Key Engineering Materials Vols. 321-323

Abstract: Analytic procedure for dissimilar materials welding problem by using developed in-house solver is applied on butt and lap jointed model. In addition, the procedure of data transferring between commercial package and in-house solver for the preparation of input data for in-house solver has been developed. Therefore we can use the commercial package as pre and post processor for in-house solver and the results from in-house solver, for example, welding residual stress can be exported to commercial package as initial value to the model and then further analysis with the application of external loading can be carried out. For the similar material welding the welding residual stress has been decided by temperature dependent material properties that are input to the source program. In the case of dissimilar welding problem due to the difference of expansion and shrinkage rate between aluminum and steel there has been a slight variation in this dependency. Since the aluminum has large thermal expansion coefficient and the mechanical melting point is lower than steel, the order and level of mechanical behavior like stress history become different. The degree of mechanical deterioration of dissimilar materials welded model has been assessed with various view aspects, namely, welding residual stress, plastic strain, equivalent plastic strain and plastic work distribution and it has been revealed that Al5052 is mechanically more sever than SPCC for same heat input.

Abstract: The application of laser welding technology has been considered to shipbuilding structure. However, when this technology is applied to primer-coated steel, good quality weld beads are not easily obtained. Because the primer-coated layer caused the spatter, humping bead and porosity which are main part of the welding defect attributed to the powerful vaporizing pressure of zinc. So we performed experiment with objectives of understanding spatter and porosity formation mechanism and producing sound weld beads in 6
t primer coated steels by a CO2 CW laser. The effects of welding parameters; defocused distance, welding speed, coated thickness and coated position; were investigated in the bead shape and penetration depth on bead and lap welding. Alternative idea was suggested to suspend the welding defect by giving a reasonable gap clearance for primer coated thickness. The zinc of primer has a boiling point that is lower than melting point of steel. Zinc vapor builds up at the interface between the two sheets and this tends to deteriorate the quality of the weld by ejecting weld material from lap position or leaving porosity. Significant effects of primer coated position were lap side rather than surface. Therefore introducing a small gap clearance in the lap position, the zinc vapor could escape through it and sound weld beads can be acquired. In conclusion, formation and suspension mechanism of the welding defects was suggested by controlling the factors.

Abstract: The Korea Electric Power Research Institute (KEPRI) has developed performance demonstration programs for non-destructive testing personnel who analyze ECT (eddy current testing) data for steam generator tubing since 2001. The purpose of these performance demonstration programs is to ensure a uniform knowledge and skill level of data analysts and contribute to safe operation of nuclear power plants. Many changes have occurred in non-destructive testing of steam generator tubing such as inspection scope, plugging criteria and qualification requirements in Korea. According to the Notice 2004-13 revised by the Ministry of Science and Technology (MOST), the analyst for steam generator tubing shall be qualified as the qualified data analyst (QDA), and the site specific performance demonstration (SSPD) program shall be implemented. KEPRI developed these performance demonstration programs and they are being successfully implemented. The analyst's performance is expected to be improved by the implementation of these programs.

Abstract:
The results of ultrasonic testing performed during in-service inspection period for main components of nuclear power plant are affected by the performance of each inspector. To enhance the inspection reliability, the more restrictive qualification is required. KEPRI established Korean performance demonstration (KPD) system for the ultrasonic testing personnel, equipment and procedure which applied to the piping examination of nuclear power plant. A round robin test was conducted to evaluate the reliability of examination result by comparing conventional (DAC) UT and performance demonstration (PD) UT method. Participants took detection/length sizing test and depth sizing test using DAC method and PD method. The results showed that the PD method was preciser and more accurate than the conventional DAC method in length and depth sizing regardless of field experience and PD qualification status.

Abstract: If an assembly is partially blocked, the temperature in the upper plenum fluctuates at an early stage without a significant temperature increase. Therefore, the temperature fluctuation in the upper plenum can detect a partial blockage of an assembly. For developing the detection algorithm for a partial blockage, we numerically analyzed the temperature fluctuation in the upper plenum due to a partial blockage in an assembly. For analyzing the time dependent turbulence variables, the LES (Large Eddy Simulation) turbulence model in a commercial code was used. After analyzing the temperature fluctuations in the upper plenum, we studied the change of its characteristics according to the blocked conditions through some FFT analyses and statistical analyses. We found that the change of the skewness of the temperature fluctuation was the most significant parameter to detect a partial blockage and that its highest frequency is about 15Hz at 10cm beyond the exit of the assembly. Also, we have suggested that the resolution of the thermocouple should be less than 2 K in order to measure the fluctuated values of the temperature and that the response time of the thermocouple was less than 30ms.