In Boiling Water Reactors, how does increasing steam voids affect reactivity?

• A. It tends to reduce reactivity by reducing moderation
• B. It increases reactivity by increasing neutron speed
• C. It has no effect
• D. It makes reactivity unpredictable

Answer: (A)

Increasing steam voids in a Boiling Water Reactor reduces moderation. The coolant also serves as the primary neutron moderator, so when steam bubbles form, the liquid water density drops and fewer neutrons are slowed to thermal energies. Thermal neutrons have the highest probability of fission in U-235, so fewer thermal neutrons means a lower fission rate and thus lower reactivity. Although neutrons may become faster, the fission cross-section ofU-235 is much smaller for fast neutrons, and other losses (like capture or non-fuel absorption) keep the overall effect as a reduction in reactivity. This is why increasing steam voids produces a negative reactivity feedback.