The test ban treaty group routinely does radiation projections in an effort to understand which of its global stations to activate for monitoring the worldwide ban on nuclear arms testing. It has more than 60 stations that sniff the air for radiation spikes and uses weather forecasts and powerful computers to model the transport of radiation on the winds.

On Wednesday, the agency declined to release its Japanese forecast, which The New York Times obtained from other sources. The forecast was distributed widely to the agency’s member states.


Earlier this week, the leading edge of the tangible plume was detected by the Navy’s Seventh Fleet when it was operating about 100 miles northeast of the reactor complex.

The United Nations has also detected radiation at its detector station in Gunma, Japan, which lies about 130 miles to the southwest.