The void coefficient is defined as the change in reactivity per percent change in void volume.
αV = dρ⁄d%void
It is expressed in units of pcm/%void. The value of void coefficient in PWRs may be of the order of -100 pcm/%void. The formation of voids in the core has the same effect as the temperature increase of the moderator (decreasing the density of the moderator) In comparison with the change in the moderator temperature, boiling minimally affects the neutron leakage, because it is unlikely that local boiling occurs at the periphery of the reactor core, where the local power drops significantly.
The magnitude and sign (+ or -) of the void coefficient is primarily a function of the moderator-to-fuel ratio. Major impacts on multiplication of the system arise from the change of the resonance escape probability. But it is also dependent on an boron concentration in the primary coolant (in case of PWRs). As was written at the beginning of the cycle (BOC), when the PWR core contains large amount boron disolved in primary coolant (chemical shim), an increase in voids causes an increase in the thermal utilization factor.
Void coefficient in pressurized water reactors
In pressurized water reactors, the void content of the core may be about one-half of one percent. It is cause by nucleate boiling, that may occur even during operational conditions. Nucleate boiling occurs when any surface of fuel cladding reach the saturation temperature (e.g. 350°C), which is determined by the pressure in the pressurizer (e.g. 16MPa). Such local nucleate boiling does not pose any problem for the reactor operation.
On the other hand during abnormal condition, boiling in the reactor core is one of the most important phenomena, that may take place in the core. From the reactivity point of view, nucleate boiling have very important consequences on the reactivity of the reactor core. Boiling affects reactivity in the same manner as the presence of voids and therefore it is characterized by the void coefficient.
Void coefficient in boiling water reactors
In systems with boiling conditions, such as boiling water reactors (BWR), the void coefficient is of prime importance during reactor operation. Boiling water reactors generally have negative void coefficients, and during normal operation the negative void coefficient allows reactor power to be adjusted by changing the rate of water flow through the core.
The negative void coefficient acts against power increase and contribute to the reactor stability. As the reactor power is raised to the point where the steam voids start to form, voids displace moderator from the coolant channels within the core. This displacement further reduces the moderator-to-fuel ratio of the core, which is under moderated. This results in a negative reactivity addition, thereby limiting reactor power rise. Major impacts on multiplication of the system arise from the change of the resonance escape probability, becauses the presence of voids causes a hardening of neutron spectrum in the reactor core resulting in higher resonance absorption (lower p).