(Adds New England operating reserves)

Jan 4 (Reuters) - Entergy Corp said on Thursday it shut its 688-megawatt Pilgrim nuclear power plant in eastern Massachusetts because of the failure of one of the two lines that connect the reactor to the New England electric grid.

The outage occurred during a winter storm that tested the power grid’s ability to keep the lights on as homes and businesses used most of the region’s natural gas supplies for heating, leaving less of the fuel available for power generation.

“There are no immediate reliability issues to the local area” after the shutdown of Pilgrim, said Marcia Blomberg, a spokeswoman at ISO New England, which operates the region’s power grid.

The lights did not go out in New England after Pilgrim shut because New England, like all U.S. power grids, keeps some generating plants in reserve in case an operating unit shuts unexpectedly. On Thursday, the grid had a reserve of over 2,100 MW. One megawatt is enough to power about 1,000 U.S. homes.

Blomberg said, however, that the loss of Pilgrim “does further challenge the region on fuel availability because we need to rely on other generating resources to meet consumer demand and meet overall grid reliability.”

The region, which usually gets about half of its power from gas-fired generators, was already receiving more electricity from oil-fired units instead of gas.

Blomberg said Pilgrim shut Thursday afternoon “due to storm conditions.”

Entergy spokesman Patrick O’Brien said the company was “working to determine the cause of the line loss, noting the plant had been operating safely for 227 days following the completion of its most recent refueling outage in May 2017.

“We will take this opportunity to conduct preventive maintenance that we could not otherwise perform with the plant operating at full power,” O’Brien said. (Reporting by Scott DiSavino and Jarrett Renshaw in New York; Editing by Tom Brown and Grant McCool)