Negative moderator temperature feedback implies reactivity changes in what way?

• A. Reactivity decreases as the moderator heats up
• B. Reactivity increases as the moderator heats up
• C. No change with moderator temperature
• D. Only occurs in fast reactors

Answer: (A)

Increasing the moderator temperature lowers reactivity. When the moderator (water) heats up, it expands and its density drops, so there are fewer moderator molecules per unit volume to slow neutrons. With less efficient slowing, fewer neutrons reach the thermal energies where fission of fuel like U-235 is most probable. Since the reactor relies on those thermal neutrons for sustaining fission, the overall neutron multiplication decreases, reducing reactivity. This negative feedback helps stabilize power after a temperature rise. In fast reactors, the moderator plays little role, so moderator temperature has little impact on reactivity.