On April 26, 1986, an accident at the Chernobyl nuclear reactor in the former Soviet Union released a full spectrum of fission products into the atmosphere. The accident was not promptly reported, and little information on the amount or character of the release was available. Within a few days of the release, ARAC produced quantitative estimates of the regional-scale contamination. For 16 days, ARAC supported the joint U.S. effort coordinated by the Environmental Protection Agency to assist western Europe's immediate response.

Also, ARAC's long-range particle-in-cell model accurately simulated the spread of the radioactive cloud over the entire northern hemisphere, as verified later by radiological measurements. A fraction of the radioactivity that was released by the initial explosion and subsequent graphite fire was apparently lofted to heights of 5 to 6 kilometers, as evidenced by its rapid transport around the northern hemisphere. The globe at the left shows the distribution of radioactivity over the northern hemisphere 10 days after the Chernobyl accident. ARAC models determined that the lower section of the radioactive cloud headed toward Scandinavia, whereas the upper section headed in an easterly direction, across Asia, Japan, the North Pacific, and the west coast of North America.

 Original source of table:  http://narac.llnl.gov/responses/chernobyl.html