Seismic Analysis of Large-Scale Piping Systems for the JNES ... - NRC

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5 CONCLUSIONS AND RECOMMENDATIONS The JNES/NUPEC Ultimate Strength Piping Test Program has provided valuable new test data on high level seismic elasto-plastic behavior and failure modes for typical nuclear power plant piping systems. The JNES/NUPEC program was a comprehensive program, which included basic piping material tests, piping component static and dynamic tests, simplified piping system seismic tests, and shaking table seismic tests of a representative large-scale piping system to levels well above the typical design earthquake. A selected number of component and piping system tests including the large-scale piping test were carried out to failure. The test specimens were heavily instrumented to record displacements, accelerations and strains versus time at critical locations. The component and piping system tests demonstrated the strain ratcheting behavior that is expected to occur when a pressurized pipe is subjected to cyclic seismic loading. All pipe failures that were observed in this test program were characterized as through-wall cracks that occurred as a result of fatigue ratcheting. The test data generated under this program augments the database of existing information on full-scale piping system and component seismic tests performed in the U.S. Although the JNES/NUPEC program was geared toward demonstrating margins in Japanese piping design, there are a number of similarities in materials and design requirements that make the test results applicable to U.S. piping designs. Technical justification for recent changes in the ASME Code piping design rules relied primarily on data collected under the EPRI/NRC Piping and Fitting Dynamic Reliability Program. The JNES/NUPEC data from the large-scale piping tests and from the previous simplified piping system tests [DeGrassi and Hofmayer 2005] may be evaluated in the same manner to provide additional technical justification from an independent test program. As reported in Section 3 of this report, BNL performed linear analyses and Code evaluations for selected large-scale piping system tests. Linear analysis methods were applied in accordance with current and earlier ASME Code requirements and in accordance with NRC guidelines. The results demonstrated that the highest level JNES/NUPEC large-scale piping seismic tests subjected the piping components to stress levels well above Code allowables for the elbow. Since the as-built elbows were measured to be significantly thicker than the design dimensions, analyses were carried out using both design and as-built piping dimensions. The Code evaluation results illustrated the differences in minimum stress margins between different Code Editions and Addenda. For the ultimate strength test, which resulted in an elbow failure, the as-built piping dimension analyses showed that the 1994 and the 2004 Code analyses with 5 percent damping provided essentially identical margins of 2.63. The NRC-endorsed 1993 Code analyses using the NRC recommended damping value of 4 percent from Regulatory Guide 1.61 Revision 1 provided about 70 percent higher margins (4.50). The 1993 Code analyses using the NRC-accepted damping values from Code Case N-411 provided about 50 percent higher margins (3.94) than the 2004 Code analyses. Analyses using the Regulatory Guide 1.61 Revision 0 damping value of 2 percent provided the largest (140 percent) margins (6.35). Since a failure actually occurred at the highest stressed elbow, these margins may be interpreted as failure margins based on stress level for this specific test. However, it should be noted that since the number of cycles in these tests far exceeded the normal number of SSE cycles and the failure was characterized as a fatigue failure, some additional margin may be available. On the other hand, since the dominant seismic input frequency for the ultimate strength test was on-resonance, it is possible that a different seismic input may result in a smaller margin. As noted above, the results of these studies and of possible further evaluation of the large-scale piping test data could be useful in resolving some of the staff’s technical concerns over the 2004 ASME Code alternative seismic piping design rules including damping values. The JNES/NUPEC Ultimate Strength Piping Test Program provided substantial test data for benchmarking elasto-plastic analysis methods and computer codes. The BNL nonlinear analyses 139
described in Sections 4 of this report provided useful insights into the application of state-of-the-art elasto-plastic analysis methods for predicting the seismic response of a piping system. The BNL nonlinear analyses attempted to further develop and refine the earlier simplified piping system analysis methods for application to the large-scale piping analyses. This included improvements in defining the material models, input motion characterization, and finite element mesh density. Numerous complex nonlinear transient analyses were carried out. Two finite element models were created for both the design method confirmation tests and the ultimate strength tests. The first was a piping system model which used plastic pipe elements and a multi-linear material model to obtain the displacement and acceleration responses for the entire piping system. The second model was an elbow model which used finite strain shell elements and the Chaboche nonlinear material model to obtain the strain responses. The displacement responses at two nodes around one elbow, generated from the piping system model, were used as the input displacement boundary conditions for the elbow model. The analyses showed that the piping system model can accurately predict the displacement and acceleration responses for low to moderate input motions and less accurately for high input motions. For the design method confirmation tests, it was noted that the plasticity accumulation in the piping system model affected the performance of the piping system model only during the early part of the input motion and did not change the overall response for the entire time histories. The displacement and acceleration responses appear to be restrained for large input motions which may imply that the multi-linear material model resulted in shakedown behavior. The elbow model predicted relatively accurate strain ratcheting histories compared to test data. However, it was noted that the level of accuracy for the analysis to test strain comparisons was not as good as for the piping system displacement and acceleration response. The comparison of the maximum strain ranges appeared to be more consistent among the four selected elements and generally better than comparisons using the entire strain ratcheting histories. For potential further studies of these test results, some additional modeling and analysis improvements can be explored. These improvements may include (1) refinement of the piping system model with smaller element sizes, (2) refinement of the elbow model locally at the strain range location, (3) characterization of the strain gradient along the hoop direction, (4) a transient analysis of the elbow model to take into account the inertia and damping effect, (5) a combined model with pipe elements for straight pipe segments and shell elements for the elbows and nozzles, (6) investigation of methods to better consider the plasticity accumulation over a series of consecutive tests, and (7) examination of the fatigue ratcheting failure mechanism. Items 4 through 7 require the use of high performance computers. Although improvements in the analytical predictions with the updated material and finite element models were observed, large variations in the test comparisons, particularly for strain and strain ratcheting were still noted. The nonlinear dynamic characteristics of a large piping system are difficult to predict with high accuracy even when state-of-the-art models and finite element codes are used. In regulatory activities related to piping systems in nuclear power plants, reviewers should be aware of such difficulties and uncertainties in any piping system seismic analysis submittals involving elasto-plastic analysis. Furthermore, continued investigation of the subject may lead to a better understanding of the analytical methods and even quantification of the uncertainties in these methods and the supporting test data. 140
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NUREG/CR-6983 BNL-NUREG-81548-2008
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Seismic Analysis of Large-Scale Pip
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FOREWORD This report documents the
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Figure 2-7 Model B Simplified Pipin
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Figure 4-64 Acceleration A2 Compari
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In the final phase of the test prog
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Continued investigation of the subj
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1 INTRODUCTION Prior to the establi
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2 DESCRIPTION OF JNES/NUPEC TEST PR
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Photographs of the DM Test Piping a
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Design Method Confirmation Test Ult
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Stress (N/mm 2 ) Figure 2-1 Monoton
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Hoop Strain (%) 18 16 14 12 10 8 6
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φ76.3 7.8 1500 Variable Hanger Sma
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Figure 2-9 Overview of Design Metho
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Figure 2-11 DM Test Nozzle 17
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Figure 2-13 DM Test Spring Hanger 1
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Figure 2-15 DM Test Accelerometer L
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Figure 2-17 DM Test Strain Gage Loc
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Figure 2-19 US Test Piping System S
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Figure 2-21 US Test Elbow 2 and Add
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Figure 2-23 US Test Accelerometer L
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Figure 2-25 US Test Strain Gage Loc
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Displacem ent(m m ) S train(%) 100
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Figure 2-30 US Test Elbow 2 Fatigue
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3 BNL LINEAR ANALYSES AND CODE EVAL
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where: PD 2t D + 0S m (3-4) 2I 0 0
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limits and specified that stresses
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Regulatory Guide 1.61 Revision 0 da
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Code Edition 1993 1993 Table 3-3 DM
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Figure 3-1 Design Method Confirmati
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Acceleration (g) Acceleration (g) 1
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Acceleration (g) 16 14 12 10 8 6 4
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g, and 0.471 g respectively in the
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A simple approach has been taken in
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so as not to prevent ANSYS from fin
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are made with time histories and Fo
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DM4-2(2) (Restart): This analysis c
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The only responses obtained from th
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elements. The final hoop (plastic)
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are somewhat smaller than but in an
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difference in the residual displace
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esponse for the entire time histori
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Figure 4-1 Horizontal and Vertical
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Figure 4-7 Comparison of Baseline-c
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Stress(MPa) Stress (MPa) 600 500 40
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Hoop and Axial Strain (%) 17 16 15
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Figure 4-16 Displacement D2 Compari
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Page 103 and 104: Figure 4-20 Displacement D4 Compari
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Page 155 and 156: U.S. Nuclear Regulatory Commission,
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Page 159 and 160: Figure A-1 Original and Modified Re
Page 161 and 162: Figure A-5 Ratio of Reaction Forces
Page 163 and 164: Figure A-9 Ratio of Reaction Force
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Seismic Analysis of Large-Scale Piping Systems for the JNES ... - NRC

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