Material properties: the specific gravity (relative density) of uranium and plutonium is approximately equal to which of the following values?

Introduction / Context:
Heavy actinide metals like uranium and plutonium are extremely dense. Recognizing their approximate specific gravity helps in estimating fuel mass, shielding considerations, and handling logistics in fuel fabrication and storage.

Given Data / Assumptions:

• Specific gravity is relative to water at 4 °C.
• We need an approximation commonly used in exams and quick estimates.

Concept / Approach:
Uranium metal has density ~19.1 g/cm^3 and plutonium (delta phase varies) is about ~19.8 g/cm^3; both round to “about 19.” This contrasts with structural steels (~7.8 g/cm^3) and copper (~8.9 g/cm^3), highlighting the exceptional density of actinide metals.

Step-by-Step Solution:
Recall densities: U ≈ 19.1 g/cm^3, Pu ≈ 19–20 g/cm^3.Map to specific gravity numbers relative to water ≈ 1 g/cm^3.Select the closest integer: 19.

Verification / Alternative check:
Engineering handbooks tabulate U and Pu densities in the 19–20 g/cm^3 range; typical practice is to quote “about 19.”

Why Other Options Are Wrong:

• 9 and 13: Far below actinide densities.
• 27: Too high; few pure metals approach this density.
• 7.8: Approximate for steel, not U/Pu.

Common Pitfalls:
Confusing oxide fuel densities (e.g., UO2 pellets ~10–11 g/cm^3 theoretical) with metal densities; mixing specific gravity with atomic weight.

Final Answer:
19