When a reactor is shut down, xenon builds up rapidly, reaching a peak after about 10 - 11 hours (refer to Figure 7). This always occurs because Xe-135 is still being produced from the decaying I-135 and it is no longer being burned out since the neutron flux has essentially dropped to zero. This Xe-135 must decay with its normal half life of 9.2 hours. This is represented as follows:

It should be obvious that the Xe-135 build-up after shutdown depends on the operating flux before shutdown; the greater the operating flux, the higher will be the xenon peak (refer to Figure 7). Notice that the xenon peak after shutdown, following operation at the higher fluxes, is many times greater than the equilibrium value. Also note that the Xe-135 peak is low and offers no problems when operating with fluxes of about 10¹³ neutrons/cm²s or less.

For reactors with higher fluxes the build-up of Xe-135 after shutdown may prevent the reactor from being started again for a considerable length of time, unless sufficient excess reactivity has been built into the reactor to override these effects. (Core life is often measured to the time at which maximum xenon poisoning can no longer be overridden by the excess reactivity still remaining in the core.)

Suppose that a reactor does not have enough reactivity for xenon override and has an excess reactivity of only 0.1, as shown in Figure 7. If the reactor had been operating at a flux level of ~9 x 10¹³ neutrons/cm²s or higher, before shutdown, Figure 7 illustrates the reactor would not be able to start up again until after a wait of ~25 - 35 hours.

If, however, the reactor did have enough excess reactivity to override xenon, then a startup could be made during periods of peak xenon concentrations. Figure 8 shows the Xe-135 build-up after shutdown and compares normal decay with the burn-out of xenon at 50% and 100% reactor power during a startup at peak xenon concentrations.

Now U-238 has some very large resonances in its absorption cross section and if the neutron is slowed to the energy of one of these resonances, then it has a high probability of being captured. This process is called resonance absorption. The number of neutrons that escape resonance absorption compared to the number of neutrons that begin to slow down is the resonance escape probability factor, p.

The absorption of neutrons by the Xe-135 nuclei, of course, causes the rapid decrease in xenon poisoning. The decrease in poisoning has the same effect as the insertion of positive reactivity. In order to prevent the reactor from increasing in power level due to the increase in reactivity, the control rods must be appropriately inserted.

NOTE: Normally a TRIGA reactor is not operated for long periods of time at a high neutron flux so that xenon override seldom represents a major operational problem nor does it represent a problem during transient operations. In addition, because of the large core excess needed to overcome the strong negative temperature coefficient in TRIGA reactors, peak xenon does not prevent reactor start-up as may be the case in many other reactors.