Control rods: Redstone no longer affects the Reactor. You will need a control rod to turn off the reactor, but it only works if the control rod touches the reactor. (moar nerfs to da reactor!!!1!)
Old school nuclear ractor config: Pretty much just the old 1.2.5 or earlier reactor functionalities for those who really want it.
Control rods: Although you can not see them it can be assumed that the control rods already exist inside a reactor block. without control rods a reactor would not be able to be moderated, controlled and "Turned off" effectively. The main control factor for a reactor is literally how far the control rods are pulled out of the nuclear pile. In minecraft this can be seen as a redstone signal activating the reactor.
Old school nuclear reactor config: Absolutely not, the reactor was changed for a reason. While I am usually a big fan of "Config all the things!" such a large feature should not have a config in my opinion.