Seismic Analysis of Large-Scale Piping Systems for the JNES ... - NRC
Seismic Analysis of Large-Scale Piping Systems for the JNES ... - NRC
Seismic Analysis of Large-Scale Piping Systems for the JNES ... - NRC
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5 CONCLUSIONS AND RECOMMENDATIONS<br />
The <strong>JNES</strong>/NUPEC Ultimate Strength <strong>Piping</strong> Test Program has provided valuable new test data on high<br />
level seismic elasto-plastic behavior and failure modes <strong>for</strong> typical nuclear power plant piping systems.<br />
The <strong>JNES</strong>/NUPEC program was a comprehensive program, which included basic piping material tests,<br />
piping component static and dynamic tests, simplified piping system seismic tests, and shaking table<br />
seismic tests <strong>of</strong> a representative large-scale piping system to levels well above <strong>the</strong> typical design<br />
earthquake. A selected number <strong>of</strong> component and piping system tests including <strong>the</strong> large-scale piping test<br />
were carried out to failure. The test specimens were heavily instrumented to record displacements,<br />
accelerations and strains versus time at critical locations. The component and piping system tests<br />
demonstrated <strong>the</strong> strain ratcheting behavior that is expected to occur when a pressurized pipe is subjected<br />
to cyclic seismic loading. All pipe failures that were observed in this test program were characterized as<br />
through-wall cracks that occurred as a result <strong>of</strong> fatigue ratcheting.<br />
The test data generated under this program augments <strong>the</strong> database <strong>of</strong> existing in<strong>for</strong>mation on full-scale<br />
piping system and component seismic tests per<strong>for</strong>med in <strong>the</strong> U.S. Although <strong>the</strong> <strong>JNES</strong>/NUPEC program<br />
was geared toward demonstrating margins in Japanese piping design, <strong>the</strong>re are a number <strong>of</strong> similarities in<br />
materials and design requirements that make <strong>the</strong> test results applicable to U.S. piping designs. Technical<br />
justification <strong>for</strong> recent changes in <strong>the</strong> ASME Code piping design rules relied primarily on data collected<br />
under <strong>the</strong> EPRI/<strong>NRC</strong> <strong>Piping</strong> and Fitting Dynamic Reliability Program. The <strong>JNES</strong>/NUPEC data from <strong>the</strong><br />
large-scale piping tests and from <strong>the</strong> previous simplified piping system tests [DeGrassi and H<strong>of</strong>mayer<br />
2005] may be evaluated in <strong>the</strong> same manner to provide additional technical justification from an<br />
independent test program.<br />
As reported in Section 3 <strong>of</strong> this report, BNL per<strong>for</strong>med linear analyses and Code evaluations <strong>for</strong> selected<br />
large-scale piping system tests. Linear analysis methods were applied in accordance with current and<br />
earlier ASME Code requirements and in accordance with <strong>NRC</strong> guidelines. The results demonstrated that<br />
<strong>the</strong> highest level <strong>JNES</strong>/NUPEC large-scale piping seismic tests subjected <strong>the</strong> piping components to stress<br />
levels well above Code allowables <strong>for</strong> <strong>the</strong> elbow. Since <strong>the</strong> as-built elbows were measured to be<br />
significantly thicker than <strong>the</strong> design dimensions, analyses were carried out using both design and as-built<br />
piping dimensions. The Code evaluation results illustrated <strong>the</strong> differences in minimum stress margins<br />
between different Code Editions and Addenda. For <strong>the</strong> ultimate strength test, which resulted in an elbow<br />
failure, <strong>the</strong> as-built piping dimension analyses showed that <strong>the</strong> 1994 and <strong>the</strong> 2004 Code analyses with 5<br />
percent damping provided essentially identical margins <strong>of</strong> 2.63. The <strong>NRC</strong>-endorsed 1993 Code analyses<br />
using <strong>the</strong> <strong>NRC</strong> recommended damping value <strong>of</strong> 4 percent from Regulatory Guide 1.61 Revision 1<br />
provided about 70 percent higher margins (4.50). The 1993 Code analyses using <strong>the</strong> <strong>NRC</strong>-accepted<br />
damping values from Code Case N-411 provided about 50 percent higher margins (3.94) than <strong>the</strong> 2004<br />
Code analyses. Analyses using <strong>the</strong> Regulatory Guide 1.61 Revision 0 damping value <strong>of</strong> 2 percent<br />
provided <strong>the</strong> largest (140 percent) margins (6.35). Since a failure actually occurred at <strong>the</strong> highest stressed<br />
elbow, <strong>the</strong>se margins may be interpreted as failure margins based on stress level <strong>for</strong> this specific test.<br />
However, it should be noted that since <strong>the</strong> number <strong>of</strong> cycles in <strong>the</strong>se tests far exceeded <strong>the</strong> normal number<br />
<strong>of</strong> SSE cycles and <strong>the</strong> failure was characterized as a fatigue failure, some additional margin may be<br />
available. On <strong>the</strong> o<strong>the</strong>r hand, since <strong>the</strong> dominant seismic input frequency <strong>for</strong> <strong>the</strong> ultimate strength test<br />
was on-resonance, it is possible that a different seismic input may result in a smaller margin. As noted<br />
above, <strong>the</strong> results <strong>of</strong> <strong>the</strong>se studies and <strong>of</strong> possible fur<strong>the</strong>r evaluation <strong>of</strong> <strong>the</strong> large-scale piping test data<br />
could be useful in resolving some <strong>of</strong> <strong>the</strong> staff’s technical concerns over <strong>the</strong> 2004 ASME Code alternative<br />
seismic piping design rules including damping values.<br />
The <strong>JNES</strong>/NUPEC Ultimate Strength <strong>Piping</strong> Test Program provided substantial test data <strong>for</strong><br />
benchmarking elasto-plastic analysis methods and computer codes. The BNL nonlinear analyses<br />
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