Effects of Alpha-Beta Transformation on High Temperature (LOCA) Creep behavior of Zr-Alloys

Abstract

The Zircaloy material models used for evaluation of the cladding behavior during a hypothetical LOCA are usually based on creep rupture tests at constant temperature and constant heat-up rates. Tests with a more realistic thermal history, including a first temperature peak, show under certain conditions significant higher creep resistance and longer time-to-rupture. To find the cause of this phenomenon, different examinations were performed. Synchrotron radiation was used to investigate the alpha to beta phase transformation kinetic. This technique allows examination of the bulk material without any preparation. In these tests, diffraction patterns were evaluated during constant heat-up rates (up to 10 K/s) and under stepped temperature changes. Phase transition was found to be delayed at fast heat-up rates. Structural changes and oxidation were examined by metallography. Changes in microstructure were seen at temperatures greater than 850°C. Significant oxidation was found at temperatures above 1000°C.

Cladding burst behavior of Fe-based alloys under LOCA

“…Creep of Zr-based alloys under a LOCA transient is rather complex since the temperature range includes the αβ transition (~750-950°C) [15,16]. Therefore separate creep rates, in the distinct α, α+β, and β regions need to be considered for accurate consideration of cladding response.…”

Thermo-mechanical behavior of Zircaloy-4 claddings under simulated post-DNB conditions

“…the thermal history [4, 5,6]. Moreover, the metallurgical state of the claddings has a strong impact on its thermo-mechanical properties [4, 7,8]. Garde for instance [9], pointed out a superplastic behavior in the dual phase domain.…”