BOSTON, MARCH 6, 2018 — A likely water leak inside the feedwater heating system prompted Pilgrim Nuclear Station in Plymouth to shut down overnight.

It’s the latest setback for a plant that has fueled up for the last time and is scheduled to close on May 31, 2019.

The duration of the shutdown is unknown.

According to Entergy Pilgrim spokesman Patrick O’Brien, the indication of the likely water leak spurred plant officials to take the station off the grid and it began powering down at 3 a.m.

“The heater with the suspected leak, known as the 5th Point, is located inside the condenser bay and due to this location, the unit needed to be shut down to allow for a prosper inspection and repair under the safest conditions possible,” O’Brien said in a statement.

O’Brien said plant employees and the public “remained safe at all times.”

The nuclear plant powered through last weekend’s nor’easter after employees at the coastal plant topped off fuel supplies, removed items that could become airborne, worked with Eversource and ISO New England to ensure the reliability of offsite power sources, and established storm coverage to ensure adequate plant staffing, according to O’Brien.

More than 21,000 electricity customers in Massachusetts remained without power Tuesday morning, including 41 percent of customers in Pembroke, 34 percent of Norwell, 34 percent of Rochester, 33 percent of Halifax, and 26 percent of Scituate.

On Friday night, during the peak of a severe nor’easter that toppled trees and featured widespread flooding, more than 450,000 customers were without power.

The second nor’easter in a week is forecast to move through the region on Wednesday.