The void coefficient is defined as the change in reactivity per percent change in the void volume.
αV = dρ⁄d%void
It is expressed in units of pcm/%void. The value of the 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). Compared 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 the multiplication of the system arise from the change of the resonance escape probability. But it is also dependent on a boron concentration in the primary coolant (in the case of PWRs). As was written at the beginning of the cycle (BOC), when the PWR core contains a large amount of boron dissolved in the primary coolant (chemical shim), an increase in voids causes an increase in the thermal utilization factor.
The 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 caused by nucleate boiling, which may occur even during operational conditions. Nucleate boiling occurs when any surface of fuel cladding reaches the saturation temperature (e.g.,, 350°C), 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 conditions, 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 has 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.
The void coefficient in boiling water reactors
The void coefficient is of prime importance during reactor operation in systems with boiling conditions, such as boiling water reactors (BWR). Boiling water reactors generally have negative void coefficients. During normal operation, the negative void coefficient allows reactor power to be adjusted by changing the water flow rate through the core.
The negative void coefficient acts against power increase and contributes to the reactor stability. As the reactor power is raised to the point where the steam voids 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 the multiplication of the system arise from the change of the resonance escape probability because the presence of voids causes hardening of neutron spectrum in the reactor core resulting in higher resonance absorption (lower p).