Pilgrim Nuclear Power Station remained shut down Tuesday after power lines that connect the plant to the region’s electric grid failed during last week’s winter storm.

Officials at Entergy Corp., a Louisiana-based company that owns the 46-year-old nuclear plant, declined to say when Pilgrim would resume producing power.

“Pilgrim’s return to 100 percent is considered business-sensitive information, and that information is not made public,’’ said Patrick O’Brien, a plant spokesman.

O’Brien said it was not unusual for the plant to remain closed for a prolonged period after a sudden shutdown. During each of the past two winters, Pilgrim went offline for five days after storms, he said.

“The time offline really relates to the amount of preventative maintenance you undertake during the outage,’’ O’Brien said.

Officials at the US Nuclear Regulatory Commission said they weren’t concerned about the shutdown.

“We would not consider this outage to be out of the ordinary,’’ said Neil Sheehan, a spokesman for the commission. “Our view is that we expect safety checks and any repair work to be successfully completed before the reactor is restarted.’’

He said nuclear plants typically take advantage of unplanned shutdowns to perform required maintenance.

Officials at Pilgrim have been repairing a safety relief pilot valve that had been leaking, Sheehan said.

Before restarting the plant, operators need to complete a safety checklist and tasks such as topping off the fuel tanks of emergency diesel generators, he said.

Commission inspectors are monitoring the maintenance, Sheehan said.

“They do not have any immediate safety concerns,’’ he said.

Control room operators at Pilgrim shut the plant down manually during the storm last Thursday after an electrical line that provides power to the plant failed, apparently because of a glitch at another plant in Sandwich, Sheehan said.

Officials at ISO New England, a company in Holyoke that operates the region’s grid, said the lack of power from Pilgrim hasn’t interrupted the supply of electricity throughout New England. Pilgrim typically provides enough power to supply more than a half-million homes and businesses.

“The regional power system is currently operating reliably,’’ said Marcia Blomberg, an ISO New England spokeswoman.

Activists who oppose the plant, however, said the shutdown should raise red flags.

“You would think at a time for high energy need. . . that they would get right back into the game,’’ said Diane Turco, director of Cape Downwinders. “It is also disconcerting that it takes [an unplanned shutdown] to initiate preventative maintenance work.’’

David Abel can be reached at dabel@globe.com. Follow him on Twitter @davabel.