Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[timeasterday]

Some video taken Sunday: http://peevee.tv/v/84w992

A few screen grabs from the video here: http://mainichi.jp/select/jiken/graph/20110327_2/

 

[RealWing]

It is maybe useful to go back to the TMI event, which is the only large scale partial core melt due to lack of cooling. It does give some insight into how rapidly the core can start slumping/melting and the reactivity effects. (TMI was a PWR, but core response would be generally similar to a BWR)
For example, some quotes from the Vessel Investigation Report http://library.iit.edu/govdocs/resources/NUREGCR6197part02.pdf

"PHASE 2 (100-174 minutes)
During this period, interactions between melted cladding and structural materials resulted in eutectic material flowing down through the central region of the core until it reached the liquid/steam interface where it solidified, forming the lower supporting crust. Analysis revealed that the lower crust was a Zr-Ag-In-Fe-Ni metallic mixture.5 Zircaloy cladding and Inconel grid spacers most likely formed the first eutectics. Stainless steel control rod cladding would melt when temperatures exceeded 1,427°C. The cladding may also have been breached due to interactions between the stainless steel cladding and molten zircaloy, since the materials form relatively low temperature eutectics. After cladding failure, molten Ag-In-Cd control material with a melting temperature of 827°C would flow down into the crust. Increased fuel rod temperatures would eventually lead to fuel melting.

Phase 4 (224 to 226 minutes)Some control material and quenched, fuel-bearing ceramic material apparently relocated to the lower head prior to 224 minutes. However, the majority of the ceramic material that was deposited on the lower head relocated from the core to the lower head beginning 224 minutes after reactor scram. Debris relocation was completed in approximately 120 seconds. The relocation was substantiated by a sharp increase in pressure and by the increase in SRM count rate"

Note the 120 seconds time frame and the increase in the "SRM count rate" The SRM is the Source Rate Monitor which is a low range fission counting device. This indicates that there was an increase in the fissioning process, (but it was nowhere near going critical.)

There has been several posts about the possibility that the reactor could achieve criticality again and my assessment is that it is possible although unlikely. If the fuel geometry is in the right configuration with some water as moderator and with insufficient poison in the water (eg boron) or the absence of control rod blades, then it could go prompt critical. That is why they are doing whatever they can to ensure boron is being added to any coolant going into the core.

The worst case event is for one (or more) reactors going critical without any control on the power increase. Having said that, BWR's -as designed- are inherently stable (ie as power increases, reactivity decreases), however I have no idea how they behave if the core geometry changes due to melting of the fuel pencils.

 

[83729780]

Some video taken Sunday: http://peevee.tv/v/84w992

I hope 3:45 to 3:47 settles the corium vs. insulation debate (even though the footage is from #3).

 

[havemercy]

I hope 3:45 to 3:47 settles the corium vs. insulation debate (even though the footage is from #3).

Could you explain that please ? (ok, thanks for the response below ;)

 

[Astronuc]

I apologize for taking up a topic already discussed thoroughly on this thread, but there's one thing I've been trying to find an answer to, thus far with no success:

As far as I understand, the US plants with Mark I containments were at some point of time refitted with what is called a "hardened venting", mentioned e.g. http://www.scribd.com/doc/51291366/Report-BWR-Mark-I-Containment-03192011-2 , page 10). Would anyone have any idea, whether the plants in Fukushima Dai-ichi had anything equivalent, or was the venting of containment steam attempted e.g. through pipe lines designed just for changing the atmosphere from air to nitrogen under normal pressure/temperature conditions?

My understanding is that Mk I do not have hydrogen recombiners - and I don't know if any have been retrofitted. I believe Mk II and Mk III do.

 

[warren_c]

I just want to thank every one who has contributed to this thread especially the PF mentors and those who have worked in the industry. I have learned a lot from all of your knowledge and I appreciate that most of the discussion has been fact based.

Thank you all. I wish the best for Japan and hope they get the situation under control soon.

 

[RealWing]

My understanding is that Mk I do not have hydrogen recombiners - and I don't know if any have been retrofitted. I believe Mk II and Mk III do.

I believe you are correct- The realtively small MK-I containment systems rely on nitrogen inerting to ensure there is a low risk of H2 explosions and the NRC accepted this in the 1980's. Powered H2 recombiners may still have been retrofitted, but withiout any power - they did nothing.

 

[Astronuc]

I think there is nitrogen in the drywell, but not on the reactor floor.

At Fukushima, it was not by design that the H₂ went into the volume above the reactor building floor. Apparently the venting system duct work (or somewhere else) ruptured/leaked. The H₂ and steam should have gone up the stack.

The point to be emphsized here is that the event is well BEYOND design basis - the combined effects of earthquake and tsunami (greater than design), the station blackout, and then the injection of seawater and boric acid.

You bet other plants are now looking at DBE and combined effects of natural phenomena, and their EOPs.

TVA is already looking at their units from a worst scenario.
http://www.bloomberg.com/news/2011-03-25/tva-plans-more-catastrophic-scenarios-after-japan-official-says.html

 

[Ivan Seeking]

They were just hit by another quake, about a 6.5, with a [small] Tsunami warning that affects the plant. Hopefully this won't make things worse.

 

[RealWing]

I think there is nitrogen in the drywell, but not on the reactor floor.

At Fukushima, it was not by design that the H₂ went into the volume above the reactor building floor. Apparently the venting system duct work (or somewhere else) ruptured/leaked. The H₂ and steam should have gone up the stack.

The point to be emphsized here is that the event is well BEYOND design basis - the combined effects of earthquake and tsunami (greater than design), the station blackout, and then the injection of seawater and boric acid.

You bet other plants are now looking at DBE and combined effects of natural phenomena, and their EOPs.

TVA is already looking at their units from a worst scenario.
http://www.bloomberg.com/news/2011-03-25/tva-plans-more-catastrophic-scenarios-after-japan-official-says.html

Both the dry well and wet well are interted with N2 - but not the reactor floor.

Agree about DBE events and the already analyzed beyond DBE events- we need to rethink this whole issue - particularly for older reactors. The new "advanced" reactor designs have what appears to be several passive cooling systems that do not rely on any power.

Just like TMI and Chernobyl - unfortunately it takes an tragic event such as this one to make all us engineers and plant operators look hard at the lessons-learned and act on them.

 

[TCups]

Could you explain that please ?

You almost had to be there. It was the beginning of the analysis of the photos coming out of Fukushima, starting with this still photo of the north wall of Bldg 4 after a fire and explosion, and the interpretation that there was a tongue of "something" hanging out of the side of Bldg 4 somewhere around post #500-600. There was debate regarding whether this was insulation or corium. The annotations in black letters were my own observations, added to the photo, and at that point in time there was speculation on my part that the SFP was on the north side of the Bldg. It wasn't

see:

http://i306.photobucket.com/albums/nn270/tcups/r735227_5964756.jpg

The new video is interesting. Notice the drywell cap in the northeast corner of Bldg 4 in the new video?

 

[Texan99]

This http://bravenewclimate.com/2011/03/13/fukushima-simple-explanation/" claims that the replacement generators couldn't be made to connect to the existing system in time:

So mobile diesel generators were trucked in. . . . This is where things started to go seriously wrong. The external power generators could not be connected to the power plant (the plugs did not fit). So after the batteries ran out, the residual heat could not be carried away any more.

 

[Astronuc]

You almost had to be there. It was the beginning of the analysis of the photos coming out of Fukushima, starting with this still photo of the north wall of Bldg 4 after a fire and explosion, and the interpretation that there was a tongue of "something" hanging out of the side of Bldg 4 somewhere around post #500-600. There was debate regarding whether this was insulation or corium. The annotations in black letters were my own observations, added to the photo, and at that point in time there was speculation on my part that the SFP was on the north side of the Bldg. It wasn't

see:

http://i306.photobucket.com/albums/nn270/tcups/r735227_5964756.jpg

The new video is interesting. Notice the drywell cap in the northeast corner of Bldg 4 in the new video?

I the drywell cap is on the NE side, I would expect that the SFP is on the south side of the building.

 

[AtomicWombat]

Actually they garnered a heck of a lot of information from Chernobyl and Three Mile Island. AT Chernobyl, the Lava that was generated from the melt down actually flowed through pipes under the reactor and out onto a basement corridor floor. Photos are available all over the Internet, but the easiest one to locate is a Wiki. Three Mile Island had accumulation at the base of the reactor vessel. That mass was analyzed for content and it consisted primarily of Zirconium and Uranium with far smaller amounts of Steel, Nickle (Inconel), and Chromium. When I read that report I was wondering what happened to the control rods. It turns out the Boron (If the control rods have melted) eventually becomes Boric Acid in solution. As the Japanese were pumping sea water into the reactor, they were diluting the Boric Acid and it was flowing out into the building. Do that long enough, and you can make your corium glow with numerous fissions, especialy if it started out as an old core operating at a high rate of power when the troubles began.

Thanks Joe,

I've posted photos of "corium" before:
https://www.physicsforums.com/showpost.php?p=3198702&postcount=580"

Five different types of corium were discovered at Chernobyl, perhaps emphasising its tendency to separate into phases:
http://en.wikipedia.org/wiki/Corium_(nuclear_reactor)#Chernobyl_accident"

My impression was that at Chernobyl, with the exception of the initial firefighting, water was not heavily used to contain the situation, mainly due to fears of a steam explosion when the corium melted through to the basement.
http://en.wikipedia.org/wiki/Chernobyl_disaster#Steam_explosion_risk"

 

[Astronuc]

This http://bravenewclimate.com/2011/03/13/fukushima-simple-explanation/" claims that the replacement generators couldn't be made to connect to the existing system in time:

That's one problem I heard about with someone with connections to Japan.

I wonder it they couldn't splice cables - or does that imply the wrong voltage?

Next time - have compatible backup generators, and don't put EDGs and fuel supply oceanside - especially not when the coast is near a major subduction zone.

And apparently, since 1990, there has been one mag 7+ earthquake between Iwaki and Tokyo, near the coast.

 

[TCups]

This http://bravenewclimate.com/2011/03/13/fukushima-simple-explanation/" claims that the replacement generators couldn't be made to connect to the existing system in time:

In medicine, after a major traumatic injury, the first hour is called "the golden hour". It is the critical time when heroic efforts to stabilize the patient, maintain an airway, replace lost blood, etc. are most effective -- before body functions start to shut down. After the disasters, the quake and the flood, it sounds like Fukushima may have had a "golden hour" -- a very short interval where the right intervention might have dramatically changed the course of events. How sad it would be if it was technology as basic as "a different type of plug" that was the "nail", as the child's story goes -- "for want of a nail the horse was lost", etc.

It would be hard for me to imagine those first minutes and hours. Power failing. No phone service. The tsunami hit. The generators go and the operators look on in horror as things go horribly wrong.

As I posted earlier -- a helicopter pad and a universal emergency power connection interface might have made for a very different outcome.

I also remember the story of TMI having only one phone line in the control room and the operators desperately trying to call for assistance, but the line was busy much of the time by people calling into see what was going on.

Hindsight is usually 20/20, and often, it is the simple stuff that leads to failures.

 

[Rational Deb8]

Hi folks,

I'm new here, first post. I'm hoping that RealWing or other experts here might be able to help with a few questions.

First, it seems that they've recovered CAMS readings on the drywell and suppression chamber. I'm interested in the significance, if any, but have no idea what normal operating CAM rad levels would be on BWR drywell, or what levels would be expected 15 days post scram (or even 15 days post normal coast down). I feel like an idiot, because I've lost the link (still have the pdf page up) to provide to you all - would have been probably either NISA, TEPCO, METI, or JAIF pdf status report... They're showing:

CAMS

Unit 1 D/W: 3.46 ×10e1Sv/h
S/C: 2.22×10e1Sv/h
(As of 9:00, March 27th)

Unit 2
D/W: 4.16×10e1Sv/h
S/C: 1.41×10e0Sv/h
( As of 9:00, March 27th )

Unit 3
D/W: 3.37×10e1Sv/h
S/C: 1.31×10e0Sv/h
(As of 10:10, March 27th)

I don't suppose anyone here knows of a good source for BWR typical instrumentation normal operating ranges, and charts or spot tables of what would be expected for the for a few days/weeks after scram (ideally), or even after normal shut down?

Thanks in advance!

 

[83729780]

Could you explain that please ?

days earlier (page 26 or so) there was discussion as to what the molten material here was (insulation? corium?):

in the new video you can see a similar formation oozing over a wall (3:45 to 3:47)

looks rather innocuous IMHO

 

[Joe Neubarth]

Thanks Joe,

I've posted photos of "corium" before:
https://www.physicsforums.com/showpost.php?p=3198702&postcount=580"

Five different types of corium were discovered at Chernobyl, perhaps emphasising its tendency to separate into phases:
http://en.wikipedia.org/wiki/Corium_(nuclear_reactor)#Chernobyl_accident"

My impression was that at Chernobyl, with the exception of the initial firefighting, water was not heavily used to contain the situation, mainly due to fears of a steam explosion when the corium melted through to the basement.
http://en.wikipedia.org/wiki/Chernobyl_disaster#Steam_explosion_risk"

At Chernobyl they found (or so I read years ago) that the water only seemed to accelerate the burning process in the pile. Could be the water was dissociating and providing oxygen to the burning Uranium and Graphite.

 

[RealWing]

Hi folks,

I'm new here, first post. I'm hoping that RealWing or other experts here might be able to help with a few questions.

First, it seems that they've recovered CAMS readings on the drywell and suppression chamber. I'm interested in the significance, if any, but have no idea what normal operating CAM rad levels would be on BWR drywell, or what levels would be expected 15 days post scram (or even 15 days post normal coast down). I feel like an idiot, because I've lost the link (still have the pdf page up) to provide to you all - would have been probably either NISA, TEPCO, METI, or JAIF pdf status report... They're showing:

CAMS

Unit 1 D/W: 3.46 ×10e1Sv/h
S/C: 2.22×10e1Sv/h
(As of 9:00, March 27th)

Unit 2
D/W: 4.16×10e1Sv/h
S/C: 1.41×10e0Sv/h
( As of 9:00, March 27th )

Unit 3
D/W: 3.37×10e1Sv/h
S/C: 1.31×10e0Sv/h
(As of 10:10, March 27th)

I don't suppose anyone here knows of a good source for BWR typical instrumentation normal operating ranges, and charts or spot tables of what would be expected for the for a few days/weeks after scram (ideally), or even after normal shut down?

Thanks in advance!

Good question!

I've been watching the Suppression Chamber or S/C (Torus area) for some time since it gives you some indication of fuel damage. When the reactor vessel was vented to relieve pressure (and allow water injection), it vents into the S/C under water. If it was just steam, the radiation levels would be quite low. - but the levels are quite high instead.

eg Unit 1 is now 22.2 Sv/hr or 2220 rem/hr or ~36rem/min. In other words , in less than 1 minute, a worker would receive their emergency dose for a year (assuming my calcs are correct)

 

[AtomicWombat]

At Chernobyl they found (or so I read years ago) that the water only seemed to accelerate the burning process in the pile. Could be the water was dissociating and providing oxygen to the burning Uranium and Graphite.

The are at least a couple of reasons I can think of. If the pile is superheated there will always be a steam interface between it and any water layer - so heat transfer will be poor. At the interface the steam temperature will approach the pile temperature.

This steam will react with
1) Zircon - producing hydrogen gas in an exothermic reaction:
Zr + 2H2O -> ZrO2 + 2H2.
2) Graphite:
C + H2O -> CO + H2
See:
http://pubs.acs.org/doi/pdf/10.1021/ja01324a015"
The 2nd reaction is likely to be restricted to Chernobyl.

 

[AtomicWombat]

days earlier (page 26 or so) there was discussion as to what the molten material here was (insulation? corium?):

in the new video you can see a similar formation oozing over a wall (3:45 to 3:47)

looks rather innocuous IMHO

And doesn't look to be the same material in my opinion. The 2nd one appears to be a muddy water stain with no real bulk. The first one is yet to be determined.

 

[Reno Deano]

That's one problem I heard about with someone with connections to Japan.

I wonder it they couldn't splice cables - or does that imply the wrong voltage?

Next time - have compatible backup generators, and don't put EDGs and fuel supply oceanside - especially not when the coast is near a major subduction zone.

And apparently, since 1990, there has been one mag 7+ earthquake between Iwaki and Tokyo, near the coast.

More than likely it was a combination of electrical: splicing/connector, voltage and phasing in of the supplied power.

 

[tortamuss]

The are at least a couple of reasons I can think of. If the pile is superheated there will always be a steam interface between it and any water layer - so heat transfer will be poor. At the interface the steam temperature will approach the pile temperature.

This steam will react with
1) Zircon - producing hydrogen gas in an exothermic reaction:
Zr + 2H2O -> ZrO2 + 2H2.
2) Graphite:
C + H2O -> CO + H2
See:
http://pubs.acs.org/doi/pdf/10.1021/ja01324a015"
The 2nd reaction is likely to be restricted to Chernobyl.

The Leidenfrost effect keeps popping into my mind. Any thoughts?

http://en.wikipedia.org/wiki/File:Heat_transfer_leading_to_Leidenfrost_effect_for_water_at_1_atm.png

 

[downwinder]

Just a quick recap on the issue of the Cl-38, I-134 and the neutron reading which could indicate a critical mass had formed somewhere in the fuel pool or reactor vessels. It appears the I-134 issue has been resolved as confusion with interpreting Gamma spectrum but the Cl-38 and periodic neutron readings still remain a puzzling issue. Could we have witnessed a reconfiguration of the fuel geometry when these results were observed? It appears from the radiation readings in the Torus and the high level of fission products that some level of fuel melt down has taken place.

 

[AtomicWombat]

http://en.wikipedia.org/wiki/File:Heat_transfer_leading_to_Leidenfrost_effect_for_water_at_1_atm.png

Great link tortamuss!

 

[Rational Deb8]

I've been trying to understand a bit better what might have happened in the two or three hours post quake - and along those lines, to also understand better just how some of the emergency and residual heat removal systems would normally be expected to function (e.g., not specific to Fukushima's post EQ situation, but how they're supposed to function or can be operated, as necessary). I'm sorry if some of these are ignorance questions - I've dug around a little on the web, and even pulled out an old texbook - and while it is easy to find basic descriptions of these systems, it seems awfully difficult to find any that answer the more specific questions below.

So I hope you'll bear with me as I try to get across what I'm interested in and/or having problems understanding... I know there may not be sufficient info available on some of what actually happened at Fukushima, but hoping that folks here might be able to fill me in if I've missed something. Please correct me if I'm wrong on any of the details below. Just to make referencing answers easier, I'll number the main questions...

So, first, for Fukushima - all three plants SCRAM'd on EQ, which would have had also immediately shut the Main Steam Isolation Valves. At all three plants, diesels started up just fine.

1) in this scenario & point in time, what should have been active - RCIC or HPI (ECCS), or both? If both, would operators have left both on, or switched to one exclusively (if so, which?).

2) before the tsunami hit, would protocols have them switch all three plants to RHR? If yes, would 45 min or so have been enough time to complete the transition?

3) theoretically - if a plant scram's, and has RCIC active already, is ANY power required to for function - e.g., is decay heat turbine power sufficient to keep that system active, or does the process require battery powered valve activation to cycle or something like that? Or is battery power only required initially, to switch into RCIC?

4) IS battery power necessary to initiate RCIC, or can the plant be gotten into RCIC manually, without any power?

5) theoretically, if a plant scram's, and RCIC is active - temperature in the RPV, torus, condenser, and suppression pool water is the limiting factor, correct? Meaning once the temp in the loop hits a certain temp, it can't any longer cool sufficiently to be of use... is that the limiting factor? Or is it pressure in either torus or suppression pool? If it's not temp, but pressure that limits, can't that be controlled by venting? Or does water level in the core also come into play before the temp is high enough for the temp to be the limiting factor in RCIC function?

6) theoretically, how long post scram would RCIC be expected to be functional before the water temp got so hot that it could no longer cool sufficiently? Hours? Days?

7) If Dai-ichi units were in RHR when the tsunami took out diesels, could they have allowed pressure to build back up, and switched into RCIC? Or do plant systems somehow preclude that?

8) Does anyone know just what core cooling systems were actually operating when the tsunami hit, then which during the time between tsunami and venting (if any)?

9) I've heard/read a couple of different stories on the replacement diesels - one I've seen mentioned here, the 'plug didn't fit' scenario. I have troubles with that one, because it seems that either an electrician would re-wire/splice, or proper ones would be brought in - which apparently they still haven't been (maybe units 5 & 6? Were those replacements, or did they just get their own working?). Then I've seen 'they weren't powerful enough' which I also have problems with, because again, it seems they'd just get the right sized units brought in - and in any case, would have gotten the 'right' ones from other Japan power stations. That would have occurred by now even if it wasn't in time to avoid the explosions. So, that implies to me that the problem wasn't 'the right' portable/replacement diesels, but almost had to be electrical/connection damage from the tsunami, right?

I'm sorry this is so long, and thanks so much in advance for any enlightenment on these issues!

 

[Rational Deb8]

And here is probably the ignorance question of the decade...

I had thought that BWR's ran with significant coolant cover over the assembly tops - but recently read that no harm would be expected in BWRs during normal operating conditions even, ran with as much as 1/3rd of the top of the rods uncovered, because of steam cooling. Is this correct? Or would it even be correct during cool down or something?

The plant status reports showing coolant levels approx 7 feet below the top of the fuel rods had really bothered me until I ran across that, but I have no idea if that is accurate or not - and don't understand why they haven't managed to cover the rods considering the amount of water they've been injecting and that the RPV's or CPV's seem to be holding pressure - unless perhaps it is correct that having the steam/water boundary below the rod tops isn't necessarily damaging and somehow running with the lower water level is actually preferable in this situation?

 

[Astronuc]

Just a quick recap on the issue of the Cl-38, I-134 and the neutron reading which could indicate a critical mass had formed somewhere in the fuel pool or reactor vessels. It appears the I-134 issue has been resolved as confusion with interpreting Gamma spectrum but the Cl-38 and periodic neutron readings still remain a puzzling issue. Could we have witnessed a reconfiguration of the fuel geometry when these results were observed? It appears from the radiation readings in the Torus and the high level of fission products that some level of fuel melt down has taken place.

The fuel doesn't necessarily have to melt. The fuel pellets only need exposure with water and the fuel will start to oxidize - usually along the grain boundaries. The more the fuel (UO₂) is exposed, the more fuel that has the potential to oxidize. Grains simply drop out - no melting. The cladding can develop guillotine breaks or axial splits, either of which result from localized hydriding of the cladding.

Small cracks allow the gases, Xe and Kr, and volatiles like I and Cs to leak out. If one is finding solid fission products, like metals and metal oxides, then that simply indicates that the breach in the fuel is significant - the larger the breach, the more solid fission products can come out. Melting is not necessary.

Melting implies a solid becomes a liquid - as in the stainless steel or Zircaloy-2 which form the bulk of the structural elements in the core. The fuel pellets are encased in the Zircaloy-2 cladding, which is sealed by end plugs at top and bottom. The fuel rods sit between upper and lower tie plates made of cast SS304 (CF3). The assembly is then jacket in a square channel (with round corners) of Zircaloy-2. The SS304 melts about 1400°C, and Zircaloy-2 melts at about 1800-1850°C. The control rods sitting between the assemblies/channels are made of SS304. So melting is a significant issue, BUT it's not clear that such temperatures have been achieved in the FK U1,2 or 3, and they are not necessary to achieve for fuel particles or solid fission products to be released.

If the core did achieve criticality, I would expect pressure spikes as the water boiled to steam. I don't think we've seen any evidence of that.

The Cl-38 is puzzling, and there is no report of Na-24 or Cl-36, which one would expect if there was Cl-38.

There maybe some spontaneous fissons from the high burnup and MOX fuel, but those should be at relatively low levels compared to fissions at power.

 

[Rational Deb8]

Good question!

I've been watching the Suppression Chamber or S/C (Torus area) for some time since it gives you some indication of fuel damage. When the reactor vessel was vented to relieve pressure (and allow water injection), it vents into the S/C under water. If it was just steam, the radiation levels would be quite low. - but the levels are quite high instead.

eg Unit 1 is now 22.2 Sv/hr or 2220 rem/hr or ~36rem/min. In other words , in less than 1 minute, a worker would receive their emergency dose for a year (assuming my calcs are correct)

Exactly - you and I are thinking on the exact same lines. But here's the thing, considering the location, and the fact that they are using CAMS, I would expect that area to always be quite toasty. You'd be getting shine from core itself, from 40 years of activation in the RV & D/W metals themselves, from the coolant water, etc. Right? Typically CAMS are used in very high dose rate areas. That's why I was wondering what the normal operating dose rate on those CAMS are, and in the days post scram or even normal shut down... because heck if I know if these dose rates, screamin' as they are, are anything out of the ordinary -- or if they're effectively 'telling' us that we've got slag on the RPV bottom.

Then I get even more confused because I have no idea how not having the normal filtering systems operating to remove activation products from the water for 15 days no less,(wouldn't affect shine from the core itself or the activated RPV & D/W), plus we've been venting off water/steam, which obviously is taking some of the radioactivity with it, but I'd still think that we've got to be getting a lot of concentration within the system too...add some sea water impurities/salts activation on top of it all... and I've no idea if these factors are even a significant factor in terms of the dose rate those CAMS would be expected to be seeing at this point...

Or am I misunderstanding here RealWing, and you're saying that you're sure that these dose rates are far higher than we ought to be seeing from those CAMs?

 

[AtomicWombat]

The questions raised on this board about why in the heck we are finding fission products that have a short term half life in the adjoining buildings. As I have posted, my conjecture is based upon the possibility of a Reactor melt down. Conjecture only as I understand this forum does not want to say that that is the only possible explanation to all of the recent findings. I am convinced that we had a full reactor melt down two weeks ago, but as I have stated to be politically correct, that is just my opinion and is not stated as known fact.

Um - we have to wait for the evidence that the CRBs and fuel melted. SS304 has a melting point of 1400 - 1455 °C, and Zircaloy-2 has a slightly higher melting point of about 1800°C. It's not clear yet that those temperatures were realized. The control rods are not strongly heated (there is some gamma heating related to decay products in the core), and they sit between fuel assemblies and their Zircaloy channels. The steam between the channels might have been somewhat superheated, but it's not clear that the steam would superheat to > 1000°C.

Joe, I largely agree. In my view the balance of evidence would indicate at least one full meltdown - producing the explosion in reactor 3. There may be others in other reactors and SFPs.

In any case there is a credible scientific case for the possibility that a meltdown has occurred. I don't think it is something you "wait for the evidence" for. Strong evidence for the TMI partial meltdown was only found years after the event when the core was sampled in 1986:
http://en.wikipedia.org/wiki/Three_Mile_Island_accident#Investigations"

In particular the emergency planners should be working on the basis that a full meltdown may have occurred in all three reactors and at least one SFP. A high degree of effort should be put into establishing the exact state of the reactor cores and SFPs and the integrity of containment vessels and piping. They should not be assuming that the battle is just to keep the fuel rods cool, as planning on that basis alone may be counterproductive if the fuel elements have largely melted.

I hope and expect they are taking this into account.

 

[Rational Deb8]

More than likely it was a combination of electrical: splicing/connector, voltage and phasing in of the supplied power.

If that were the case, however, I would have thought they would have gotten the right ones brought in long before now, wouldn't they? I mean, even if it wouldn't have been in time to prevent the hydrogen explosions in the first 2 reactors, they would desperately have liked to have had power of any sort back to these reactors - and diesel power might have avoided the last two hydrogen explosions. So, why haven't they managed to get diesel's working anywhere other than unit's 5 & 6? To me that implies damage to the electrical systems at or between the connection point and the plant systems themselves - severe enough that they can't reconnect diesels...but this is NOT my area and I could certainly be wrong!

 

[Rational Deb8]

...It appears the I-134 issue has been resolved as confusion with interpreting Gamma spectrum... It appears from the radiation readings in the Torus and the high level of fission products that some level of fuel melt down has taken place.

Hi Downwinder,

Where did you find these facts please?

 

[Astronuc]

And here is probably the ignorance question of the decade...

I had thought that BWR's ran with significant coolant cover over the assembly tops - but recently read that no harm would be expected in BWRs during normal operating conditions even, ran with as much as 1/3rd of the top of the rods uncovered, because of steam cooling. Is this correct? Or would it even be correct during cool down or something?

The plant status reports showing coolant levels approx 7 feet below the top of the fuel rods had really bothered me until I ran across that, but I have no idea if that is accurate or not - and don't understand why they haven't managed to cover the rods considering the amount of water they've been injecting and that the RPV's or CPV's seem to be holding pressure - unless perhaps it is correct that having the steam/water boundary below the rod tops isn't necessarily damaging and somehow running with the lower water level is actually preferable in this situation?

BWRs start boiling water as low as 3 or 4 meters in the core. But this is done at ~72 atm at saturated conditions. There is some water, but the exit void fraction can be as high as 75-80%. However, there is usually some moisture in the steam flow, and the coolant velocity is quite high. In the liquid phased, the coolant velocity is about 5-7 m/s, and the steam has a high velocity because it has a much lower specific volume. Under normal conditions, the flow is forced, as in forced convection.

Normally when the reactor is shutdown, there is about 30 feet (~10 m) of water above the top of the core. There is a heat removal system that allows the hot water to be cooled, so that boiling does not occur. Typically the water above the core should be no hotter than one's bathtub or shower. Afterall, people work over the core in order to service the reactor, including removing and inserting fuel.

The problem with the current situation is low flow or lack of flow, and low flow means very low heat transfer coefficients, which means the heat is not getting out of the fuel fast enough to maintain low temperatures. In a stagnant steam environment, without sufficient heat removal, the fuel heats up until radiative heat transfer becomes substatial. If that is not enough to prevent the cladding from reaching melting temperatures, then obviously the cladding melts in which case is would flow downward with gravity until it enounters something like water or other solid which would cool it, i.e., solidify it. Steel melts at temperatures below that of the melting point of Zircaloy-2. Before melting, metals creep or flow under pressure or gravity.

It's not clear that the fuel melted however.

 

[PietKuip]

Just a quick recap on the issue of the Cl-38, I-134 and the neutron reading which could indicate a critical mass had formed somewhere in the fuel pool or reactor vessels. It appears the I-134 issue has been resolved as confusion with interpreting Gamma spectrum but the Cl-38 and periodic neutron readings still remain a puzzling issue.

I am quite convinced that the chlorine-38 number was due to the same error as the spurious I-134 activity.

It takes time for them to get the samples to the gamma spectrometer. That gamma spectrometer uses smart software to identify peaks in the spectrum, using a database with the kind of stuff that may be present in a nuclear power plant.

But there are lots of peaks. Because of the high count rates, there will also be accidental summing peaks of two strong gamma energies. The program then identifies some small spurious peak as Cl-38.

What is more, the program will calculate numbers for the time the sample was taken. It corrects for decay. In the case of these rather short-lived isotopes, those corrections may be as large as a factor 1000.

Such extrapolations backward are nonsense of course, but the lab guy just takes the computer output for granted.

I do not think there ever was any Cl-38 in the spectrum.

 

[downwinder]

The fuel doesn't necessarily have to melt. The fuel pellets only need exposure with water and the fuel will start to oxidize - usually along the grain boundaries. The more the fuel (UO2) is exposed, the more fuel that has the potential to oxidize. Grains simply drop out - no melting. The cladding can develop guillotine breaks or axial splits, either of which result from localized hydriding of the cladding.

Thanks for that explanation it helps with the understanding of what is happening. In the case of Three Mile Island as I recall there was actual melting of the fuel? With that in mind it may be possible to recover from this accident if the situation can be stabilized where the fuel assemblies can eventually be removed from the reactor. I am sure there will still be issues of assembly “swelling” but at least not a Chernobyl.

 

[Rational Deb8]

Another question...

Where is hydrazine stored (e.g., any stored in the reactor building where it might have been involved in the explosions)? How is it inserted into the coolant? Would it have been inserted after the scram in each unit (before the explosions)?

 

[downwinder]

Rational Deb8 see post #1588

 

[Astronuc]

Thanks for that explanation it helps with the understanding of what is happening. In the case of Three Mile Island as I recall there was actual melting of the fuel? With that in mind it may be possible to recover from this accident if the situation can be stabilized where the fuel assemblies can eventually be removed from the reactor. I am sure there will still be issues of assembly “swelling” but at least not a Chernobyl.

If the fuel melted, then removing fuel will be very difficult due to dimesional distortion. Also, BWR fuel assemblies are normally lifted by the handle on the upper tie plate, and 8 of the fuel rods form the mechanical connection between the upper and lower tie plates. If those tie rods all failed, then one cannot raise the entire fuel assembly. Special tools would be need to extract fuel rods, or peices of fuel rods and fuel assembly. The same goes for the control rods, which are lifted with the handle at the top.

 

[Astronuc]

Another question...

Where is hydrazine stored (e.g., any stored in the reactor building where it might have been involved in the explosions)? How is it inserted into the coolant? Would it have been inserted after the scram in each unit (before the explosions)?

I'm not aware that hydrazine is injected into the coolant.

If the FK units were using hydrogen water chemistry (HWC) then I would expect it to be in the reactor water clean up system outside of containment.
See -Hydrogen Water Chemistry During Start-Up (HDS)
http://www.jsm.or.jp/ejam/Vol.1.No.3/NT/10/article.html

[strike]At the time of the explosions, I believe they were pumping borated seawater into the reactors.[/strike]

Explosions, Seawater injection

March 12
http://www.world-nuclear-news.org/rs_Battle_to_stabilise_earthquake_reactors_1203111.html

Television cameras trained on the plant captured a dramatic explosion surrounding Fukushima Daiichi 1 (Unit 1) at around 6pm.
. . . .
The injection of seawater into parts of the building near the reactor started at 8.20pm and this is planned to be followed by addition of boric acid, which is used to inhibit nuclear reactions. Tepco had to put the operation on hold for a time when another tsunami was predicted, but work recommenced after the all-clear.

Unit 1 explosion occurred before seawater and boric acid injection.


http://www.world-nuclear-news.org/RS_Explosion_rocks_third_Fukushima_reactor_1402111.html
Unit 3, March 14, Explostion 1101 am.

Not clear from the article if seawater and boric acid were injected before or after the explosion.

 

[Rational Deb8]

...Normally when the reactor is shutdown, there is about 30 feet (~10 m) of water above the top of the core. There is a heat removal system that allows the hot water to be cooled, so that boiling does not occur. Typically the water above the core should be no hotter than one's bathtub or shower. Afterall, people work over the core in order to service the reactor, including removing and inserting fuel.

Astronuc, thank you for the reply. I confess I wasn't thinking about the velocities involved during normal ops (duh! facepalm). But on shut down, there must be a transition period between boiling with rods partially/mostly uncovered to +10m bathtub water. I'm sure that would be far far more rapid with normal system available than the Fukushima situation... but under normal circumstance even, how long does a BWR have to coast down before they can open up for fuel ops or something of that nature? I know Daiini has achieved cold shutdown as of a few days ago - at that point can a core be opened up & recirc or RHR still runs below the top of the water level to keep temps down? Or is it awhile yet?

The problem with the current situation is low flow or lack of flow...

Ok, that all makes sense to me - but what still doesn't is why are they running with water below the top of the fuel in all three reactors then?

 

[AntonL]

Some video taken Sunday: http://peevee.tv/v/84w992

A few screen grabs from the video here: http://mainichi.jp/select/jiken/graph/20110327_2/

Any idea where the steam at video 3:25 to 3:29 originates from
later again at 3:49 to 4:04
seems to come from a crack in the concrete and quiet forcefully
any ideas?

 

[Pheesh]

you tube version of video dated 3-27-2011 of another flyover of the reactor.

Edit: just realized this was posted earlier but the youtube 480p looks higher quality than the earlier posted clip

 

[downwinder]

But there are lots of peaks. Because of the high count rates, there will also be accidental summing peaks of two strong gamma energies. The program then identifies some small spurious peak as Cl-38.

It is also possible that the compton edge or some other distortion may have slightly shifted the peak. I have seen that situation before and may be a good explanation of the Cl-38. That would only leave the neutron readings to resolve. I am sure in all the tension at the site there may also be issues with these readings. Might it be possible that these readings were the result of gamma bias on the neutron measuring device?

 

[PietKuip]

It is also possible that the compton edge or some other distortion may have slightly shifted the peak. I have seen that situation before and may be a good explanation of the Cl-38.

That can happen in a NaI scintillation detector, but in a germanium detector the photopeaks are really very narrow, and do not look like Compton edges. The Cl-38 gammas are at quite high energies. That is where one gets the accidental coincidence sums of strong peaks at lower energies.

That would only leave the neutron readings to resolve. I am sure in all the tension at the site there may also be issues with these readings. Might it be possible that these readings were the result of gamma bias on the neutron measuring device?

I do not know what kind of neutron detector they have at the gate. It might be better to use the activation of indium foils closer to any suspected sources of neutrons.

 

[Bodge]

...

Or am I misunderstanding here RealWing, and you're saying that you're sure that these dose rates are far higher than we ought to be seeing from those CAMs?

I too would appreciate an answer to this question.

 

[Astronuc]

Astronuc, thank you for the reply. I confess I wasn't thinking about the velocities involved during normal ops (duh! facepalm). But on shut down, there must be a transition period between boiling with rods partially/mostly uncovered to +10m bathtub water. I'm sure that would be far far more rapid with normal system available than the Fukushima situation... but under normal circumstance even, how long does a BWR have to coast down before they can open up for fuel ops or something of that nature? I know Daiini has achieved cold shutdown as of a few days ago - at that point can a core be opened up & recirc or RHR still runs below the top of the water level to keep temps down? Or is it awhile yet?

It would normally take several days - perhaps 5 to 7, perhaps less. On critical path the idea is to get things moving as quickly as possible. Some plants have outages down to about 15 days, and I believe the TVO BWRs have done outages in about 7 days, IIRC.

I know of mid-cycle outages where the head comes off after a few days in order to remove failed fuel. Such an outage might only last 7 days.

Ok, that all makes sense to me - but what still doesn't is why are they running with water below the top of the fuel in all three reactors then?

As far as I know, they are trying to fill the reactor to the extent possible. Ideally, the core would be covered and they'd have a closed loop cooling system running through normal heat exchangers - especially if they now have power. If they had fuel uncovered, I would think that is simply due to the limitations of the equipment they had - hoses(?). If they could not achieve the proper pressure (head) of water, then the water level would be limited.

 

[jarvik]

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.

As 134I has a gamma emission at 847.0 kev (95.4% yield) and 56Co 846.7 kev (100% yield) I could see it being very easy for an operator / software error getting them mixed up.

Assuming gamma spectrometry is the analysis being done of course.

http://ie.lbl.gov/toi/Gamma.asp?sql=&A1=134&A2=134&Z=53

 

[Joe Neubarth]

If that were the case, however, I would have thought they would have gotten the right ones brought in long before now, wouldn't they? I mean, even if it wouldn't have been in time to prevent the hydrogen explosions in the first 2 reactors, they would desperately have liked to have had power of any sort back to these reactors - and diesel power might have avoided the last two hydrogen explosions. So, why haven't they managed to get diesel's working anywhere other than unit's 5 & 6? To me that implies damage to the electrical systems at or between the connection point and the plant systems themselves - severe enough that they can't reconnect diesels...but this is NOT my area and I could certainly be wrong!

The diesels were running for about 35 minutes and up to operating temperature when they were douched. They probably received massive damage from the instant immersion in cold water. The AC replacement generator that was ordered had to have been up to the specifications of the condensate pump and the sea water pump. There was no phase required as they were not trying to sync with any existing power supply as long as Red went to Red and Black went to Black and Green went to Green. If they say the did not have the right plug, what they really meant to say was that they didn't have the right electrician, or perhaps did not have an electrician at all. Most unusual for an Electric Power Company.

 

[AntonL]

URGENT: Radioactive water at No. 2 reactor due to partial meltdown: Edano
TOKYO, March 28, Kyodo

The government believes highly radioactive water detected at the No. 2 reactor of the Fukushima Daiichi nuclear power plant is due to a partial meltdown of fuel rods there, its top spokesman said Monday.

Chief Cabinet Secretary Yukio Edano told a news conference that the government believes that the meltdown was only temporary.

==Kyodo

Partial Meltdown? does he mean the initial destruction on 11th/12th when core was damaged

Asahi.com report the same as water in contact with molten core

So I reckon it is not a new meltdown, but the original partial meltdown

but the bad news - highly contaminated water leaking from reactor and if delivery of water to reactor breaks down the level will drop faster due to leak ...