Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Xenon-135
Iodine-135 is a fission product of uranium with a yield of about 6% (counting also the iodine-135 produced almost immediately from decay of fission-produced tellurium-135). This I decays with a 6.57 hour half-life to Xe. Thus, in an operating nuclear reactor, Xe is being continuously produced. Xe has a very large neutron absorption cross-section, so in the high neutron flux environment of a nuclear reactor core, the Xe soon absorbs a neutron and becomes almost-stable Xe. Thus, in about 50 hours, the Xe concentration reaches equilibrium where its creation by I decay is balanced with its destruction by neutron absorption.