In a reactor nuclear energy is produced in the blank

In a reactor, nuclear energy is produced in nuclear fissions.

Explanation:

A nuclear reactor is a device used in power plants to control the reaction of nuclear fission. This reaction occurs in an uncontrolled way, for example, in the explosion of atomic bombs; but the reactors have mechanisms that prevent this, causing the reaction to be controlled and reused to generate electricity.

This is achieved because the reactor is mounted in a way that intersperses bars of fissile fuel - which is usually enriched uranium (uranium with a large amount of uranium 235) or plutonium 239 -; with neutron moderator bars.

Nuclear energy originates from the splitting of uranium atoms - a process called fission. This generates heat to produce steam, which is used by a turbine generator to generate electricity. Because nuclear power plants do not burn fuel, they do not produce greenhouse gas emissions.