Reactor Theory (Nuclear Parameters)
Production and Removal of Xenon-135
Xenon-135 has a 2.6 x 106 barns neutron absorption cross section. It is produced directly by
some fissions, but is more commonly a product of the tellurium-135 decay chain shown below.
The fission yield (g) for xenon-135 is about 0.3%, while g for tellurium-135 is about 6%.
The half-life for tellurium-135 is so short compared to the other half-lives that it can be assumed
that iodine-135 is produced directly from fission. Iodine-135 is not a strong neutron absorber,
but decays to form the neutron poison xenon-135. Ninety-five percent of all the xenon-135
produced comes from the decay of iodine-135. Therefore, the half-life of iodine-135 plays an
important role in the amount of xenon-135 present.
The rate of change of iodine concentration is equal to the rate of production minus the rate of
removal. This can be expressed in the equation below.
rate of change of iodine concentration = yield from fission - decay rate - burnup rate
g I S
f lI NI s
fission yield of 135I
macroscopic fission cross section fuel
thermal neutron flux
decay constant for 135I
microscopic absorption cross section 135I
is very small, the burn up rate term may be ignored, and the expression for the rate
of change of iodine concentration is modified as shown below.
g I S
f lI NI
When the rate of production of iodine equals the rate of removal of iodine, equilibrium exists.
The iodine concentration remains constant and is designated NI(eq). The following equation for
the equilibrium concentration of iodine can be determined from the preceding equation by setting
the two terms equal to each other and solving for NI(eq).