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Out-of-pile experiments have been performed to investigate the escalation in temperature of Zircaloy-clad fuel rods during heatup in steam due to the exothermal Zircaloy steam reaction. In these tests single Zircaloy/uranium dioxide (UO2) fuel rod simulators surrounded with a Zircaloy shroud—simulating the Zircaloy of neighboring rods—were heated inside a fiber ceramic insulation. The initial heating rates were varied from 0.3 to 2.5 K/s.
In every test an escalation of the temperature rise rate was observed. The maximum measured surface temperature was about 2200°C. The temperature decreased after the maximum had been reached without decreasing the input electric power. The temperature decreases were due to inherent processes including the runoff of molten Zircaloy. The escalation process was influenced by the temperature behavior of the shroud, which was itself affected by the insulation and steam cooling.
Damage to the fuel rods increased with increasing heatup rate. For slow heatup rates nearly no interaction between the oxidized cladding and UO2 was observed, while for fast heatup rates the entire annular pellet was dissolved by molten Zircaloy.
Zircaloy, zirconium alloys, nuclear industry, ultrahigh temperature, Zircaloy-steam reaction, temperature escalation, fuel-cladding interaction, uranium dioxide, Zircaloy-4 fuel degradation, degraded core, severe fuel damage
Hauptabteilung Ingenieurtechnik, Kernforschungszentrum Karlsruhe, Karlsruhe,