The fission product behavior during two postulated loss-of-flow accidents (leading to high- and low-pressure core degradations) in the Advanced Test Reactor (ATR) has been analyzed. These transients are designated ATR transients LCP15 (high pressure) and LPP9 (low pressure). Normally, transients of this nature would be easily mitigated using existing safety systems and procedures. In these analyses, failure of these safety systems was assumed so that core degradation and fission product release could be studied. A probabilistic risk analysis was performed that indicated that the probability of occurrence for these two transients is on the order of 10−5 and 10−7per reactor year for LCP15 and LPP9, respectively. The fission product behavior analysis included calculations of the gaseous and highly volatile fission product (xenon, krypton, cesium, iodine, and tellurium) inventories in the fuel before accident initiation, release of the fission products from the fuel into the reactor vessel during core melt, the probable chemical forms, and transport of the fission products from the core through the reactor vessel and existing piping to the confinement. In addition to a base-case analysis of fission product behavior, a series of analyses was performed to determine the sensitivity of fission product release to several parameters including steam flow rate, (structural) aluminum oxidation, and initial aerosol size. The base-case analyses indicate that the volatile fission products (excluding the noble gases) will be transported as condensed species on zinc aerosols. Approximately 40% of the initial inventories of volatile fission products will be deposited within either the reactor vessel or the intact piping in the confinement, and 60% of the inventory will be released into the open confinement during transient LCP15. For transient LPP9, ∼15% of the initial inventories of cesium and iodine and 30% of the tellurium fission products are deposited within either the reactor vessel or the intact piping in the confinement, and the remaining inventory would be released into the open confinement. The different tellurium behavior within the reactor vessel during transient LPP9 was caused by chemical absorption of tellurium on the stainless steel structural surfaces.