Void Coefficient

In nuclear engineering, the void coefficient (more properly called "void coefficient of reactivity") is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids (typically steam bubbles) form in the reactor moderator or coolant. Reactivity, in the nuclear engineering sense (not to be confused with chemical reactivity), measures the degree of change in neutron multiplication in a reactor core. Reactivity is directly related to the tendency of the reactor core to change power level: if reactivity is positive, the core power tends to increase; if it is negative, the core power tends to decrease; if it is zero, the core power tends to remain stable. The reactivity of the core may be adjusted by the reactor control system in order to obtain a desired power level change (or to keep the same power level). It can be compared to the reaction of an automobile as conditions around it change (for instance, wind intensity and direction or road slope), and therefore the corresponding counter-measure that the driver applies to maintain road speed or execute a desired manoeuvre.

Reactivity is affected by many factors, including coolant/moderator temperature and density, fuel temperature and density, and structural temperature and density. Net reactivity in a reactor is the sum total of all these contributions, of which the void coefficient is but one. Reactors in which either the moderator or the coolant is a liquid typically will have a void coefficient value that is either negative (if the reactor is under-moderated) or positive (if the reactor is over-moderated). Reactors in which neither the moderator nor the coolant is a liquid (e.g., a graphite-moderated, gas-cooled reactor) will have a void coefficient value equal to zero.

Read more about Void CoefficientExplanation, Reactor Designs

Other articles related to "coefficient, void coefficient, void coefficients, voids":

Pressurized Water Reactors - PWR Reactor Design - Moderator
... boiling water as the coolant, has a large positive thermal coefficient of reactivity, that increases heat generation when coolant water temperatures increase ... This property is called the void coefficient of reactivity, and in an RBMK reactor like Chernobyl, the void coefficient is positive, and fairly large, causing rapid transients ... very low neutron absorption, so heavy water reactors such as CANDU reactors also have a positive void coefficient, though it is not as large as that of ...
Void Coefficient - Reactor Designs
... Boiling water reactors generally have negative void coefficients, and in normal operation the negative void coefficient allows reactor power to be ... However, the negative void coefficient can cause an unplanned reactor power increase in events (such as sudden closure of a steamline valve) where the reactor pressure is suddenly increased ... In addition, the negative void coefficient can result in power oscillations in the event of a sudden reduction in core flow, such as might be caused by a ...
Loss-of-coolant Accident
... its power output in the event of a LOCA or of voids appearing in its coolant system (by water boiling, for example) ... This is measured by the coolant void coefficient ... Most modern nuclear power plants have a negative void coefficient, indicating that as water turns to steam, power instantly decreases ...

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