Void coefficient

The void coefficient (including coolant loss coefficient or Voidkoeffizient called ) is a measure of the change in the reactivity of a nuclear reactor for the formation of vapor bubbles in the coolant or moderator. A reactivity change, which is not compensated, for its part, changes in the thermal output of the reactor result. Therefore, the void coefficient is important for the safety of the reactor.

The term void coefficient is mainly used in boiling water reactors, wherein the steam generating operation purpose. For all other types of reactors bubbles or cavitation is a deviation from normal operation, and here is the loss of coolant or Voidkoeffizienten spoken. Physically, it is about the same size.

Declaration and definition

As the reactor coolant is used in most types of reactors, pressurized water, liquid metal or other gas. Once the core temperature is raised far enough to boil a liquid coolant, whereby vapor bubbles are created, that the cavities in the coolant ( in the boiling water reactor is the normal operating state). Due to loss of coolant may also result in the formation of cavities ( LOCA ).

The liquid coolant is usually used as host, and also acts to some extent is inevitable as a neutron absorber. The gas bubbles, that is, the vapor refrigerant, because of their much lower density, show much less effect than the liquid refrigerant, thereby reducing the neutron multiplication factor k changes (see criticality). For a small change of the void fraction of the total coolant volume, the corresponding change in the reactivity = (k-1 ) / k is proportional to the percentage change of volume; the proportionality constant is the void coefficient


The void coefficient of a reactor with liquid moderator and / or coolant, depending on the design, positively in all operating conditions or under all operating conditions be negative or even change depending on the operating condition of its sign. In reactors, the core does not contain liquids, there is naturally no blistering and no void coefficient.

Typical numerical values ​​of the coefficients for a boiling water reactor steam bubbles are, for example:

-1.2 · 10-3/Vol % at 20 % void fraction,

-1.6 · 10-3/Vol % at 40 % void fraction.

Positive void coefficient

This case occurred in the reactor accident at Chernobyl, where a nuclear power plant reactor of the RBMK came with a positive void coefficient due to incorrect operation of control. As moderator serves in this type not the coolant, but outside of the pressure tubes mounted blocks of graphite. The heating power has become too high, leading to increased evaporation of the water. Since water vapor has a much lower density than water, less the back-diffusing from the graphite thermal neutrons were now absorbed on the way to the fuel, that the reactivity increases. In conjunction with other design features, this led to prompt Überkritikalität of portions of the reactor core and thus to the disaster.

Also in the CANDU reactor coolant loss coefficient is positive, but so small that corresponding power changes are easily dominated by the reactor control.

Negative void coefficient

A negative void coefficient means that the thermal performance decreases in the normal case where form in cooling water or moderator cavities. This also means that the reactivity increases when the size of the voids decreases, what happens when a sudden pressure increase, for example, in a boiling water reactor, such as when accidentally steam pipes are sealed off.

"Normal" light-water reactors, the coolant also serves as a moderator. The reactors are designed to easily undermoderated, that is, a reduction in the amount Moderator reduced under all circumstances the reactivity. Such reactors with always negative void coefficient are sometimes referred to as inherently stable or intrinsically stable.

This stability is not to be confused with so-called " inherent safety " of the reactor. For example, a negative void coefficient does not change the fact that bubble -strewn water cools less effectively, and especially not alter the decay heat, which can lead to meltdown at a large LOCA and failure of any emergency cooling. Thus, a partial meltdown is the fault in March 1979 at the nuclear power plant at Three Mile Iceland, a plant with a negative void coefficient, takes place.