Patent Number: 055770835
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIGS. 1 and 1A, a self-contained electrical system is depicted which supplies electrons to the surface of a sensitized metallic structural member 2, as in the case of the heat affected zone 6 of a weld 4. This is used as a typical example of a thermally sensitized metal zone, but sensitization could result from any known cause other than thermal sensitization. FIG. 1 shows a small, inexpensive, self-powered electrical system capable of supplying sufficient electrons to the metal surface to inhibit the corrosion reaction due to local ECP above the threshold value for IGSCC. The electronic components are fabricated from radiation-hardened semiconductors capable of withstanding relatively high .gamma.-radiation fields such as those encountered inside the reactor pressure vessel, but outside the core region. The device is not intended for use in the core, where neutron fluxes are sufficiently high to destroy the components. In accordance with the present invention, the source of electrons is the decay of a radioactive isotope, depicted in FIG. 1 as a current source 22. For ease of handling and fabrication, the isotope should be a .beta.-emitter (nuclear electrons) without decay .gamma.-radiation. A partial list of emitter isotopes includes H-3, C-14, Si-32, Sr-90 and Ru-106, with Ru-106 preferred because of its 368-day half-life and 39.4 keV .beta.-ray. This type of radiation is easily stopped by a thin metal wall, for example. All these isotopes have a single decay mode with no y radiation. About 1 gram of emitter material is sufficient to supply enough electrons for more than 2 years of operation. Referring to FIG. 1A, the source assembly is a flat disk 24 of metal, containing the emitter isotope 26 electrically isolated from the metallic collector 24 by ceramic stand-off 28 and ceramic feed-through 30. The narrow space 32 between the emitter 26 and the collector 24 is evacuated during fabrication to efficiently collect electrons formed in the nuclear decay of Ru-106. A small pump-out tube (not shown) is used to evacuate the source enclosure after a ceramic-to-metal seal of the feed-through is made. The collector material could be nickel, or a nickel-base alloy, and the ceramics could be alumina to thermally match the metal. These are typical materials, but other possible combinations exist, which allow the device to reliably operate at reactor temperature. Referring to FIG. 1, the power supply 20 requires no external power source, since it is energized by the nuclear decay electrons. Therefore, the device is self-contained, small and rugged when properly packaged using radiation-hardened integrated circuit technology (e.g., SiC semiconductor devices). The source current I.sub.S arises from the collection of nuclear decay electrons and produces a voltage across the source resistor R.sub.S which is a slowly decreasing function of time (because of the emitter decay). The Zener diode 24 and load resistor R.sub.L stabilize and limit the output voltage B.sub.+, since the voltage drop across the diode is essentially the same for all reverse currents I.sub.Z flowing through it in the breakdown region of the device. The voltage B.sub.+ is regulated and stabilized, since large changes in diode current produce small changes in diode voltage. The resulting voltage across the load resistor R.sub.L, due to the load current I.sub.L, is insensitive to the .beta.-emitter decay and can be used to power the active components in the control circuit. Evidently, this low-voltage DC power supply has a finite lifetime, since the emitter eventually decays to such a low activity level that it cannot produce a supply voltage sufficient to power the control circuit. This lifetime is determined by the type of emitter isotope, its activity and the design requirements of the operational amplifiers in the control circuit. Typically, it is about 2 years, or longer, if the emitter is Ru-106. The center electrical conductor of a small mineral-insulated steel sheathed cable 16 is attached to the metal surface and connected to an electrical control circuit 10 that operates off the low-voltage DC power supply 20. The control circuit 10 and DC power supply 20 are enclosed in a housing 8 made of material able to withstand thermal and radiological conditions inside a boiling water reactor. The passive conductor of a twisted-shielded pair of cable conductors is connected to a reference electrode 18 located in the oxidizing coolant near the metal surface and to a terminal of the control circuit. The current collected at the metal surface is controlled by the applied voltage on the load resistor R via an electrical conductor connected to the surface of the metal to be protected and to another terminal of the control circuit. This current I is converted to a voltage drop across R, which is input to a differential amplifier 12 of gain G. The differential amplifier output is the effective voltage "error signal", which is integrated by the operational amplifier 14 with time constant .tau.=R.sub.1 C. The small stand-off resistor R.sub.2 depletes excess charge build-up on the feedback capacitor C to eliminate any possibility of integrator malfunction. In accordance with the preferred embodiment of the control circuit 10, a first junction J1 is electrically connected to a first input terminal of the differential amplifier 12, to an output terminal of the operational amplifier 14 by way of a resistance R, and to the structural member 2 by a first electrical conductor; a second junction J2 is electrically connected to a first input terminal of the operational amplifier 14, to a first terminal of the resistance R.sub.2, and to the reference electrode 18 by a second electrical conductor; a third junction J3 is electrically connected to a second input terminal of the differential amplifier 12, to a second terminal of the resistance R.sub.2, and to a first terminal of the capacitance C; and a fourth junction J4 is electrically connected to a second terminal of the capacitance C, to the output terminal of the differential amplifier 12 byway of a resistance R.sub.l, and to a second input terminal of the operational amplifier 14. If the integrator output voltage V is positive, the current through the load resistor R is driven negative. If V is negative, the current through the load resistor R is driven positive. When the voltage drop is zero (i.e., the metal surface is at floating potential V.sub.f), the current to the surface is zero, the desired operating point. A simple circuit analysis shows that the current through R decays, from a non-zero value to zero, exponentially with time. The net effect is that the collection voltage "hunts" for the zero-current condition at the surface, sliding along the curve shown in FIG. 2. The collected current is dissipated in the load resistor R, which is sized to dissipate the small amount of energy. The gain and time constant are chosen so that the temporal behavior of the circuit averages any rapidly fluctuating changes in surface current arising in plant operations, further stabilizing operation. Therefore, no surface charge density can form, and no space-charge zone of either sign can exist near the metal surface for very long. Electron depletion of the metal and IGSCC are defeated by the invention, since electrons are forced to flow into the metal to compensate for those that would be lost by oxidation. In other words, the effective ECP of the metal/coolant couple is reduced below the local threshold automatically, no matter how the local water chemistry changes during reactor operations. There is no need to make a global change in ECP, since a small circuit "mouse" like that shown in FIG. 1 can be attached to a sensitized metal anywhere that can be accessed by the small MI cable. A multiplicity of such devices can be distributed around a weld, for example, to protect the entire heat-sensitized zone from IGSCC. The devices can be tack-welded to the metal member and all referenced to one, or a few, reference electrodes, as desired. The IGSCC suppression system in accordance with the invention provides means for electrically minimizing net electron transfer at the surface of sensitized metals exposed to a coolant electrochemical potential, thereby inhibiting the basic physical process of intergranular attack of metal alloys, such as austenitic stainless steels. It utilizes an adaptive DC circuit with active element feedback to adjust the surface potential to minimize the current to the metal surface, thereby minimizing corrosion, such as IGSCC, independently of the details of the coolant chemistry. The invention further provides means for supplying sufficient electron current to minimize, or eliminate, local IGSCC through the application of a regulated nuclear decay power supply, thereby giving the device a stand-alone capability useful in nuclear reactor applications. The invention utilizes a unique, compact configuration that is specific to the local minimization of IGSCC and is a low-cost answer to crack initiation and growth in reactors, caused by intergranular attack of austenitic stainless steels used in reactor internals and piping. In accordance with a further aspect of the invention, it provides means for essentially eliminating undesirable IGSCC without impacting the operating performance of the nuclear plant. Lastly, the unique IGSCC suppression system of the invention can be retrofitted into existing plants as a field replacement, with minimal time and cost, thereby reducing future maintenance and repair costs attributable to IGSCC in operating BWR plants. The invention is expected to be especially useful in operating plants where the water chemistry in vessels and piping can be variable, unpredictable and plant-specific. The invention also has application in other contexts, such as corrosion control in petrochemical or industrial environments. The preferred embodiments have been disclosed for the purpose of illustration only. Variations and modifications of those embodiments will be readily apparent to electronics engineers of ordinary skill. All such variations and modifications are intended to be encompassed by the claims appended hereto.