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The leak-before-break (LBB) design of piping system for nuclear power plant has been based on the premise that the leakage due to the through-wall crack can be detected using leak detection systems before a catastrophic break. The piping materials are required excellent J-R fracture characteristics. However, ferritic steels for reactor coolant piping system have operated in a range of dynamic strain aging (DSA) temperature, so that the fracture resistance could be reduced by the influence of DSA under dynamic loading. Therefore, in order to apply the LBB design concept in piping system under seismic loading, both static and dynamic J-R characteristics must be evaluated. The materials used in this study are SA 516 Gr.70 for elbow pipe, SA 508 Cl.1a for main pipe and welding joints of main pipe. The crack extension during dynamic and static J-R tests were measured by the direct current potential drop and the compliance method, respectively. This paper describes the influences of dynamic strain aging on the J-R fracture characteristics with loading rate of the pipe materials and their welding joints.

The present work is concerned with the steelmaking practices which improve the mechanical properties of the A 508 class 3 steel for reactor pressure vessel. Three kinds of steelmaking practices were applied to manufacture the forged heavy wall shell for reactor pressure vessel, that is, the vacuum carbon deoxidation(VCD), modified VCD containing aluminum and silicon-killing. The segregation of the chemical elements through the thickness was quite small so that the variations of the tensile properties at room temperature were small and the anisotropy of the impact properties was hardly observed regardless of the steelmaking practices. The Charpy V-notch impact properties and the reference nil-ductile transition temperature by drop weight test were significantly improved by the modified VCD and silicon-killing as compared with those of the steel by VCD. Moreover, the plane strain fracture toughness values of the materials by modified VCD and silicon-killing practices was much higher than those of the steel by VCD. These were resulted from the fining of austenite grain size. It was observed that the grain size was below 20μm (ASTM No. 8.5) when using the modified VCD and silicon-killing,compared to 50μm (ASTM No. 7.0)when using VCD.

In order to improve the toughness in the welded heat affected zone (HAZ) of the SA508 Grade 3 Class 1 steel, the intercritical heat treatment (IHT) has been applied before the welding. The application of the IHT before welding resulted in the decrease of fracture appearance transition temperature (FATT50) and 68Joule transition temperature (vTr68). The improved effects of the IHT on the mechanical properties of this steel have been consistently preserved in the HAZ and the effect of the IHT before welding on the transition temperatures in the HAZ of this steel was much better than those of base metal. These positive results were closely related with the newly formed sub-grains, the spheoridized carbides precipitated at sub-grain boundaries and lath boundaries which would have been acted as obstacles to the cleavage propagation.

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