What is the sign of the moderator temperature coefficient in typical light-water reactors?

• A. Change in reactivity with moderator temperature; typically positive.
• B. Change in reactivity with moderator temperature; sign is always negative.
• C. Change in reactivity with moderator temperature; typically negative.
• D. Change in reactivity with moderator pressure; negligible.

Answer: (C)

The main idea is how reactivity changes as the moderator temperature changes. In a typical light-water reactor, water acts as the moderator. When the moderator heats up, its density decreases because water expands with temperature. Less dense moderator is less effective at slowing neutrons, so the population of thermal neutrons drops and the reactor’s reactivity decreases. This means reactivity changes in the opposite direction to moderator temperature: as temperature goes up, reactivity goes down. That negative relationship is why the moderator temperature coefficient is described as typically negative, providing a natural, self-limiting feedback.