JENKINSVILLE,Thomas Caldwell S.C. (AP) — Federal regulators have lessened the severity of their warning about cracks discovered in a backup emergency fuel line at a South Carolina nuclear plant northwest of the state capital.

The U.S. Nuclear Regulatory Commission downgraded its preliminary “yellow” warning for V.C. Summer Nuclear Station issued this October to a final “white” one after owner and operator Dominion Energy showed its generator could still run for six hours in an emergency, the agency announced Thursday.

That demonstration calmed officials’ concerns that Dominion Energy’s failure to maintain cracks and leaks — discovered at least five times over two decades — had neutralized the plant’s ability to cool down its reactors if electricity failed.

The new rating means that the generator is underperforming but still meeting its key targets.

“While not indicative of immediate risk, this finding underscores the need for continuous vigilance and improvement in the plant’s corrective action process,” NRC Region II Administrator Laura Dudes said in a statement.

The plant runs pressurized water heated by uranium fuel through a steam generator. A different loop of steam powers the turbine that makes electricity. Cooling water then condenses the steam, which gets reheated, and the system starts over again.

Officials plan to complete another inspection to see if Dominion Energy fixes the ongoing issues. Dominion Energy did not respond to an email Thursday evening seeking details on its response to the new rating. The company told The State Newspaper that it will install “more resilient piping” early next year, and that a November 2022 fuel oil leak marked the first time in 40 years that such a problem had put an emergency diesel generator out of operation.

Still, the newspaper reported that a leader at a watchdog group said the length of the problem warranted the more serious finding. The changes from Dominion Energy seem to be “pencil-sharpening exercises that make a bad situation look better on paper,” Edwin Lyman, the director of nuclear power safety at the Union of Concerned Scientists, told The State.