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

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.
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