Natural uranium consists almost entirely of the isotope $U_{238}$.

Only $U_{235}$ is fissile, meaning it can sustain a nuclear chain reaction with thermal neutrons.

Enrichment processes are required for most commercial nuclear reactors to achieve a higher $U_{235}$ concentration.

Natural uranium is abundant in the Earth's crust.

Used directly in CANDU-type heavy water reactors without enrichment.

Low concentration of fissile $U_{235}$ necessitates enrichment for light water reactors.

Requires complex chemical separation/enrichment processes for industrial use.

Nuclear power plant fuel cycle.

Research reactors.

Nuclear weapon development (highly enriched).

Standard value: Natural uranium contains ~99.27% $U_{238}$ and ~0.72% $U_{235}$.

Option B (0.7) is the standard approximation used in nuclear engineering textbooks.