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Tuesday, December 20, 2011

"Cold Shutdown" controversy at Fukushima Daiichi

There has been a lot of bad press about the announcement by the Japanese Government that the reactors at Fukushima Daiichi have achieved a state of cold shutdown. I have tried to clear this up in a previous post, but since there does appear to be continued confusion in some media, I will make a couple of additional points.

It was stated (and continues to be, in places) that the owner-operator of the Fukushima Daiichi site (TEPCO) and that the Japanese government have been less than forthcoming with information, and that operations on the site and effects off site have been covered up. This could not be further from the truth; simply look at all the posts on this site and all the videos on the APR YouTube Channel for this proof. To this end (that is, for the sake of transparency) a very detailed recovery timeline was developed and published widely by all concerned parties and continues to be used to this day.

The achievement of 'cold shutdown' is one of the steps on this timeline, and as such when it's reached, it's announced. This is simple enough for anyone to understand. But who made this determination? Let's look at a quote from NISA's site (Nuclear and Industrial Safety Agency) to get the facts:

December 19, 2011... NISA website. "The Nuclear Emergency Response Headquarters evaluated that the reactors at Fukushima Daiichi NPS, TEPCO, were brought to a condition equivalent to "cold shutdown" and, even if an unforeseeable event occurs, the exposure dose at the site boundaries can be kept at a sufficiently low level as a result of the evaluation of the securing of safety at nuclear reactor facility of Fukushima Daiichi NPS, TEPCO, at the completion phase of Step 2, stating that a safe condition has been achieved and the accident at the NPS itself has come to be settled."

So, the Nuclear Emergency Response Headquarters (this is a special construct of the Japanese Government) decided that the condition had been achieved, and it seems obvious to me that NISA and TEPCO would be included in this decision making simply by virtue of the complicated manner in which the Japanese nuclear industry operates. Thus, this is neither a unilateral nor simply a political announcement - it's technical. And it's mapped out on the timeline as a required step.

What does this mean? Well, "cold shutdown" is a term normally used for intact reactors in normal conditions; the term is adapted here to indicate (as I've posted before) that the reactors are cooled down sufficiently and that release of radionuclides to the atmosphere is minimal or zero. Here are some further specific facts:

-No further core damage is occurring (or has for some time.)
-No condition of explosive hydrogen accumulation exits, with nitrogen purge to the reactors and primary containments and gas handling systems operating.
-No condition of decay heat buildup in the reactors exists, with both feedwater system cooling feed and core spray ring feed in progress.
-No condition of dose above 1 mSv/yr exists outside the nuclear power station.
-No condition of wide uncontrolled atmospheric release exits with the reactors cooled down below 100C and thus no boiling occurring to disperse materials with steam.

In the final analysis, this announcement is kind of like saying "I think I have caught a cold." You already had the cold before you knew it; the symptoms accumulated until as a group you put them all together to determine you were sick. This announcement is very similar; each of the contributory factors was reached independently of the others in ways, but tied together in ways so that the actual state of "cold shutdown" has existed for weeks. Only now did the nuclear response headquarters unit, NISA and TEPCO collaboratively decide that it was time to announce without doubt that this status had been achieved. What this means is detailed above; it could not be more clear what this means and does not mean. It also could not be more clear that there is no political advantage in making this announcement for anyone - it's a matter of plant status, and that's all.

8:15 AM Eastern Tuesday December 20, 2011



Here are some links that readers might find to be of interest.

NRC Definition of "Cold Shutdown"

Nuclear Energy Institute's statement - December 19, 2011

IAEA welcomes news of cold shutdown at Fukushima Daiichi

Link to PDF file from TEPCO to IAEA mentioning previous "stable cooling" threshold and the later, now achieved "cold shutdown" status

NRC Chairman Jaczko's comments after a visit to Fukushima Daiichi, on the cold shutdown announcement and plant conditions

Below, a graph from Tokyo Electric Power Company showing core decay heat generation for all three Fukushima Daiichi reactors. In reality, "Fukushima Daiichi" is "Fukushima Number One" nuclear power plant site, and 1F is the TEPCO code for Fukushima Daiichi. Thus, 1F1 is Fukushima Daiichi No. 1 reactor plant, 1F2 is No. 2 and 1F3 is No. 3. Note that core power due to decay heat as calculated by ORIGEN2 analysis code as of September 2011 is already down to roughly one megawatt for No. 2 and No. 3 plants, and down to roughly 0.8 MWt for No. 1 plant.


  1. Good and useful summary, Thanks Will.

  2. I think the term 'Cold shutdown' is sufficiently inappropriate in this case to be actively damaging to the credibility of the industry spokespeople that embrace it.
    Afaik, the fuel is generally believed to have collapsed into a heap and melted through the RPV into the containment.
    We now have three slugs of fuel that are still generating megawatts of decay heat, somewhere near the bottom of the RPVs and/or the containment. I believe that the reactors are still producing some steam from these piles because the interior of these fuel piles is still white hot due to the decay heat.
    That cannot plausibly be called a 'Cold shutdown', at least imho.
    In fairness, this is an unprecedented situation and the reactors have been brought to a state of reasonable stability, but we should be frank enough to admit that this is not a 'Cold shutdown'.
    We will regret misusing the term the next time we need to reassure the public if we allow it to be applied to cover trashed reactors which are no longer in immediate danger of emitting massive radioactivity.

  3. @netudiant: The term "cold shutdown" was deliberately chosen as a parallel to normal practice, so that some parallel to normal practice could be drawn. As such, as an expressive term for a generally unfamiliar and mostly (in nuclear engineering) uneducated public it's quite appropriate. The output of each core is well below one megawatt by now; I had that ORIGEN derived graph on the site a long while back. The whole "egg shaped corium deposit encased in salt" theory seems not to have gained any traction when posited some time back. It seems from the best evidence now that the condition of the plant internals is probably more along conventionally expected lines; I don't believe that the core masses are as you say "white hot." "No longer in immediate danger of emitting massive radioactivity" is essentially one of the cold shutdown criteria established by the Japanese government, NISA and TEPCO to declare cold shutdown. As I said- the term is very clearly defined, and my article makes perfectly clear all of what this term is and is not. Attempting to convey mileposts along the recovery to the public is a good thing, even if at times it's a bit clumsy. Just so long as it's honest. What is DIShonest is when the mainstream media tries to say that the Japanese government is trying to say something that isn't true; what the Japanese ARE saying is really clear when you read the quote in the article above.

  4. Will,
    Thank you for your helpful note.
    Your comment about the cores producing much less than a megawatt each surprised me, but you are entirely correct, under half a megawatt each even for the bigger cores using the MIT data shown here: http://mitnse.com/2011/03/16/what-is-decay-heat

    That indicates the current water injection levels are sufficient to absorb all the heat without boiling, which was not the case earlier and which had been the source of my diatribe. So I stand corrected, or at least a little abashed, but I still would have much preferred to use a different term to describe the situation such as 'stabilized' rather than 'cold shutdown'.
    I don't think a term that is in routine use to describe the status at operating nuclear plant should be poisoned by applying it to this disaster. No one would say that a plant that burned down was 'retired from service'.
    My $0.02.
    Using 'cold shutdown' here is a misuse of the language that does a disservice to the industry. Some language is too clumsy to be useful, no matter how good the intention.

  5. Will,
    Sorry, I dropped a decimal point, the cores are still generating several megawatts each according to the MIT data. By March 2012, MIT estimates the decay heat will be about one five hundredth of the maximum thermal power, so about 4 megawatts each for the two big reactors and 3 for the littler one. Unless I've dropped another decimal point, that is too much heat to absorb without boiling at least some of the water based on current injection rates.
    So the situation is far from 'cold shutdown' because uncontrolled boiling continues, even excluding other considerations.

  6. @netudiant-- The graph I consulted is published by TEPCO, and is dated May 26 2011; it used the ORIGEN2 evaluation code. This shows that as of exactly six months after the accident, No. 1 plant (1F1) would be developing about 0.8 MW of heat, while the other two (1F2, 1F3) would be about one megawatt each. That was as of mid-September. I wonder why there is such a disparity with the MIT page, which I have just looked at. Regardless of this disparity, it is the data I quote here which the Japanese are using and which is being backed up by actual experience on the site in order to declare this step complete. I will look at some other materials I have and see what their quotes for decay heat are.. if they go out this far after scram.

  7. Note that I have added onto the original post today (December 21, at about 1:20 PM) with additional links supporting general acceptance in legitimate circles regarding the "cold shutdown" declaration, and regarding the terminology. Note also the decay heat graph.

  8. Will,
    Imho, the TEPCO charts are inconsistent internally. It makes no sense for the decay heat from reactors 2 and 3, which are each 60% bigger than reactor 1, to be only 25% bigger. In any event, I will continue to believe the MIT figures, rather than the data I get from TEPCO, even though they are the people on the spot, because MIT's comments do not come through the haze of translation.
    Separately, thank you for the TEPCO comments, which seem very carefully measured. They only say:
    'The release of radioactive materials from the PCV is under control and the public radiation exposure resulting from additional releases is being significantly held down.' (P III-4 TEPCO release)
    I see that as an admission that reactors 2 and 3 are still producing radioactive steam. That is probably also why they are not enclosing reactors 2 and 3 as yet.
    Radioactive steam condensing everywhere inside the covering would make work on the reactors entirely impossible.
    I thought TEPCO's release was much more honest, albeit much longer, than Mr Jaczko's statement, which studiously avoided saying anything other than to call the Japanese declaration a tremendous milestone.

  9. There is a footnote to the MIT calculations...

    "* Values for the decay heat were calculated based on assuming an infinite reactor operation time prior to shutdown. Infinite operation is a conservative assumption, and actual values may be significantly lower than those that are shown in the figure and table."

    I don't know whether "significantly lower" would include the difference seen here.

  10. @joffan: Right you are. There is some discussion actually going on about this behind the scenes in the ANS Social Media forum, and if and when anything concrete gets hammered out I will let you know. Right now it is not looking good for the TEPCO graph but it is not looking good for the M.I.T. numbers either; the final calculated value will probably be between them somewhere. What's important to understand though is that these are just calculations - remember that the measured temperatures keep lowering at Fukushima Daiichi's three reactors and that's what counts.

  11. Will,
    Thank you for staying on this topic, it is important to get the facts right.
    Joffan's contribution is a valuable addition, I had not caught that, it means the MIT estimates are high, although it should not be more than 50% off versus the Fukushima actuals, which afaik reflect some years of operations for the cores involved.
    The key measure is of course the actual data from the site and it is good the temperatures are all comfortably below 100 degrees C, but of course the worry is that these may be misleading, if the readings are taken on an empty shell, with the hot fuel actually outside the RPV.

  12. @Everyone: I have here now from Dr. John Bickel, PhD some manual calculations he ran as a result of my query to the American Nuclear Society list I'm a member of. His method shows about 1.5 Mw maximum for No. 1 and something less than 3 for the other two, but he notes that the method employed when compared to the ANS standard curve "over predicts the amount of decay heat by a factor of about two times." Thus it does appear at this moment that the TEPCO figures ARE LOW, but that the MIT figures are FAIRLY HIGH and are probably more inaccurate to the high side than TEPCO's are to the low side.

  13. Will,
    This is a classic example of why humility is an essential part of good engineering. Here we have three world class entities, MIT, TEPCO and the ANS, producing estimates for nuclear decay heat from a known amount of fuel that vary by a factor of 4.

    I cannot think of a better illustration of the painful truth that models and calculations must always be verified against real life experience, otherwise big errors can occur.
    Clearly there are many considerations that influence the result in this instance beyond merely the size of the original fuel load. Your work in digging out a more correct estimate is a real contribution to our understanding of the current situation.

  14. @netudiant: Thank you very much! I have someone else as well who has provided some input, from whom I expect to hear more and I'll also pass that along if / when something more concrete is yielded.

  15. Link to document from NISA, stating that the amount of decay heat generated in the cores is being removed successfully. Much useful data in here. Note on Page 6 the chart giving a 0.88 MW decay heat value -- correlating well with the value from the ORIGEN-2 derived graph for Reactor 1F1.