The number one reactor at the site was shut down in May for maintenance and tests.
Faults were uncovered in the reactor’s pressure vessel in July.
Axpo said it must “prove that the integrity of the reactor pressure vessel has not been compromised despite the irregularities that were found”.
Based on the current results, the energy company said it remains convinced that the number one reactor will restart and go back on power grid after completion of the testing phase.
However, it acknowledged that the analyses and required certificates will take a great deal of time so that a restart approval is not expected before the end of July 2016.
The number two reactor, meanwhile, has been shut down since August for a change of parts.
Axpo said it expects that it should be able to restart at the end of the month, depending on the findings from supervisory authorities.
The Beznau plant is located in the municipality of Döttingen on an artificial island in the Aar River.
It has produced electricity since 1969 but Axpo has faced demands from environmentalists to close down the reactors on safety grounds.
In March 2014, 100 Greenpeace activists broke into the Beznau site and erected banners calling for the end of the plant.
The federal government decided to stop building new nuclear power plants and to phase out existing ones in the wake of the 2011 Fukushima nuclear disaster in Japan.