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6.2 Dynamics of xenon-135 63

8

Reactivity loss ($) 4
6

25% step change in reactor power
2
50% step change in reactor power
100% step change in reactor power
0
0 10 20 30 40 50 60 70 80 90 100
Time (h)
FIG. 6.3
Xe-135 poisoning transients for step changes in reactor power from zero power.

Fig. 6.3 shows Xe-135 reference reactor poisoning transients for several step changes
in reactor power from zero power.

6.2.7 Xe-135 after Shutdown
Consider a reactor in which I-135 and Xe-135 have reached their steady state levels
as given by Eqs. (6.7) and (6.8), respectively. Now assume that the neutron flux sud-
denly goes to zero. The transient responses of I-135 and Xe-135 are given by
0 0
I tðÞ ¼ I exp λ I tÞ (6.21)
ð
ss

λ I
0 0 0
ð
X tðÞ ¼ X exp λ X tð Þ + I exp λ X tðð Þ exp λ I tÞÞ (6.22)
ss
ss
λ I λ X
Fig. 6.4 shows Xe-135 poisoning after shutdown of the reference reactor for four dif-
ferent initial power levels.
The data for these simulations are the same as for the reactor described in
Section 6.2.6. The results show that the level of poisoning increases with initial reac-
tor power and is large. The ability to override this poisoning depends on the amount
of reactivity that can be added by other means (usually control rods or diluting dis-
solved neutron poisons). For large initial flux levels, the poisoning reaches levels
that exceed the ability to override with available operator actions. Therefore, restart
is impossible until the Xe-135 falls low enough to permit restart. Fig. 6.5 shows a
hypothetical case. The reference reactor shuts down from full power and operator
actions can add no more than 9 dollars of reactivity. The figure shows the time inter-
val during which restart is impossible, and is from around 1.5 to 21h.
Fig. 6.6 shows I-135 as well as Xe-135 for a step change in reference reactor
power from zero to 100%, then a step decrease back to zero power. Note that at
steady state I-135 level at 100% power is larger than the Xe-135 level. As the I-
135 decays after shutdown, it causes the peak in Xe-135 level.