Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[Joe Neubarth]

My understanding is based on an analogy with optics. A white material such as milk, chalk, white paint, or clouds looks white because all the light that falls on it is scattered back to the open space on the illuminated side, in a random direction --- instead of being absorbed or transmitted to the other side. However, at a microscopic scale, most white materials consist of transparent particles in a transparent medium with a different index of refraction. So, a large part of the incident light is not scattered off the material 's surface; instead it enters the material, and is then refracted or reflected by its particles many times, random-walk style. The internal scattering has no preferred direction; but if there is no absorption and the material is thick enough, most of the photons will eventually come out of it *towards the same side they entered*. In other words, their U-turn is not deterministic but only a statistical event --- due to the fact that there is no scattering on one side of the surface.

By the same token, if one were to suspend a light bulb by a thin wire in the middle a thick fog bank, much of the light emitted by it will eventually come back to it --- simply because the wandering photons cannot end their trip in any other way. This is more obvious if the lamp is suspended in milk; but it should seem reasonable also for fog, when one considers that a thick enough cloud bank at noon will look black from below.

The same should happen to neutrons that escape a fuel mass completely surrounded by a large body of water, or any other moderating material: as long as we can ignore absorption and spontaneous decay in the water, most of them will eventually come back to the fuel, with thermal energies.

However, if there is some absorption in the surrounding material, not all neutrons will come back. This is analog to what happens to photons in materials like smoke or cappuccino, that contain light-absorbing substances together with light-scattering grains: the material will look gray or colored, instead of white. Also, if there are other absorbing objects (such as a concrete floor) immersed in the same medium and not far from the fuel, part of the neutrons will end up there before coming back to the source.

In the case of Fukushima, I believe that they are using borated water to cool the corium, instead of plain water; which in the optical analogy should be equivalent of suspending a lamp in china ink instead of milk, or in tire smoke instead of fog. However the uranium in the molten mass may be covered by some other material (iron and zirconium oxides?) whose "neutron color" I cannot guess.

As I said, my understanding is all based on this optical analogy. It may not apply at all to the situation.

I note your tremendous artistic talent in replying. Thank you.

I have always thought of is as ping pong balls and bowling balls. Ping pong balls (neutrons) will bounce off of heavier atoms (bowling balls) without losing energy, but will bounce off of Hydrogen atoms and impart some of their energy to the Hydrogen. I guess eventually they can be turned around.

 

[MadderDoc]

Yesterday, March 12th, past noon JST, there appears to have been further clearing of trees to the southwest of the plant, see attachment of photos of the plant looking in from the south (from 12:00, 13:00 and 14:00)

 

[Jorge Stolfi]

For whatever it is worth, I have updated my plots of reactor variables for Fukushima Daiichi #1--#3:
http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/Main.html

I added markers to the plots showing the core meltdown times, as guessed in
the June 2011 report by the Japanese Government to IAEA
http://fukushima.grs.de/sites/default/files/NISA-IAEA-Fukushima_2011-06-08.pdf
They seem to coincide with sudden and definitive depressurizations of the RPV, as expected.

Reading the report, I was disappointed at the lack of any mention to the hiccup in #3's RPV pressure, on march 21 02:00, which is conspicuous in TEPCO's spreadsheets but apparently was not reported to NISA.

For over a month, the pressure readings for #1's RPV had been rising, and on june 3 they were over 600 kPa ("A") and 1600 kPa ("B"). On june 4 the "A" reading dropped suddenly to 128 kPa (27 kPa more than atmospheric) and the "B" sensor went dead dead. Presumably the sensors were recalibrated between june 3 and june 4.

The CAMS readings for the drywells are about 250 Sv/h (#1), 15 SV/h (#2), and 4 Sv/h (#3). That is sieverts, not microsieverts or millisieverts. Presumably all three reactors are leaking steam from the containment to the atmosphere. Are the "steam" plumes above the reactors made of this stuff?

 

[tsutsuji]

The test run of the water purifying facility starts tomorrow (June 14th) (instead of today as I previously read in the news), the launching of the facility is expected on June 17th or later according to nuclear department acting general director Jun'ichi Matsumoto : http://www.bloomberg.co.jp/apps/news?pid=90920019&sid=alOpyQW842bg ; Around June 18th according to http://www.chunichi.co.jp/s/article/2011061390141100.html

Low concentrations of curium were found in soil samplings in Okuma town 2 or 3 km away from the plant. This is the first time curium is found outside of the plant. It is a by-product of plutonium. The Education and Science ministry says it is a concern for internal exposure. Americium was also found, but it is a low and old deposit from nuclear weapon tests : http://www.tokyo-np.co.jp/s/article/2011061301000491.html

Strontium was found in ground water near unit 1 and unit 2, and in sea water at the intake of unit 3 where it amounted to 240 times the allowed concentration : http://www.fnn-news.com/news/headlines/articles/CONN00201327.html . 7,300 Bq/l (240 times the 30 Bq/l limit) for Sr 90 according to http://www.meti.go.jp/press/2011/06/20110612001/20110612001-7.pdf (Japanese)

The Japanese food and drug administration approved on June 6th two drugs used to cure ingestion of transuranic elements such as plutonium, but insists that it is not connected with the present accident as the approval had been requested before the earthquake. These medicines, which have already been approved in the US, France or Germany use a chemical property called chelation : http://www.yakuji.co.jp/entry23266.html

 

[htf]

As I said, my understanding is all based on this optical analogy. It may not apply at all to the situation.

I don't think we get far with qualitative pictures. The whole thing is a race between different processes. To make the high energy neutrons thermal collisions are required. Neutrons will on the average travel their mean free path between these collisions and they will be scattered in all directions and some of them will get absorbed, some of them will never come back to the corium. Surely, some of them come back to the corium as thermal neutrons and have a high probability of causing a fission reaction. Is the fraction of these lucky neutrons high enough for a chain reaction? I am not an expert but I doubt you can say anything without a quantitative calculation.

 

[clancy688]

I somewhere read that the tsunami at Onagawa NPP reached the footing of the plant, but not more.

I don't think that there would've been troubles as big as in Fukushima if the plant would indeed have been inundated.
First it's newer - from the early nineties. It's highly probable that it has more fancy security systems than Fukushima AND better tsunami protections.
But second you need more than a tsunami to kill a NPP. It's often overseen, but Fukushima Daiichi didn't lose cooling capability because of the tsunami. The station blackout happened because offsite power was lost due to collapsing electricity lines which were damaged by the earthquake.
If those towers would've withstood the earthquake, perhaps emergency cooling could've been sustained.
Did something similar happen in Onagawa? I'm not sure. But they provided shelter for tsunami refuges, so I don't think that they had no electricity.

 

[MiceAndMen]

Yesterday, March 12th, past noon JST, there appears to have been further clearing of trees to the southwest of the plant, see attachment of photos of the plant looking in from the south (from 12:00, 13:00 and 14:00)

Fascinating. If we saw what happened at 11:00 and later on at 15:00 it might be even more insightful. How many trees have been removed? 10? 100? What does it matter?

 

[MiceAndMen]

This discussion reflects on the poor credibility record laid out by TEPCO, any apparent inconsistency needs to be explored to the limit to be sure we understand what is happening and the thermal images of Unit 4 have shown the inconsistency of higher temperatures at the site of the RPV/PCV for a long time.

TEPCO's inconsistency needs to be explored to the limit. What is the limit?

I could point to lots of conspiracy theories created around that inconsistency.

Please don't.

 

[clancy688]

How many trees have been removed? 10? 100? What does it matter?

Exactly. Or rather - why did TEPCO cut them down? There must be a reason connected to the status of the plant, to contamination or something similar.
You just don't cut off trees to beautify the environment when you're threatened by four crashed nuclear reactors. Recent sat images of the perimeter would be helpful.

 

[Borek]

Please follow tsunami walls discussion here: https://www.physicsforums.com/showthread.php?t=506517

 

[joewein]

Exactly. Or rather - why did TEPCO cut them down? There must be a reason connected to the status of the plant, to contamination or something similar.
You just don't cut off trees to beautify the environment when you're threatened by four crashed nuclear reactors. Recent sat images of the perimeter would be helpful.

If they need more space for water tanks, the wooded area uphill from units 1-4 would be a prime candidate.

 

[tsutsuji]

Exactly. Or rather - why did TEPCO cut them down? There must be a reason connected to the status of the plant, to contamination or something similar.
You just don't cut off trees to beautify the environment when you're threatened by four crashed nuclear reactors. Recent sat images of the perimeter would be helpful.

The newly created waste storing areas are shown on the map at the bottom of http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110609e8.pdf

 

[clancy688]

The newly created waste storing areas are shown on the map at the bottom of http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110609e8.pdf

Thanks. So they are cutting wood down to get space for their planned water processing plants and storage tanks.

 

[roliqq]

Excessive levels of strontium detected in seawater, NHK, June 13, 2011:

[...] TEPCO announced that strontium-90 was also detected for the first time in ground water near the reactors’ buildings.

A ground water sample taken on May 18, around reactor number 2, measured 6,300 becquerels per liter. And for reactor number one, the sample showed 22 becquerels.

TEPCO explained it usually takes about 3 weeks to analyze the samples.

Why exactly does it take so long to analyse water samples for strontium? Can't really think of an analyzing process that lasts three weeks.

 

[Astronuc]

Excessive levels of strontium detected in seawater, NHK, June 13, 2011:

[...] TEPCO announced that strontium-90 was also detected for the first time in ground water near the reactors’ buildings.

A ground water sample taken on May 18, around reactor number 2, measured 6,300 becquerels per liter. And for reactor number one, the sample showed 22 becquerels.

TEPCO explained it usually takes about 3 weeks to analyze the samples.

Why exactly does it take so long to analyse water samples for strontium? Can't really think of an analyzing process that lasts three weeks.

If the activity is very low, the counting must be increased to obtain reliable statistics, and usually one allows the shorter-lived radionuclides to decay away.

 

[causeceleb]

If the activity is very low, the counting must be increased to obtain reliable statistics, and usually one allows the shorter-lived radionuclides to decay away.

unless, of course, there is a reasonable way of chemically isolating a
particular isotope from the rest of the sample.

 

[MadderDoc]

Let's not jump too fast here.
Forget the steam explosion for a moment, let's try and figure what vectored energy upwards.
I see only two possibilities:
1. a shaped charge sort of effect which I see as very unlikely because it would have had to have formed such a perfect 360 degree containment (in 3 dimensions)
or
2. the blast was contained by the walls of the fuel pond.
I can think of nothing except the two I mentioned above.

As regards the vectoring I think we could make a distinction between the vectoring of the ejected large objects, and the vectoring of the rising column and mushroom cloud.

Some of the ejected large objects do appear to have been vectored close to vertically upwards, however most of the ejected objects seem to have been vectored in a more southerly direction, some landing as far away as the 4-piped exhaust stack south of unit 4. Few if any objects were vectored significantly to the north of unit 3. So if there was a shaped charge effect or anything of that kind, the vectoring was not vertically up, but rather tilted to the south.

Otoh, it is clear from the video that the cloud _itself_ was vectored strongly vertically upwards. However I think this is just what one would expect from an expulsion of a mixture of hot water vapor and dust. For a start water vapor has -- other things being equal -- a much lower density than air, and this produces buoyancy, direction up. With hot water vapor this buoyancy is only further enhanced. Adding to this, in a rising and cooling column of water vapor filled with dust, water vapor will have easy access to the condensation nuclei, which are necessary for its condensation. Condensation of the water vapor will lead to intermixing of more air into the rising column, and the intermixed air will be heated by the release of the latent heat of condensation, again this produces more buoyancy for the rising cloud.

 

[robinson]

I find it difficult to even believe that anyone with knowledge of physics looks at the blast and believes it was simply hydrogen burning. Or that it didn't damage the reactor, the fuel ponds, and almost everything else.

I mean, most of the building is simply gone. Not damaged, it's gone. The blast was so loud it was heard 40km away.

It should be possible to calculate exactly how much TNT it would take to create a blast like that. If Hydrogen alone can do that kind of damage, it might be time to change the physics books.

 

[Jorge Stolfi]

It should be possible to calculate exactly how much TNT it would take to create a blast like [#3's]. If Hydrogen alone can do that kind of damage, it might be time to change the physics books.

This has been discussed before in this thread, and the amount of H2 needed was estimated. There were also photos of other industrial insallations wrecked by gas leaks.

Gas explosions are powerful indeed. In São Paulo (> 10 million hab) practically all kitchen stoves run on propane from portable tanks. There used to be ~1 explosion per day due to leaks from cracks in old hoses, worn-out valves, or improperly fitted connections. Typically the explosion would wreck the house or apartment. Now scale up from the volume of an average kitchen to that of the reactor's service space...

 

[zapperzero]

I note your tremendous artistic talent in replying. Thank you.

I have always thought of is as ping pong balls and bowling balls. Ping pong balls (neutrons) will bounce off of heavier atoms (bowling balls) without losing energy, but will bounce off of Hydrogen atoms and impart some of their energy to the Hydrogen. I guess eventually they can be turned around.

I'm afraid it could be even simpler. The geometry of the blob may be such that it provides its own neutron reflection. In enriched Uranium, you can do fission with fast neutrons if you have enough of them.

 

[SteveElbows]

I find it difficult to even believe that anyone with knowledge of physics looks at the blast and believes it was simply hydrogen burning. Or that it didn't damage the reactor, the fuel ponds, and almost everything else.

I mean, most of the building is simply gone. Not damaged, it's gone. The blast was so loud it was heard 40km away.

Large amounts of the building remain intact.

I am no expert in explosions, but I see plenty of impressive hydrogen blasts on youtube, and I know that one of the issues with using hydrogen as a fuel is safety.

People can talk up the explosion at 3 all they like, and attempt to discredit thy physics knowledge of others, it does not prove anything or help their case.

Science is being done a disservice via a disproportionate amount of focus on visual evidence. Estimated total radiation release into the environment, as per the calculations used to come up with an INES level, suggests that the vast majority of contamination came from reactor 2. Yet this is hardly discussed, perhaps because there is too much judging of reactors by their cover.

 

[clancy688]

You guys do realize that there's a thread which deals specifically with the Unit 3 explosion? How about you talk about that topic over there?

 

[tsutsuji]

Thanks. So they are cutting wood down to get space for their planned water processing plants and storage tanks.

Yes. It had been announced in late April in the Japanese media that the "Forest of Wild Birds" (1) was considered for cutting down to allow tank storage, calculating that a 30,000 ton facility requires a 15,000 m² area of land : https://www.physicsforums.com/showthread.php?p=3262303&highlight=forest#post3262303

(1) as translated into English on the soil sampling map: http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/110422e16.pdf

 

[Pu239]

Large amounts of the building remain intact.

I am no expert in explosions, but I see plenty of impressive hydrogen blasts on youtube, and I know that one of the issues with using hydrogen as a fuel is safety.

People can talk up the explosion at 3 all they like, and attempt to discredit thy physics knowledge of others, it does not prove anything or help their case.

Science is being done a disservice via a disproportionate amount of focus on visual evidence. Estimated total radiation release into the environment, as per the calculations used to come up with an INES level, suggests that the vast majority of contamination came from reactor 2. Yet this is hardly discussed, perhaps because there is too much judging of reactors by their cover.

I'm afraid to ask this question, having not assiduously followed previous threads, but will try to phrase the question in the most learned and scientific way I can think of:

Why is Reactor #4 smoking?

http://www.tepco.co.jp/en/nu/f1-np/camera/index-e.html

(LARGE amounts of steam, possible smoke, with possible tongues of flame (unverified)).

 

[SteveElbows]

Would anybody care to join me in thinking about reactor 2 more this week?

Looking at most of the documents & attachments of the report to IAEA, there are a few things which suggest that reactor 2 was responsible for a very large percentage of the radioactive release into the environment.

OK we already knew the water was very bad at reactor 2, and that it leaked into the sea, and that we could estimate that something in the approximate region of 40,000-50,000TBq INES-converted release got out that way.

But on April 12th when the INES 7 rating was announced, it was also suggested that reactor 2 was responsible for most of this, and for this rating we are talking about air rather than sea. See for example the following news story:

http://english.kyodonews.jp/news/2011/04/84888.html

A considerable amount of the radioactive materials emitted is believed to originate from the plant's No. 2 reactor, whose containment vessel's pressure suppression chamber was damaged by an explosion on March 15, said Kenkichi Hirose, a Cabinet Office adviser serving for the safety commission, at a news conference.

''Our estimates suggest the amount of radioactive materials released into the air sharply rose on March 15 and 16 after abnormalities were detected at the No. 2 reactor,'' Hirose said. ''The cumulative amount of leaked radiation has been gradually on the rise, but we believe the current emission level is significantly low.''

Having checked back to this time period on this thread, and looked at various other discussions and news articles on the net, I do not feel that this detail received sufficient attention. For sure the sombre way in which reactor 2 suspected suppression chamber damage was announced, and the highly contaminated water found there, already indicated to people that reactor 2 was causing great harm, but it still feels to me like focus on this is frequently lost, often in favour of reactors where the visual is more alarming.

Some of the issues surrounding reactor 2 and its environmental consequences probably belongs in a different thread, but as INES ratings stuff has often been discussed in this thread I would like to at least draw attention to one aspect here.

Report to IAEA, attachment IV-2 http://www.kantei.go.jp/foreign/kan/topics/201106/pdf/attach_04_2.pdf

This is where the NISA 370000 TBq got upgraded to 840000 TBq.

On page 7 we see Table 5, which shows possible release to environment of a variety of substances, with values for each reactor. Substances such as Cs-137 and I-131 are orders of magnitude higher for reactor 2.

Now these numbers are based on analysis, and there are multiple analysis cases for reactor 2 which could have been chosen to use in this table, many of which would have given much lower numbers (see a little earlier in that doc for some detail). But I guess they picked this case for a reason, I imagine they swayed towards it because scenario used for this case seems likely, as well as actual radiation data from around the time period that reactor 2 was failing badly.

So, worthy of more attention?

 

[Borek]

I'm afraid to ask this question, having not assiduously followed previous threads, but will try to phrase the question in the most learned and scientific way I can think of:

Why is Reactor #4 smoking?

http://www.tepco.co.jp/en/nu/f1-np/camera/index-e.html

(LARGE amounts of steam, possible smoke, with possible tongues of flame (unverified)).

Conditions in the night are such that it looks steaming every day. My bet is that is mostly a matter of local weather - humidity and temperature changes. Fog clouds are moving in and out of the light, which makes the picture dramatic.

I have looked at the live feed at several occasions, never longer than just a few minutes, usually after someone raises alarm - and each time it looks the same to me.

Edit: it occurred to me that compression artifacts look like a dynamic smoke/steam cloud as well. Thats typical in low light conditions - dark, large spots with slight color gradient look like moving even if the image is still. It can look as a dynamic steam motion, even if the real situation is quite stable.

 

[daveb]

http://www.nrc.gov/public-involve/public-meetings/webcast-live.html"

The above link is a public webcast about the status of the joint NRC/Japanese task force investigating the evens at the plant.

 

[jim hardy]

before becoming alarmed by clouds coming out of the plant, one should google terms
'dewpoint fukushima'.

every time temperature and dewpoint get real close, 'net boards light up with reports of fires.

other night it happened even though temp and dewpoint were both 19C, humidity 100%, and local weather channel reported dense ocean fog.

old jim

 

[etudiant]

As the SPF for reactor 5 is above 80*C, it will steam copiously under most conditions.
Until the structural reinforcements under the pool are completed, that temperature will get higher still, as it is unreasonable to expect structural work to get done while radioactive water overflows from above.

 

[MadderDoc]

Would anybody care to join me in thinking about reactor 2 more this week?

Certainly I am game, just wondering which aspects you want there to be thought more about :-). It appears to me that you are particularly interested in the air release from unit 2, which you think might account for the majority of the total air release, so perhaps that could be the first aspect to think about.

Looking at most of the documents & attachments of the report to IAEA, there are a few things which suggest that reactor 2 was responsible for a very large percentage of the radioactive release into the environment.

<..> on April 12th when the INES 7 rating was announced, it was also suggested that reactor 2 was responsible for most of this, and for this rating we are talking about air rather than sea. See for example the following news story:
http://english.kyodonews.jp/news/2011/04/84888.html

The argument made there by the cabinet secretary seems pitifully inadequate, and indeed has a form that makes it look like a 'post hoc ergo propter hoc' fallacy. Very _much_ had been going on before those high levels were measured on the 15th and the 16th of March, and during this period smoke and steam were reported billowing out from several sources within the plant, not only, and not even mainly, from unit 2.

Report to IAEA <..>
On page 7 we see Table 5, which shows possible release to environment of a variety of substances, with values for each reactor. Substances such as Cs-137 and I-131 are orders of magnitude higher for reactor 2.

Now these numbers are based on analysis, and there are multiple analysis cases for reactor 2 which could have been chosen to use in this table, many of which would have given much lower numbers (see a little earlier in that doc for some detail). But I guess they picked this case for a reason, I imagine they swayed towards it because scenario used for this case seems likely, as well as actual radiation data from around the time period that reactor 2 was failing badly.

There must be more than guessing and imagining that they picked this scenario for sound reasons.Taking things from this particular report on trust alone is for me quite out of bounds.

So, worthy of more attention?

Perhaps. By which route/medium/mechanism should the majority of total air releases from the plant have escaped from unit2?

 

[NUCENG]

My nuclear power theory was all taught to me in naval nuclear power school half a century ago so I am asking a question that we were not taught an answer to. We were taught to operate plants, not deal with theory of glowing radioactive blobs.

At Fukushima we have heard talk of recriticality.

How is it possible for those melted cores to go critical again when the normal operating process was for neutrons from fission to slow down in water and thus interact with Uranium 235. If the water ain't there, are the neutrons becoming thermal (lower energy state) by bouncing off of the entrained impurities in the melted blob? Does the size of the blob provide for reflection of energy (Neutrons) back towards the center?

We know that Reactor One is melted down, yet it sure looks like it is "breathing" (as some call it) with increases in temperature coming in long waves that would seem to indicate increases in criticality and then decreases.

NucEng, can you or somebody else with experience answer this?

Has any institution done any research in Blob criticality theory?

I am looking at everything I can find on this event. I just checke my download directory related to Fukushima and I am approaching 7 GB. I've at least skimmed most of it, and I just don't know whether the remaining core, debris, corium, blobs are or have been critical since rods were inserted on 3/11.

My position is that Recriticality is possible. I have seen anomalies in sub-drain I-Cs ratios that are exactly that - anomalies. The data limitations to just a few isotopes make it impossible to disprove recriticality, but the same limitations prevent proving recriticality has occurred. As a Navy nuc, you probably remember the term subcritical neutron amplification as the reactor approaches criticality. Oscillating generation of neutrons could explain detection of increased neutron "beams" and blue flashes of light. But criticality is a self-sustaining condition and I haven't seen proof of that.

A hot glowing blob is an interesting image as in imagination. If a hot blob exists it is deficient in moderator so would be difficult to support criticality. At the edges of the blob the presence of water moderator would likely be oscillating between a steam layer and rewetting, that again could produce an oscillating neutron amplification effect, but again, that is not my understanding of criticality.

Destruction of boral or boraflex inserts in a dense packed fuel pool without soluble boron could cause criticality, but at least pictures from unit 4 and pool radioactivity levels are inconclusive.

Part of wisdom is alleged to be knowing what you don't know and I am much wiser now than on March 11. It amazes me how many people can be so certain of what is going on inside all that concrete and steel. We are like six blind men "seeing" the elephant. It is only when we share information and listen to each other that a clear picture will emerge.

We will all now sit in a circle, hold hands and sing "Kumbaya!" ;-}

 

[NUCENG]

But if they are going out of the core to hit the water, who sends the neutrons back into the core?

I remember in Nuc Pwr Sch they did tell us that the reactor vessel does act to redirect the neutrons back in the direction of the core if they try to escape. Gotta herd them little doggies in the right direction or they might get lost.

But if the Blob is out of the reactor, the neutrons would be free to wander in the cement and mortar around them until the blob hits bedrock.

Hummmmm? Blob Criticality Theory. Somebody has to be writing a thesis on this for a doctorate. Right?

"It's Alive! It's Alive!"

If the neutron comes out of the blob its direction is "outward", if there is water around the blob the neutron begins to collide or bounce off the hydrogen nuclei losing a lot of energy in each collision or bounce. Hitting something heavier like an Oxygen nucleus also deflects the neutron, but does not absorb as much of the energy as would a target about the same size as the neutron. In any case the neutron gradualy slows down and due to allthose deflections is now going in a completely random direction (even inwards back into the blob).

Thus water can be both a moderator and a reflector. At some point the neutron will hit a target at the right energy and be absorbed. If that happens to be a fissile atom, the story starts all over again. And they all lived happily ever after.

 

[Quim]

.....criticality is a self-sustaining condition

Is a criticality which causes its own end by rearranging the physical proximity of the supporting material still a criticality? even if it only lasts a couple of milliseconds?

 

[Borek]

Thus water can be both a moderator and a reflector. At some point the neutron will hit a target at the right energy and be absorbed. If that happens to be a fissile atom, the story starts all over again. And they all lived happily ever after.

I don't doubt it is possible that a single neutron gets back, but the main question is whether it can be a self sustaining process? k close to 1?

 

[GJBRKS]

I just saw a white and black spotted DOG walk past the TEPCO webcam , it stopped and even looked into the cam for a sec !

( unfortunately no screenshot)