Definition check: what is a “fertile” nuclear material?

Introduction / Context:
Reactor physics distinguishes between fissile and fertile isotopes. Fissile isotopes sustain a chain reaction upon absorbing a thermal neutron, while fertile isotopes are not immediately fissile but can be transmuted into fissile nuclides via neutron capture and subsequent decay. Properly identifying fertile materials is foundational to understanding breeding and fuel cycles.

Given Data / Assumptions:

• Typical fertile isotopes: U-238 and Th-232.
• Typical fissile isotopes: U-235, U-233, Pu-239, Pu-241.
• Definition-based question, not application-specific.

Concept / Approach:
When a fertile material absorbs a neutron, it becomes a heavier isotope that beta decays into a fissile nuclide. Examples: U-238 → (n,γ) → U-239 → β → Np-239 → β → Pu-239; Th-232 → (n,γ) → Th-233 → β → Pa-233 → β → U-233. These transmutation chains underpin breeder reactor concepts and closed fuel cycles.

Step-by-Step Solution:
Recall the definition of fertile: convertible to fissile after neutron absorption.Recognize that direct fission by thermal neutrons is a property of fissile, not fertile, materials.Select the option matching the conversion definition.

Verification / Alternative check:
Charts of the nuclides and standard reactor textbooks classify U-238 and Th-232 as fertile, with the above breeding paths to Pu-239 and U-233 respectively.

Why Other Options Are Wrong:
“Fissioned by slow neutrons” describes fissile behavior.“Fissioned by fast neutrons” may occur for some isotopes but does not define “fertile.”“Fissioned by either slow or fast neutrons” likewise describes fissile or fissionable behavior, not fertility.

Common Pitfalls:
Confusing “fissionable” (capable of fission under some conditions) with “fissile” (efficiently fissions with thermal neutrons); forgetting that fertile materials require transmutation to become fissile.

Final Answer:
Converted into fissile material on absorption of a neutron.