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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).
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.
Fission product release tests were performed with light water reactor (LWR) fuel rod segments containing large amounts of cesium and iodine in the pellet-to-cladding gap space in order to check the validity of the previously published Source Term Model for this type of fuel. The model describes the release of fission product cesium and iodine from LWR fuel rods for controlled loss-of-coolant accident (LOCA) transients in the temperature range 500 to 1200°C. The basis for the model was test data obtained with simulated fuel rods and commercial fuel irradiated to high burnup but containing relatively small amounts of cesium and iodine in the pellet-to-cladding gap space.
Models for cesium and iodine release from light-water reactor (LWR) fuel rods failed in steam were formulated based on experimental fission product release data from several types of failed LWR fuel rods. The models were applied to a pressurized water reactor (PWR) undergoing a hypothetical loss-of-coolant accident (LOCA) temperature transient. Calculated total iodine and cesium releases from the fuel rods were 0.053 and 0.025% of the total reactor inventories of these elements, respectively, with most of the release occurring at the time of rupture. These values are approximately two orders of magnitude less than releases used in WASH-1400, the Reactor Safety Study.
A series of experiments was conducted with highly irradiated light-water reactor fuel rod segments to investigate fission products released in steam in the temperature range 500 to 1200°C. (Two additional release tests were conducted in dry air.) The primary objectives were to quantify and characterize fission product release under conditions postulated for a spent-fuel transportation accident and for a successfully terminated loss-of-coolant accident (LOCA). In simulated, controlled LOCA-type tests, release at the time of rupture proved to be more significant than the diffusional release that followed. Comparison of the release data for the dry-air tests with the release data of similarly conducted tests in steam indicated significant increases in the releases of iodine, ruthenium, and cesium in air. Various parameters that affect fission product release are discussed, and experimental observations and analysis of the chemical behavior of releasable fission products in inert, steam, and dry-air atmospheres are examined.
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.
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.