Temperature Escalation in PWR Fuel Rod Simulator Bundles Due to the Zircaloy/Steam Reaction: Post Test Investigations of Bundle Test ESBU-2A

摘要

This KfK report describes the post test investigation of bundle experiment ESBU-2a. ESBU-2a was the second of two bundle tests on the temperature escalation of Zircaloy clad fuel rods. The investigation of the temperature escalation is part of the program of out-of-pile experiments performed within the frame work of the PNS-Severe Fuel Damage program. The bundle was composed of a 3 x 3 fuel rod array of our fuel rod simulators (central tungsten heater, UO/sub 2/-ring pellet and Zircaloy cladding). The length was 0.4 meter. The bundle was heated to a maximum temperature of 2175/sup 0/C. Molten cladding which dissolved part of the UO/sub 2/ pellets and slumped away from the already oxidized cladding formed a lump in the lower part of the bundle. After the test the bundle was embedded in epoxy and sectioned with a diamand saw, in the region of the refrozen melt. The cross sections were investigated by metallographic examination. The refrozen (U,Zr,O) melt consists variously of three phases with increasing oxygen content (metallic alpha -Zry, metallic (U,Zr) alloy and a (U,Zr)O/sub 2/ mixed oxide), two phases ( alpha -Zry, (U,Zr)O/sub 2/ mixed oxide), or one phase ((U,Zr)O/sub 2/ mixed oxide). The cross sections show the increasing oxidation of the cladding with increasing elevation (temperature). A strong azimuthal dependency of the oxidation is found. In regions where the initial oxidized cladding is contacted by the melt one can recognize the interaction between the metallic melt and ZrO/sub 2/ of the cladding. Oxygen is taken away from the ZrO/sub 2/. If the melt is in direct contact with steam a relatively well defined oxide layer is formed. (ERA citation 12:026820)