Void coefficient: what is it and why is it important in BWRs?

• A. Change in reactivity due to voids in the moderator; can increase or decrease reactivity depending on core design; becomes important in boiling-water reactors.
• B. Change in reactivity due to voids in the moderator; always decreases reactivity.
• C. Change in reactivity due to voids caused by fuel voiding; always increases reactivity.
• D. Void coefficient describes temperature coefficient of fuel.

Answer: (A)

The main idea being tested is how the reactor’s neutron population responds to the presence of steam voids in the moderator. The void coefficient is the change in reactivity (often expressed as dk/k or in dollars/pcm) that results from a change in the fraction of voids, i.e., the amount of steam occupying the space that would otherwise be filled by moderator.

In a boiling-water reactor, the coolant is also the moderator, so when the coolant boils and steam voids form, the density of moderating material drops. That change alters the neutron spectrum and the balance of neutron production and absorption. Depending on the core design and neutron economy, this can push the reactor toward higher or lower reactivity. In many BWR designs the interplay of fewer slow (thermal) neutrons and changes in absorption/leakage leads to a notable reactivity change with voids, and thus the void coefficient becomes a prominent feedback mechanism during operation and transients.

So the correct way to describe it is: the difference in reactivity caused by voids in the moderator, which can either increase or decrease reactivity depending on the specific reactor design, and this effect is especially important in boiling-water reactors because voids are intrinsic to their operation and transients.