Release of Iodine and Noble Gas Fission Products from Defected Fuel Elements During Reactor Shutdown and Start-Up

Data from a single defected fuel element operating in-reactor are used to develop a physically based model for describing the increased release of iodine and noble gas fission products to the primary coolant following a reactor shutdown and start-up. Iodines are only released on reactor shutdown, whereas both species are observed on start-up. A dependence on the decay constant of λ^-3/2 is typically observed for these transient releases, indicative of a source of release from fuel cracking with little holdup due to transport in the fuel-to-sheath gap.