FISSION-PRODUCT TRANSPORT AND BEHAVIOR DURING 2 POSTULATED LOSS-OF-FLOW TRANSIENTS IN THE ADVANCED TEST REACTOR

The fission product behavior during two postulated loss-of-flow accide nts (leading to high- and low-pressure core degradations) in the Advan ced Test Reactor (ATR) has been analyzed. These transients are designa ted ATR transients LCP15 (high pressure) and LPP9 (low pressure). Norm ally, transients of this nature would be easily mitigated using existi ng safety systems and procedures. In these analyses, failure of these safety systems was assumed so that core degradation and fission produc t release could be studied. A probabilistic risk analysis was performe d that indicated that the probability of occurrence for these two tran sients is on the order of 10(-5) and 10(-7) per reactor year for LCP15 and LPP9, respectively. The fission product behavior analysis include d calculations of the gaseous and highly volatile fission product (xen on, krypton, cesium, iodine, and tellurium) inventories in the fuel be fore accident initiation, release of the fission products from the fue l in to the reactor vessel during core melt, the probable chemical for ms, and transport of the fission products from the core through the re actor vessel and existing piping to the confinement. In addition to a base-case analysis of fission product behavior, a series of analyses w as performed to determine the sensitivity of fission product release t o several parameters including steam flow rate, (structural) aluminum oxidation, and initial aerosol size. The base-case analyses indicate t hat the volatile fission products (excluding the noble gases) will be transported as condensed species on zinc aerosols. Approximately 40% o f the initial inventories of volatile fission products will be deposit ed within either the reactor vessel or the intact piping in the confin ement, and 60% of the inventory will be released into the open confine ment during transient LCP15. For transient LPP9, approximately 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 b e released into the open confinement. The different tellurium behavior within the reactor vessel during transient LPP9 was caused by chemica l absorption of tellurium on the stainless steel structural surfaces.