Improve the grammaticality of this sentence: Current reactors in used around the world are considered 2nd or 3rd generation systems, with the 1st generation systems having been retired long time ago.
The 1st generation of nuclear reactors is the 2nd generation reactor.
The 2nd generation of nuclear reactors is the 3rd generation reactor.
The 3rd generation of nuclear reactors is the 4th generation reactor.
The 4th generation of nuclear reactors is the 5th generation reactor.
The 5th generation of nuclear reactors is the 6th generation reactor.
The 6th generation of nuclear reactors is the 7th generation reactor
The 2nd generation of nuclear reactors is the 1st generation of nuclear reactors.
The 1st generation of nuclear reactors is the 1st generation of nuclear reactors.
The 2nd generation of nuclear reactors is the 2nd generation of nuclear reactors.
The 3rd generation of nuclear reactors is the 3rd generation of nuclear reactors.
The 3rd generation of nuclear reactors is the 3rd generation of nuclear reactors.
The 4th generation