Which reactor type produces more Pu-239 than it consumes and uses no moderator?

Which reactor type produces more Pu-239 than it consumes and uses no moderator?

• A. Fast breeder reactor
• B. Pressurized water reactor
• C. Boiling water reactor
• D. Heavy water reactor

Answer: (A)

Breeding Pu-239 requires a fast neutron spectrum and fertile material around the core. A fast breeder reactor operates with little to no neutron moderation, so neutrons stay fast. Its design typically includes a blanket of U-238 around the core; fast neutrons convert U-238 to Pu-239 in that blanket, while Pu-239 within the core is consumed by fission. Because the neutron economy is arranged so that more Pu-239 is produced from U-238 than is burned, the reactor achieves a breeding ratio greater than one. That combination—no moderator and net Pu-239 production—defines this reactor type. In moderated reactors like those using light water or heavy water, neutrons are slowed, which changes how efficiently Pu-239 is produced versus consumed, so they don’t typically achieve net Pu-239 production.

Breeding Pu-239 requires a fast neutron spectrum and fertile material around the core. A fast breeder reactor operates with little to no neutron moderation, so neutrons stay fast. Its design typically includes a blanket of U-238 around the core; fast neutrons convert U-238 to Pu-239 in that blanket, while Pu-239 within the core is consumed by fission. Because the neutron economy is arranged so that more Pu-239 is produced from U-238 than is burned, the reactor achieves a breeding ratio greater than one. That combination—no moderator and net Pu-239 production—defines this reactor type.

In moderated reactors like those using light water or heavy water, neutrons are slowed, which changes how efficiently Pu-239 is produced versus consumed, so they don’t typically achieve net Pu-239 production.