Fissile vs fertile: which of the following is a non-fissile (fertile) material under thermal-neutron conditions?

Introduction / Context:
Nuclear fuel classification distinguishes fissile isotopes (which readily fission with thermal neutrons) from fertile isotopes (which do not, but can be converted into fissile species by neutron capture and subsequent decay). Recognizing which is which is fundamental for reactor fuel-cycle questions and breeder concepts.

Given Data / Assumptions:

• Thermal-neutron regime is implied unless otherwise noted.
• Common fissile isotopes include U-235, U-233, Pu-239, Pu-241.
• Common fertile isotopes include Th-232 and U-238.

Concept / Approach:
Thorium-232 is fertile: it captures a neutron to become Th-233 → Pa-233 → U-233 (fissile). In contrast, U-235, U-233, Pu-239, and Pu-241 are fissile and support thermal-neutron chain reactions. Therefore, within the given list, Th-232 is the non-fissile (fertile) choice.

Step-by-Step Solution:
List fissile isotopes: U-235, U-233, Pu-239, Pu-241.Identify Th-232 as fertile, not fissile, under thermal conditions.Select Thorium-232 accordingly.Note that fertile isotopes are essential in breeding strategies to produce new fissile fuel.

Verification / Alternative check:
Fuel-cycle diagrams universally show Th-232 breeding to U-233 and U-238 breeding to Pu-239 via neutron capture and beta decays; both are classic fertile pathways.

Why Other Options Are Wrong:
Pu-239 / Pu-241: well-known fissile isotopes.U-235: primary fissile constituent of natural uranium.U-233: fissile product bred from Th-232.

Common Pitfalls:
Confusing “fissile” with “fertile” due to similar wording; remember fertile needs conversion, while fissile sustains thermal-neutron fission directly.

Final Answer:
Thorium-232