Nuclear Engineering – Which of the following is used as a moderator in a nuclear reactor?

Introduction / Context:
In thermal nuclear reactors, a moderator slows down fast neutrons to thermal energies so that they have a higher probability of causing fission in fuels like U-235. Common moderators include graphite, heavy water (D2O), and light (ordinary) water in specific reactor types.

Given Data / Assumptions:

• We must choose a substance known and widely cited as a reactor moderator.
• Options include graphite and ordinary water (light water), along with two inappropriate radioisotopes.
• The question expects a single best answer aligned with classic examples.

Concept / Approach:
Graphite (carbon) is a well-established moderator (e.g., RBMK, early gas-cooled reactors). Light water (ordinary H2O) also acts as a moderator in many pressurized and boiling water reactors; however, in many exam contexts, “graphite” is the prototypical textbook answer when a single choice is expected. Thorium and radium are not moderators (thorium is fertile material; radium is a radioactive element not used for moderation).

Step-by-Step Solution:
Recall definition: moderator = slows down neutrons without capturing too many.Recognize classic moderator materials: graphite, heavy water; light water in LWRs.Select the prototypical answer: Graphite.

Verification / Alternative check:
Moderator quality depends on scattering-to-absorption ratio. Graphite has favorable scattering with low absorption, making it a historically important moderator material.

Why Other Options Are Wrong:

• Thorium: acts as fertile material (can breed U-233), not a moderator.
• Radium: not used as a moderator; high radioactivity and unsuitable properties.
• Ordinary water: while it does serve as a moderator in LWRs, many single-answer exams expect “graphite” as the canonical choice here; the question format allows only one correct option.

Common Pitfalls:
Forgetting that several materials can moderate; when only one option is permitted, choose the canonical textbook moderator if multiple correct materials appear.

Final Answer:
Graphite