The Influence of Water Radiolysis on Corrosion By Supercritical Water

Development of is attractive because of the high thermal efficiency afforded by operating at supercritical temperatures.  The nuclear core materials to be used in supercritical water reactors (SCWR) will be exposed to the aqueous coolant at supercritical temperatures and under high radiation fields.  Understanding the response of the structural alloys to the aggressive chemical conditions generated by supercritical water radiolysis is important to ensure the integrity and enhance the usable life of SCWRs.  This paper reports on the corrosion behavior of Alloy 625, Incoloy® 800H, and Type-310 stainless steel, the candidate materials proposed for use in a Canadian pressure-tube SCWR heat transport system.  These materials were exposed to simulated reducing and oxidizing conditions in a Hastelloy® autoclave containing subcritical (saturated steam at 285°C) and supercritical water (at 400°C and 250 bar) in the absence of radiation, and while being subjected to gamma radiation at 285°C under saturated conditions, thereby simulating the in situ generation of water radiolysis products.  After exposure, the alloy specimens were retrieved and characterized by SEM-EDX, Raman spectroscopy, XPS, and AES depth profiling.  On all of these materials, the oxide films formed in the presence of radiation were thinner than those on unirradiated surfaces, and those formed in the presence of oxygen were thinner than those grown in the absence of oxygen.  The kinetics of oxide growth were found to be slower at the lower temperature.  The nature of the corrosion products formed and the role of various alloying elements in influencing the protection offered by the corrosion product oxide are discussed in detail.