Fission product release tests with uranium dioxide fuel and lead under conditions related to reactor accidents for lead-cooled reactors.

• Ba release was observed in Pb-covered sample exposed to air. • Onset of Cs and Ru release was delayed for Pb-covered sample in air. • Cs and Ru were retained in Pb-covered sample exposed to steam. • Pb-covered samples in oxidizing environments have comparatively lower release rates of I. Tests of fission product release under accident conditions were performed on three samples of irradiated UO 2 fuel fragments immersed in molten Pb. Two tests of bare irradiated UO 2 fragment samples were conducted under similar conditions for direct comparison. The irradiated fuel samples had been cooled for several years, then encapsulated and trace-reirradiated to populate limited inventories of short-lived fission products (such as 133Xe and 131I). The tests were conducted with sample temperatures between 1570 and 1590 K. The releases from Pb-covered samples under inert conditions were similar to those from bare UO 2 under inert conditions. Greater 133Xe releases were observed from Pb-covered samples in oxidizing environments. Release of 140Ba was detected for the Pb-covered sample exposed to air. Under oxidizing conditions, Pb-covered samples retained more 137Cs and 103Ru (delayed onset of release in air and complete retention in steam) as well as 131I. [ABSTRACT FROM AUTHOR]