What are the primary sources of neutron generation in a nuclear reactor?

In a nuclear reactor, the primary sources of neutron generation include fission reactions and neutron sources used for calibration.

Fission reactions are fundamental to the operation of a reactor because they involve the splitting of heavy atomic nuclei, typically uranium-235 or plutonium-239. This process releases a significant number of neutrons, which in turn can initiate further fission events in a self-sustaining chain reaction, providing the essential fuel for the reactor's power generation.

Additionally, neutron sources for calibration are utilized in reactors to ensure the monitoring and control systems are functioning accurately. These sources can be used to establish a known neutron flux, which helps in the calibration of measuring instruments and in neutron flux mapping within the reactor core.

The other options do not serve as primary sources of neutron generation. Fission reactions are crucial, but chemical reactions within the coolant and thermal energy from decay heat do not generate neutrons in the manner required for sustaining a nuclear chain reaction. Chemical reactions in the coolant typically involve heat transfer and do not produce neutrons, while thermal energy from decay heat refers to the residual heat generated after fission reactions have ceased, which is important for cooling but not for neutron generation during normal operations.