Pilgrim Nuclear Power Station was shut down for the second time in roughly three weeks following a mechanical issue at the Plymouth plant.
A U.S. Nuclear Regulatory Commission spokesman says the facility experienced an "unplanned shutdown" around 8:35 a.m. Tuesday after water levels unexpectedly spiked in the reactor pressure vessel.
Water is pumped into the reactor vessel so it can be boiled and converted into steam to generate electricity.
Officials with Entergy Corp., the company that owns and operates Pilgrim, say the safety of the public and the plant's employees was never at risk during the incident.
Operations at the facility were stopped for four days in mid-August for repairs following the failure of a main steam isolation valve.
The 44-year-old plant is scheduled to close in May 2019.