Plutonium facts for nuclear engineering: which statements about origin and material properties are correct?

Introduction / Context:
Plutonium (principally Pu-239 and Pu-240 in civilian fuel cycles) is produced in nuclear reactors when U-238 captures neutrons and undergoes a sequence of beta decays. Understanding where plutonium comes from and how its physical properties compare to other actinides is fundamental in fuel-cycle engineering and materials handling.

Given Data / Assumptions:

• Commercial thermal reactors irradiate uranium oxide fuel containing U-235 and U-238.
• Thorium is considered as a high-melting-point fertile material for comparison.
• We focus on broad, widely-used values rather than phase-specific quirks.

Concept / Approach:
In-reactor transmutation converts some U-238 to Pu isotopes through neutron capture and decay: U-238 → U-239 → Np-239 → Pu-239. Spent fuel reprocessing (chemical separation) can recover plutonium for potential recycle (e.g., as MOX fuel). Regarding material properties, plutonium exhibits complex allotropy and a comparatively low melting point (~640°C) among actinides, while thorium’s melting point is much higher (~1690°C), making the statement about relative melting points true.

Step-by-Step Solution:
1) Confirm origin: plutonium forms in thermal reactors and is extracted from spent fuel → statement (a) is correct.2) Compare melting points: Pu ≈ 640°C; Th ≈ 1690°C → statement (b) is correct.3) Therefore, the combined option (c) is correct.

Verification / Alternative check:
Standard nuclear data tables list these melting points and document plutonium’s presence in discharged fuel assemblies; industrial reprocessing routes (PUREX and variants) are designed around this fact.

Why Other Options Are Wrong:
Selecting only (a) or only (b) ignores that both statements are simultaneously true.“Neither” contradicts established reactor physics and materials data.

Common Pitfalls:
Confusing metallic plutonium with plutonium dioxide (PuO2) ceramic fuel forms; overlooking plutonium’s multiple allotropes that complicate metallurgy but not the basic melting-point comparison.

Final Answer:
Both (a) and (b).