Laboratory studies in search of the critical hydrogen concentration $ Kotchaphan Kanjana, Kyle S. Haygarth, Weiqiang Wu, David M. Bartels n University of Notre Dame, Department of Chemistry and Biochemistry, Notre Dame Radiation Laboratory, Room 203C, Notre Dame, IN 46556, USA HIGHLIGHTS c The Critical Hydrogen Concentration was investigated up to supercritical temperature. c Steady state H 2 was measured for high dose rate conditions. c Virtually no O 2 was measured even at very high dose rates. c Several rate constants of the ’’accepted’’ kinetics model must be changed. article info Article history: Received 22 June 2012 Accepted 13 September 2012 Available online 27 September 2012 Keywords: Nuclear reactor Hydrogen water chemistry Water radiolysis Kinetics model Supercritical water Corrosion abstract Addition of H 2 to primary coolant water is widely used in the nuclear reactor industry to suppress water radiolysis and lower the corrosion potential. The critical hydrogen concentration (CHC) – the minimum concentration of excess H 2 that can completely suppress O 2 ,H 2 O 2 , and H 2 formation from water radiolysis – is an important quantity for the management of reactor water chemistry. For the design of future supercritical water cooled reactors, we have investigated whether water radiolysis can be suppressed with a reasonable overpressure of H 2 . Experiments were carried out using 2.5–2.8 MeV electrons from a van de Graaff accelerator, which can easily produce dose rates on the order of one kilogray/second, typical of power reactors. Radiolytic H 2 and O 2 production was measured as a function of excess dissolved H 2 . The results indicate that net radiolysis of water can be suppressed in supercritical water. Anomalous high H 2 concentrations were obtained using metal (hastelloy or titanium) irradiation tubing rather than sapphire or silica. We ascribe these results to a radiation- stimulated corrosion process at high temperature. Throughout the subcritical temperature regime, almost no oxygen is measured, even though kinetic modeling suggests there should be concentrations well above our detection threshold. To explain this result we recommend that unmeasured rate constants for  H þO 2  and (e  ) aq þO 2  should be considered completely diffusion-limited. At 300 1C, the (high dose rate) steady state H 2 concentration in pure water is almost completely determined by the equilibrium H 2 þ  OH3  HþH 2 O. The measured steady-state H 2 is in good agreement with the recent equilibrium constant estimate of Bartels (Radiation Physics and Chemistry 78(3): 191–194, (2009)). & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Addition of H 2 to primary coolant water (i.e., hydrogen water chemistry) is widely used in the nuclear reactor industry to suppress water radiolysis and lower the corrosion potential of stainless steel (Bilanin et al., 1987; Macdonald, 1992). Maintenance of the corrosion potential below  230 mV (SHE) has been shown to minimize the rate of stress corrosion cracking (Was et al., 2011). Water radiolysis increases the corrosion potential by producing free radicals and hydrogen peroxide (Macdonald, 1992): H 2 O ) e (aq)  ,  OH,  H, H þ , OH  ,H 2 ,H 2 O 2 (R1) The key effect of hydrogen addition in radiolysis occurs via the chain reaction (reaction numbers taken from Table 2):  OH þ H 2 3 H 2 O þ  H (R32b, R32f)  H þ H 2 O 2 ) H 2 O þ  OH (R12) In reaction (R32b) the principle oxidizing free radical (  OH) is converted into a reducing radical (  H). Oxidation products (H 2 O 2 and eventually O 2 ) are reduced back to water (reaction (R12) and others), and the net result is no chemical change (Elliot and Bartels, 2009; Elliot and Stuart, 2008). Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/radphyschem Radiation Physics and Chemistry 0969-806X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2012.09.011 $ The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. This is manuscript number 4920 of the Notre Dame Radiation Laboratory. n Corresponding author. Tel.: þ1 574 631 5561; fax: þ1 574 631 8068. E-mail address: bartels.5@nd.edu (D.M. Bartels). Radiation Physics and Chemistry 82 (2013) 25–34