Development Of The LBB Calculation Programs For Nuclear Pipes And Constraint-dependent LBB Analysis

The LBB design and assessment of nuclear pipes are very complicated. Advanced methods need to be used and calculation programs need to be developed to enhance accuracy and efficiency of the calculations. In this paper, LBB calculation and analysis programs for nuclear pipes were developed which based on the enhanced reference stress method (ERS method) and the J-integral stability assessment diagram method. The accuracy of calculation in the programs was verified through comparing the results with those calculated by 3D finite element method for nuclear pipes and the test data of pipes in the literature. The effect of crack-tip constraint for pipes on the LBB analysis has been researched based on the results of calculation programs and the 3D finite element analysis. The constraint-dependent LBB curves for pipes with circumferential cracks have also been constructed. By comparing the constraint parameters from the crack tip of a specimen and a cracked pipe through finite element method, transferability of specimen J-R curves to pipes was investigated. The main achievements are as follows:(1) LBB calculation programs for nuclear pipes were developed which based on the acknowledged and advanced calculation method for fracture parameters. This program can be used in the calculation of J-integral, crack opening displacement (COD)、crack opening area (COA) and leak rate, and the LBB crack growth stability analysis for nuclear pipes. The results of the programs agreed very well with that of the FE analysis and the test data. So the programs developed in this paper have sufficient accuracy.(2) According to 3D finite element model, the load-independent crack-tip constraint τ* and it’s distribution of pipes with through wall circumferential flaws were obtained. The empirical calculation equations of τ* for pipes were also obtained. The calculation results show that the τ* increases with increasing crack length 6, Rm and Rm/t and the internal pressure has little influence on the τ* for pipes under bending.(3) In LBB analyses, the critical crack length and the results of LBB analysis calculated by the J-R curve of the standard high constraint specimen for pipes with shorter cracks is over-conservative. To increase accuracy of LBB assessments, the crack-tip constraint effects should be incorporated. The constraint-dependent LBB curves were constructed based on the τ* and the J-R curve of A508 steel.(4) For pipes with Rm<450mm, Rm/t>5, by comparing the constraint parameters τ* and stress field from the crack tip of specimens and cracked pipes, the results show that the crack-tip constraint of shallow cracked SEN(B) or deep cracked SNE(T) specimens agree very well with that of the most pipes. The application of J-R curves of those specimens will lead to conservative results.