Uranium fission chain reaction — how can a sustained chain be developed? For a reactor-grade uranium system, which measures help initiate and sustain a fission chain reaction so that the system becomes critical?

Introduction / Context:
Nuclear reactors rely on neutron economy to maintain a self-sustaining chain reaction. Two powerful levers are fuel enrichment (increasing the fissile isotope U-235 fraction) and moderation (slowing down neutrons), both of which raise the probability that a neutron will induce fission rather than be absorbed or leak from the core.

Given Data / Assumptions:

• Fuel contains uranium isotopes U-238 and U-235.
• Objective: achieve k-effective equal to 1 (criticality).
• Neutron losses occur by leakage and non-fission absorption.

Concept / Approach:
Criticality requires that the average number of neutrons from each fission that cause another fission equals one. Increasing U-235 content raises the macroscopic fission cross-section. Moderation slows down neutrons to thermal energies where the U-235 fission cross-section is much higher, greatly improving the likelihood of fission and reducing the required enrichment.

Step-by-Step Solution:
Recognize that U-235 is the primary fissile isotope in low-enriched uranium.Slower (thermal) neutrons have a larger probability of inducing U-235 fission.Therefore, enrichment (a) and moderation (b) both move the system toward k-effective = 1.Combining both is the standard approach in most power reactors.

Verification / Alternative check:
Pressurized and boiling water reactors use light-water moderation with low-enriched uranium; heavy-water reactors use stronger moderation allowing natural uranium; both exemplify how (a) and/or (b) achieve criticality.

Why Other Options Are Wrong:
Choosing only one factor ignores the complementary role of the other. “None of these” contradicts reactor design practice.

Common Pitfalls:
Confusing moderators with coolants or assuming enrichment alone is sufficient without considering leakage and absorption in structural materials.

Final Answer:
Both (a) and (b)