Prompt Neutron Spectrum

• Nutrons released from fission have an average Energy = 1.98 MeV.
• A large fraction of neutron is above the threshold energy of 0.6 MeV (refer table in previous page) needed to fission U²³⁸.
• Thus some U²³⁸ is also directly fissioned in reactors.
• It has been noticed that as the energy of the colliding neutron is high, the probability of reaction is poor.
• This is attributed to the reduced time a neutron spends in the zone of interaction.
• Hence the probability of reaction is inversely proportional to the speed of the neutron.
• As the interaction decreases rapidly with increase in energy, and that the energy of the prompt neutrons are high, we need to slow them down.
• This is done by the use of moderator.

Moderators and Coolants

• Hydrogen in the form of water (H₂O) and heavy hydrogen in the form of heavy water  (D₂O) are the most common moderators.
• In gas cooled reactors graphite (C) is used as the moderator.
• Coolant is employed to transport the heat generated in the core to the steam generator, where steam is generated.
• In water moderated reactors the same fluid also serves as coolant.
• There are some safety advantages if we use gas as a coolant. In such cases CO₂ or He is used.