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Experiments conducted at Oak Ridge National Laboratory both with fission product simulants and with irradiated commercial fuel have been utilized to develop a semi-empirical model of fission product release from defected Light Water Reactor (LWR) fuel rods. At fuel temperatures less than 1200°C, releases occur from fission products previously accumulated in the pellet-to-cladding gap region. In this temperature range, the release of species of moderate volatility is postulated to result from two processes. The first of these, which occurs during the period of fuel clad rupture, is due to the transport of the fill and fission product gases as they are vented through the cladding defect. The second mechanism for release, which is time-dependent, involves the diffusional transport of the semi-volatile species to the point of clad rupture through the interconnected voids (the pellet-to-cladding gap and cracks in fuel pellets) within the fuel rod.
For practical purposes the release of cesium and iodine from LWR fuel rods in the temperature range 500 to 1600°C can be considered to originate from three sources: (1) the gap inventory with release by both burst (vented gas) and diffusion; (2) grain boundary with release by tunnel formation; and (3) UO2 matrix with release by solid state diffusion. The chemical behavior of released iodine and cesium (at least after contact with steam) is predominantly that of CsI and CsOH. Fission gas release is the sum of the plenum inventory, gas embedded in the fuel and cladding surface layers and that released by tunnel formation, and solid state diffusion from the UO2 matrix. A small amount of large particle-sized fuel dust is ejected at time of rupture.
For practical purposes the release of cesium and iodine from LWR fuel rods in the temperature range 500 to 1600/sup 0/C can be considered to originate from three sources: (1) the gap inventory with release by both burst (vented gas) and diffusion; (2) grain boundary with release by tunnel formation; and (3) UO/sub 2/ matrix with release by solid state diffusion. The chemical behavior of released iodine and cesium (at least after contact with steam) is predominantly that of CsI and CsOH. Fission gas release is the sum of the plenum inventory, gas embedded in the fuel and cladding surface layers and that released by tunnel formation, and solid state diffusion from the UO/sub 2/ matrix. A small amount of large particle-sized fuel dust is ejected at time of rupture.
The principal objectives of the fission product release program are to determine the quantity of radiologically significant fission products released from defected LWR fuel rods under accident conditions, identify their chemical and physical forms, and interpret the results for use as input to computer models of postulated transportation and loss-of-coolant accidents. Experimental work with flowing steam in the temperature range 500 to 1200°C and with dry air at 500°C and 700°C has been completed. One series of tests, the Implant Test Series, employed simulated fission products which were coated on unirradiated UO2 fuel pellets; a second series, the Low Burnup Fuel Test Series, used fuel capsules irradiated to 1000 MWd/MT at high heat rating (560 to 660 W/cm), and a third series of experiments, the High Burnup Test Series, used fuel irradiated to 30,000 MWd/MT in the H.B. Robinson reactor at low heat rating (175 to 320 W/cm). Sufficient analytical results have been obtained to permit the formulation of a preliminary empirical model for cesium release in steam. The model assumes that cesium release is the sum of two components: burst release (that carried out with escaping plenum gas when the rod ruptures) and diffusion release (that diffusing from the gap space after the plenum gas has vented).
This paper discusses fission product release from light-water-reactor-type fuel rods to the coolant loop during design basis accident tests. One of the tests was a power-cooling-mismatch test in which a single fuel rod was operated in film boiling beyond failure. Other tests discussed include reactivity initiated accident (RIA) tests, in which the fuel rods failed as a result of power bursts that produced radial-average peak fuel enthalpies ranging from 250 to 350 cal/g. One of the RIA tests used two previously irradiated fuel rods. On-line gamma spectroscopic measurements of short-lived fission products, and important aspects of fission product behavior observed during the tests, are discussed. Time-dependent release fractions for short-lived fission products are compared with release fractions suggested by: the Reactor Safety Study; NRC Regulatory Guides; and measurements from the Three Mile Island accident. Iodine behavior observed during the tests is discussed, and fuel powdering is identified as a source of particulate fission product activity, the latter of which is neglected for most accident analyses.
The principal objectives of the fission product release program currently in progress at Oak Ridge National Laboratory are to determine the quantity of radiologically significant fission products released from defected light water reactor (LWR) fuel rods under accident conditions, identify their chemical and physical forms, and interpret the results for use as input to computer models of postulated spent fuel transportation accidents (SFTAs) and loss-of-coolant accidents (LOCAs). The purpose of the paper is to summarize the source term models which have been developed for cesium and iodine by this program, and to demonstrate the application of the source term models to the analysis of cesium and iodine release during a Pressurized Water Reactor (PWR) LOCA.