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Electron probe microanalysis (EPMA) and optical microscopy results were obtained on a disc from the melted fuel bundle of the first Phebus test (FPTO). In this test, 20 lightly irradiated UO(2) fuel rods and a central Ag-In-Cd control rod were heated by fission power to an estimated 3000 C. The disc was cut from the bottom of the molten pool in the lower part of the test assembly. The corium matrix of the solidified molten pool was a single homogeneous phase of composition (U(0.5) Zr(0.47) Fe(0.02) Y(0.01)) O(2). It had reached a temperature of at least 2900 C, but at the axial location of the disc the molten pool had cooled from a lower temperature. The rate of cooling had been sufficient to inhibit separation of the Zr-rich tetragonal and monoclinic (U,Zr) O(2) phases. Inclusions in the corium were Relr alloys, W oxide and sesquioxides based on Cr and ferric iron, Attack of the ZrO(2) thermal shrouds by liquid corium was increased by the presence of Fe, Ni and Cr in the melt. Appreciable concentrations of Ag and In from the central control rod were found only in a region of the disc where limited melting had occurred, discovered combined with Zr in two intermetallic compounds. In the same region, severe steam oxidation of Zircaloy had resulted in the rejection of the alloying element tin which was also a component of the two intermetallic compounds. Cadmium, the control rod constituent with the lowest concentration was not detected.

Additional information

Authors: BOTTOMLEY P D W, JRC Karlsruhe (DE);MONTIGNY F, JRC Karlsruhe (DE);STALIOS A D, JRC Karlsruhe (DE);WALKER C T, JRC Karlsruhe (DE)
Bibliographic Reference: Paper presented: EMAS '97: 5th International Workshop on Modern Developments and Applications in Microbeam Analysis, Torquay (GB), May 11-15, 1997
Availability: Available from (1) as Paper EN 40445 ORA
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