The plume is projected to expand inland and overlap a broad area of the east coast of Japan through the afternoon and evening of Sunday April 10 Japan Standard Time (JST).
A WSJ story reports: “The government’s top spokesman, Chief Cabinet Secretary Yukio Edano, said Thursday the current 20-kilometer (12-mile) evacuation zone around the Fukushima Daiichi plant may need to be enlarged, because the original parameters were established to protect against too much exposure in the short term and radiation continues to emanate from the complex.”
This low-level emission of radiation is worrisome almost four weeks into the crisis. There is much uncertainty about the actual conditions at the Fukushima site in terms of the status of the reactors. The New York Times had an excellent analysis today about the divergent interpretations expressed by the NRC and Japanese Nuclear and Industrial Safety Agency (NISA) in the absence of much concrete information.
The Naval Research Laboratory’s skillful data-assimilating triply nested (45 km/15 km/5 km) mesoscale meteorological model COAMPS produced the fields above, 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.)
We have utilized COAMPS to make detailed and accurate forecasts of various coastal regions, including large-scale (synoptic) weather patterns, but it also is skillful at simulating local coastal processes such as the influence of cold upwelled ocean waters, or sea breezes, when there is no strong synoptic forcing. We have previously used the model to simulate the movement of hypothetical releases along the east coast of Japan.