Patent Number: 053923251
Section: summary

FIELD OF THE INVENTION This invention generally relates to the catalytic reaction of two or more molecular species having dilute concentrations in fluids flowing in pipes at elevated temperatures. In particular, the invention relates to reducing the corrosion potential of components exposed to high-temperature (i.e., about 150.degree. C. or greater) water. BACKGROUND OF THE INVENTION In a boiling water reactor ("BWR"), the high-temperature (.about.288.degree. C.) water coolant is highly oxidizing due to dissolved radiolytically produced chemical species, such as oxygen and hydrogen peroxide. These molecules and/or compounds are generated as water passes through the reactor core and is exposed to very high gamma and neutron flux levels. Because of the resultant high electrochemical potential ("ECP"), reactor structural materials, such as stainless steels and nickel-based alloys, can suffer stress corrosion cracking ("SCC"). It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 5 ppb or greater. Stress corrosion cracking is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals are produced from radiolytic decomposition of the reactor water. Such oxidizing species increase the ECP of metals. Electrochemical corrosion is caused by a flow of electrons from anodic and cathodic areas on metallic surfaces. The ECP is a measure of the thermodynamic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of, e.g., SCC, corrosion fatigue, corrosion film thickening, and general corrosion. The useful lifetime of reactor components, such as piping and pressure vessel internal structures, can be limited by SCC. To date, SCC has resulted in a large inspection and repair cost in the nuclear industry and could eventually lead to premature decommissioning of BWR plants due to economic considerations. A number of countermeasures have been developed to mitigate SCC in BWRs by sufficiently reducing either the stress level, the material susceptibility to cracking, or the "aggressiveness" of the environment. Of the various mitigation strategies, reducing the environmental aggressiveness (i.e., oxidizing potential) can provide the broadest, most comprehensive approach, since the environment contacts all the potentially susceptible surfaces of interest. The primary strategy to reduce the ECP of the water to some benign value has been to add hydrogen gas to the reactor feedwater in sufficient quantity that hydrogen is available to chemically recombine, in the presence of a radiation field, with dissolved oxygen and hydrogen peroxide to form water. This process is called hydrogen water chemistry (HWC). If the hydrogen concentration is sufficient, the resultant ECP can be reduced below the SCC threshold value. As used herein, the term "threshold value" means a corrosion potential at or below a range of values of about -230 to -300 mV based on the standard hydrogen electrode ("SHE") scale. Stress corrosion cracking proceeds at an accelerated rate in systems in which the ECP is above the critical potential, and at a substantially lower rate in systems in which the ECP is below the critical potential. Water containing oxidizing species such as oxygen increases the corrosion potential of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in corrosion potentials below the critical potential. In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2 and O.sub.2. For steady-state operating conditions, equilibrium concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are established in both the water which is recirculated and the steam going to the turbine. This concentration of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 is oxidizing and results in conditions that can promote SCC in susceptible structural materials. When HWC is used to make the bulk coolant sufficiently reducing, the nitrogen isotope .sup.16 N, which is normally present in the water phase during reactor operation, partitions into the steam phase. This results in an increase in .sup.16 N gamma activity in the steam lines and turbine systems, which can exceed regulatory personnel radiation exposure limits at hydrogen addition levels needed for broad SCC protection. To reduce the .sup.16 N gamma activity to acceptable levels at these plants, it is now necessary to add shielding at strategic locations, which can be costly. In most cases, this consideration has limited use of HWC to protection of only those components where the ECP can be reduced below the SCC threshold without a significant increase in steam-phase .sup.16 N. SUMMARY OF THE INVENTION The present invention is a method and apparatus for protecting selected high-cost-impact reactor systems, such as piping, against SCC by reducing the ECP of these systems without an unacceptable increase in steam-phase .sup.16 N. The apparatus is a catalytic recombiner installed upstream of the piping or other system requiring SCC protection. The recombiner will facilitate the reaction of a small (stoichiometric) hydrogen addition with the dissolved oxygen and hydrogen peroxide present in the water entering the selected system. Thus, the ECP of the water exiting the recombiner will be reduced below the SCC threshold value and SCC will be prevented downstream of the recombiner at all system locations up to the point where the water either mixes with coolant containing higher concentrations of oxygen and/or hydrogen peroxide, or again passes through the reactor core, where radiolysis reoccurs. The instant invention consists of a catalytic recombiner constructed from a material with a catalytically active surface that facilitates the recombination of dissolved oxygen and hydrogen peroxide with hydrogen which is added as a gas to the water upstream of the recombiner. To increase the catalytic recombiner efficiency, the active surface area must be maximized per unit flow volume, consistent with the allowed system pressure drop. For any given system being protected, the allowable pressure drop increase due to the presence of the recombiner is set by the system design. To accomplish this chemical reaction process, the invention utilizes a recombiner having a high surface-to-volume ratio and constructed from a noble-metal alloy known to be an efficient catalyst in high-temperature water. The recombiner is constructed of relatively thin metal sheets of noble-metal alloy (e.g., at least about 1 wt. % palladium in stainless steel), fabricated into shells, plates, or continuous strips and configured to allow insertion into the upstream portion of the system to be protected. The recombiner may take the form of a cartridge of compact, rugged, modular design that can be designed for a wide range of pipe sizes and flow velocities. The recombiner structure includes means for mixing reactants between channels of the mass exchanger, thereby minimizing segregation and enhancing overall efficiency. The catalytic recombiner is arranged and situated such that all (except perhaps a small leakage flow) water phase which ultimately flows through the component to be protected will first flow over the surfaces of the catalytic material. The catalytic surfaces react with the water radiolysis products O.sub.2 and H.sub.2 O.sub.2 in the liquid phase to form H.sub.2 O in accordance with reactions such as (but not limited to) the following: ##STR1## Reaction (3) is followed by reaction (1) to produce water. Thus, the catalytic recombiner of the invention provides a means for substantially reducing oxygen concentration in fluids, thereby lowering the ECP and reducing the likelihood of SCC in susceptible downstream components. Although the invention is disclosed in the context of BWRs, it can be used with compressible or incompressible fluids, such as air or water at elevated temperatures, in a variety of technical applications that involve the catalytic reaction of undesirable compounds in solution. For example, the principle of the invention can be applied in systems for converting CO in exhaust systems (such as catalytic converters on automobiles) or systems for converting CO.sub.2 into H.sub.2 O (such as scrubbers in fossil fuel plants).