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Friday, May 13, 2011

NISA responds to TEPCO plan to fill dry well at No. 1 plant

NISA has published its response to TEPCO regarding the plan to flood No. 1 plant's dry well to a height above active fuel. While present events may certainly change this policy, it might be important both for the other plants on site and for future reference to note the results of this quick but apparently thorough evaluation.

And the findings are simply this: While NISA thinks TEPCO needs to ensure that no further water leakage (that is, more rapid) will result from filling the drywell and wants it to monitor injection closely, and also thinks that TEPCO needs to evaluate some strengthening of torus support in the future, it has SIGNED OFF TOTALLY on TEPCO's evaluation of the situation, of the plan, of the effects and results.

In other words, the operation would have (would have had?) the desired effect. It would not have resulted in further increased threat of steam explosion or hydrogen burn. TEPCO's analysis of the seismic resistance in the condition stipulated was correct and proper. So long as no further flood exited the reactor building, and flood height did not exceed the vent line, the water was not a problem.

Although I've already noted them, here are the only two special orders NISA issued to TEPCO regarding this plan:

1. Regarding the water level inside the PCV and the leakage from the PCV,
in addition to carefully monitoring the process, implement appropriate
response measures determined in advance such as controlling the
injection amount of water, etc.

2. Regarding the support columns of the suppression chamber, which have
little leeway with respect to the acceptable criteria values, from the
presumption that aftershocks will continue to arise from now on, review
how to further strengthen the seismic resistance of the NPS and make
improvements for its operational environment.

So, while it appears that this plan probably will not be used in its original sense (in that the present estimate is that even if flooding the dry well is performed, it won't have to be nearly as high) we do see that in the previously assumed core condition both TEPCO on honest evaluation, and NISA on its review, found the plan to be workable and safe.

9:10 PM Eastern Friday 5/13

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