Patent Number: 052572960
Section: summary

BACKGROUND OF THE INVENTION Field of the Invention. This invention relates generally to chemical solvent cleaning systems and methods and, more particularly, to chemical solvent cleaning systems and methods for use in steam generators used in nuclear power generation to monitor the corrosive effects of chemical solvent cleaning of steam generators. Prior Art. Certain nuclear power stations using pressurized water reactor technology utilize a large heat exchanger, known as a steam generator, to transfer heat from the reactor coolant system (i.e. heat generated during nuclear fission) to a secondary system. The heat transfer results in the boiling of secondary steam feedwater, which produces steam to drive the plant's turbine, which in turn powers the electric generator. The steam is then condensed and returned to the steam generator in a continuous recirculation loop. The secondary feedwater contains trace quantities of contaminants, primarily iron oxide, which tend to plate out inside the steam generator, eventually resulting in flow blockage, corrosion, and/or loss of heat transfer capability. An emerging mitigation method for removing such contaminants involves the use of a chemical solvent, usually during unit shutdown, to dissolve the contaminants. The thermal efficiencies of steam generators are reduced due to accumulations of iron and copper corrosion deposits. In actual operation at Arkansas Nuclear One--Unit 1 (ANO-1) at Russelville, Ark., it was observed that such deposits occurred primarily between the fourth and ninth tube support plates of the once through steam generators (OTSG's) at ANO-1. Highly effective, low corrosive solvents for removing iron and copper deposits have been developed through a program sponsored by the Steam Generator Owners Group (SGOG), and the Electric Power Research Institute (EPRI). Prior art chemical cleanings of OTSG's utilize a process known as a "fill, soak and drain" (F/S/D) process. Some recirculation-type steam generators are cleaned using a circulation method, but only to clean the very lowest parts of the generator. F/S/D was developed to assure adequate mixing of solvent between the inner tube bundle of an OTSG and the annulus region (between the outer shell and inner shroud of the generator). This method, while effective for mixing, increased the overall time and thus the cost of the cleaning process by requiring additional steps to be performed throughout the cleaning process. Further, temperature differentials in the solvent between the top and bottom of the steam generator, as well as solvent stagnation in the annulus region of the generator created concerns about the effectiveness of F/S/D. F/S/D requires additional intermediate heating steps due to the time required to empty the generator. These intermediate steps further increased the overall time for the cleaning process as well as the amount of waste generated by the process. Further, F/S/D methods often use a high initial solvent temperature to offset cooling which takes place during the process, which can result in increased corrosion of internal steam generator components. A cleaning system and method was needed to assure adequate mixing of fresh solvent throughout the relevant portions of the steam generator. SUMMARY OF THE INVENTION Therefore, it is an object of this invention to provide a steam generator chemical solvent mixing system and method which increases mixing of chemical solvent within the steam generator It is another object of this invention to provide a steam generator chemical solvent mixing system and method which maintains a more constant solvent temperature within the steam generator. It is still another object of this invention to provide a steam generator chemical solvent mixing system and method which is easily operable within a steam generator without substantial modifications to the steam generator. It is a further object of this invention to provide a steam generator chemical solvent mixing system and method which is economical and efficient to operate and administer. It is still a further object of this invention to provide a steam generator chemical solvent mixing system and method which accomplishes various combinations of the above objects. Accordingly, a chemical solvent mixing system is provided in combination with a steam generator. The steam generator generally includes a plurality of generator tube support plates, each generator tube support plate having a plurality of openings. A plurality of generator tubes form a tube bundle, with each generator tube passing through aligned openings in the support plates. An outer casing surrounds the tube bundle and the generator tube support plates, the outer casing having an upper end and a lower end, upper access openings between the upper and lower ends, and lower access openings near the lower end. A tube bundle shroud, having an upper end and a lower end and a plurality of passageways in the lower end surrounds the tube bundle between the outer casing and the tube bundle. The tube bundle shroud forms an annulus between the outer casing and the shroud. The chemical solvent mixing system includes a pump, having a discharge port fluidly connected to a discharge line and a suction port fluidly connected to a suction line. An upper level injector, including an injector nozzle extends through one upper access opening in the outer casing and extends into the annulus, the upper level injector being fluidly connected to the discharge port of the pump. A lower access eductor is provided, including an eductor nozzle extending through one lower access opening and adapted to discharge solvent through one passageway in the tube bundle shroud and into the tube bundle, the lower access eductor being fluidly connected to the discharge port of the pump. A gas injection line is provided, fluidly connected at one end to a gas source and at the other end to the lower access eductor. A return line fluidly connects at one end to a second lower access opening and at the other end to the suction port of the pump. A method for chemically cleaning a steam generator is also provided utilizing the system to fill the steam generator with solvent and circulate the solvent within the generator.