The dominant production route of Xe-135 is as a daughter product of a radioactive decay chain starting with Те-135 as follows:

Te-135 -> 1-135 — Xe-135 -* Cs-135 — Ba-135(stable)
2 min 6.7 h 9.2 h 2 x 104 у

As the half-life of Te-135 is very short it may be neglected. Iodine has a low neutron absorption cross — section and its concentration is therefore determined

The xenon concentration at equilibrium can be found from Equations (3.7) and (3.8) by putting 6N/ot and 5N[/5t equal to zero. This gives:

X + ] cra(E)o(E)6E

It is seen from Equation (3.9) that the saturation level depends on neutron flux via the fission rate term on the top and the neutron capture in xenon term on the bottom of this expression. As the flux is in­creased the saturation level increases until the neutron capture term dominates the radioactive decay term X. Neutron flux levels differ in the different thermal reactor systems, for example, typical total fluxes in magnox, AGR and PWR are 0.5, 1.7, 3.0 in units of 101J neutrons/crrrs, and the saturation xenon con­centration for operation is the reactivity effect of the xenon which depends on the neutron absorption rate in xenon relative to other materials. The resulting differences in xenon reactivity-worth between reactor systems is much less, typical figures for magnox, AGR and PWR for full power xenon-worth being 1.7, 2.1 and 2.5 Niles.

In order to provide sufficient reactivity control to accommodate the change in reactivity due to xenon build-up a coarse control system is required. This is provided in magnox and AGR reactors by bulk rods in banks. The use of partially-inserted bulk rods during reactor start-up causes distortion of the axial and radial flux shape. These distortions may prevent full reactor power output being achieved because of the need to stay within operating rules limits on can temperature. In PWR the xenon reactivity poisoning is counteracted by reduction of boron level in the water moderator which provides a uniform reactivity control and therefore does not itself cause significant flux distortions. The distribution of xenon throughout the reactor is not uniform since it depends on the neutron flux distribution; hence changes in reactor power level can lead, via xenon level distribution changes, to flux distortion, particularly in the axial direction. These distortions are controlled in the PWR by balancing movements of control assembly banks against boron concentration control.

When a reactor has been operating at power for some time and is then shut down, the removal term in Equation (3.8) relating to neutron absorption in xenon goes to zero but the production term, XiN;, due to radioactive decay of iodine, does not imme­diately reduce and the net result is an increase in xenon level. A peak in xenon level is reached after about ten hours and then decays away over two or three days as shown in Fig 3.3.

In order to override the xenon build-up extra re­activity is needed. Without it the reactor may not be capable of achieving criticality and starting up dur­ing the few days immediately following a reactor shutdow n.

Fic. 3.3 Xenon and iodine concentrations following
shutdown from equilibrium conditions