Page 80 - Dynamics and Control of Nuclear Reactors

CHAPTER

7
Reactivity feedbacks

7.1 Basics
Changes in power reactor temperature or pressure cause reactivity changes. The
causes for these reactivity changes are the temperature dependence of nuclear prop-
erties, changes in the quantity of material present in the reactor core because of den-
sity changes or changes in the dimensions or shape of core components. Evaluating
the feedback reactivity involves the use of feedback coefficients (change in reactivity
per unit change in a process variable). Feedback reactivity effects are very important
in achieving suitable reactor performance.
This chapter addresses a qualitative description of relevant effects. Quantitative
evaluation of feedback coefficients requires use of detailed neutronic models. These
methods are described in books on reactor physics [1] and are beyond the scope and
purpose of this book. Here, it will be assumed that analysts will be provided with
reactivity coefficients needed to perform a dynamic simulation.
The following sections address reactivity feedbacks in general for thermal reac-
tors, specifically for thermal reactors that are so-called Generation II and Generation
III reactors.

7.2 Fuel temperature feedback in thermal reactors
Fuel temperature affects reactivity through changes in Doppler broadening of
absorption resonances. Some fission neutrons are lost during slowing down as a
result of absorption in heavy isotopes (principally U-238 and Pu-240 in thermal reac-
tors with low-enrichment uranium fuel). Resonances in their absorption cross sec-
tions cause a decrease in the number of neutrons reaching thermal energy (the
resonance escape probability decreases).
Changes in temperature change the relative motion between the resonance
absorber and a neutron. This causes a reduction in the peak of the absorption cross
section and a broadening over a wider range of energies. See Fig. 7.1.
Since the cross section is still very large in the broadened region, the heavy iso-
tope absorbs more neutrons than would be absorbed in an unbroadened resonance.
This increased resonance absorption causes reactivity to decrease. The effect
occurs in U-238 and is even stronger in Pu-240 as it builds up by absorptions in
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