The reactor core temperature of the reactor rises for initially unknown reasons and Charlie Isaacs fears a meltdown might occur. They theorize that Xenon-135 is produced within the reactor as a decay product of one of the fission products of the uranium fuel, and that this isotope then "poisons" the nuclear reaction due to its large neutron capture "cross-section". This simply means that the Xenon captures a lot of neutrons which are then no longer available for the fission chain reaction of the uranium, slowing it down. That indeed did historically occur in the semi-experimental X-10 reactor. However, if there is less fission, there is also less energy output and the temperature goes down, not up. So there is no risk of a meltdown in that scenario.