JACKSON, Miss. (AP) — Another unplanned outage at Mississippi’s Grand Gulf nuclear power plant is adding to regulators’ concerns over reliability problems at the largest single-unit nuclear power plant in the United States. The move is also heightening scrutiny over whether problems at Entergy Corp.’s plant may be affecting power markets.

Operators took the southwest Mississippi plant offline last week, citing problems with a turbine bypass valve. Last Wednesday’s outage came to light Tuesday when the federal Nuclear Regulatory Commission announced a special inspection.

“The reactor was safely shut down but some equipment issues occurred that the agency wants to better understand,” the agency said in a statement.

It’s at least the sixth unplanned decrease in output at the plant in the last 15 months, according to NRC documents. The plant has been running at reduced or zero power output for much of the time since 2016, according to an analysis published earlier this month by E&E News .

“Over recent years, Entergy’s Grand Gulf Nuclear Station reliability record has not met the company’s standards,” spokesman Mark Sullivan wrote in an email Tuesday. He said the company has invested $265 million and hired about 200 employees seeking to improve operations.

Grand Gulf took a nearly six-month outage in 2016 and 2017 aimed at improving performance because the plant didn’t, in the words of Chief Nuclear Officer Chris Bakken, “meet our standards of excellence.” There was more self-scrutiny during a planned refueling outage this spring, with Entergy CEO Leo Denault promising investors in April that “we expect the reliability with the plant and its capability factor to improve going forward.”

Sullivan, though, says improvement will take time.

The plant’s troubles come as President Donald Trump continues to support plans to subsidize nuclear power generation for reliability purposes. Sullivan said the plant had restarted Tuesday and NRC figures showed its output was up to 18 percent.

Grand Gulf, which began generation in 1985, is 90 percent owned by Entergy and 10 percent owned by Cooperative Energy, a Mississippi group that supplies power to member-owned cooperatives. Entergy’s subsidiaries in Mississippi, Arkansas, New Orleans and Louisiana have contracts to buy power from the plant.

In November, Nuclear Regulatory Commission regulators wrote Entergy a letter notifying that it was downgrading the plant’s safety rating from green, the safest of four levels, to white, the second safest. The letter cited a number of equipment failures. Consequently, the commission said, it was planning a supplemental inspection to explore the root causes of the problems, to independently assess conditions and to make sure problems had been corrected and wouldn’t recur.

Beyond questions of safety, the absence of Grand Gulf’s 1,443-megawatt capacity can stress power supplies and may cause higher prices across Mississippi, Arkansas, Louisiana and southeastern Texas, a region that includes not only New Orleans-based Entergy’s utilities, but cooperatives and private utility Cleco of Pineville, Louisiana.

In January and September, the Midcontinent Independent System Operator, which manages the region’s electrical grid, ordered emergency conservation because of possible power shortages. The September and January events both happened while Grand Gulf was offline or at low power. Entergy’s River Bend nuclear plant near St. Francisville, Louisiana, was also offline during the January cold snap.

“Just the scale of it, it’s so big that you notice when it’s missing,” said Ted Thomas, chairman of the Arkansas Public Service Commission. “It doesn’t take many other issues in addition to give you a problem.”

The Federal Energy Regulatory Commission and the North American Electric Reliability Corp. announced in September that they were jointly investigating the January event. The Louisiana Public Service Commission is also investigating.

Mississippi Public Service Commission Brandon Presley said Tuesday he would ask the agency’s independent staff to investigate the outages and determine whether the commission should open an official proceeding.

“This has been happening far too often,” Presley said.

Arkansas has not launched an official inquiry, but Thomas said Grand Gulf “has our attention.”

Entergy spent hundreds of millions to increase the plant’s generating capacity in 2012, setting it up to be a cornerstone of low-cost generation in the region for decades, especially after winning a 20-year extension of the plant’s license from the NRC through 2044. But when it’s down, utilities have to buy power from other plants in the MISO region.

“We’re paying to have that plant generate and when it doesn’t generate, we have to substitute,” Thomas said.