What the first nucleus hit with to initiate the fission reaction? is?
In a controlled nuclear fission reaction, the number of neutrons released is controlled to maintain the desired level of reaction.
Strictly speaking, “fission” is not necessarily initiated by a particle impact. It is the result of an unstable atom spontaneously splitting with the release of energetic neutrons (and other particles).
These are used in a controlled reactor to enhance the probability of other, more stable atoms becoming unstable due to the adsorption of the released energetic neutrons.