ACS SASSI (Sargent & Lundy Program No. 03.7.848-2.21) by Ghiocel Predictive Technologies Inc. (GPT) is a program which performs seismic soil-structure interaction (
SSI) analysis. Sargent & Lundy (S&L) purchased licenses to use ACS SASSI NQA Version 2.2.1 from GPT in 2008 and verified and validated the software under S&L's
QA program.
S&L used ACS SASSI for seismic analysis activities for South Texas Project Units 3 & 4 (STP 3&4).
On July 14, 2009, during SSI analysis of the Control Building performed to address the STP 3&4 Shear Wave Velocity Departure, irregularities were observed in the results obtained using ACS SASSI and the analysis was halted. The irregularities in turn led us to question the results for the Ultimate Heat Sink / Reactor Service Water Pump House (UHS/RSWPH) completed earlier.
Our investigations, including a review of other uses of the software, indicate that numerical instability may occur with high numbers of soil layers, even when soil layer properties and number of soil layers (less than or equal to 100) are within the parameters stated in the User's Manual. Our investigations indicate that the results for the Control Building with 94 layers, the Reactor Building with 84 layers, and the UHS/RSWPH with 100 layers are not reliable. Some individual results are conservative, others are not.
S&L recommended to the South Texas Project Nuclear Operating Company (STPNOC) that the work performed using ACS SASSI NQA Version 2.2.1 relied upon to support licensing positions and to support the design basis be reanalyzed using SASSI 2000, Version 3.0. STPNOC accepted this recommendation.
S&L is no longer using ACS SASSI for production work. In addition, we have investigated whether SASSI 2000 has similar irregularities. We concluded that SASSI 2000 does not have this condition and is fundamentally sound.