The test plan called for a gradual decrease in reactor power to a thermal level of 700–1000 MW and an output of 720 MW was reached at 00:05 on 26 April. However, due to the reactor's production of a fission byproduct, xenon-135, which is a reaction-inhibiting neutron absorber, power continued to decrease in the absence of further operator action; a process known as reactor poisoning. In steady-state operation, this is avoided because xenon-135 is "burned off" as quickly as it is created from decaying iodine-135 by the absorption of neutrons from the ongoing chain reaction, becoming highly stable xenon-136. With the reactor power reduced, previously produced high quantities of iodine-135 were decaying into the neutron-absorbing xenon-135 faster than the reduced neutron flux could burn it off.