Reactor Theory (Nuclear Parameters) DOE-HDBK-1019/2-93REACTIVITYReactivityCoefficientsand ReactivityDefectsThe amount of reactivity (r) in a reactor core determines what the neutron population, andconsequently the reactor power, are doing at any given time. The reactivity can be effected bymany factors (for example, fuel depletion, temperature, pressure, or poisons). The next severalchapters discuss the factors affecting reactivity and how they are used to control or predictreactor behavior.To quantify the effect that a variation in parameter (that is, increase in temperature, control rodinsertion, increase in neutron poison) will have on the reactivity of the core, reactivitycoefficientsare used. Reactivity coefficients are the amount that the reactivity will change fora given change in the parameter. For instance, an increase in moderator temperature will causea decrease in the reactivity of the core. The amount of reactivity change per degree change inthe moderator temperature is the moderator temperature coefficient. Typical units for themoderator temperature coefficient are pcm/oF. Reactivity coefficients are generally symbolizedby ax, where x represents some variable reactor parameter that affects reactivity. The definitionof a reactivity coefficient in equation format is shown below.ax D rD xIf the parameter x increases and positive reactivity is added, then ax is positive. If the parameterx increases and negative reactivity is added, then ax is negative.Reactivity defects(Dr) are the total reactivity change caused by a variation in a parameter.Reactivity defects can be determined by multiplying the change in the parameter by the averagevalue of the reactivity coefficient for that parameter. The equation below shows the generalmethod for relating reactivity coefficients to reactivity defects.Dr= axDxExample: The moderator temperature coefficient for a reactor is -8.2 pcm/oF. Calculate thereactivity defect that results from a temperature decrease of 5oF.Solution:D r aT D T8.2pcmF5 F41 pcmThe reactivity addition due to the temperature decrease was positive because of thenegative temperature coefficient.Rev. 0NP-03Page 21
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
