Advanced Corrosion-Resistant Zr Alloys for High Burnup and Generation IV Applications

Abstract

Research ObjectivesThe objective of this collaboration between four institutions in the US and Korea is to demonstrate a technical basis for the improvement of the corrosion resistance of zirconium-based alloys in more extreme operating environments (such as those present in severe fuel duty cycles (high burnup, boiling, aggressive chemistry) and to investigate the feasibility (from the point of view of corrosion rate) of using advanced zirconium-based alloys in a supercritical water environment. This technical basis is to be obtained through the comparison of the corrosion kinetics and the examination of the fine structure of oxide layers formed in model alloys. These model alloys are designed to isolate specific features of the microstructure thought to affect the formation of the protective oxide layer so that their effect on the corrosion rate can be studied individually. The key aspect of the program is to rationalize the differences in corrosion kinetics between alloys through the differences in the structure and evolution of the protective oxide formed on each alloy. The structural differences in the oxides were studies using advanced characterization techniques (including submicron-beam synchrotron radiation diffraction and fluorescence, cross-sectional transmission electron microscopy (TEM), transmitted light optical microscopy, and nano-indentation) to characterize both the metal and the oxide so that we can also relate these differences in oxide structure to the original microstructure of the alloy. Results and ConclusionsA detailed study has been conducted to address the issue of susceptibility of model Zr alloys to uniform corrosion in the proposed supercritical water reactor and of the specific role of alloying elements. Thirty model alloys were corrosion-tested for periods over 400 days in 360ºC water (as well as limited testing in lithiated water) and from 150 to over 300 days in 500ºC supercritical water. The corrosion results show a wide range of corrosion behavior with the data included in Appendices A, B, and C and discussed in Tasks 2 and 3.The general agreement between the three different tests performed at 500°C was quite good. In particular, little difference was seen between the results from static and dynamic autoclave testing. The results from lower pressure steam testing agreed with the supercritical water results but showed slightly less oxide growth. Since the rankings of the alloys were preserved from test to test, it is possible the steam test could serve as a preliminary screening test for supercritical water behavior.1. Corrosion tests for the model Zr alloys showed a wide variety of corrosion resistance depending on the alloy composition during both 360ºC and at 500º testing. These differences manifested themselves in the difference in pre-transition kinetics (as characterized by values of A and n) and in the different tendencies for breakaway.2. In comparison with other alloys being considered for the SCWR, the Zr alloys showed higher corrosion rates than austenitic alloys and...