Nuclear fuels — Which of the following isotopes is NOT a naturally occurring nuclear fuel found in nature (as mined), considering that naturally occurring fuels include uranium-235/238 and thorium-232?

Introduction:
This question tests basic nuclear fuel-cycle knowledge. Naturally occurring nuclear fuels are those present in mineable ores in appreciable amounts and usable directly as fissile or fertile materials, notably uranium-235, uranium-238, and thorium-232. Identifying which listed isotope is not naturally occurring helps learners avoid common isotope-number mix ups.

Given Data / Assumptions:

• Naturally occurring fuels include U-235/U-238 and Th-232.
• Isotopes produced only through neutron reactions in reactors (e.g., U-233, Pu-239) are not naturally abundant.
• The options mention thorium-233, plutonium-239, and uranium-238.

Concept / Approach:
We separate isotopes into: (1) naturally abundant (ore-grade) vs. (2) reactor-bred. Fertile isotopes (U-238, Th-232) are not fissile at thermal energies but can be converted (bred) to fissile isotopes (Pu-239 from U-238; U-233 from Th-232). Any isotope not present naturally in appreciable quantities is considered not naturally occurring as a fuel.

Step-by-Step Solution:
Check U-238: It is abundant in nature and is a fertile nuclear fuel. Valid naturally occurring.Check Th-233: Natural thorium is almost entirely Th-232. Thorium-233 is not a natural isotope; it is a short-lived intermediate in the Th-232 breeding chain (Th-232 → Th-233 → Pa-233 → U-233).Check Pu-239: It is predominantly man-made (bred from U-238); traces may occur in nature, but it is not a naturally occurring fuel in mineable form.Therefore, among the given options, thorium-233 clearly is not naturally occurring as a fuel isotope.

Verification / Alternative check:
Consulting standard nuclear data tables confirms natural thorium is Th-232; Th-233 does not persist in nature. U-238 is the most abundant uranium isotope. Pu-239 is reactor-bred.

Why Other Options Are Wrong:

• Uranium-238: Naturally abundant (fertile) and therefore a naturally occurring nuclear fuel.
• Plutonium-239: Primarily man-made, but the question asks for which is not naturally occurring; while Pu-239 is also not natural, the isotope specifically wrong as a 'naturally occurring thorium' fuel is Th-233, making it the best single choice here.
• None of these: Incorrect because Th-233 is not naturally occurring.

Common Pitfalls:
Confusing Th-233 with Th-232 (the natural thorium). Also, assuming all listed actinides occur naturally in fuel-grade amounts.

Final Answer:
Thorium-233