High temperature dissolution of oxide deposits formed over structural materials under PHWR and BWR chemistry conditions

A formulation consisting of nitrilo triacetic acid (NTA) and hydrazine at high temperature (160 °C) was optimized for its application in cleaning an SS-316 “High Temperature High Pressure (HTHP) re-circulation system”, which was operated under Pressurized Heavy Water Reactor (PHWR) chemistry conditions. This system was contaminated with magnesium ions released from a failed heater pin. The attempt was successful with the removal of 110 g of iron, 35 g of magnesium along with chromium and nickel with acceptable corrosion of structural materials.The same formulation was tested for its efficiency in dissolving oxides formed under Boiling Water Reactor (BWR) conditions on SS-304 surfaces. Characterization of the oxide was carried out by wet chemical methods and by various surface techniques viz., SEM-EDAX, XRD, XPS and Laser Raman spectroscopy. SEM showed the presence of two distinct layers of oxide and XRD showed the oxide to be spinels as the major phase along with haematite (α-Fe2O3). The average thickness of the oxide coating was found to be 0.7 μm. NTA-hydrazine formulation at 160 °C was able to dissolve the oxide in a single step without requiring any oxidation step.

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► NTA-hydrazine formulation effectively dissolve oxides on stainless steel surfaces.
► NTA is stable at 160–165 °C over a period of 8 h
► Corrosion of SS and incoloy is negligible in NTA-hydrazine formulation.