Zirconium Cladding Deformation in a Steam Environment with Transient Heating

Zircaloy-4 cladding is being tested under loss-of-coolant accident (LOCA) conditions to determine its deformation behavior and to provide data for verification of analytical models. Data obtained thus far from 34 single-rod tests imply less ballooning, and consequently less flow restriction, than would be predicted from earlier data. The instrumented simulators, consisting of unirradiated Zircaloy-4 cladding with an internal heater to simulate fuel pellet heating, are tested to failure in superheated steam over a wide range of internal pressures at a temperature increase rate of about 28°C/s. Cladding surface temperature and internal pressure data are recorded during the transient and deformation measurements are obtained posttest. An analytical expression is given for the burst temperature as a function of burst pressure. The experimental results show excellent correlation between cladding deformation and surface temperature distribution. Deformation is extremely sensitive to small temperature variations.