Analysis of Reaction between UO_2 Pellet and Zircaloy Cladding Under Severe Fuel Dmage(SFD) Conditions

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One of the purposes of this work was to obtain information on the effect of the initial oxygen concentration of -Zr on the morphology of the interfacial zone resulting from the reaction with UO_2. From the reactions of UO_2 with -Zr(o) of seceral different oxygen concentrations,it was found that the reaction zone varies with the oxygen concentration. More specifically, the three-layer structure develops at low oxygen concentrations, and as the concentration increases,the zone exhibits a less-layered character until a point is reached where no reaction occurs between UO_2 and -Zr(O).In the equilibrium study, a considerable amount of work has been expended in an effort to construct the isothermal sections of the U-Zr-O system at 1000 and 1400゜C. The section at 1400゜C differs from that at 1000゜C in that the two additional phase regions containing a liquid-uranium-rich (U,Zr) alloy occur at 1400゜C. It was confirmed that the liwquid phase exists in the isothermal section at 1400゜C. It should be noted that at 1400゜C UO_2 can equilibrate with -Zr(O) of oxygen content between 25 and 30 at%. Zirconium can reduce UO_2 and can produce liquid uranium as long as the oxygen content of Zr is less than about 25 at%. However, above 25 at% oxygen, Zr can no longer take up oxygen from UO_2. Tentative tie lines were determined in the two-phase region of -Zr(O) and (U,Zr) at 1000 and 1400゜C. A calculated isothermal section at 1000゜C based on a thermodynamic analysis agrees well with the experimentally determined section.