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The Fukushima nuclear plant has us on tenterhooks as we watch the ongoing efforts to contain the radioactive contamination from the damaged reactors. Insight into what is happening there and and what might happen there depend on understanding how the accident happened and how it unfolded in the early days. A recent summary* of the incident put forth by the energy company Areva puts together in one concise statement those early events. What I read there clarified this sequence for me and I thought it might help others too. If you would like to follow a timeline join me below the swirl of orange.

*The Areva pdf is available at many sites but I could not find the original Areva link. This new link is from a neutral site and the most recent (April 15) but there are many others across the spectrum.

11.3.2011 Friday


14:46 - Earthquake - Magnitude 9
The power grid in northern Japan fails but the reactors are mainly undamaged. The SCRAM was successful and shut down the power generation due to chain reactions.  Heat generation from radioactive decay of fission products continues at a much lower rate:
· After Scram ~6%
· After 1 Day ~1%
· After 5 Days ~0.5%

Additional automatic responses to the earthquake include closing all non-safety related penetrations of the containment and starting the diesel generators. Emergency Core cooling systems were supplied and the plant was in a stable save state.

15:01(?) Tsunami hits plant
About fifteen minutes after the earthquake the tsunami hits. Because it exceeds the plant design, there is flooding of some or all of these: diesel; switchgear building; fuel tanks; essential service water buildings.

15:41 Station Blackout
About an hour after the earthquake and SCRAM, there is a common cause failure of the power supply. This leaves only batteries for power and leads to failure of all but one Emergency core cooling system.

16:36 Unit 1
Nearly two hours after the SCRAM Unit 1, the oldest of the reactors, has its isolation condenser stop, perhaps because the tank that feeds it is now empty.

13.3.2011 Sunday

2:44 Unit 3
About a day and a half after the SCRAM in Unit 3 the reactor isolation pump stops because the batteries that powered it are drained.

14.3.2011 Monday

 13:25 Unit 2
About two days after the SCRAM the last of the three reactors that were powered up before the earth quake has pump failure and the reactor isolation pump stops.

At this point all three reactors have no means to remove heat from the cores (now putting out about ~1% the power that was generated during full power).

This residual decay heat still produces steam in the reactor pressure vessel. Rising pressure opens the steam relieve valves. Steam discharges into the Wet-Well (Torus). As the reactor water turns into steam that is vented into the torus, the liquid level in the reactor pressure vessel (RPV) falls.

As the liquid level falls, at some point ~50% of the core is exposed. The residual heat causes the cladding temperatures to rise, but at this level of exposure no significant core damage happens.

When the liquid level falls further, ~2/3 of the core is exposed and the cladding temperature exceeds ~900°C. This temperature leads to balooning or breaking of the cladding. Without this covering the fuel rods release fission products.

When the liquid level falls further still and ~3/4 of the core is exposed, the temperature of the cladding exceeds ~1200°C. At this high temperature, the zirconium in the cladding starts to burn under the steam atmosphere in a reaction with the steam:
 

 
Zr + 2H20 ->ZrO2 + 2H2

This is an exothermal reaction (chemical reaction that releases heat). The oxidation of the zirconium further heats the core and produces hydrogen gas.

Effect of high temperatures on the core



It is expected that all three units reached ~1800°C.   This temperature causes melting of the cladding and of steel structures.

It is expected that Units 1 and 2 reached ~2500°C   which would cause breaking of the fuel rods and could leave a debris bed inside the core.

Maybe Unit 1, the oldest unit and the one that lost cooling first, reached ~2700°C. If it did, this temperature is high enough to cause significant melting of Uranium-Zirconium-oxides.

Restoring the water supply brings the temperatures down and stops zirconium oxidation in all 3 Units:
 Unit 1: 12.3. 20:20 (27h w.o. water)
 Unit 2: 14.3. 20:33 (7h w.o. water)
 Unit 3: 13.3. 9:38  (7h w.o. water)


The Areva report goes on in much detail into aspects such as how the hydrogen produced by the zirconium fire reached  the upper story where it exploded and reconstructions of the spent fuel pool events. The diagrams throughout are very helpful; I am hoping that by confining myself to a partial summary of the narrative that this is still fair use.


For ongoing coverage of the Fukushima incident, check out and follow Japan Nuclear Incident Liveblogs - the ROVs and individual diaries.

Originally posted to Wee Thoughts on Fri Apr 08, 2011 at 10:51 AM PDT.

Also republished by SciTech and Japan Nuclear Incident Liveblogs.

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Comment Preferences

  •  Corrections welcomed. (18+ / 0-)

    I have attempted to convey the Areva report accurately and all the numbers are from it. If you see a place where I have made an error in summarizing from the report let me know so that I can correct it. If you see places where you believe the Areva report itself is in error, let me know that with citations and I will either correct this account or indicate that there is a diversity of opinions.

    •  Great work, Wee One! /nt (10+ / 0-)

      It rubs the loofah on its skin or else it gets the falafel again.

      by Fishgrease on Fri Apr 08, 2011 at 11:32:50 AM PDT

      [ Parent ]

    •  You left out the (1+ / 0-)
      Recommended by:
      Wee Mama

      rather important hydrogen explosions that blew the secondary containments all to hell and dumped tons of debris onto already overheated spent fuel pools, throwing hot fuel rods willy-nilly around the site and up to a mile away.

      Unit 1 blew its lid on March 12. Unit 3 destroyed itself on March 14. Unit 2 blew its torus March 15. On the same day the zirconium cladding on spent fuel assemblies in the Unit 4 fuel pool started burning, and soon whatever water was left in the pool was boiling. On the 15th TEPCO admitted the spent fuel could go critical, all workers were evacuated on the 16th as smoke was rising. By evening the 50 martyrs were back, spraying water on Units 5 & 6. On the 17th National Defense Forces were dropping water from helicopter bladders onto the plants, and TEPCO began work on the World's Biggest Extension Cord. Firefighters and police joined the martyrs to help hose the wreckage down from tank trucks...

      Now, more than ever, we need the Jedi.

      by Joieau on Fri Apr 08, 2011 at 04:09:54 PM PDT

      [ Parent ]

      •  This is specifically an early chronology, to sort (3+ / 0-)
        Recommended by:
        Joieau, peraspera, Fishgrease

        out for myself as much as for anybody else those early events that set the boundaries for meltdowns and possible corium formation. As I say in the diary,

        The Areva report goes on in much detail into aspects such as how the hydrogen produced by the zirconium fire reached  the upper story where it exploded and reconstructions of the spent fuel pool events.

        I think these later events are also very much worth close study to understand the cascade of failures and how early problems turned into the later ones.

        •  Cool. (2+ / 0-)
          Recommended by:
          Wee Mama, Ignacio Magaloni

          It's just that the plants blew up on Saturday, Sunday and Monday. And Areva's playing TEPCO here claiming it was zirconium that released all that hydrogen. This, I think, is an important note, because NRC (that I know for sure) has known since the beginning that radiolysis helped. Quite a lot. While TEPCO, Areva and the entire pro-nuclear lobby worldwide had been insisting since Day-1 that it doesn't happen when ~2000 degree melting metal fuel rods - live or spent - get dowsed with occasional cold water.

          Now that the RST report is out and TEPCO's been forced to understand that flooding the drywells with water wasn't that great of an idea at all, radiolysis becomes a very, very significant threat when the corium starts coming through those used-to-be control rod holes. If those containments blow sky high, God only knows where the reactors end up and the spent fuel pools - all of 'em - are complete toast.

          So on this, applying a little hindsight helps to round out what was really going on during those first four days. IMHO, of course. You and the Live Blog/ROV gang are doing a great job.

          Now, more than ever, we need the Jedi.

          by Joieau on Fri Apr 08, 2011 at 04:35:00 PM PDT

          [ Parent ]

          •  I'm sure radiolysis contributes some hydrogen - do (3+ / 0-)
            Recommended by:
            Joieau, peraspera, Fishgrease

            you happen to have a source for the relative proportions of it vs. the zirconium burn?

            The Areva report seems to think that the only reactor that might have reached the temperatures needed to melt the uranium oxide was reactor 1, and that that reactor only possibly, not certainly experienced melting of the uranium oxides. If you think there was melting for sure there or in 2 or 3, do you happen to have a source? As I said in my tip jar I will revise with new sources as available.

            •  Here's an article (3+ / 0-)

              about Unit 2. Based on the RST report. At least 1/3 of the Unit 3 core is molten, per the same report. They determine this via the isotopes being measured in releases (which is why TEPCO is now only reporting on 3 in their sea water assays).

              Bear in mind that the "possible" breach to the torus is underground and thickly shielded all around, at the center of what's left of the reactor building. It is breached to primary containment and the vessel itself, not significantly (beyond vessel venting) to the atmosphere. Thus it is unlikely to be the source of major atmospheric releases beyond the other units, though it might be a source for underground tunnel water releases. Depends on whether or not the core has already melted through to the catacombs to give it a route of release.

              Now, more than ever, we need the Jedi.

              by Joieau on Fri Apr 08, 2011 at 05:00:32 PM PDT

              [ Parent ]

              •  Before I plow through it, is "the RST report" the (1+ / 0-)
                Recommended by:
                Joieau

                one that is dated March 26th but released recently, like this one? If there is a newer one I'd appreciate a pointer to it. Since your link goes to the NYT article that is about the NRC press statement, if the NRC is based on the RST report it makes more sense to go to that. I have some issues with the Times article but what's more important is what the source says.

                •  No, far as I can tell (0+ / 0-)

                  it's the same one you guys linked to previously. An interesting explanation (of expectations) from more than a week ago, that can help contextualize the thermal imaging from yesterday as to where the 'hot spots' are located. The worst of them are not where the reactors are located.

                  Now, more than ever, we need the Jedi.

                  by Joieau on Sat Apr 09, 2011 at 02:28:50 PM PDT

                  [ Parent ]

            •  It can help to envision (4+ / 0-)

              the sheer size of these beasties. You don't get that from distance photos. I knew an hp tech who was providing coverage to a work crew in the torus of a plant just like these once a long time ago. They climbed in through a hatch via ladder onto a mesh metal catwalk above the water level of the suppression pool. No crouching. Were doing just fine when some glitch or other caused an emergency scram, which sent reactor water forced out of the reactor by thermal transient into the torus, filling it really fast. He got the crew out and ended up with soaked legs before just barely managing to not get sucked away and drowned.

              The suppression pool at Diachi's Unit 2 had to already be hot before the explosion, as were they all. The explosion had to happen in the overflow piping leading TO the torus to represent a breach of the vessel. Hydrogen in the reactor vessel from melting zirconium and radiolysis would go out the steam piping or relief valves (by thermodynamic design, hence the blown upper stories of units 1 and 3), not into the torus. Or the others would have blown their toruses as well, and they so far have not. To get into the torus there had to be huge "bubbles" enough to lift the water, steam and gases to the overflow, because water was at the time seriously low in the vessel.

              No operator mistakenly or purposely routed any of this, because there were no operators and no operational controls at the time. This is pure thermodynamics, complicated by a runaway heat source that is practically unimaginable and isn't going to cool on its own.

              Now, more than ever, we need the Jedi.

              by Joieau on Fri Apr 08, 2011 at 05:27:36 PM PDT

              [ Parent ]

    •  Several items in response, wee one: (0+ / 0-)

      1.Yesterday in Arnie Gunderson's dramatic video he said there were several errors in that first Areva report and that his next video would address those errors. (He implied that the truth was even worse.) I see no new vid up yet but you might want to keep yr eyes peeled: http://www.fairewinds.com/

      2. Here's the thread from where I posted the report two weeks ago (ish) and some comments in response. (I had particularly asked for kbman to look at it) http://www.dailykos.com/...

      3. There is a newer Areva document from yesterday. Again, I have no idea if the original event has been edited to reflect new information or not. My eyes are a bit fried.

      Newer AReva doc (this pdf is hosted at Fairewinds Assoc)  http://www.fairewinds.com/...

      4. Keep up the good work!!!!

  •  Major design flaw (6+ / 0-)
    15:01(?) Tsunami hits plant
    About fifteen minutes after the earthquake the tsunami hits. Because it exceeds the plant design, there is flooding of some or all of these: diesel; switchgear building; fuel tanks; essential service water buildings.

    Why oh why was this critical equipment not designed/sited properly? If not for this flood damage, the cores would have never overheated.

    And secondly, what is being done to ensure this same equipment is safe & secure from tidal surges at other oceanfront plants - particularly in the USA?

    There should never be a tax benefit for companies that screw over American workers.

    by bear83 on Fri Apr 08, 2011 at 12:58:02 PM PDT

  •  Looks to me as if (4+ / 0-)

    most of the radioisotopes released can be attributed to Unit 2 and the Unit 4 spent fuel fire.

    Hopefully the studies that will follow will dedicate some significant effort into finding out why the hydrogen explosions at units 1 and 3 were at the service floor, but the explosion at unit 2 was in the torus. That torus damage looks to me to be the single worst source of contamination, presumably responsible for the highly radioactive water that's hampering efforts to bring the cooling systems back online.

  •  Thank you wee mama! (2+ / 0-)
    Recommended by:
    Fishgrease, peraspera
  •  Minor update: the link to the Braun/Areva (0+ / 0-)

    presentation is the newest I could find. The original and intermediate ones do not differ on the chronology in the body of the diary.

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