Some fission products like 135Xe and [47]Sm have very large absorption cross­sections for thermal neutrons. They are not produced directly by fission but by beta decay of precursor fission fragments. The decay chains by which they are produced as

135Xe has an absorption cross-section of 2.7 x 106 barns for thermal (0.025 eV) neutrons, while that of 149Sm is 40 800 barns. Because of the dominant effect of 135Xe we give the derivation of the xenon effect. The evolution equations read

where we have neglected captures by iodine. yI ‘ 0.06 is the yield of mass 135 in fission. At equilibrium

yI^f’

nI

yA’

nXe =t—— :———

AXe + °Xe’

for a flux of 4 x 1013 n/cm2/s, o-Xe’ = 10—4 AXe = 2 x 10—5. Thus

It follows that

у = -0.03.

After a reactor shut-down, the evolution of xenon is given by

which yields

Figure 8.3 shows the evolution of the xenon-induced reactivity decrease after shut-down of a thermal reactor at two flux levels: 4 x 1013 and 2 x 1014n/cm2/s.

It is remarkable that, as apparent from equation (8.19), the initial xenon concentration is independent of the neutron flux, at least for fluxes that are not too small. In critical reactors the reactivity decrease prevents restarting of the reactor if a large enough positive reactivity reserve is not available. Hybrid systems can be restarted at any time, although the gain will be smaller if the xenon concentration is high. However, if the reactor is

stopped long enough, the xenon concentration vanishes and thus the reactivity is larger by 0.0035 than the reactivity during operation. This is true for any thermal reactor, and has to be added to the protactinium — related reactivity increase, in the case of thorium reactors. For fast reactors the xenon effect is negligible.