Moderator used in nuclear power plants: identify a common choice Which of the following is commonly used as a neutron moderator in power reactors? Choose the correct option.

Concept
A moderator slows down fast neutrons to thermal energies to sustain fission efficiently. Common moderators used in power reactors include heavy water (D₂O) and graphite. Concrete is primarily a shielding material, not a moderator.

Why choose heavy water here
Among the listed choices, both graphite (A) and heavy water (B) are valid moderators; with a single answer MCQ and typical power-plant emphasis (e.g., PHWRs), heavy water is the most fitting selection. Options involving concrete are incorrect as moderators.

Final Answer
Heavy water