MONTPELIER -- The U.S. Nuclear Regulatory Commission is rejecting calls from watchdog groups and some U.S. senators to speed up moving spent nuclear fuel from the pools where most of it is stored.

An NRC memo provided to The Associated Press on Tuesday shows the commission accepted its staff’s recommendation to stop considering expedited transfer of the spent fuel to dry cask storage, weighing in on a debate over whether transfers should be made for safety concerns and not just logistics.

The NRC’s decision comes after years of increasing clamor to move spent fuel from thinly sheltered spent fuel pools into concrete and steel casks. Many plants have begun the transfers because they are running out of room in their spent fuel pools without the long-promised national waste dump.

The memo, issued Friday, says the NRC doesn’t want to deal with the issue again.

"The Commission has approved the staff’s recommendation that this ... activity be closed and that no further generic assessments be pursued related to possible regulatory actions to require the expedited transfer of spent fuel to dry cask storage," said the memo from Rochelle Bavol, acting secretary of the commission to Mark Satorius, the commission’s executive director for operations.

Bill Dean, the NRC’s Northeast Regional Administrator, said in an interview that the agency had concluded that both spent fuel pools and dry casks were "adequate storage processes for spent fuel, and there is not a significant safety benefit to requiring full transfer to dry cask storage."

Five U.S. senators, including Democrat Patrick Leahy and independent Bernie Sanders of Vermont, wrote to NRC Chairwoman Allison Macfarlane earlier this month to complain about relaxed emergency and security safeguards at closed nuclear plants. They focused mainly on the risks posed by spent fuel pools, swimming pool-like structures where highly radioactive nuclear fuel rods are moved after serving their time as reactor fuel.

"We are one natural disaster, mechanical failure or terrorist attack away from a disaster," Sanders said in a statement Tuesday. "The sooner we get the spent (fuel) out of the pools and into dry casks, the better, and if the NRC will not change the rules, I will continue to work with my colleagues to change the rules through legislation."

The letter, also signed by Democratic Sens. Barbara Boxer, of California, Edward Markey, of Massachusetts, and Kirsten Gillibrand, of New York, cited NRC and other studies on the risks.

They wrote that studies have concluded that "draining a spent nuclear fuel pool can lead to fires, large radioactive releases and widespread contamination" and that research has found spent fuel pools "could not be dismissed as potential targets for terrorist attacks."

Boxer, who chairs the Senate Environment and Public Works Committee, said in a statement Tuesday that she was "deeply troubled" by the NRC’s action.

"Earlier this month, a wildfire came within a half mile of the now-closed San Onofre nuclear plant, which is storing most of its spent fuel in pools rather than in dry cask storage," Boxer said.

David Lochbaum, the Chattanooga, Tennessee-based director of the nuclear safety project at the Union of Concerned Scientists, denounced the NRC’s work on the issue.

"We’re very disappointed by that decision, because it’s wrong," he said. He called it "very shoddy work. It was incomplete, inaccurate."