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

The main purpose of a moderator in a thermal nuclear reactor is to decrease neutron speeds. This is essential because slower (thermal) neutrons are more effective at inducing fission reactions in certain types of fuel, such as uranium-235. By using materials like water or graphite as a moderator, the reactor can maintain a sustained chain reaction, ensuring efficient energy production.