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Tuesday, May 17, 2011

Fukushima Daiichi update: Tuesday 5/17

As expected, there are a number of news developments from TEPCO and the Fukushima Daiichi site.

TEPCO has revealed the fact that the isolation condenser cooling system (fitted to the oldest plant, No. 1) was shut down within ten minutes of having been actuated automatically after the earthquake. The shutdown of the system is suspected to have been done manually. TEPCO is investigating to discover if this is actually the case, and if so, why the system was shut down. There is speculation that the operators found its operation suspect or unsatisfactory, or were alarmed by the pressure drop in the system.

The water injection rate at No. 1 plant has been reduced back to six cubic meters per hour. TEPCO is analyzing the results obtained during the period of higher injection rate.

As one might expect, there is wide speculation that the cores at No. 2 and No. 3 plant aren't in any better condition than that at No. 1 plant.

For the second time in a week or so, TEPCO has injected borated water into No. 3 plant. This plant has been using two different simultaneous injection paths for a number of days now to try to drive temperatures back down - one firefighting connection and the normal feed system piping. This is of some interest, since the only reason to inject borated water would be due to a fear that increased fission rate is occurring. Considering that in these plants, from what information we have specifically been able to gain, the control rods would melt BEFORE the fuel would melt, it's possible that TEPCO is thinking the rod material is perhaps more at the bottom of a mixed mass of corium.

Finally (for this post, that is) TEPCO is apparently roughly sticking to its original timeline for site recovery.. with plants stable and cooled in six to nine months. A number of additional areas of concern have been merged into the recovery plan, but the overall time expectations are stated by TEPCO as workable.

We have more information and updates in progress.

2:25 PM Eastern Tuesday 5/17


  1. Apart from small confounding factors, the fission rate should be proportional to the water temperature / flow rate, right?

  2. @iya: There should be a very tiny fission rate right now, at this time period after the full scram insertion, in a reactor whose core is not seriously deranged, partly or fully melted. If the core is seriously damaged to the point of melting into a mass of what we call "corium," which is a mixture of the uranium fuel pellets, zirconium cladding, melted control rod material and so forth then you really get into a gray area as to what the fission rate.. if any .. really is. Without a lot of water to moderate neutrons there may not be a large inventory of thermalized neutrons to induce fission, but then again with the new and highly irregular geometry in the core area it's hard to guess whether neutrons could thermalize in enough numbers to cause a sustained, instead of continuously lowering, rate. Recriticality has been the subject of much conjecture in accident studies. The results from this accident in Japan will greatly enhance future studies by allowing us to modify theoretical analysis with actual events. Hopefully there is data being collected on neutron leakage to get a handle on fission rate.

    To me it seems more likely that exposed fuel was superheating water, causing the serious rise in feed nozzle temperature... but temperature increases at other times at the other plants in recent weeks didn't trigger the use of boron. We must wait for more facts.

  3. mention of recriticality a while back here:

    "... NUREG/CR-5653 the "Recriticality in a BWR Following a Core Damage". That's a great read because they consider a station blackout as an initiating scenario...."


  4. Another article:

    Recriticality Energetics of a Hypothetical Water Reflood Accident in a Damaged Light Water Reactor

    K. N. Schwinkendorf

    Nuclear Science and Engineering / Volume 132 / Number 1 / May 1999 / Pages 118-126


    (I suspect the word "dollars" in the abstract is a copy/paste/spellcheck error, where it says:

    "Reactivity insertion rates analyzed in this study generally are smaller than in LMR CDAs (tens of dollars per second versus up to hundreds of dollars per second) ....")
    Technical Paper

  5. You can't blame them for being cautious - even if there was only a very small chance of fission, injecting boron couldn't hurt and might help.

    I doubt it, after so long, but we may never know.

  6. @hank: No, "dollars" is a very old-fashioned and waaaay out of date unit for making reference to reactivity.