Page 65 - Dynamics and Control of Nuclear Reactors

CHAPTER

6
Fission product poisoning

6.1 The problem
Fission reactions produce primary fission products directly (immediately when a fis-
sion reaction occurs) and through radioactive decay of primary fission products.
Some of these isotopes have very large absorption cross sections. Their presence
in a reactor causes a substantial reactivity decrease that must be canceled by adding
reactivity (as by withdrawing a control rod or diluting a dissolved neutron poison in
the reactor coolant). There are many fission products, but two are especially impor-
tant because of their impact on reactor operation. These fission products are Xenon-
135 and Samarium-149.
The effect of Xe-135 has three components: steady-state global poisoning,
transient global poisoning, and spatial oscillations in reactor power.

6.2 Dynamics of xenon-135
Xenon-135 has a very large absorption cross section for thermal neutrons
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(σ a 3.5 10 b).

6.2.1 Xe-135 production
Xenon-135 is produced directly as a fission product and by decay of another fission
product, Iodine-135. Xenon-135 has a fission yield (atoms produced per fission) of
0.003. That is, a Xe-135 atom is produced in 0.3% of fissions. I-135 is the result of
decay of another fission product, Tellurium-135. But Te-135 decays very rapidly, so
I-135 appears at essentially the same time as the Te-135 atom. I-135 has a fission
yield of 0.063. I-135 decays into Xe-135 with a half-life of 6.7h (decay constant
of 2.87 10 5 1
s ). The I-135 absorption cross section is small, resulting in negli-
gible absorption losses compared to decay losses.

6.2.2 Xe-135 losses
Xe-135 disappears as a result of radioactive decay and by neutron absorption. The
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half- life of Xe-135 is 9.2h (decay constant of 2.09 10 s ). Neutron absorptions
in Xe-135 cause disappearance of Xe-135 atoms at a rate of X σ aX Φ, where X is the
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Dynamics and Control of Nuclear Reactors. https://doi.org/10.1016/B978-0-12-815261-4.00006-8
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