What is a defining feature of a fast reactor compared to a thermal reactor?

What is a defining feature of a fast reactor compared to a thermal reactor?

• A. It uses fast neutrons and operates with little or no moderator.
• B. It relies on heavy water moderation to slow neutrons.
• C. It uses a solid fuel form exclusively.
• D. It cannot use liquid metal coolants.

Answer: (A)

Fast reactors are defined by their neutron spectrum: they operate with fast neutrons and use little or no neutron moderator. In thermal reactors, neutrons are slowed by a moderator (like light or heavy water) to thermal energies, which increases the fission probability for many fuels. By keeping neutrons fast, a fast reactor changes the neutron economy and fuel behavior, enabling different breeding possibilities and often using liquid metal coolants to handle heat and maintain the fast spectrum. The other statements aren’t defining features: heavy water moderation would slow neutrons; solid fuel form is not exclusive to fast reactors; and many fast reactors actually use liquid metal coolants, not the opposite.

Fast reactors are defined by their neutron spectrum: they operate with fast neutrons and use little or no neutron moderator. In thermal reactors, neutrons are slowed by a moderator (like light or heavy water) to thermal energies, which increases the fission probability for many fuels. By keeping neutrons fast, a fast reactor changes the neutron economy and fuel behavior, enabling different breeding possibilities and often using liquid metal coolants to handle heat and maintain the fast spectrum. The other statements aren’t defining features: heavy water moderation would slow neutrons; solid fuel form is not exclusive to fast reactors; and many fast reactors actually use liquid metal coolants, not the opposite.