Fission product phases, irradiated fuel

The Purex process is used for almost all fuel reprocessing today. Irradiated UO2 fuel is dissolved in HNO3 with the uranium being oxidized to U02(N03)2 and the plutonium oxidized to Pu(NC>3)4. A solution of TBP in a high-boiling hydrocarbon, such as n-dodecane, is used to selectively extract the hexavalent U02(N03)2 and the tetravalent Pu(NC>3)4 from the other actinides and fission products in the aqueous phase. The overall reactions are... 

After dissolution of the chopped irradiated fuel in boiling 3 M HNO3, the resultant mixture contains mainly U022+, Pu4+ and a small amount of Pu022+ that are extractable into the organic phase, fission products including inex-... 

The other possibility is determination of the concentration of appropriate fission products in the irradiated fuel. The fundamentally large number of fission products that can be chosen from is reduced by several conditions that have to be satisfied in order to obtain reliable values on the fuel burnup. First, the halflife of the nuclide must be sufficiently long to allow integration over the entire operation period therefore, its halflife should be longer by at least a factor of about three than the irradiation period of the fuel to preclude an overestimation of the final phase of irradiation. (On the other hand, measurement of short-lived nuclides can... 

Despite the limited solubility of BaO in UO2 (quite in contrast to SrO, which is highly soluble), barium seems to be homogeneously distributed in the irradiated fuel matrix, presumably as Ba(II) in the UO2 lattice. Barium can also be incorporated into the perovskite-type grey phase which was detected predominantly in high-burnup fast breeder reactor fuels however, the significance of this phase in irradiated LWR fuels seems to be questionable. The chemical state of fission product barium in the oxide fuel apparently depends strongly on the stoichiometry of... 

Kleykamp, H. (b) Post-irradiation examination and composition of the residues from nitric acid dissolution experiments of high-bumup fuel. J. Nucl. Materials 171, 181 — 188 (1990) Kleykamp, H., Paschoal, J. O., Pejsa, R., Thiimmler, F. Composition and structure of fission product precipitates in irradiated oxide fuels Correlation with phase studies in the Mo-Ru-Rh-Pd and BaO-U02-Zr02-Mo02 systems. J. Nucl. Materials 130, 426-433 (1985) Kopp, D., Munzel, H. Release of volatile carbon-14 containing products from Zircaloy. J. Nucl. Materials 173, 1-6 (1990)... 

For this experiment, the reactor core had been equipped with a central fuel module containing 100 pre-pressurized fuel rods, the UO2 fuel of which had been enriched to 9.7% and which had been pre-irradiated to a bumup of about 450 MWd/t U. The transient phase started with the reactor scram and was terminated about 30 minutes later when the external temperatiure on the surface of the shroud of the central fuel module reached 1517 K at this time, the highest measured cladding temperature reached 2100 K. When reflooding of the reactor core with emergency coolant was started, a rapid temperature excursion occurred within the central fuel module which was caused by the enhanced metal—water reaction. The transient was followed by a post-transient period of 44 days during which the reactor core was cooled by recirculating coolant and the concentrations of fission products deposited in the primary coolant system as well as their behavior in the blowdown suppression tank were measured. 

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