Patent Number: 039765415
Section: summary

BACKGROUND OF THE INVENTION This invention relates to the purification and decontamination of the secondary coolant of nuclear steam supply system. In a pressurized water reactor equipped with steam generators that have both liquid and vapor phases, any impurities entering the generators in the feedwater have a tendency to concentrate in the liquid phase. Contaminants in the secondary coolant originate from a number of sources including: feedwater makeup water, corrosion of the secondary system, condenser leakage, steam generator tube leakage, and residue from manufacture and assembly and subsequent cleanups of the steam generator. Several problems occur when contaminants are concentrated to an excessive degree in the steam generator. One danger is that the contaminants tend to plate out on the heat transfer surfaces and tend to settle out on horizontal areas. This results in the formation of scale and layers of crud which decrease the efficiency of heat transfer processes thereby decreasing the heat transfer performance of the steam generator. In addition the formation of scale and crud deposits can have an even greater effect on the steam generators. Recent industry experience and laboratory test results indicate that impurity deposits can act to accelerate the corrosion of steam generator heat exchange tubes. Corrosion of the steam generator tubes is undesirable since excessive corrosion results in steam generator tube failure which allows the leakage of the pressurized radioactive primary coolant into the secondary system thereby contaminating the secondary coolant. Therefore, it is desirable to exclude feedwater impurities from the steam generator liquid phase. It is also desirable to have a system which can remove both radioactive and non-radioactive contaminants from the secondary coolant without incurring excessive cost. Since solid contaminants have such a pronounced effect on the integrity of the steam generator heat exchange tubes, every attempt should be made to exclude all solids from the steam generators. It has been conventional in the past to attempt to prevent impurity collection in the steam generator by periodically blowing down the steam generator liquid to a holding tank for delivery to a waste treatment facility. Another conventional prior art method has been to add solid chemicals to the coolant for the purpose of combining with the dissolved impurities in the steam generator feedwater to produce a particulate precipitant. The particulate was then later removed by filtration and/or ion exchange. Neither of these prior art systems has proved to be successful in preventing steam generator deposits and subsequent steam generator corrosion. Both approaches allow the collection of solid impurities in the steam generators for at least a short period of time. Therefore, plating out and crud deposition occur regardless of the clean up efforts. SUMMARY OF THE INVENTION The present invention combines the attributes of volatile chemical addition, continuous blowdown, and full flow condensate demineralization to yield a greatly simplified and economically attractive system for the control of steam generator contaminants. During normal plant operation (defined as no primary to secondary leakage) condensate from the condenser is pumped through a full flow condensate demineralizer system by the condensate pumps. Volatile chemical additions are made to the secondary coolant to control both the pH and oxygen concentration of the coolant without the unnecessary formation of solids. Dissolved solids resulting from condenser leakage and suspended solids resulting from system corrosion are removed in the condensate polishers by ion exchange and/or filtration. At the same time a continuous blowdown of approximately 1 percent of the main steaming rate of the steam generators is maintaned. Radiation detectors monitor the secondary coolant for radioactive contaminants which indicate the presence of a primary to secondary leak in the steam generator. If these monitors indicate no primary to secondary leakage, the blowdown is cooled and returned directly to the condensate pump discharge. Thus, any contaminants in the blowdown fluid are removed when the condensate passes through the condensate demineralizers. If one of the radiation monitors should indicate a primary to secondary leak, several operations are initiated. First, a signal is sent diverting the condensate flow from the condensate demineralizer system to a bypass line around the condensate demineralizers. Next, the flow control valves to an auxiliary blowdown heat exchanger are opened, thereby, delivering the blowdown flow to an auxiliary blowdown ion exchanger system. A temperature controller at the outlet of the heat exchanger controls a second bypass valve which diverts blowdown flow from the blowdown ion exchanger. When the temperature of the effluent exiting from the blowdown heat exchanger is compatible with the resin specifications of the ion exchangers, the bypass valve causes the blowdown flow to pass through the blowdown ion exchangers, thereby effectively cleaning the blowdown flow and eliminating serious contaminant concentration in the fluid phase of the steam generator.