**\[DNFSB LETTERHEAD\]**

January 4, 2005

The Honorable Linton Brooks

Administrator

National Nuclear Security Administration

U.S. Department of Energy

1000 Independence Avenue, SW

Washington, DC 20585-0701

Dear Ambassador Brooks:

The Defense Nuclear Facilities Safety Board (Board) encloses for your
information and use a report on the structural design of the Pit
Disassembly and Conversion Facility. The report indicates that, as a
general matter, the design is progressing well and the approach is
sound. The Board is particularly encouraged by the design team's
technical competence and its willingness to respond to questions about
the design raised by the Board's staff.

As noted in the enclosed report, several ideas and suggestions have been
made by the Board's staff regarding analysis and design. We will
continue to follow the implementation of these ideas and suggestions.

Sincerely,

***John T. Conway***

***Chairman***

c: Mr. Mark B. Whitaker, Jr.

Enclosure

**DEFENSE NUCLEAR FACILITIES SAFETY BOARD**

**Staff Issue Report**

November 23, 2004

**MEMORANDUM FOR:** J. K. Fortenberry, Technical Director

**FROM:** B. Jones

**SUBJECT:** Structural Design of Pit Disassembly and Conversion
Facility

This report documents the results of a series of structural design
reviews of the Pit Disassembly and Conversion Facility (PDCF) conducted
from June 2003 to October 2004. Members of the staff of the Defense
Nuclear Facilities Safety Board (Board) A. Hadjian, B. Jones, and H.
Massie conducted the reviews with support from outside experts J.
Stevenson and P. Rizzo.

**Background.** The primary mission of the PDCF is to (1) receive
surplus weapons plutonium in the form of pits and other plutonium
metals, (2) convert the plutonium metal to plutonium oxide, and (3)
remove any residual classified attributes through blending of the
converted plutonium oxide. Safety-class controls are part of the
confinement strategy, which includes an active ventilation system and
five Performance Category (PC)-3 structures: the Plutonium Processing
Building, Mechanical and Support Equipment Building, sand filter
structure, fan house, and exhaust stack.

Washington Group International (WGI) is the lead design contractor and
is performing the structural design work, while the soil-structure
interaction (SSI) work is subcontracted to Simpson Gumpertz & Heger
(SGH). The structures are in the final design phase, nearing completion
of the SSI analyses. The static and dynamic analysis work is ongoing and
will be finalized once the SSI results are available. Construction is
scheduled to begin in 2007.

**General Observations.** The project organization and planning, as well
as the design, are in good order. The Board's staff is pleased with the
technical competence of WGI and its subcontractors. Initial indications
are that the quality of the work being performed is at a high level. The
Board's staff is encouraged by WGI's use of outside consultants R. P.
Kennedy and I. M. Idriss, whose comments are respected and
well-received. Likewise, WGI is receptive to issues raised by the
Board's staff and is willing to adjust its approach when technically
sound arguments are presented. The healthy working relationships
established by the project's structural engineering team with outside
consultants and the Board's staff will help promote the project's
success.

**Summary of Involvement of the Board's Staff.** The Board's staff and
outside experts have made a number of comments and observations
throughout the reviews, some of which are being tracked as the design
progresses. A summary of the more significant issues raised is provided
below.

*Structural Configuration*―The Mechanical and Support Equipment Building
has a concrete portion classified as PC-3 and a steel portion with a
lower, PC-2 classification. Early in the design process, the Board's
staff urged that special attention be paid to the interface between
these two portions. Following the suggestion of the Board's staff, WGI
decided to design the main steel framing elements to the higher, PC-3
requirements. Taking this approach has significantly reduced the risks
associated with one portion of the structure adversely affecting the
behavior of the other in a design basis seismic event.

*Load Path*―The two portions of the Mechanical and Support Equipment
Building share a structural wall. The roof elevation drops by more than
5 feet at this common wall. The Board's staff pointed out that this
represents an undesirable design detail with the potential for adverse
behavior in a seismic event. Given the location of the lower roofs
connection to the common wall, a brittle shear failure could occur. This
situation is contrary to the above positive decision to design the steel
framing elements to the higher, PC-3 requirements. The Board's staff
urged that this design detail be modified to allow a continuous load
path at a common roof elevation that would not rely on out-of-plane
loading of the common wall. WGI is currently considering this design
modification.

*Time-History Characteristics*―For use in the SSI analyses, SGH
developed time histories to match the design response spectrum. The
Board's staff reviewed these time histories and noted unconservative
characteristics. Specifically, the power of the artificial earthquake
was distributed throughout 13 second strong-motion duration. This
duration was chosen on the basis of a magnitude 7.0−7.5 event
(Charleston earthquake) and guidance from American Society of Civil
Engineers (ASCE) 4-98, *Seismic Analysis of Safety-Related Nuclear
Structure*. However, it is noted that the lower-magnitude local events
will be of shorter duration, but likely would produce high frequency
"spikes." The Board's staff noted that the effects of this type of
ground motion should be considered in addition to a distant higher
magnitude event such as a re-occurrence of the Charleston Event.

Further, the time histories were generated using software developed in
the 1970s. Since then, programs have been developed that would be more
appropriate for use on the project. As a result of the issues raised by
the Board's staff, the time histories were modified to represent a
magnitude 6.0−6.5 event with a 7 second strong motion duration, using
more advanced software.

*SSI Modeling*―The Board's staff made a number of observations
concerning the adequacy of the SSI modeling, including the following:

-   An inner corridor wall in the Plutonium Processing Building is
    detailed such that it does not transmit in-plane shear forces to the
    roof. It does, however, transmit out-of-plane loads to the roof and
    columns. SGH had modeled the wall by lumping its mass at its base,
    which does not reflect the load transfer mechanism of the
    roof-to-wall connection detail. The modeling error was acknowledged
    and corrected.

-   SGH had originally planned to model the soil cover of significant
    depth by simply lumping the soil mass at the roof nodes. The Board's
    staff noted that this approach would neglect the overturning effects
    that the soil imposes on the structure. In response, SGH modified
    its model to include rigid links to account for the overturning
    effects. The rigid links connect the center of mass of the soil to
    the roof slab.

-   The PDCF structures have a number of separation joints that must be
    modeled adequately in the SSI analyses. The Board's staff worked
    with SGH and WGI to ensure that the modeling technique would
    accurately depict WGI's structural details.

*SSI Model Validation*―Two independent three-dimensional finite-element
models have been developed for each PC-3 structure. WGI developed models
with sufficient detail to enable the design of individual structural
elements, while SGH developed simplified models to simulate the dynamic
behavior of the structures. WGI performed a mesh size sensitivity study
to justify the use of standard 2.5 foot square elements.

The Board's staff emphasized the need to compare the response of the two
models in a fixed-base environment to confirm that the models are
similar and to increase confidence in the modeling. In response, an
extensive comparison of the two models was performed. For each
structure, WGI was able to show that the simplified SSI model closely
emulates the dynamic behavior of the detailed design model, and
therefore is acceptable for use in the SSI analysis.

*Process for Determining Design Loads*―In the preliminary design phase,
the Board's staff raised issues concerning the process by which SSI
results are interpreted and processed to arrive at design loads. WGI was
planning to use an equivalent static analysis. The Board's staff
stressed that averaging of peak accelerations from the SSI analysis may
be inappropriate.

The Board's staff proposed an alternative approach with the potential to
both simplify the design process and increase confidence in the final
design. The staff suggested obtaining an enveloped acceleration response
spectrum from the SSI results at the base of the structure and
performing a fixed-base response spectrum analysis of the building using
the more detailed model. SSI results would be used to generate
in-structure response spectra. Not only would this approach capture the
dynamic behavior of the building, but a rigorous validation of the
design process would be avoided.

After investigating its feasibility and benefits, WGI decided to adopt
this approach. Soil springs were included to directly obtain forces and
moments in the base mat. Also, WGI is designing the in-plane shear walls
to carry all the lateral loads while still designing the out-of-plane
walls to carry the design loads from the finite element model.

*Design Inconsistency*―The Board's staff noted an inconsistency in WGI's
structural analysis and design methodology. WGI was planning to use 7
percent of critical damping for design of the PC-3 structures, while not
using cracked reinforced concrete section properties in the
analysis―this despite the fact that concrete structures generally need
to undergo significant cracking before 7 percent damping is mobilized.

The Board's staff noted that if the analysis is based on a nearly
elastic response, the higher forces associated with 4 percent damping
will need to be used in the design.

Alternatively, if the lower forces associated with 7 percent damping are
to be used in the design, the structural analysis must consider the
material properties of cracked flexural members, which would result in
load redistribution to the more rigid resisting elements. WGI, as well
as its outside consultants, agreed that its analysis and design were
inconsistent and will modify its methodology accordingly.

*Soft-Zone Settlements*―WGI considers the soft zone subsidence issue as
a separate load case in which both the live load and dead load effects
are included. The structural analyses rely on softening or cracking of
the concrete structures, and as a result, are heavily dependent on the
geometry and amplitude of the design settlement profiles. Because of
this dependency, the Board's staff is evaluating the surface subsidence
calculations, including the following:

The adequacy of the soft-zone consolidation estimates during the design
basis earthquake, including a numerical assessment of uncertainties and
dependency on earthquake duration.

-   The adequacy of FLAC (Fast Lagrangian Analysis of Continua) software
    as an analytical tool for calculating ground surface subsidence
    given a specified displacement at depth.

-   User competency and proper use of past FLAC results in calibrations.

-   The adequacy of the tunneling approach, which is calibrated to FLAC
    results, for calculating ground surface subsidence at PDCF.

-   Details of the inelastic structural response calculations given a
    surface subsidence due to multiple soft zones and adherence to code
    requirements.

*Test Fill*―The issue of determining the properties of the structural
backfill prior to construction has been discussed. There is no clear
plan for how WGI will ensure that the design values of the structural
backfill properties, such as shear wave velocities and strain-dependent
degradation curves, will be validated prior to the start of backfill
operations.

*Ductile Detailing*―As a result of comments made by the Board's staff,
WGI has adopted the ductile detailing requirements of ASCE 43-05,
*Seismic Design Criteria for Structures, Systems, and Components in
Nuclear Facilities*. Although the Department of Energy's (DOE) standards
require ductile detailing, an industry gap formerly existed in the
requirements for rigid shear wall buildings typical of those in the
complex. ASCE 43-05 fills this gap and represents a major step forward
in the design of ductile buildings in the DOE complex.