Nuclear power plant capacity was offline or otherwise not generatingelectricity 50% more during the June 1 to Aug. 31 period this year than a year ago,according to an analysis by SNL Energy.

Cumulative offline nuclear capacity, determined from reportednuclear unit daily power levels and those unit's generating capacity, increasedfrom 266 GW in 2015 to 398 GW in 2016 for the period. This is the largest amountof nuclear capacity offline during the summer since 2013. Nuclear outages duringthe summertime months are typically avoided due to the increased need for theirsteady baseload generation, with refueling periods usually scheduled for other timesof the year.

Through the summer of 2016, the average daily amount of nuclearcapacity offline was 4.3 GW, up from 3.0 GW in the summer of 2015. This high averagewas driven by a large number of both forced and maintenance outages in June, whichsaw an offline daily average of 6.3 GW for the month. The Salem PWR 1 unit, whichwas taken offline in April, came back online in July and, subsequently, nuclearunits posted an offline daily average of 4.4 GW for that month. August had the lowestoffline daily average of the summer, at 2.4 GW. The large amount of outages in Junealso coincided with a decrease in nuclear generation year over year to 19,133 TWhin June 2016 from 19,560 TWh in June 2015, a difference of 427 TWh.

Ten units accounted for 76% of the 398 GW in offline capacityduring the 2016 summer; these 10 units operated on an average daily power levelof 71.24%. Six of the 10 units underwent a forced outage, and four underwent a maintenanceoutage. Forced outages are critical errors with the operation of a nuclear powerplant and are often unplanned and unpredictable, while maintenance outages are usuallyplanned and often coupled with time to refuel the plant.

Unit 1 at the Salemnuclear plant had the largest amount of offline capacity during the summer, postingan average daily power level of 33.09%. Salem's unit 1 outage started in April andlasted for 106 days, including both a refueling period and a maintenance periodrequired to replace reactor core baffle bolts. The single unit at the nuclear plantwent offline in March and had to make similar repairs, then subsequently refueledin June, with time offline totaling 100 days. Its average daily power level thissummer was 72.66%.

Unit 2 at the Salem nuclear plant had the second-lowest averagedaily power level, at 62.60%. This was caused by the need for condensate pump repairs,which then caused the unit's daily power level to be at or below 60% for 35 daysin July and August.

Unit 1 at the GrandGulf nuclear plant experienced forced outages causing it to be at apower level of zero for 25 days, and below 100% for 36 days. This led to it havingthe third-lowest average power level over the summer period, at 66.46%.

These three units — Salem unit 1 and unit 2 and Grand Gulf unit1 — accounted for 39% of the offline capacity during the summer.

About the data

For the purpose of thisanalysis, a one-to-one relationship was assumed between the daily power level (%)reported and the actual generating unit's daily operating level (%). This analysisonly includes currently operating nuclear units. This data is received directlyfrom the Nuclear Regulatory Commission.