Correct option is B
Moderators in nuclear reactors are used to slow down fast neutrons to thermal energies to increase the probability of fission in fissile materials like uranium-235. Common moderators include materials with low atomic mass and high scattering cross-section but low absorption cross-section.
· Heavy water (D₂O) is an excellent moderator due to its low neutron absorption.
· Beryllium (Be) is also used as a moderator because of its low atomic mass and good neutron slowing-down properties.
· Zirconium is used as a cladding material (not a moderator) due to its low neutron absorption.
· Cadmium is used as a control rod material (neutron absorber, not moderator).
Thus, the correct pair is beryllium and heavy water.
