The Influence of Dissolved Hydrogen on the Solubility and Transport of Iron and Nickel in Reactor Coolant Systems

Paul Sherburne, AREVA

In Pressurized Water Reactors (PWRs), hydrogen is added to the reactor coolant to suppress the formation of oxidants produced in the core region by radiolysis. Currently, the concentration of dissolved hydrogen in the reactor coolant is restricted to a range of 25 – 50 cc/kg (STP) which is much higher than required to suppress radiolysis. At the same time, the incidences of stress corrosion cracking of structural nickel-based alloys; viz., Alloy 600 and weld metal Alloys 82 and 182, has increased, leading to repairs by weld overlay and sleeving, or to replacement of pressurizers and reactor vessel heads. It has been shown in the laboratory that increasing the dissolved hydrogen to concentrations greater than 50 cc/kg (STP) will decrease the growth rate of preexisting cracks and may delay the initiation of new cracks. These results have led to an initiative in the nuclear power industry to consider increasing the concentration of dissolved hydrogen in the reactor coolant to 80 cc/kg (STP) or even higher. While it is generally accepted that increasing reactor coolant hydrogen decreases crack growth rates, there are potentially unfavorable side effects. Increased hydrogen concentrations can, for example, result in increased release of dissolved base metals (iron, nickel and chromium) from the steam generator tubing (Alloy 600 in original units and Alloy 690 in replacement units) and from reactor coolant piping (austenitic stainless steel). These species can then deposit in-core on the fuel elements, leading to increased personnel dose rates, increased fuel crud levels, neutron flux imbalance (Axial Offset Anomaly), and possibly damage to the Zircaloy cladding. The likelihood of these negative consequences occurring is currently the subject of intense debate in the industry. Existing thermodynamic models and limited test data have been used to study the solubilities of nickel, nickel oxide, and iron in the reactor coolant as a function of dissolved hydrogen, pH and temperature. In this study, StreamAnalyzer™ (Mixed Solvent Electrolyte (MSE) model) was used to calculate the solubility of nickel and its oxides and their behavior in the reactor coolant system. Results of the study are summarized in this paper. Comparisons with available test data and other model results are also presented.

Paul A. Sherburne, AREVA

Paul Sherburne is an Advisory Engineer in the Plant Chemistry and Corrosion Engineering group of AREVA NP Inc. where he is currently leading the effort to establish primary and secondary water chemistry programs for the U.S. EPR (Evolutionary Pressurized Water Reactor). He has worked in the nuclear industry for more than 35 years, primarily in the areas of failure analysis, materials performance, corrosion testing, and plant chemistry. Paul has a B.S. in Aerospace Engineering from Iowa State University and an M.S. in Engineering from the University of Akron. He is a registered Professional Engineer in the Commonwealth of Virginia and a member of NACE International.

Return to Speakers Gallery

FAX
973.539.5922

ADDRESS
OLI Systems, Inc.
108 American Road
Morris Plains NJ 07950

EMAIL