Which type of reaction occurs in the core of a nuclear reactor in a nuclear power plant?

Nuclear fission

Explanation:

In the core of nuclear reactors, fission of heavy nuclides occur.

A nuclear fission is a reaction which involves the disintegration of a heavy nucleus spontaneously into two lighter nuclei and several neutrons with the release of large amount of energy.

In this reaction, a chain reaction develops through which daughter nuclides are bombarded with the neutrons and they in turn continue to break down. If the chain reaction is controlled, the released energy is used to generate electricity. This is the crux of what occurs in the core of nuclear reactors.