Japan Earthquake: nuclear plants

Borek

That's what I was aiming at - as the pressure is unlikely to be that high, if the sensor is under water it must be wrong.

Yes, but what is the water level and is the TC under water or above? Or do we simply not know?

tsutsuji

http://www3.nhk.or.jp/news/genpatsu-...0_ondokei.html The NISA instructed Tepco to write a report by 15 February, on such things as the cause of the high thermometer values and the way to measure temperatures by other methods.

http://www.mbs.jp/news/jnn_4953273_zen.shtml Tepco tested 15 of the 41 thermometers installed inside unit 2's PCV, and found that two more thermometers were broken. Nothing abnormal was found with the two thermometers located at the same height as the one that had abnormal values.

http://www.47news.jp/CN/201202/CN2012021401002271.html 8 thermometers are broken out of a total of 41 at unit 2's RPV. Adding to the thermometer that momentarily reached 400°C, two other thermometers were found with an abnormal electric resistance. Tepco had judged that 5 other thermometers were broken. Tepco is judging the RPV's temperature trend with the remaining 33 thermometers.

elektrownik

Xenon detected in unit 2: http://enenews.com/tepco-press-hando...hour-half-life
http://www.tepco.co.jp/en/nu/fukushi...20214_07-e.pdf

tsutsuji

These results are hardly different from those of 6 February, are they?

Yamanote

Right. That's why I ignore all the stuff on enenews.

etudiant

Well, ENENews has been in the forefront of the disaster focused reporting, so they deserve to be taken with more than a pinch of salt.
That said, we've not had afaik a nuclear reactor of this size experience as catastrophic a failure, so Fukushima is writing new chapters in the book.
The layman's understanding is that there is no plausible mechanism to achieve recriticality in the damaged fuel, but that residual decay heat is a serious issue. Is this incorrect and are there plausible ways the damaged fuel can be brought back to criticality in any substantial way as a consequence of this accident?

Rive

ftp://ftp.cordis.europa.eu/pub/fp5-e...cs/09-sara.pdf

So they think it's likely happens in some circumstances.

But Tsutsuji is right about the Xenon levels, so in this particular case it's not likely happened.

Yamanote

Thanks Rive, very interesting!

Enenews compares two different measurement anyway...
So they seem to be more into headlines than into explanations and solutions. Not helpful to me.

etudiant

Thank you, Rive, for a useful link.
This is an informative study, but dealing with a much lesser accident than Fukushima.
They posit a situation after an accident where the control rods have melted but the fuel is largely intact, and resume cooling water injections with unborated water. Obviously the fuel geometries maximize the neutron capture once the water moderates the neutrons emitted.
Here we have damaged/destroyed fuel, maybe in heaps, maybe melted into corium, under a shower of borated water, so we have fast neutrons, with no moderator around and bad geometries.
How is recriticality possible in those conditions?

Jim Lagerfeld

How about this paper - it seems to me to better match the current situation, although apologies in advance if I have missed something obvious. They suggest that based on a simulation similar to three mile island, cycling criticality events are quite possible in a damaged fuel debris bed, although they conclude that they should be self-regulating and not large enough to destroy the containment (average 5-7GJ):

"Recriticality Energetic of a Hypothetical Water Reflood Accident in a Damaged Light Water Reactor"

http://www.osti.gov/bridge/purl.cove...R/webviewable/

As far as the boration is concerned, if the water is flowing through the primary containment like a sieve at many tons per hour, I can not see how sporadically injecting large amounts of boron would maintain any kind of stable boration - I think the media people have misread the boron injections.

During the last injection of boron, the TEPCO daily update hinted that their motivation was actually that by rapidly increasing the injection rate they might raise the level of the water in the reactor and reflood a damaged section of the core and inadvertently increase reactivity:

http://www.tepco.co.jp/en/press/corp...2020805-e.html

"In order to avoid the increasing
possibility of re-criticality to occur, which might be brought up by
sudden cold water injection, resulting in the rise of water density
inside the reactor, we injected boric acid into the reactor as a safety
countermeasures against the re-criticality from 0:19 am to 3:20 am on
February 7, which was before increasing the amount of injection water,
and changed the amount of the core spray system injection water from 3.7
m3/h to 6.7m3/h at 4:24 am (the amount of the continuing feed water
system injection is 6.8m3/h)."

So rather than temp going up -> better add boron, it seems like temp going up -> better increase water injection -> better borate reflood water in case more reactive fuel is flooded. Looking at it that way, the boron injections suddenly seem less knee-jerk.

etudiant

Thank you for a very relevant insight.
This paper certainly seems to fit the bill pretty well. It suggests that recovering the fuel with water has in fact quite a powerful impact on the potential for criticality, which is certainly news to me. That implies that the future performance of this facility is in fact somewhat unpredictable, rather than being bounded by the decay heat curve. It also means that getting a good handle on the distribution and location of the fuel in these reactors is of more than academic importance.

Atomfritz

Yes.
There was a large thread about the topic "what caused the explosion of reactor 3" or the like which got deleted by the moderators.

My impression is that this happened because this thread threatened to become blasphemic in the way that it could show that recriticality issues are in fact possible.

It showed the neutral and attentive reader the thinking taboos of the nuclear "professionals".
I remember the <edit> user "Morbius" multiply stating apodictically that a recriticality would be impossible, implying that the corium is present in the form of one big, contiguous blob and basing his "calculations" on this assumption.
Three Mile Island proved that this is unlikely, but this belief appears to be still firm at most nuclear "professionals".

The user "Jim Hardy" pointed several times to the fact that he considers reflooding a reactor that has run dry as a very dangerous thing.
How true, as the control/absorber rods are going to melt away long before the fuel rods, and the water moderator would fill that void and create the risk of an uncontrolled chain reaction.
But nobody seemed to understand what "Jim Hardy" shyly pointed at and there was no discussion about this problem.

Anyway, can we really exclude the possibility that the "reordering" of the corium particles due to the changes of the cooling by Tepco could create critical configurations when they randomly form "lattices" of fuel material and moderating water?
The fact Tepco felt the need to use boron again proves that they are actually concerned.

I know that what I said is pure blasphemy to some nuclear "professionals".

But can their over-simplified models like MELCOR, MAAP etc really cope with the complex reality in an actual core-melt situation and its aftermath?

These models grossly failed already at analyzing the Three Mile Island event.
So, should we sheeple still believe and trust in models the nuclear industry propagates even though they have been proven to be inadequate since decades?

I would really appreciate if there would be a discussion that goes beyond some particular dogmata like the categoric stating that (re)critical configurations would be impossible, towards a discussion that tries to find out what configurations could possibility lead to re-criticality, and then to find out how to avoid such configurations occurring.
This would be way more constructive than that flaming I am used from some residents here.

zapperzero

I was an avid participant in that discussion.

Afaik the thread got locked for cleanup, there was a rash of trolling and hyperventilating n00bs near the end.

Morbius is a bomb-maker. He doesn't really do (or indeed grok) slow-neutron, moderated criticality stuff. His area of expertise is with reflectors and neutron initiators and high keff reactions in highly enriched material. Understandable bias.

I understood. Debris beds look a lot like working reactors - a roughly 50/50 mix of solids and water, in a steel can. Worse, the metal (structural steel, control blades etc) separates from the oxide (the contents of the fuel rods, basically) during the melt formation phase.

Not in my opinion.

Yes, which is why I, just as yourself, find the insistence that it can't happen faintly ridiculous. It is not happening now, though, or there would be Xe and I and other fission products released again.

elektrownik

New temperature data for unit 2, it is interesting that CRD sensor value is increasing and that at some points it was close to second sensor value (before >200C readings):

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Shinjukusam

They announced that three thermocouples were broken. Would that be one of them?

zapperzero

It's maddening that they track up to a point. Makes it seem like something is happening and they're failing because of it. Simplest thing I can think of is corrosion due to water. But I don't know if failure due to corrosion is as fast as all that.

Another thing that's clouding the issue is that some of these are actually normalized readings - there has been some corrosion since day one and TEPCO said they are adjusting for this in their reporting - up to a point.

elektrownik

The problem is that they are close to each other so this suggests that something is going on there
here is plot:

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