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

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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 higher) margins (6.35). Since a failure actually occurred at the highest stressed elbow in the ultimate strength test, 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 design 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. 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 second major objective of the BNL analyses was to perform nonlinear analyses to investigate and evaluate the adequacy of state-of-the-art methods for predicting the elasto-plastic response of piping systems subjected to large earthquake loads. BNL used the ANSYS computer program for performing these analyses. ANSYS is a computer program widely used by the nuclear industry that has been enhanced in recent years to include a number of improvements in its nonlinear analysis capability. 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 for the large-scale piping tests built upon the nonlinear analysis methods applied to the simplified piping system tests reported by DeGrassi and Hofmayer [2005] and attempted to further improve and refine those earlier 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 that used a finite strain shell element and the Chaboche nonlinear material model to obtain the strain response. The displacement responses at two nodes around one critical 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 but less accurately for high input motions. For the design method confirmation tests, it was noted that the plasticity accumulation in the piping system model only affected the performance of the piping system model 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. 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. xiii
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. 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. xiv
Page 1 and 2: NUREG/CR-6983 BNL-NUREG-81548-2008
Page 3 and 4: Seismic Analysis of Large-Scale Pip
Page 5 and 6: FOREWORD This report documents the
Page 7 and 8: Figure 2-7 Model B Simplified Pipin
Page 9 and 10: Figure 4-64 Acceleration A2 Compari
Page 11: In the final phase of the test prog
Page 15 and 16: 1 INTRODUCTION Prior to the establi
Page 17 and 18: 2 DESCRIPTION OF JNES/NUPEC TEST PR
Page 19 and 20: Photographs of the DM Test Piping a
Page 21 and 22: Design Method Confirmation Test Ult
Page 23 and 24: Stress (N/mm 2 ) Figure 2-1 Monoton
Page 25 and 26: Hoop Strain (%) 18 16 14 12 10 8 6
Page 27 and 28: φ76.3 7.8 1500 Variable Hanger Sma
Page 29 and 30: Figure 2-9 Overview of Design Metho
Page 31 and 32: Figure 2-11 DM Test Nozzle 17
Page 33 and 34: Figure 2-13 DM Test Spring Hanger 1
Page 35 and 36: Figure 2-15 DM Test Accelerometer L
Page 37 and 38: Figure 2-17 DM Test Strain Gage Loc
Page 39 and 40: Figure 2-19 US Test Piping System S
Page 41 and 42: Figure 2-21 US Test Elbow 2 and Add
Page 43 and 44: Figure 2-23 US Test Accelerometer L
Page 45 and 46: Figure 2-25 US Test Strain Gage Loc
Page 47 and 48: Displacem ent(m m ) S train(%) 100
Page 49 and 50: Figure 2-30 US Test Elbow 2 Fatigue
Page 51 and 52: 3 BNL LINEAR ANALYSES AND CODE EVAL
Page 53 and 54: where: PD 2t D + 0S m (3-4) 2I 0 0
Page 55 and 56: limits and specified that stresses
Page 57 and 58: Regulatory Guide 1.61 Revision 0 da
Page 59 and 60: Code Edition 1993 1993 Table 3-3 DM
Page 61 and 62: 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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Figure 4-18 Acceleration A2 Compari
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Figure 4-39 Relative Deformation Hi
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Figure 4-47 Strain Ratcheting Compa
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Figure 4-60 Horizontal Input Motion
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described in Sections 4 of this rep
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U.S. Nuclear Regulatory Commission,
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nominal dimensions for the straight
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Figure A-1 Original and Modified Re
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Figure A-5 Ratio of Reaction Forces
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Figure A-9 Ratio of Reaction Force
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Figure A-13 Ratio of Top Half Signi
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NRC FORM 335 U.S. NUCLEAR REGULATOR
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Seismic Analysis of Large-Scale Piping Systems for the JNES ... - NRC

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