| EPR European Pressurized Water Reactor is the third generation of nuclear power technology with its operation power and design pressure among world's top league.Steam generator,as the major facility of the primary circuit of nuclear power plant,has great differential pressure on both sides of its tube-to-tubesheet sealing weld.If there is any leakage on the weld left undetected,the radioactive materials from the primary circuit will leak into the second circuit,breaking the second nuclear barrier and disrupting the security and stability in the operation of nuclear power plants.To ensure the quality of the steam generator and meet the technological requirements,it is necessary to overcome the difficulty of leakage detection on the sealing welds of EPR steam generators.Firstly,this paper,based on the fact that when the facility is under the fine and rough leakage detection of airtightness,gas flowing inside and outside of the leak point is following the characteristics of molecular flow,deduces the comparison formula of the helium leakage rate in theory standard condition and actual condition,and determines the two key parameters of detection pressure and helium concentration by calculation and analysis.On this basis,the overall design of the helium leakage detection system is carried out while clarifying the functions of the three main sub-systems,including the vacuum sucking cup system,sealing tooling and the auxiliary leakage detection system.The space between the tube-to-tubesheet welds of the EPR steam generator is small,and the welds are intensively distributed in a triangular shape,with high acceptance leakage detection rate.According to the characteristics of the sealing welds,a 9-hole vacuum suction cup helium leakage detection device has been developed.This detection device,driven by air cylinder,can create sealing inside the tube through compressing the dynamic sealing ring to clamp the inner wall of the heat exchange tube.The sealing grooves of the end face are designed as curve-shaped according to the shape of the sealing welds,so as to form an end face sealing with O-shape static sealing ring in the sealing grooves.After vacuum pumping of the sealing area surrounding the sealing welds with the helium leak detector,helium will enter the vacuum area through the leakage point due to the differential pressure,and be detected by the detector,thus completing the quantitate leakage detection of the sealing weld.This device can detect nine sealing welds simultaneously,so the working efficiency is remarkable.The openings of the secondary side shell of EPR steam generator feature a profiled structure.Based on the characteristics of the openings of various nozzles and taps,this paper designs a set of special sealing tooling,which has passed the overall airtight test of the secondary side shell.This has proved that the sealing tools have good sealing performance and is suitable for taking into practice.The paper has conducted the design of the auxiliary leakage detection system including vacuum-pumping system of the secondary side shell of steam generators,pressure and helium concentration monitoring and inspection system,as well as rupture disc and auxiliary tooling.As a result,the monitoring of key parameters and index of the leakage detection system and the safety under inspection condition can be achieved.In the end,the research undertakes tests on calibration blocks and sealing weld test coupon of the helium leakage detection system consisting of the vacuum suction cup,vacuum bellows and helium leakage detector.Based on the test results of key parameters and index including sensitivity,background values,response time,clearance time,etc.,together with the analysis of the uncertainty of measurement,it is concluded that the helium leakage detection system is sensitive,reliable,convenient and practical.In conclusion,it meets the helium leakage detection requirements of the intensive tube-to-tubesheet sealing welds on EPR steam generators. |
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