HANFORD DOUBLE SHELL TANK (DST) THERMAL & SEISMIC PROJECT ESTABLISHMENT OF METHODOLOGY FOR TIME DOMAIN SOIL STRUCTURE INTERACTION ANALYSIS OF HANFORD DST

Abstract

M&D Professional Services, Inc. (M&D) is under subcontract to Pacific Northwest National Laboratories (PNNL) to perform seismic analysis of the Hanford Site Double-Shell Tanks (DSTs) in support of a project entitled ''Double-Shell Tank DSV Integrity Project-DST Thermal and Seismic Analyses''. The overall scope of the project is to complete an up-to-date comprehensive analysis of record of the DST System at Hanford in support of Tri-Party Agreement Milestone M-48-14. The thermal and operating loads analysis of the DSTs is documented in Rinker et al. (2004). The work statement provided to M&D (PNNL 2003) required that the seismic analysis of the DST assess the impacts of potentially non-conservative assumptions in previous analyses and account for the additional soil mass due to the as-found soil density increase, the effects of material degradation, additional thermal profiles applied to the full structure including the soil-structure response with the footings, the non-rigid (low frequency) response of the tank roof, the asymmetric seismic-induced soil loading, the structural discontinuity between the concrete tank wall and the support footing and the sloshing of the tank waste. The seismic analysis considers the interaction of the tank with the surrounding soil, and the effects of the primary tank contents. The DST and the surrounding soil are modeled as a system of finite elements. The depth and width of the soil incorporated into the analysis model are sufficient to obtain appropriately accurate analytical results. The analyses required to support the work statement differ from previous analysis of the DSTs in that the soil-structure interaction (SSI) model includes several (nonlinear) contact surfaces in the tank structure, and the contained waste must be modeled explicitly in order to capture the fluid-structure interaction behavior between the primary tank and contained waste. Soil-structure interaction analyses are traditionally solved in the frequency domain, but frequency domain analysis is limited to systems with linear responses. The nonlinear character of the coupled SSI model and tank structural model requires that the seismic analysis be solved in the time domain. However, time domain SSI analysis is somewhat nontraditional and requires that the appropriate methodology be developed and demonstrated. Moreover, the analysis of seismically induced fluid-structure interaction between the explicitly modeled waste and the primary tank must be benchmarked against known solutions to simpler problems before being applied to the more complex analysis of the DSTs. The objective of this investigation is to establish the methodology necessary to perform the required SSI analysis of the DSTs in the time domain. Specifically, the analysis establishes the capabilities and limitations of the time domain codes ANSYS and Dytran for performing seismic SSI analysis of the DSTs. The benchmarking of the codes Dytran and ANSYS for performing seismically induced fluid-structure interaction (FSI) between the contained waste and the DST primary tank are documented in Abatt (2006) and Carpenter and Abatt (2006), respectively. The results of those two studies show that both codes have the capability to analyze the fluid-structure interaction behavior of the primary tank and contained waste. As expected, Dytran appears to have more robust capabilities for FSI analysis. The ANSYS model used in that study captures much of the FSI behavior, but does have some limitations for predicting the convective response of the waste and possibly the response of the waste in the knuckle region of the primary tank. While Dytran appears to have somewhat stronger capabilities for the analysis of the FSI behavior in the primary tank, it is more practical for the overall analysis to use ANSYS. Thus, Dytran served the purpose of helping to identify limitations in the ANSYS FSI analysis so that those limitations can be addressed in the structural evaluation of the primary tank. The limitations of ANSYS for predicting the details of the convective (sloshing) response of the waste are not considered critical due to the large structural margins that exist in the upper portion of the primary tank. However, the analysis of the waste response in the upper portion of the primary tank and the lower knuckle of the primary tank will be supplemented by a more refined ANSYS sub-model to aid in the structural evaluation. The results of all three investigations are used to support the detailed seismic analysis of the DTSs that is reported in Carpenter et al. (2006). The results of the more detailed seismic analysis will be used to provide seismic demands that will be combined with non-seismic demands from the thermal and operating loads analysis (Rinker et al. 2004) to determine the structural integrity of the DSTs.