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VERNON — The Nuclear Regulatory Commission announced Thursday it was bringing enforcement action against the manufacturer of the steel and concrete casks used at the Vermont Yankee nuclear plant to store its high-level radioactive fuel.

The problem with the large 18-foot tall casks, which surfaced earlier this year at the San Onofre nuclear power plant in California, revolved around a loose bolt, which regulators feared at one point could affect the ability of the casks to effectively cool the still-hot nuclear fuel.

Since the California plant shared a similar cask design with Vermont Yankee, it prompted a two-month halt earlier this year at Vermont Yankee, where Entergy Nuclear was having the plant's spent nuclear fuel transferred from the spent fuel pool to the air-cooled storage canisters. The transfer resumed in May and was completed this summer.

Neil Sheehan, a spokesman for the federal regulators, said the action against Holtec International dealt with a new design it had adopted for its casks before getting federal approval. The design has since been approved, Sheehan said.

During the two-month hiatus at Vermont Yankee, Entergy inspected the empty Holtec Hi-Storm 100 casks that Holtec had already provided for the fuel transfer and storage project, and found no problems with its casks. Entergy used a total of 58 casks to store its thousands of spent fuel assembly but it could not inspect the canisters already filled with fuel.

Sheehan said that Holtec had changed the design of the casks, specifically four-inch stainless steel pins that hold the basket, which in turn holds the spent fuel.

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Holtec determined it didn't need to conduct a written evaluation, which was a violation of NRC safety regulations, Sheehan said. In the notice of violation from the NRC, the agency referred to "nonconforming and degraded conditions at both SONGS (San Onofre Generating Station) and VY, respectively."

In addition to San Onofre and Vermont Yankee, the Holtec casks in question were also used to store spent fuel at nuclear plants in Dresden, Ill., Grand Gulf in Mississippi, Hatch in Georgia, Columbia in Washington, Watts Bar in Tennessee and Callaway in Missouri.

Entergy official Joseph Lynch refused to say Thursday how many of Holtec's original designs were used before they started using the new design, and he also declined to say whether the company was seeking to recoup its expenses from Holtec. Sheehan said that 31 of the 58 casks at the Vernon site were of the new Holtec design.

The transfer of the fuel was a key condition of the pending sale of Vermont Yankee by Entergy to NorthStar Holding Co. The sale, which has state and federal approval, is expected to be completed early next year.

Sheehan said the NRC had conducted an inspection at the company's offices in Camden, N.J., and as a result had proposed there were two apparent violations that were being considered for escalated enforcement. Holtec is challenging the proposed enforcement action, and asked for a conference with the NRC regulators, Sheehan said. That conference, which is open to the public, is being held Jan. 9.

Caitlin Marmion, a spokeswoman for Holtec International, didn't respond to an email on Thursday.