WASHINGTON — The threat of a catastrophic release of radioactive materials from a spent fuel pool at Japan’s Fukushima Daiichi plant is dwarfed by the risk posed by such pools in the United States, which are typically filled with far more radioactive material, according to a study released on Tuesday by a nonprofit institute.

The report, from the Institute for Policy Studies, recommends that the United States transfer most of the nation’s spent nuclear fuel from pools filled with cooling water to dry sealed steel casks to limit the risk of an accident resulting from an earthquake, terrorism or other event.

“The largest concentrations of radioactivity on the planet will remain in storage at U.S. reactor sites for the indefinite future,” the report’s author, Robert Alvarez, a senior scholar at the institute, wrote. “In protecting America from nuclear catastrophe, safely securing the spent fuel by eliminating highly radioactive, crowded pools should be a public safety priority of the highest degree.”

At one plant that is a near twin of the Fukushima units, Vermont Yankee on the border of Massachusetts and Vermont, the spent fuel in a pool at the solitary reactor exceeds the inventory in all four of the damaged Fukushima reactors combined, the report notes.