What is Pu-239 production and its effect on reactivity in a thermal reactor?

• A. Pu-239 is formed by alpha decay of Pu-238.
• B. Pu-239 production reduces reactivity due to poison.
• C. Pu-239 is produced by neutron capture by U-238; Pu-239 production can offset burnup depletion of U-235 and modify fuel composition.
• D. Pu-239 is produced by gamma activation.

Answer: (C)

Pu-239 is produced in a thermal reactor when U-238 captures a neutron to become U-239, which beta decays to Np-239 and then to Pu-239. Pu-239 is fissile with thermal neutrons, so it can fission and contribute to the reactor’s reactivity. As fuel is irradiated, U-235 is burned away while Pu-239 accumulates, providing an additional source of fissile material and changing the fuel composition toward plutonium isotopes. This breeding effect helps offset the depletion of U-235 and can sustain or even enhance reactivity during burnup. It’s not produced by alpha decay or gamma activation, which are not the mechanisms at work here.