The Three Mile Island nuclear facility in Pennsylvania suffered a partial meltdown in 1979, leading to the worst nuclear disaster in the United States.
Damaged fuel rods from the Three Mile Island plant are stored at a facility for the U.S. Department of Energy at the Idaho National Laboratory.
The U.S. Nuclear Regulatory Commission in a letter to the Energy Department called for an outline of the corrective measures needed to ensure the integrity of the concrete storage containers used to house the damaged fuel rods.
The NRC said large pieces of concrete cracked off the main structures likely due to annual freezing-and-thawing cycles, the agency added.
The NRC found cracks in 2000 that it said were considered cosmetic, though the Department of Energy determined the concrete had degraded to the point that safety was called into question.
NRC spokesman David McIntyre told the Platts news service the Idaho storage site was safe, though it was a concern that warranted monitoring.
The Department of Energy in a letter to Platts said the concrete damage has "no impact" on the ability to store spent nuclear fuel.
The concrete surrounds stainless steel containment vessels.
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