The Pilgrim nuclear power plant in Plymouth has permanently shut down after 47 years of generating electricity.
The final shutdown occurred at 5:28 p.m. Friday.
Entergy, which owns the plan, announced in 2015 it would retire Massachusetts' only remaining nuclear reactor, citing competition from less expensive energy sources.
Entergy had said the shutdown would take about five hours, with technicians reducing Pilgrim's power output from the current 40% to 26% to about 8% before launching the final step of inserting control rods into the fuel assembly to seal off further nuclear reactions.
The focus now turns to cleaning up and dismantling the plant. The Nuclear Regulatory Commission is reviewing Entergy's proposal to sell Pilgrim to a private nuclear waste management firm that promises a speedier decommissioning.
What Does the Shutdown Mean for Regional Energy Supplies?
Pilgrim's retirement and the loss of its 680 megawatts of power isn't likely to disrupt overall regional energy supplies or the reliability of the electrical grid, according to experts.
ISO-New England, which operates the region's bulk power system, says three new power plants that can burn either natural gas or oil will be online by this summer and several small solar facilities and a new wind farm will also help absorb the impact.
"We've actually seen more megawatts coming in than the megawatts of Pilgrim that are leaving,'' said Anne George, a vice president for ISO.
The shutdown could, however, cause carbon emissions from the gas and oil-burning plants to increase short-term, she added, even while longer range trends point to growth in cleaner, renewable energy sources.
It's unclear if Pilgrim's closing could affect consumer prices. George noted that nuclear power generally enters the electricity market at lower prices than most other fuels.
What Happens Next?
It's far from certain how decommissioning will unfold.
Entergy announced plans last year to sell Pilgrim, along with several other retiring U.S. reactors, to Holtec International for decommissioning. The deals are awaiting NRC approval.
Holtec is promising an aggressive schedule for cleaning up and removing the physical plant within eight years at an estimated cost of $1.1 billion. Absent the sale, Entergy would place Pilgrim into so-called ``safe storage'' mode under a $1.6 billion decommissioning plan that could last up to 60 years.
Massachusetts Attorney General Maura Healey has raised several concerns about the proposed sale and has requested NRC hold public hearings before signing off.
Holtec has never owned a commercial nuclear plant.
Where Does Radioactive Fuel Go?
Pilgrim's more than 4,000 spent fuel rod assemblies will initially be placed in "wet storage'" - essentially large pools of water where the superhot fuel is cooled for at least two years.
Next comes the "dry storage" phase: The spent fuel is transferred into giant metal and concrete-reinforced cylinders that stay at the site unless or until a national nuclear waste storage facility is created.
Existing storage casks are kept on a pad adjacent to the reactor, roughly 300 feet (91 meters) from ocean's edge and 25 feet (7.6 meters) above sea level. Plans are in the works to build a larger pad and relocate it about 75 feet above sea level, in part as protection from flooding or erosion induced by climate change.
Who Pays for Decommissioning?
U.S. reactors are required to maintain decommissioning trust funds to cover the facilities' eventual decontamination and dismantling.
Public records show Pilgrim's fund was most recently valued at just over $1 billion.
Unused portions of the fund can continue to grow during decommissioning, but it's unclear what might happen should the money run out before the job is complete. Holtec has offered assurances it has financial resources to cover unexpected delays or cost overruns.
How is Plymouth Impacted?
Decommissioning requires fewer employees and Pilgrim's workforce is expected to fall from the current 580 to about 270 by March 2020.
In addition to losing jobs, Plymouth will also receive less revenue from the plant, even as the historic town prepares for celebrations marking the 400th anniversary of the Mayflower landing in 1620.
Town Meeting voters recently approved an agreement calling for payments in lieu of taxes of $7 million next fiscal year and $6.5 million the following year. Plymouth is getting $8.5 million from Pilgrim in this fiscal year.
Associated Press writer Steve LeBlanc contributed to this story.