Why are evacuation zones circles?

Today’s expansion of the evacuation zone by the Japanese government to include certain hotspot areas is the latest wrinkle in the evolving quest to keep the public safe from radiation.  What is the basis for the shape of these evacuation zones and how can they be improved using meteorological data?


On March 11 immediately following the earthquake and tsunami the Japanese government issued an evacuation order for a 6-mile radius, then on March 15, a 12-mile radius around the plant.   From 12 to 19 miles the government advised sheltering in place.  On March 17, the U.S., amid fears of a worst-case reactor meltdown (and catastrophic radiation release) fueled by a dearth of information about the conditions at the site, advised U.S. citizens to evacuate out to 50 miles The U.S. NRC admitted the zone was conservative but that it was reasonable given the uncertainties of the situation. Other countries followed the U.S. or advised voluntary evacuation of all citizens in Japan.  On March 25, Japan subsequently advised voluntary evacuation out to 19 miles.

Establishing circular evacuation zones implies that wind direction is not explicitly incorporated in the calculation.  The winds could equally likely be from any direction.  Yet we know that the winds blow in preferential directions, although they can be highly variable in coastal zones.  How can we take account of the direction of the winds to refine the calculation of evacuation zones?  Or as a Japanese radiation expert put it:

“The government should tell people in advance about probable wind directions and other related information, just like weather forecasts. Evacuation instructions should be issued specifically to areas where significant radiation increases are anticipated.”

CBRNE (chemical/biological/radiological/nuclear/explosive) scenarios for which emergency responders train typically unfold on timescale of hours to days.  The crisis at Fukushima is unfolding over weeks to months.    And so as we contemplate how meteorological information can help refine the determination of evacuation zones, we have to bear in mind these two different timescales.

We’ve seen in the high-resolution plume forecasts for Japan that the wind direction at the Fukushima site has been frequently offshore, sparing the urban areas of much of the radiation fallout in the early days of the tragedy.  Yet shortly after the destruction at Fukushima there were several episodes of significant release of radiation, some of which moved toward the northwest over the towns that the Japanese government is only now contemplating evacuating (see DOE airborne-sensed radiation map above).  Ideally, we would want to warn and evacuate the population in advance of a release, especially if there is potential risk of a catastrophic release.   This is where high-resolution operational forecast models (especially model ensembles) that predict out to 2-3 days can help.  Although in actuality, the airborne release of radiation at Fukushima was highly intermittent in time, the model source term could be treated as a continuous release to map out the widest area that the plume could potentially cover.   Then when the possible health effects (dosages) are factored in, rational evacuation strategies can be generated several days ahead of time.

As the weeks wore on and radiation releases continued, there were days when the winds blew toward Tokyo and the Boso Peninsula, as well as to the north.  And so when one contemplates whom to evacuate to avoid long-term exposure to radiation, having some guidance from the climatology is useful.  Looking back at the real-time high-resolution plume forecasts over the past month (details here), an evacuation zone that extends in a band north and south along the coast and is circular around the site (to take account of explosive blast release) would be a reasonable approach.  The zone might also include areas on the Boso Peninsula that were shown to be in the path of the plume on multiple days, although the possible radiation exposure would be diminished due to the distance from the source.

The actual width of the evacuation zone could be calibrated to take account of health effects including the international standard of maximum exposure, the potential risk of a catastrophic release, difficulty of evacuating areas, and a whole host of other considerations.  But the basic shape, as informed by the typical local weather conditions for that time of year, might look more like an aggregate of the images below and less like a circle.

Common plume patterns over the past month from a high-resolution forecast model.

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