It is convenient first to outline the behavior of a poison that the fast reactor does not experience. In large thermal systems xenon produced from 135Te by way of 135I is now a very important poison. Because it is a byproduct of a fission product, it is flux-induced rather than temperature- induced. It is not an important poison in the fast reactor system.

The effect of xenon is complicated by its production from iodine and its destruction by a combination of neutron absorption and natural decay to 135Cs and by the various time delays involved. The production and destruc­tion is illustrated in Fig. 1.16.

n c

The relevant equations descriding the production and decay of iodine and production and destruction of xenon are:

dljdt = ахф — AjI dX/dt = AjI — AxX — ахфХ

After shut-down, the xenon concentration does not see the lack of pro­duction of tellurium for a time delay of several hours due to the 6.7-hr iodine decay time. However it does see an immediate reduction in removal by neutron capture; thus the xenon concentration grows. This increase in xenon concentration complicates the subsequent start-up of some small thermal reactors due to the poison increase.

Further, xenon poisoning could produce spatial instabilities in the very large thermal systems, as some regions of the core could see poison changes on different time scales from others. This effect is corrected by regional control systems.

In the fast spectrum, xenon is not important since its parasitic absorption is very low (Table 1.4) and the fast cores are too small for spatial instabili­ties. However, see the thorium cycle reactivity change in the next section, as it is a similar though opposite effect.

Other fast reactor absorbers are given in Table 1.6.