Case Study on LBB Application for Field Fabrication Weld Metal of Nuclear Primary Piping Systems

Abstract

In order to apply leak before break (LBB) design for nuclear primary piping systems, dynamic and static J-R tests of field fabrication weld metal were carried out to determine mechanical properties at 316°C. For the reactor coolant piping system made by SMAW (Shielded Metal Arc Welding) process of the SA508 Cl.1a, the variation of J-R fracture characteristics with the loading rate of 1mm/min and 1,000mm/min was examined to prevent the catastrophic break under seismic loading. In the J-R test results, the J-R curves at 1,000mm/min are about 60% higher than those at 1mm/min. It suggests that the welding joints of the reactor coolant piping may be susceptible to dynamic strain aging at 316°C. For the surge line piping made by GTAW (Gas Tungsten Arc Welding) process of the SA312 TP347, excellent static J-R properties are required because the nominal diameter of the pipe is relatively small size of 12 inch. In order to examine the effect of carbon content in the filler metal on the fracture toughness of its welded metal, weld metal specimens were made by using 3 kinds of filler metals whose carbon contents were 0.050, 0.030 and 0.025%, respectively. In the static J-R test results, weld metal made by one of three electrodes satisfied the LBB acceptance criteria. Much better J-R fracture characteristics with decreasing carbon content of filler metal can be shown.