Which one oT the following is not needed in a nuclear fission reactor ?

A conventional nuclear fission reactor requires three primary components to maintain and regulate a self-sustained chain reaction: a moderator, a coolant, and a control device. The moderator (such as water or graphite) slows down fast neutrons to sustain the reaction [t2]. The coolant (such as water or liquid metal) removes the heat generated by fission to produce steam for electricity [t2]. Control devices, typically control rods, absorb excess neutrons to regulate the reaction rate [t2][t3]. An accelerator is not a standard component of a conventional fission reactor. While accelerators are used in specialized 'Accelerator-Driven Systems' (ADS) to provide external neutrons for subcritical reactors or waste transmutation, they are not needed for the operation of standard critical fission reactors used in commercial power plants [t1][t7][t9].

Sources

1. [1] https://en.wikipedia.org/wiki/Subcritical_reactor
2. [2] https://www.energy.gov/ne/articles/nuclear-101-how-does-nuclear-reactor-work