Some radioactive isotopes of xenon , for example , 133Xe and 135Xe , are produced by neutron irradiation of fissionable material within nuclear reactors . 135Xe is of considerable significance in the operation of nuclear fission reactors . 135Xe have a huge cross section for thermal neutrons , 2 @.@ 6 × 106 barns , so it act as a neutron absorber or " poison " that can slow or stop the chain reaction after a period of operation . This was discovered in the earliest nuclear reactors built by the American Manhattan Project for plutonium production . Fortunately the designers had made provisions in the design to increase the reactor 's reactivity ( the number of neutrons per fission that go on to fission other atoms of nuclear fuel ) . 135Xe reactor poisoning played a major role in the Chernobyl disaster . A shutdown or decrease of power of a reactor can result in buildup of 135Xe and getting the reactor into the iodine pit .