Increasing, but still low, radiation fields due to a release of fission products have been observed in the light-water-filled reactor container of SLOWPOKE-2 reactors fueled with a highly enriched uranium alloy. To investigate this phenomenon, samples of water coolant and headspace gas from the reactor container have been examined by gamma spectroscopy methods for several reactors with various burnup. A model has been developed to describe the kinetic behavior of the activity concentrations of the short-lived iodine and noble gas species in the reactor container water, and the noble gas concentrations in the reactor container headspace. The most likely source of the fission product release is an area of uranium-bearing material exposed to the coolant at the end weld line of the fuel elements that originated at the time of fuel fabrication. The fission product release analysis is consistent with observations from an underwater visual examination of a high-burnup core and a metallographic examination of archived fuel elements.