In general, after a nuclear plant is shut down, it has to be kept off for a few days to let Xe-135 completely decay. Quoting from Wikipedia [1] " After about 3

In general, after a nuclear plant is shut down, it has to be kept off for a few days to let Xe-135 completely decay. Quoting from Wikipedia [1] " After about 3 days of shutdown, the core can be assumed to be free of 135Xe, without it introducing errors into the reactivity calculations" 1. https://en.wikipedia.org/wiki/Iodine_pit#Fission_products_decay_and_burnup https://en.wikipedia.org/wiki/Iodine_pit#Fission_products_de...