Discussion of the MARCH 16, 2011 PRESS RELEASE DOSE ASSESSMENT ASSUMPTIONS
There are two dose assessments attached to the March 16 ” press release. Both assessments are worst case hypothetical, computer model analyses of consequences for releases from the Fukushima site. The first assessment assumed a Unit 2 reactor 100% core melt as an unfiltered release from a totally failed containment and actual meteorological conditions during early morning hours of the date indicated. The low dispersion characteristics included low wind speeds, relatively stable air, and light precipitation.
The second assessment represented multiple unit failures: 1) Unit 2 with 33% reactor core damage as an unfiltered release from a totally failed containment; 2) Unit 3 spent fuel pool with 50% damage (with 180 bundles of spent fuel discharged 105 days ago); and 3) Unit 4 spent fuel pool with 100% damage (with 550 bundles of spent fuel discharged 30 days ago) .
To account for the combined inventories of the three units, the staff adjusted the reactor power level, fuel burnup and number of assemblies in the calculation. The meteorological conditions for the second assessment also assumed actual conditions, but no precipitation, greater wind speeds, and less stable atmospheric conditions, result in greater atmospheric dispersion.
In addition, the source term included two additional days of decay before release. For the multi-unit assessment, the increased decay time before release and the greater atmospheric dispersion significantly reduced the resultant dose estimate.
Although the dose projections for the first assessment are somewhat higher than the second assessment, the differences in the modeling assumptions did not affect the overall conclusion that protective action guides would be exceeded beyond fifty miles. Both assessments are highly speculative, given the lack of actual (representative) site data and assumed no mitigation of the current situation at the time of the press release.
Although there is postulated reactor core damage in Unit 1 and Unit 3, the primary containment structure is reported to be intact.
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