High temperature dissolution of oxide deposits formed over structural materials under PHWR and BWR chemistry conditions V.S. Sathyaseelan, A.L. Rufus, P. Chandramohan, H. Subramanian, T.V.K. Mohan, S.V. Narasimhan, S. Velmurugan * Water and Steam Chemistry Division, Bhabha Atomic Research Centre Facilities, Kalpakkam 603 102, TN, India article info Article history: Received 29 November 2011 Accepted 2 April 2012 Keywords: Oxide Dissolution Corrosion NTA Hydrazine abstract 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 (a-Fe 2 O 3 ). The average thickness of the oxide coating was found to be 0.7 mm. NTA-hydrazine formulation at 160  C was able to dissolve the oxide in a single step without requiring any oxidation step. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Periodic decontamination of nuclear power plants; wherein contaminated corrosion product oxides deposited on the system surfaces are dissolved by chemical formulations, is carried out to reduce the radiation exposure hazard to operating and mainte- nance personnel. Dissolution of magnetite (Fe 3 O 4 ), the predomi- nant oxide formed over carbon steel surfaces of Pressurized Heavy Water Reactors (PHWRs) operating under reducing and alkaline conditions is quite easy. There are many studies citing high disso- lution efficiency for magnetite and substituted magnetite at lower temperatures below 100  C(Rodenas et al., 2008; Manjanna et al., 2001). Various chemical reagents viz., mineral acids and organic acids have been employed for chemical decontamination (Rufus et al., 2004; Velmurugan et al., 1995; Ayers, 1970). However, the use of dilute solutions of carboxylic and amino poly carboxylic acids offer many advantages such as low base metal corrosion, high oxide dissolution efficiency, no re-precipitation and so on. The oxides found on the surfaces of Boiling Water Reactors (BWRs) working under oxidizing conditions viz., chromium rich spinels in the inner layer and a mixture of ferrites and haematite (a- Fe 2 O 3 ) in the outer layer are not that easy to dissolve (Chien et al., 1996). Mixed ferrites and chromites are also found on non-carbon steel surfaces viz., stainless steel, incoloy and inconel of PHWRs. While haematite and ferrites require strong reducing conditions for their effective dissolution, chromites are better dissolved in a multi- step multi-cycle procedure involving alternate oxidation and reduction processes. Thus, there is a need for effective process for dissolving the above mentioned oxides, which can be achieved either by employing better reagents and/or manipulating the process parameters viz., temperature. There are several studies reported in literature on employing high temperature processes for the disso- lution of oxides (Sellers and Williams, 1984; Turner, 1978; Odar and Kuhnke, 1992). Some of our earlier studies on the influence of temperature on the dissolution of magnetite have been published else where (Sathyaseelan et al., 2011). A High Temperature High Pressure (HTHP) stainless system that was operated under PHWR conditions got contaminated with magnesium oxide (insulator in heater pins), which was released into the system due to the failure of the heater pins within two * Corresponding author. Tel.: þ91 44 27480097; fax: þ91 44 27480203. E-mail address: svelu@igcar.gov.in (S. Velmurugan). Contents lists available at SciVerse ScienceDirect Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene 0149-1970/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.pnucene.2012.04.003 Progress in Nuclear Energy 59 (2012) 100e106