon factor (DF) associated with suppression pool scrubbing can range anywhere from one (no scrubbing) to well over 1000 (99.9 % effective). This wide band is a function of the acciaent scenario and composition of the fission products, the pathway to the pool (through spargers, downcomers, etc.), and the conditions in the pool itself. Conservative DF values of five for scrubbing in MARK I suppression pools, and 10 for MARK II and MARK III suppression pools have recently been proposed for licensing review purposes. These factors, of course, exclude considerations of noble gases, which would not be retained in the pool.

Venting procedures as used within the EPGs are intended as a "last resort" operator action. Uncontrolled increases in the containment temperature or pressure will result in containment failure with unknown results. Therefore, it is felt that a controlled action with defined consequences is preferable to no operator action. The methodology to establish the venting pressure is an equally important consideration. Ideally, the venting pressure could be established solely on the actual pressure capability of the
containment. That would delay venting until the last possible time and minimize unnecessary releases. However, considerations associated with actual operating plant constraints tend to reduce the venting pressure (based on the PCPL). As a result, there are plant-to-plant differences in EOPs. This is best demonstrated by looking at the selection of the valves that are in the flow paths to be used for venting. Plants have provided a table of penetrations that will be used in the event of a serious accident. The accepted philosophy is to begin opening valves in the smallest flow path, starting with wetwell penetrations. Failing successful control of the transient, the operator is to increase the diameter of valves that are opened sequentially until even drywell valves (resulting in an unscrubbed release) would become candidates. One licensee has also proposed venting the wetwell through the spent fuel pool to enhance fission product scrubbing after core damage.

The first vent system was assumed to be similar to the 18" wetwell hard pipe to ductwork system at Peach Bottom [6].
Similar to the Peach Bottom plant, the pressure is relieved through the nitrogen purge system ductwork, which is expected to fail. For the purpose of a general evaluation of venting strategies, it was assumed that the vent system can be safely opened both before and after vessel failure and without existing on-site AC or DC power. Basically, this was a vent system which discharges into the reactor building (RB) upon actuation. Expert evaluation in NUREG/CR-4551 [7] indicated that the reactor building decontamination factors (DFs) probably range from 1.5 to 2.5 in the absence of a hydrogen burn. Although these DFs are small, source term studies indicated that they do play a role in the offsite consequences. Conversely, hydrogen burns in the RB were hypothesized to sweep out the fission products rapidly with little or no DF. Expert opinion solicited in NUREG/CR-4551 estimated a 20% probability of complete bypass of the RB (DF=1.0) during hydrogen burns. As shown in Table 1, another disadvantage of discharging directly into the RB is the potentially adverse effect on recovery equipment.

This report was published one year prior to the http://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1989/gl89016.html requiring the installation of hardened vents in BWRs with MARK I containment.

 

[zapperzero]

http://www.gengikyo.jp/english/shokai/Tohoku_Jishin/article_20110620.html

Is there a point you're trying to make?

 

[etudiant]

Is there a point you're trying to make?

Not to speak for labratSR, but I was struck by Dr Ishikawa's willingness to to accept much higher exposure levels for the general public. It is true that there are areas with 30 or more mSv/yr of natural radioactivity in India where people have lived for centuries with no apparent damage. That does not make it wise to just up the permissible exposure levels for a new population. Add to this his idea that the government should buy their output and you can see the nuclear ghetto being created.
Imho, Dr Ishikawa is not providing the level of actionable guidance that one might have expected from a man of his expertise and background. Perhaps because he is so aware of the horrendous risks that may befall a country based on his own wartime experience, he appears to be more comfortable with risks than the Japanese public. That is causing a widening disconnect between his comments and actuality, leaving him increasingly isolated.

 

[hellbet]

I hadn't read this. I have now, due to alpi's post. Here's an excerpt:

Umm... wait, what? Is this guy for real?

M. Ishikawa is really a very smart guy, now what if the "melted core" has already dropped to the confinement vessel and one stop the water? Now the corium will really be out of control... Maybe Tepco/Nisa know that evidence and consequently do "nothing" because there is nothing to do?

Whatever M. Ishikawa has a long and strong background in industry i invite anyone to ponder about... He might be just missing some crucial information about the corium but whoever knows about?

 

[LabratSR]

Is there a point you're trying to make?

Just trying to show the whole story.

 

[LabratSR]

If anyone is interested, I stumbled across an ORNL paper that I have not seen posted here before (sorry if it has been) and I find to be very informative(at least to me). In particular, Appendix D.

Human Factors Review for Severe Accident Sequence Analysis

http://pbadupws.nrc.gov/docs/ML1031/ML103140169.pdf

 

[MiceAndMen]

EDIT: And thank you, tsutsuji. You are really doing a great job to keep many people up-to-date of the data that sometimes is quite difficult to obtain from other sources here at the other side of the world.

My thanks to tsutuji as well. Without his valuable contributions this thread would be dead and buried by now. Instead, it continues to be a source of good up-to-date information regarding the Fukushima NPPs.

 

[MiceAndMen]

EDIT: To add: if there was ongoing criticality, it should lead to a wide number of short-lived isotopes being generated, such as I-135 and Xe-135, and their presence would be a certain indication of a recent criticality. On the other hand, I am not sure how TEPCO:s analysis and reporting routines would contribute to presence of such isotopes becoming reported. It appears that only I-131, Cs-134 and Cs-137 are routinely reported, but I don't know it a more complete gamma analysis is made on routine samples.

I think one of the "lessons learned" that everyone should take away from this mess going forward, is that regular "more complete gamma analyses" need to be done and the results publicized in any future incidents. TEPCO has routinely provided monitoring reports ONLY on I-131, Cs-134 and Cs-137 for months now. That is not enough, and in the future competent authorities and responsible agencies and business interests must not be allowed to point to TEPCO's example as justification for limited reporting. I would hope scientists and engineers (and lay people as well) both in and outside the nuclear power industry begin to champion this view. TEPCO is setting a bad precedent here and should not be allowed to get away with it, so to speak.

 

[gmax137]

... It is true that there are areas with 30 or more mSv/yr of natural radioactivity in India where people have lived for centuries with no apparent damage. That does not make it wise to just up the permissible exposure levels for a new population...

Are you suggesting that the Indians in that area are somehow adapted to the higher dose rate? Is there any evidence of that?

 

[etudiant]

Are you suggesting that the Indians in that area are somehow adapted to the higher dose rate? Is there any evidence of that?

Absolutely no idea on adaptation and evidence would not be easy to find..
Afaik, there have been no studies on the topic and even the basic public health data is pretty spotty, as this is a pretty poor area. There are similar high natural radioactivity sites also in parts of southern China and Brazil, likewise in fairly impoverished areas.
That said, it just seems imprudent to say that because we cannot see impact at 30mSv/yr, it is reasonable to use that as an incremental threshold.

 

[NUCENG]

Absolutely no idea on adaptation and evidence would not be easy to find..
Afaik, there have been no studies on the topic and even the basic public health data is pretty spotty, as this is a pretty poor area. There are similar high natural radioactivity sites also in parts of southern China and Brazil, likewise in fairly impoverished areas.
That said, it just seems imprudent to say that because we cannot see impact at 30mSv/yr, it is reasonable to use that as an incremental threshold.

See:
http://www.angelfire.com/mo/radioadaptive/ramsar.html

 

[htf]

Let's see if this posting is also deleted ...

TEPCO management knew about the tsunami threat and did nothing:

http://mdn.mainichi.jp/mdnnews/news/20110825p2g00m0dm050000c.html

I said this in a posting two weeks ago and received a warning from Astronuc to "refrain from such noise" and my posting was deleted. Now it is official "noise".

 

[etudiant]

See:
http://www.angelfire.com/mo/radioadaptive/ramsar.html

Thank you for the link.
A very interesting document. It seems the topic is getting some study after all.
That said, I doubt most Western research protocols would allow a 1 Gray dose to be administered to unrelated subjects for baseline comparison purposes. That is probably why similar work has not been done by Western institutions, at least afaik. We are probably mostly using the database created from prompt exposure measurements during the nuclear testing era, rather than from chronic exposure.