EFFECT OF THE INTERMEDIATE COOLING PROCESS ON THE BEHAVIOR OF ZIRCALOY-4 CLADDING AFTER A HIGH-TEMPERATURE OXIDATION

Studies were conducted to investigate the effect of the intermediate cooling process on the thermal shock behavior of Zircaloy-4 fuel cladding under a simulated loss-of-coolant accident condition and to analyze the related mechanical and microstructural properties. The Zircaloy-4 specimen was oxidized at the desired temperature and time, then various cooling processes were applied such as the direct water quench, the intermediate cooling at 700 degrees C for 200 and 2000 s, and the successive cooling from 950 to 700 degrees C. The results showed that the direct water quenching without any intermediate cooling process reduced the cladding ductility in that it reduced the minimum equivalent cladding reacted from 20 to near 17%. Ring compression ductility decreased, and the minimum thickness of the prior-beta layer thickness that causes brittle failure increased from 0.3 to 0.4 mm in the case of the direct water quench condition. As the cooling rate increased, the size of the plate inside the prior-beta phase decreased so that it induced an increase in the residual dislocation density to result in a decrease of the cladding ductility. Additional oxidation effect during a slow cooling below 950 degrees C had little influence on the cladding behavior.