Reactor Theory (Nuclear Parameters)
DOE-HDBK-1019/2-93
REACTIVITY COEFFICIENTS
Pressure Coefficient
Figure 3 Effect of Fuel Temperature on Resonance Absorption Peaks
The reactivity in a reactor core can be affected by the system pressure. The pressure coefficient
of reactivity is defined as the change in reactivity per unit change in pressure. The pressure
coefficient of reactivity for the reactor is the result of the effect of pressure on the density of the
moderator. For this reason, it is sometimes referred to as the moderator density reactivity
coefficient. As pressure increases, density correspondingly increases, which increases the
moderator-to-fuel ratio in the core. In the typical under moderated core the increase in the
moderator-to-fuel ratio will result in a positive reactivity addition. In reactors that use water as
a moderator, the absolute value of the pressure reactivity coefficient is seldom a major factor
because it is very small compared to the moderator temperature coefficient of reactivity.
Void Coefficient
In systems with boiling conditions, such as boiling water reactors (BWR), the pressure
coefficient becomes an important factor due to the larger density changes that occur when the
vapor phase of water undergoes a pressure change. Of prime importance during operation of
a BWR, and a factor in some other water-moderated reactors, is the void coefficient. The void
coefficient is caused by the formation of steam voids in the moderator. The void coefficient of
reactivity is defined as the change in reactivity per percent change in void volume. As the
reactor power is raised to the point where the steam voids start to form, voids displace moderator
from the coolant channels within the core. This displacement reduces the moderator-to-fuel
ratio, and in an under moderated core, results in a negative reactivity addition, thereby limiting
reactor power rise. The void coefficient is significant in water-moderated reactors that operate
at or near saturated conditions.
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