In nuclear power reactors worldwide, uranium fuel is mainly used in which of the following chemical forms inside the fuel rods?

Introduction / Context:
Nuclear power plants use uranium as fuel, but not in the same form that comes out of the mine. This question tests whether you know the practical chemical form of uranium that is packed into fuel rods in most light water reactors around the world.

Given Data / Assumptions:
• The focus is on uranium fuel actually loaded into the core of nuclear reactors.
• Several common uranium compounds are listed, including oxides and a fluoride.
• We are concerned with the stable, solid ceramic form used inside fuel pellets.

Concept / Approach:
Uranium passes through many processing stages. After mining and milling, uranium is converted into yellowcake, mainly U3O8. For enrichment, it is converted into uranium hexafluoride, UF6, which is a gas at moderate temperatures and can be used in centrifuges. However, the final fuel used in reactor cores is manufactured as hard ceramic pellets of uranium dioxide, UO2, which are stacked inside zirconium alloy tubes to form fuel rods. Therefore UO2 is the standard fuel form in most thermal reactors.

Step-by-Step Solution:
Step 1: Recall that UF6 is used in the enrichment process because it can exist as a gas and be separated in centrifuges. Step 2: Recognise that gaseous UF6 is not suitable as a stable solid fuel inside a reactor core. Step 3: Note that U3O8 is an intermediate product called yellowcake and is not the final fuel form in fuel rods. Step 4: Nuclear fuel fabricators convert purified uranium into ceramic pellets of uranium dioxide, UO2, because it has good thermal and chemical stability under reactor conditions. Step 5: Therefore, the correct answer is UO2, uranium dioxide.

Verification / Alternative check:
Technical descriptions of pressurised water reactors and boiling water reactors always describe the fuel rods as containing small cylindrical pellets of uranium dioxide. Regulatory documents and training material also refer to nuclear fuel as UO2 fuel, confirming that this is the globally standard form.

Why Other Options Are Wrong:
U3O8: Yellowcake is a processed concentrate used for transport and storage, not packed directly into fuel rods.
UF6: This is a volatile gas used for enrichment, not a safe or stable ceramic for reactor cores.
Metallic uranium: It was used in some early reactors but has disadvantages such as corrosion and lower dimensional stability, so it is not the main form used worldwide.
UO3: Another oxide that appears in some processing steps but not as the standard reactor fuel form.

Common Pitfalls:
Students may confuse the different stages of the nuclear fuel cycle and select UF6 because it sounds technical or U3O8 because they remember the term yellowcake. To avoid this, always think about the physical requirements of the final fuel inside the reactor: it must be a dense, solid, stable ceramic, which points clearly to uranium dioxide, UO2.

Final Answer:
The uranium fuel used worldwide in nuclear power reactors is mainly in the form of UO2 (uranium dioxide) ceramic pellets.