Interaction between aluminium oxide pellets and Zircaloy tubes in steam atmospheres at temperatures above 12000C

The burnable poison rods in light water reactors (LWR) consist of Al2O3/B4C pellets surrounded by Zircaloy-4 cladding tubes. In the Al2O3/B4C pellets of a LWR rod alumina is the main constituent (98.6 wt.-%) whereas boron carbide acts as neutron absorber. Failure of the Al2O3/Zircaloy test rods started at 13500C when first droplets of molten material were observed running down the test bundle forming bundle blockages upon solidification. Post test examinations revealed that the process of liquefaction was initiated by a reduction of alumina by Zircaloy resulting in a (Zr, Al, O) melt which decomposed on cooldown into two metallic phases, a (Zr, Al) alloy and oxygen-stabilized a-Zr(O). The components of an extremely porous ceramic melt were also Zr, Al, and oxygen but with a higher oxygen content compared to the metallic melt. The ceramic melt decomposes on cooldown into an Al2O3/ZrO2 eutectic with various amounts of primary constituents. Other types of relocated material were due to melting of essentially unreacted Zircaloy cladding and to debris formation by fracturing of oxidized cladding and Al2O3 pellets stack residues. The interactions between Al2O3 and Zircaloy occurring in a burnable poison rod are furthermore important for the behavior of the entire LWR core because the generated metals are able to attack the UO2 chemically and dissolve or liquefy the fuel even below the melting point of Zircaloy (17600C). As a result, fuel elements which contain burnable poison rods are expected to fail under severe accident conditions at about 15000C. (orig./HP)