US nuclear power plants in 2014 had an average capacity factor of 91.8 per cent, the highest level ever recorded, after a series of extended repair outages ended and plants shortened the time reactors shut for refueling, the Nuclear Energy Institute said Thursday. US nuclear plants benefited from fewer and shorter refueling and maintenance outages and did a better job of avoiding unplanned outages, NEI data showed. The performance was boosted by a particularly strong output in December, when the capacity factor of the US reactor fleet was 98.9 per cent, the data showed. The capacity factor in 2014 was higher than the 90.9 per cent figure in 2013 and well above the 2012 capacity factor of 87 per cent, which was the lowest in a decade. The capacity factor is based on the 100 nuclear units that operated during 2014.