COLUMBIA, S.C. — One of South Carolina’s nuclear power reactors remained shut down Thursday for what utility executives call a minor, non-nuclear issue.

The 33-year-old reactor at V.C. Summer Nuclear Station, about 30 miles (50 kilometers) north of Columbia, is safe and stable while teams assess repairs. There was a minor issue with the main transformer, and the plant’s safety systems responded exactly as designed said Rhonda O’Banion, spokeswoman for SCANA, parent company of South Carolina Electric & Gas.

The reactor automatically shut down Monday morning, according to the Nuclear Regulatory Commission.

There is no estimation for when it will be back online, said SCANA spokesman Patrick Flynn.

“Our focus is on completing work activities safely,” he said. “Our employees are working diligently to address the issue.”

South Carolina’s seven existing reactors supply nearly 60 percent of the state’s electricity. The state ranks third nationwide in nuclear energy production.

When operational, the V.C. Summer unit is the utility’s single biggest energy producer. But Flynn said the utility doesn’t expect to have a problem meeting customers’ electricity needs while it’s down, as the other plants will fill the gap.

The utility also owns coal, natural gas and hydroelectric plants.

SCE&G, along with state-owned Santee Cooper, decided last month to abandon construction on two additional nuclear reactors at V.C. Summer after jointly spending nearly $10 billion. The utilities’ customers have already paid more than $2 billion on the now-scuttled project through a series of rate hikes since 2009, which covered interest costs on financing.

The companies don’t expect to refund anything. Customers could end up paying off that debt over decades, as well as replacement generators.

SCE&G says it will ask state regulators at some point for permission to build another natural gas plant to meet customers’ demands.

The project’s abrupt end left nearly 6,000 people jobless and brought a backlash from legislators, who began holding hearings last week about the debacle. Legislators question why the utilities didn’t change course sooner.

Documents requested by legislators, but not yet provided by the utilities, include a copy of a 2015 report by Bechtel, which conducted an independent analysis on the project’s status. Utility executives testified they never had a complete schedule from their lead contractor Westinghouse, which declared bankruptcy in March, and concerns led to the hiring of Bechtel.

SCANA executives have said there are confidentiality issues with handing over the report.

A Senate panel has set a Sept. 7 deadline for getting the requested documents before issuing subpoenas.