Prediction of Margins in the TBS Seismic Considerations Analysis for Circumferential Surface-Cracked Piping Under Beyond Design Basis Seismic Loading

The Transition Break Size analysis is given in Draft Regulatory Guide 1216 for assessment of ECCS requirements. There are additional analyses required for seismic considerations for beyond design basis seismic loading. The beyond design basis loading was for seismic event with a probability of occurrence of 1e−6 per year, whereas safe-shutdown earthquake loading is typically closer to a probability of 1e−4 events per year. The peak-ground accelerations for US plants are typically in the 0.1 to 0.2 g’s range for SSE loading, while the 1e−6 seismic loading may be about 3 times higher (depending on site specific seismic hazard curves).A simplified method was created for the TBS seismic consideration analysis, which is given in Appendix A and B in the Draft Reg Guide. The technical basis is in NUREG-1903. The TBS analysis approach utilizes a simple method for scaling the seismic stresses from the SSE to 1e−6 loading, allowing for a bilinear stress correction (linear to yield of the material and an ultimate point consistent with uncracked seismic tested pipe results). A best-estimate fracture analysis is then conducted using the ASME Section XI Service Level D flaw size, but a best-estimate fracture analysis uses more realistic material properties and more accurate fracture analyses than the ASME Code.In this paper, the TBS flaw size was calculated by the Draft Regulatory Guide approach for the Atucha II nuclear plant in Argentina that is about to start up. Additionally a full 3D FE model of the plant including the whole NSSS, containment building, and supports between the building and the NSSS components was developed. Circumferential surface cracks were put in the nonlinear time-history FE model at the highest stressed locations in the primary pipe loop to determine the depth of the flaw that would fail at the 1e−6 seismic excitation to the plant building. This was done to assess the margins in the flaw size for the TBS analysis, and also characterize the magnitude of the LOCA.Where the TBS simplified model showed that a surface flaw of 0.709 and 0.669 of the thickness for the RPV/hot-leg and pump/cold-leg (respectively) and 270-degrees around the circumference could be tolerated, the full 3D FE analysis showed that even a surface crack of 90-percent of the thickness and 270-degrees around the circumference would not reach crack initiation for the material used in this plant with its seismic hazard curve. The information developed here may also be useful for assessing the piping integrity of a plant once it has exceeded the Service Level D limits of the ASME Code.Copyright © 2014 by ASME