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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3.3.2 Finite Element Models and Analyses<br />
The selected large-scale piping systems were analyzed and evaluated using ASME Code seismic criteria<br />
by applying linear analysis methods and assumptions in accordance with nuclear industry practice. The<br />
ANSYS [2007] general purpose finite element analysis program was used to develop <strong>the</strong> models and<br />
analyze <strong>the</strong> selected load cases. The Design Method Confirmation (DM) test specimen was modeled<br />
using ANSYS three-dimensional linear elastic straight pipe (PIPE16) and curved pipe (PIPE18) elements<br />
as shown in Figure 3-1. MASS21 elements were used to represent <strong>the</strong> 1000 kg added mass attached to<br />
<strong>the</strong> pipe. The mass <strong>of</strong> <strong>the</strong> water in <strong>the</strong> pipes was included in <strong>the</strong> pipe elements. The internal pressure <strong>of</strong><br />
10.7 MPa was applied to all pipe elements. The spring hanger was represented by a constant vertical<br />
<strong>for</strong>ce balancing <strong>the</strong> gravity load. Boundary conditions included full translational and rotational restraints<br />
at <strong>the</strong> nozzles and anchor, and horizontal and vertical translational restraints at <strong>the</strong> supports.<br />
The Ultimate Strength (US) test specimen was modeled in a similar manner using <strong>the</strong> same types <strong>of</strong><br />
elements and including <strong>the</strong> water mass and internal pressure. The spring hanger was represented by a<br />
constant vertical <strong>for</strong>ce balancing <strong>the</strong> gravity load. Support, anchor and nozzle restraints were represented<br />
in <strong>the</strong> same manner. The configuration <strong>of</strong> <strong>the</strong> piping system was identical to <strong>the</strong> DM test specimen. The<br />
only differences were <strong>the</strong> addition <strong>of</strong> a second 1000 kg mass, <strong>the</strong> removal <strong>of</strong> a lateral support, and <strong>the</strong><br />
addition <strong>of</strong> masses representing pipe bands around <strong>the</strong> pipe at <strong>the</strong> support locations. The US Test Linear<br />
ANSYS model is shown in Figure 3-2<br />
Linear response spectrum analyses were per<strong>for</strong>med in accordance with industry practice and regulatory<br />
guidelines <strong>for</strong> piping design with only two exceptions: (1) since <strong>the</strong> input motions were measured with<br />
no uncertainty and <strong>the</strong> objective was to identify minimum margins to failure, unbroadened instead <strong>of</strong><br />
broadened response spectra were applied in <strong>the</strong> analyses; and (2) since actual piping component<br />
dimensions had been provided by <strong>JNES</strong>/NUPEC, two sets <strong>of</strong> analyses were per<strong>for</strong>med using both design<br />
dimensions and as-built dimensions. In reviewing <strong>the</strong> as-built measurements, it was noted that <strong>the</strong> elbows<br />
were approximately 25% thicker than <strong>the</strong> design dimensions <strong>for</strong> 200A Schedule 40 pipe. This difference<br />
had a significant effect on <strong>the</strong> piping system frequencies and <strong>the</strong> system responses, and was <strong>the</strong>re<strong>for</strong>e<br />
believed to be an important consideration in <strong>the</strong> evaluation <strong>of</strong> stress margins.<br />
The response spectra <strong>for</strong> each load case were generated from <strong>the</strong>ir respective table motion acceleration<br />
time history records using <strong>the</strong> P-CARES program [Nie, et al 2007]. The horizontal and vertical response<br />
spectra <strong>for</strong> <strong>the</strong> large-scale piping DM load cases are shown in Figure 3-3 through Figure 3-6. The<br />
horizontal response spectra <strong>for</strong> <strong>the</strong> US test shown in Figure 3-7. The spectra were generated <strong>for</strong> 2 percent,<br />
4 percent and 5 percent modal damping values. These damping values cover <strong>the</strong> range <strong>of</strong> <strong>NRC</strong>-accepted<br />
and ASME-proposed values <strong>for</strong> nuclear power plant piping systems that have been applied in <strong>the</strong> past<br />
three decades or proposed in more recent years. Two percent damping was specified in <strong>NRC</strong> Regulatory<br />
Guide 1.61 [1973] <strong>for</strong> safe shutdown earthquakes (SSE) <strong>for</strong> piping systems with pipe diameter equal to or<br />
less than 12 inches (300 mm). This damping value had been applied in piping system seismic design<br />
analyses <strong>for</strong> most operating nuclear plants. ASME Code Case N-411 [1986] allowed frequency<br />
dependent damping as an alternative <strong>for</strong> all earthquakes and all pipe sizes. The damping values are 5<br />
percent <strong>for</strong> modes with frequencies less than 10 Hz and 2 percent <strong>for</strong> modes with frequencies greater than<br />
20 Hz. For frequencies between 10 Hz and 20 Hz, <strong>the</strong> damping values are linearly interpolated between 2<br />
and 5 percent. The staff accepted N-411 damping with certain limitations specified in Regulatory Guide<br />
1.84 and used it primarily to review operating reactor issues. This Code case expired in 2000, but was<br />
incorporated into ASME Code Section III Appendix N. In 2007, <strong>NRC</strong> issued Revision 1 to Regulatory<br />
Guide 1.61 [2007] which specified 4 percent damping <strong>for</strong> SSE <strong>for</strong> all pipe sizes and analysis methods. It<br />
also allowed <strong>the</strong> use <strong>of</strong> N-411 frequency dependent damping as an alternative <strong>for</strong> piping systems analyzed<br />
by <strong>the</strong> uni<strong>for</strong>m support motion response spectrum method and subject to <strong>the</strong> restrictions that had been<br />
specified in RG 1.84. The ASME Code 1994 Addenda significantly revised <strong>the</strong> piping seismic stress<br />
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