Page 83 - Dynamics and Control of Nuclear Reactors

76 CHAPTER 7 Reactivity feedbacks

FIG. 7.3
Fission cross sections of fissile isotopes, U-235 and Pu-239 (ENDF, open access, evaluation
number ENDF/B-VIII.0).

For 1/v absorbers, the v in the denominator (σ constant/v) cancels the v in the
numerator, causing the reaction rate to be independent of moderator temperature.
However, some important reactor constituents are not 1/v absorbers. Fig. 7.3
shows cross sections for the fissile isotopes, U-235 and Pu-239.
The U-235 cross section shows a slight negative departure from 1/v behavior.
This means that the spectrum effect causes U-235 absorptions and fissions to
decrease when moderator temperature increases (a negative component of moderator
temperature coefficient of reactivity). This effect is small compared to other effects
caused by moderator temperature changes.
The Pu-239 effect is a different story. Pu-239 has a low energy (around 0.3 eV)
capture and fission resonance. This causes a positive departure from 1/v behavior.
This means that the spectrum effect causes Pu-239 captures and fissions to increase
when moderator temperature increases (a positive component of moderator temper-
ature coefficient of reactivity). This effect is significant in reactors with significant
Pu-239 inventories (for example in thermal reactors that are fueled with low-
enrichment uranium and consequent Pu-239 production by neutron captures in
U-238).
Coolant (or moderator/coolant) temperature changes cause density changes that
cause changes in neutron absorptions in the coolant. Since increased coolant temper-
ature always causes fluid expansion and consequent removal of a neutron absorber
from the core, the neutron absorption component of the coolant temperature coeffi-
cient is always positive.
The change in moderator density also changes neutron scattering which, in turn,
changes neutron mean-free paths. Increase in mean free paths enables more neutrons