What is the main purpose of a moderator in a thermal nuclear reactor? A to shield the surroundings from ionising radiations B to decrease the number of fission chain reactions C to decrease neutron speeds D to prevent the core from overheating - AQA - A-Level Physics - Question 29 - 2021 - Paper 2

This is not correct. The moderator's purpose is to help sustain the fission chain reaction rather than decreasing it.

This is the correct answer. The main purpose of a moderator in a thermal nuclear reactor is to slow down fast neutrons, increasing the probability of fission when they collide with fissile material like Uranium-235.

While it is crucial to manage the reactor's temperature, this is not the primary role of the moderator itself.