The 60 Terawatt hours of low carbon electricity produced was almost 50% higher than the last year before the stations were acquired by EDF Group in 2009.
EDF said in a statement that performance has improved due to capital investment in power station, investment in training as well as the application of EDF Group's global expertise in operating nuclear power stations.
"The exceptional performance is a result of the £300 million annual investment in the power stations which is in addition to £350 million spent on plant operations every year," a statement said.
"Investment in skills and training has also helped maintenance teams plan and carry out work more effectively, cutting the number of unplanned outages."
The plants' safety record in 2012 also remained among the best yet, underlining the strong link between safety and operational efficiency.