DOE-HDBK-1019/2-93 Reactor Theory (Nuclear Parameters)
The important information in this chapter is summarized below.
Reactivity Coefficients Summary
The temperature coefficient of reactivity is the change in reactivity per degree
change in temperature.
A reactor is under moderated when a decrease in the moderator-to-fuel ratio
decreases keff due to the increased resonance absorption. A reactor is over
moderated when an increase in the moderator-to-fuel ratio decreases keff due to
the decrease in the thermal utilization factor.
Reactors are usually designed to operate in an under moderated condition so that
the moderator temperature coefficient of reactivity is negative.
Increasing the moderator temperature will decrease the moderator-to-fuel ratio.
Decreasing the moderator temperature will increase the moderator-to-fuel ratio.
A negative temperature coefficient of reactivity is desirable because it makes the
reactor more self-regulating. An increase in power, resulting in an increase in
temperature, results in negative reactivity addition due to the temperature
coefficient. The negative reactivity addition due to the temperature increase will
slow or stop the power increase.
The fuel temperature coefficient is more effective than the moderator temperature
coefficient in terminating a rapid power rise because the fuel temperature
immediately increases following a power increase, while the moderator
temperature does not increase for several seconds.
The Doppler broadening of resonance peaks occurs because the nuclei may be
moving either toward or away from the neutron at the time of interaction.
Therefore, the neutron may actually have either slightly more or slightly less than
the resonant energy, but still appear to be at resonant energy relative to the
Uranium-238 and plutonium-240 are two nuclides present in some reactor fuels
that have large resonance absorption peaks.