 Some radioactive isotopes of xenon , for example , 133Xe and 135Xe , are produced by neutron irradiation of <unk> material within nuclear reactors . 135Xe is of considerable significance in the operation of nuclear fission reactors . 135Xe has a huge cross section for thermal neutrons , 2 @.@ 6 × 106 <unk> , so it acts as a neutron <unk> 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 <unk> disaster . A <unk> or decrease of power of a reactor can result in buildup of 135Xe and getting the reactor into the iodine pit . 
