How does Pu-239 production occur in a reactor, and what is its effect on reactivity?

• A. Pu-239 is produced from Pu-240 decay
• B. Neutron capture by U-238 forms Pu-239, affecting reactivity
• C. Pu-239 only reduces reactivity
• D. Pu-239 production has no effect on reactivity

Answer: (B)

Pu-239 production in a reactor happens when a neutron is captured by U-238, turning it into U-239. That new nucleus beta decays first to Np-239 and then to Pu-239. Pu-239 is fissile with thermal neutrons, so it can fission and release energy and more neutrons, helping sustain the chain reaction. This breeding process adds fissile material to the fuel, which tends to increase reactivity or help maintain it as the original fuel is burned. The buildup occurs over time because the intermediate isotopes have their own decay half-lives, so Pu-239 accumulates and then contributes to the neutron economy. While some neutrons are consumed in forming Pu-239 and later Pu-240 and other isotopes, the net effect of forming Pu-239 is typically an increase in reactivity due to the added fissile material.