Electric generation capacity losses due to nuclear plant outages were relatively low during much of the 2018 summer, averaging 2.8 gigawatts (GW) from June through August.

The Energy Department's Energy Information Administration (EIA) said that this year’s seasonal maintenance and refueling cycle began earlier than in recent years. Total nuclear outages averaged 14.5 GW in the last week of September. The earlier-than-expected retirement of the Oyster Creek Generating Station and a temporary plant shutdown related to Hurricane Florence lifted outages in September.

Source: Energy Information AdministrationNuclear capacity outages are typically lowest during the summer and winter months when electricity consumption is high to meet cooling and heating demand. EIA said that nuclear power plant outages can be either scheduled or unscheduled, and they can range from a partial outage, where only some of a plant’s capacity is offline, to a full outage, where the entire plant is shut down.

EIA says that a scheduled shutdown is generally timed to coincide with the plant’s refueling cycle. Nuclear power plants typically refuel every 18 to 24 months, often during the fall and spring.

Forced Shutdowns

An unscheduled, or forced, shutdown can result from equipment failure, operational error or environmental conditions. Most unplanned outages occur because of non-reactor core issues — including external plant conditions, such as severe weather, or non-nuclear internal plant conditions, such as those involving steam turbine and electricity generation sub-systems.

For example, on October 12, St. Lucie Unit 2 experienced a reactor trip due to a bus fault during a transfer of the bus power supply from an auxiliary transformer to a startup transformer. The bus fault caused a fire that took the plant's fire brigade 30 minutes to put out.

Following the reactor trip, both steam generators were supplied by main feedwater and all control rods were inserted into the core. The operator, Florida Power & Light, told the Nuclear Regulatory Commission (NRC) that the unit was on hot standby while an investigation into the mishap was under way.

Source: EIAAnd, in late September, the 1,300 MW Unit 3 at Exelon Nuclear's Peach Bottom nuclear station in Pennsylvania experienced a hot shutdown following the loss of two condensate pumps. All control rods were inserted into the reactor and decay heat was being removed via the main condenser.

In a filing with the NRC, the nuclear operator said that following the reactor scram, water level fell from a normal level of 23 in to below 1 in. Reactor water level fell to -48 in, which resulted in the start of the high-pressure coolant injection and reactor core isolation cooling systems.

During the past summer, three unscheduled outages in Tennessee, Arkansas and Arizona affected available summer capacity in 2018, EIA said. Tennessee’s Watts Bar Unit-2 shut down for six days after a main turbine and generator tripped on June 22. Arkansas Nuclear One Unit-1 shut down from June 23 to June 26 to repair a leakage in the reactor coolant system. And Palo Verde Nuclear Generating Station Unit-3 in Arizona shut down for three days beginning June 27 because of low steam generator water levels.

Storms and Retirement

Hurricane Florence caused a nine-day unscheduled outage at the Brunswick Nuclear Plant near Wilmington, North Carolina, in mid-September. EIA said that the 1,870 MW facility is located about four miles from the Atlantic coast and was shut down by plant operators before hurricane force winds and flood waters reached the site.

Source: EIAAlso in mid-September, the 625 MW Oyster Creek Nuclear Generating Plant in New Jersey permanently shut down. The plant was previously expected to retire in late 2019, but the timeline was accelerated by more than a year to coincide with the end of the plant’s final fuel and maintenance cycle. The retirement is considered an unscheduled shutdown in EIA data until the NRC amends its outage data to reflect the closure.

EIA said that total unscheduled outages as a result of these plant shutdowns decreased from 85 in 2015, to 78 in 2016 to 55 in 2017, and to 37 so far in 2018. Outages can fluctuate year to year based on plant technical issues and external circumstances.

Shorter Refueling Cycles

During the past six years, average refueling outages have become shorter, falling from an average of 46 days in 2012 to 34 days in 2018. The decrease is a result of greater operator experience in regulated markets and competition from other generators in unregulated or wholesale electricity markets, EIA says.

In 2017, six U.S. reactors had refueling outage times of less than 20 days: Peach Bottom Unit-3 (15 days), Vogtle Unit-1 (16 days), Nine Mile Point (17 days), Quad Cities (18 days), Dresden (18 days) and Three Mile Island Unit-1 (19 days).