Page 76 - Dynamics and Control of Nuclear Reactors

68 CHAPTER 6 Fission product poisoning

1

0

–1
Reactivity ($) –2

–3

–4
–5

–6
0 5 10 15 20 25 30
Time (h)
FIG. 6.11
Xe-135 poisoning after 1 cent reactivity step.

6.2.11 Xenon-induced spatial oscillations
Xe-135 can cause spatial oscillations in reactors. Consider a case in which reactivity
is simultaneously increased in one region and decreased in the other. The following
scenario would ensue.

Increased Power Region Decreased Power Region
P " P #
I " (increase future Xe) I #
Xe # (initial burnout) Xe "
ρ " (reduced Xe poisoning) ρ #
P " (caused by initial Xe burnout) P #
Xe " (from increased decay of I) Xe #
ρ # ρ "
P # P "

Note that power oscillates and the regional powers are 180 degrees out of phase. The
oscillations continue because each region now finds itself where the other
region began.
In a power reactor, negative reactivity feedback from temperature changes and
control action would dampen or eliminate the oscillations. Operating procedures
serve to suppress any Xenon oscillations in power reactors.