Several northward wind shifts draw the plume north of Fukushima and also move earlier plume remnants over the Boso Peninsula in the late evening of Thursday April 7 and morning of Friday April 8 Japan Standard Time (JST).
No radioactive release has been reported. The passive tracer maps show a possible trajectory of contamination, if it were released, given the predicted meteorological conditions.
Plume remnants that had drifted off the grid to the south are projected to blow north. The right panel below shows the source of this older plume on the middle (15 km resolution) grid nest.
Concerns mount that reactor vulnerabilities may be exacerbated by the sustained efforts to stabilize the site as detailed in a report by the U.S. Nuclear Regulatory Commission. On Thursday nitrogen gas is pumped into reactor 1 to head off hydrogen explosions.
The Naval Research Laboratory’s skillful data-assimilating triply nested (45 km/15 km/5 km) mesoscale meteorological model COAMPS produced the fields, using a passive tracer to map the expected plume trajectory. Lighter gray contours show more concentrated material. Each shade of gray represents a factor of 10 difference in tracer concentration. [Plume images are a research tool; contours are arbitrary units for visualization purposes.]
The modeled “plume” is a tracer that is released at a constant concentration, simply follows the air flow, and (a) does not settle due to gravity, (b) does not get deposited with rain, and (c) does not have any radioactive decay. These neglected factors typically dramatically reduce the effects of radiation when it travels over distances of hundreds of miles. There is much uncertainty in the source term, as the releases of radiation that occurred in the past were highly intermittent. (So although modeled as such, the plume in actuality was not a continuous release.)