Question

what is thermal neutron ?

Answer 1

A thermal-neutron reactor is a nuclear reactorthat uses slow or thermal neutrons.
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Answer 2

Thermal neutrons are those which have energy about (1/40) eV or 0.025 eV. They have this name because they would be in thermal equilibrium with atoms at room temperature, 300 K.

Fast neutrons are those which have energies of a few MeV, such as are produced in a fission reaction.

Slow neutrons are the same as thermal neutrons.

The function of a moderator such as heavy water or graphite in a nuclear reactor is to slow down fast neutrons till they become slow or thermal neutrons. The cross-section for fission for U235 with thermal neutrons is many times larger than with fast neutrons.