Reactor Theory (Nuclear Parameters)DOE-HDBK-1019/2-93REACTIVITY COEFFICIENTSPressureCoefficientFigure 3 Effect of Fuel Temperature on Resonance Absorption PeaksThe reactivity in a reactor core can be affected by the system pressure. The pressure coefficientof reactivity is defined as the change in reactivity per unit change in pressure. The pressurecoefficient of reactivity for the reactor is the result of the effect of pressure on the density of themoderator. For this reason, it is sometimes referred to as the moderator density reactivitycoefficient. As pressure increases, density correspondingly increases, which increases themoderator-to-fuel ratio in the core. In the typical under moderated core the increase in themoderator-to-fuel ratio will result in a positive reactivity addition. In reactors that use water asa moderator, the absolute value of the pressure reactivity coefficient is seldom a major factorbecause it is very small compared to the moderator temperature coefficient of reactivity.VoidCoefficientIn systems with boiling conditions, such as boiling water reactors (BWR), the pressurecoefficient becomes an important factor due to the larger density changes that occur when thevapor phase of water undergoes a pressure change. Of prime importance during operation ofa BWR, and a factor in some other water-moderated reactors, is the void coefficient. The voidcoefficient is caused by the formation of steam voids in the moderator. The void coefficient ofreactivity is defined as the change in reactivity per percent change in void volume. As thereactor power is raised to the point where the steam voids start to form, voids displace moderatorfrom the coolant channels within the core. This displacement reduces the moderator-to-fuelratio, and in an under moderated core, results in a negative reactivity addition, thereby limitingreactor power rise. The void coefficient is significant in water-moderated reactors that operateat or near saturated conditions.Rev. 0NP-03Page 27
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