Patent Number: 052873927
Section: summary

FIELD OF THE INVENTION This invention relates to reducing the corrosion potential of components exposed to high-temperature water. As used herein, the term "high-temperature water" means water having a temperature of about 150.degree. C. or greater, steam, or the condensate thereof. High-temperature water can be found in a variety of known apparatus, such as water deaerators, nuclear reactors, and in steam-driven central station power generation. BACKGROUND OF THE INVENTION Nuclear reactors are used in central-station electric power generation, research and propulsion. A reactor pressure vessel contains the reactor coolant, i.e. water, which removes heat from the nuclear core. Respective piping circuits carry the heated water or steam to the steam generators or turbines and carry circulated water or feedwater back to the vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288.degree. C. for a boiling water reactor, and about 15 MPa and 320.degree. C. for a pressurized water reactor. The materials used in both boiling water and pressurized water reactors must withstand various loading, environmental and radiation conditions. Some of the materials exposed to high-temperature water include carbon steel, alloy steel, stainless steel, nickel-based alloys, and cobalt-based alloys. Despite the careful selection and treatment of these materials for use in water reactors, corrosion occurs on the materials exposed to the high-temperature water. Such corrosion contributes to a variety of problems, e.g., stress corrosion cracking, sticking of pressure relief valves, buildup of the gamma radiation emitting .sup.60 Co isotope and erosion corrosion. Stress corrosion cracking is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. As used herein, the term "stress corrosion cracking" (hereinafter "SCC") means cracking propagated by static or dynamic stressing in combination with corrosion at the crack tip. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC. It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 5 parts per billion (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 electrochemical corrosion potential of metals. Electrochemical corrosion is caused by a flow of electrons from anodic and cathodic areas on metallic surfaces. The corrosion potential 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. Stress corrosion cracking in boiling water nuclear reactors and the associated water circulation piping has historically been reduced by injecting hydrogen in the water circulated therein. The injected hydrogen reduces oxidizing species in the water, such as dissolved oxygen, and as a result lowers the corrosion potential of metals in the water. However, factors such as variations in water flow rates and the time or intensity of exposure to neutron or gamma radiation result in the production of oxidizing species at different levels in different reactors. Thus, varying amounts of hydrogen have been required to reduce the level of oxidizing species sufficiently to maintain the corrosion potential below a critical potential required for protection from SCC in high-temperature water. As used herein, the term "critical potential" 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 for the case of pure water. Stress corrosion cracking proceeds at an accelerated rate in systems in which the electrochemical potential is above the critical potential, and at a substantially lower rate in systems in which the electrochemical potential 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 boiling water reactor (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 of susceptible materials of construction. One method employed to mitigate SCC of susceptible material is called hydrogen water chemistry, whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding hydrogen gas to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species to reform water, thereby lowering the concentration of dissolved oxidizing species in the water. The rate of these recombination reactions is dependent on local radiation fields, flow rates and other variables. Corrosion potentials of stainless steels in contact with reactor water containing oxidizing species can be reduced below the critical potential by injection of hydrogen into the water in a concentration of about 50 to 100 ppb or greater. For adequate feedwater hydrogen addition rates, the conditions necessary to inhibit SCC can be established in certain locations of the reactor. These conditions are an electrochemical potential of less than -0.230 V.sub.she. Different locations in the reaction system require different levels of hydrogen addition, as shown in FIG. 2. Much higher hydrogen injection levels are necessary to reduce the corrosion potential within the high radiation flux of the reactor core, or when oxidizing cationic impurities, e.g., cupric ion, are present. However, feedwater hydrogen additions, e.g., of about 200 ppb or greater, that reduce the corrosion potential below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived .sup.16 N species, as shown in FIG. 3. For most BWRs, the amount of hydrogen addition required to provide mitigation of SCC of pressure vessel internal components results in an increase in the main steam line radiation monitor ("MSLRM") by a factor of greater than about four. This increase in main steam line radiation can cause high, even unacceptable, environmental dose rates that can require expensive investments in shielding and radiation exposure control. Accordingly, although the addition of hydrogen lowers the corrosion potential of reactor water, it is also desirable to limit the amount of hydrogen in reactor water, while maintaining the corrosion potential below the critical potential. The primary products of water radiolysis in the core are H.sub.2, H.sub.2 O.sub.2, OH, H and the hydrated electron. In irradiated water, O.sub.2 and H.sub.2 O.sub.2 are in a state of dynamic equilibrium. During HWC, the computed ratio of H.sub.2 O.sub.2 to O.sub.2 in the downcomer annulus is large. The reason reported by M. Ullberg et al., "Hydrogen Peroxide in BWRs", Water Chemistry for Nuclear Reactor Systems 4, BNES, London, 1987, pp. 67-73, is that the H.sub.2 added during HWC initially slows down the oxidation of H.sub.2 O.sub.2 to O.sub.2, speeds up the reduction of O.sub.2 to H.sub.2 O.sub.2 and has little effect on the reduction of H.sub.2 O.sub.2 to H.sub.2 O. Thus, hydrogen peroxide is relatively stable in the recirculation water of a BWR. It is further known from the Ullberg et al. article that H.sub.2 O.sub.2 in water will decompose on a heterogeneous solid surface at elevated temperatures by the reaction: EQU 2H.sub.2 O.sub.2 +Surface.fwdarw.2H.sub.2 O+O.sub.2 This decomposition of H.sub.2 O.sub.2 is referred to as heterogeneous decomposition. The rate of decomposition can be increased through the use of decomposition catalysts and will also be dependent on the temperature and the ratio of surface area to volume. SUMMARY OF THE INVENTION The present invention improves upon the conventional BWR operated in accordance with the HWC principle by incorporating a passive structure immediately downstream of the steam separator assembly which catalyzes the decomposition of hydrogen peroxide only or which catalyzes both the decomposition of hydrogen peroxide and the recombination of water. The only difference in the structure of the respective catalyzers is that the catalyzing structure which recombines water includes a water recombination catalyst, such as a noble metal, whereas the catalyzing structure which decomposes hydrogen peroxide without recombining water includes no water recombination catalyst. The present invention improves upon known HWC techniques by allowing the achievement of specified conditions at key locations in the reactor system by addition of relatively lower levels of hydrogen to the feedwater. Thus, he negative side effect of high main steam line radiation increase can be avoided. In addition, the amount of hydrogen required and associated costs will be reduced significantly. One preferred embodiment of the invention is a passive recombiner operating in the water phase of the BWR vessel immediately downstream of the steam/water separator location. This recombiner comprises a catalytic material arranged and situated in an open structure having a high surface area-to-volume ratio such that all (except perhaps a small leakage flow) water phase exiting the steam/water separator device flows over the surface of the catalytic material. The catalytic recombining surfaces react with the water radiolysis product species H.sub.2, O.sub.2 and H.sub.2 O.sub.2 in the liquid phase to reform water in accordance with reactions such as (but not limited to) the following: ##STR1## Reaction (3) is followed by reaction (1) to produce water. The passive catalytic recombiner of the invention is constructed to ensure that the pressure drop of the reactor water across the device is very small (less than 5 psi). In addition, the catalytic material must be corrosion resistant in pure water under BWR conditions and have structural strength at reactor temperatures. The recombiner includes a stainless steel flow-through housing packed with catalytic recombiner material, which could take the form of tangled wire or strips, crimped ribbon, porous sintered metal composite, a honeycomb structure or any other structure having a high surface area-to-volume ratio. The preferred catalytic recombiner material is stainless steel plated or alloyed with a noble metal. In accordance with another preferred embodiment of the invention, a passive catalytic decomposer is provided in a conventional BWR by installing the same flow-through structure as that used for the recombiner, except that the material making up the high surface area-to-volume structure does not incorporate a water recombination catalyst. The decomposer is made of a solid material having surfaces which cause heterogeneous decomposition of hydrogen peroxide, but which do not catalyze water recombination. The preferred catalytic decomposer material is stainless steel because of its predictable performance in a BWR environment. However, other solid materials which cause heterogeneous decomposition and which have structural strength and corrosion resistance suitable for the BWR environment can be used. The catalytic surfaces of the decomposer react with the water radiolysis product H.sub.2 O.sub.2 in the liquid phase to decompose H.sub.2 O.sub.2 in accordance with reaction (3).