OFFICE OF RIVER PROTECTION
June 28, 2006
The Honorable A. J. Eggenberger
Defense Nuclear Facilities Safety Board
625 Indiana Avenue, NW, Suite 700
Washington, DC 20004-2901
Dear Mr. Chairman:
STATUS AND PATH FORWARD FOR THE SEISMIC GROUND MOTION ISSUE AT THE WASTE TREATMENT AND IMMOBILIZATION PLANT (WTP)
This letter provides the status of the seismic ground motion issue, and U.S. Department of Energy’s (DOE) plan for the future seismic design at the WTP at Hanford. In February 2005, DOE developed significantly more demanding ground motion criteria based on the implementation of a site-specific seismic site response model for the WTP. These updated ground motion design response spectra are approximately 40% higher than the original 1996 design basis spectra over a range of frequencies from 3 Hz to 8 Hz, and are designated as the revised ground motion (RGM). In 2004, the Defense Nuclear Facilities Safety Board (DNFSB) and DOE Peer Review Team (PRT) identified a number of concerns regarding the mesh densities in the models used for the finite element analysis of the WTP facilities. A six-month lead time was anticipated to complete the dynamic analyses to develop facility loads and the in-structure response spectra. In order to perform design and procurement in the interim period, with minimal risk of rework, bounding interim seismic design criteria was developed in April 2005 for the design of the facilities using the RGM spectra, which was designated as the Interim Seismic Criteria (ISC). The ISC incorporated a 40% increase in loading at all frequencies, as well as bounding mesh amplification factors. Dynamic analyses of the key WTP facilities were completed in September 2005.
In addition, to minimize the impact of the sizeable increase in the spectra to the already constructed, fabricated, and future designs, DOE, in conjunction with the PRT and Bechtel National, Inc. (BNI), developed a list of areas of design and analysis where significant conservatism existed, which could be reduced while providing adequate safety. These changes were evaluated to ensure that they were justified. DOE-PRT, DNFSB staff, and the U.S. Army Corps of Engineers extensively reviewed these changes. Detailed criteria were developed based on parametric studies to ensure that sufficient meshes are incorporated in the finite element modeling of facilities. Finite element mesh densities were significantly increased for the facility structural models, and the original GTStrudl software was changed to more capable SAP2000 software. The Structural Design Criteria (SDC) was revised (Revision 10) and issued in December 2005 that incorporated all of the above changes (Attachment 1). Completion of the dynamic analyses and the approval of the updated SDC allowed the use of ISC to be terminated.
Since then, BNI has been designing the facility structures using the RGM spectra and the revised SDC. Procurement of equipment has incorporated revised in-structure response spectra based on the RGM.
Good geotechnical and geological site profile information was available for the site soils encountered to a depth of several hundred feet for the development of the RGM. However, uncertainty remains as a result of lack of direct shear wave velocity data for the interbed sequence at deeper depths. To compensate for this uncertainty in interbed material properties, the recommended surface design response spectra utilized the 84th percentile of site amplification response to define a conservative representation of the mean surface ground motion (DOE-STD-1020-94, and DOE-STD-1023-95). Due to concerns over the lack of sufficient site data in the development of the RGM, DOE has made the decision to perform deep bore drilling at the site to enhance direct estimates of subsurface dynamic properties. This decision was made largely to confirm that the RGM spectra are a conservative representation of the mean spectra. It is anticipated that with the improved definition of the properties of the site profile from the deep drilling program, the mean spectrum will be less than the current revised design spectra based on the 84th percentile relative amplification function.
In addition to the conservatism that exists in the development of the RGM, DOE considers the ongoing WTP design still maintains other conservatisms. The demand-to-capacity ratios in many of the major walls are significantly less than 1. This allows accommodation of load transfers without exceeding allowable code criteria, which provides added assurance of acceptable structural behavior during an earthquake, even if the future ground motion exceeds the current RGM. The combination of multiple lateral load path capability in the design, together with the use of ductile detailing and the availability of untapped inelastic energy absorption characteristics of the structural elements suggest that the WTP facilities can absorb a significant increase in seismic ground motion without jeopardizing the facility safety.
Also attached is the DOE report Seismic Ground Motion Issue Report for the Waste Treatment and Immobilization Plant (WTP) Hanford, Washington, Revision 2 (Attachment 2). This report summarizes the evolution of the seismic ground motion issue, the impact of this issue on the project, and the actions taken to mitigate its impact. The document identifies the remaining uncertainty and the path forward for completion of the design and construction of the WTP.
Based on the extensive technical reviews of the RGM and associated SDC described above, DOE considers using these criteria to be justified as the basis for facility design, and revalidation of existing designs. DOE also considers these criteria to be conservative and to provide adequate safety margin. The Board is requested to provide acknowledgement that the issuance of SDC, Revision 10, resolves two of the issues (Seismic Ground Motion and Structural Engineering) outlined in your letter of October 17, 2005, to the Secretary of Energy. The other two issues (Chemical Process Safety and Fire Protection) will be resolved in future correspondence.
If you have any questions, please contact me, or your staff may contact John R. Eschenberg, Project Manager, WTP Project, (509) 376-3681.
Roy J. Schepens, Manager
Office of River Protection