In a PWR, injecting a slug of cold water into the core will have what effect on reactivity?

• A. Decrease reactivity
• B. Increase reactivity
• C. No effect
• D. Immediate reactor scram

Answer: (B)

Injecting a slug of cold water into the core increases the local moderator density. Denser moderator slows neutrons more effectively, boosting the probability that they cause fission and raising the effective multiplication factor momentarily. So the immediate effect is a positive reactivity insertion, causing a temporary rise in power. As the cold water heats and mixes with the surrounding coolant, its density decreases and the reactivity effect fades, consistent with the coolant’s negative temperature coefficient. This is not a scram and it is not a permanent decrease in reactivity.