[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:
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.
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.