Correct Option
Heavy water (D₂O) functions as a neutron moderator in certain types of nuclear reactors. Its primary role is to slow down the high-energy (fast) neutrons produced during nuclear fission. By reducing their speed to thermal energies, the probability of these neutrons being absorbed by fissile nuclei, such as uranium-235, is significantly increased. This moderation process is essential for sustaining a controlled nuclear chain reaction. Heavy water is chosen for this purpose in some reactors because it has a lower neutron absorption cross-section compared to light water, meaning it absorbs fewer neutrons while effectively slowing them down.
Incorrect Options
Option 2: The objective in a nuclear reactor is to slow down neutrons, not increase their speed. Fast neutrons are less likely to cause further fission in uranium-235 compared to thermal neutrons.
Option 3: While heavy water can also serve as a coolant in some reactor designs (e.g., CANDU reactors), its fundamental and primary function in the context of enabling the nuclear reaction is as a moderator to slow down neutrons. Cooling is a separate, though often complementary, function.
Option 4: The function of a moderator is to facilitate and sustain the nuclear chain reaction by making neutrons more effective. Stopping the nuclear reaction is achieved by inserting control rods, which are made of neutron-absorbing materials like cadmium or boron, into the reactor core to absorb excess neutrons.