Question

which process occurs in a fission nuclear reactor?
nuclei combine.
nuclei split apart.
neutrons combine.
neutrons split apart.

Explanation:

In nuclear fission, heavy - nuclei are split into lighter nuclei. This is the fundamental process in a fission nuclear reactor. The other options are incorrect as combining nuclei is fusion, and neutrons do not combine or split apart in the context of fission reactions in the way described.

Answer:

Nuclei split apart.