**\[DNFSB LETTERHEAD\]**

December 31, 2002

The Honorable Everet H. Beckner

Deputy Administrator for Defense Programs

National Nuclear Security Administration

U.S. Department of Energy

1000 Independence Avenue, SW

Washington, DC 20585-0104

Dear Dr. Beckner:

The Defense Nuclear Facilities Safety Board (Board) is continuing to
review the geotechnical investigation and proposed foundation design for
the Highly Enriched Uranium Materials Facility (HEUMF) at the Y-12
National Security Complex. On March 25, 2002, the Board sent a letter to
the Department of Energy (DOE) regarding foundation alternatives being
considered for the HEUMF. On November 5-7, 2002, the Board's staff met
with representatives of DOE and its contractor to discuss the foundation
design. In general, the Board agrees with the chosen foundation
alternative, but believes some aspects of the design require additional
consideration to ensure that possible deficiencies are not overlooked
during the ongoing facility design. The enclosed report, prepared by the
Board's staff, summarizes observations and concerns noted during the
staffs review.

Sincerely,

***John T. Conway***

***Chairman***

c: Mr. William J. Brumley

Mr. Mark B. Whitaker, Jr.

Enclosure

**DEFENSE NUCLEAR FACILITIES SAFETY BOARD**

**Staff Issue Report**

December 4, 2002

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

**COPIES:** Board Members

**FROM:** W. Linzau

**SUBJECT:** Geotechnical and Foundation Considerations for the Highly

> Enriched Uranium Material Facility

This report documents observations made by the staff of the Defense
Nuclear Facilities

Safety Board (Board) regarding the Highly Enriched Uranium Material
Facility (HEUMF) at the Y-12 National Security Complex (Y-12). The
Board's staff member, W. Linzau, and outside expert P. Rizzo
participated in discussions at the site and observed the geological
testing being conducted.

**Background**. The HEUMF is being built to store highly enriched
uranium as part of the Y- 12 Site Integrated Modernization Program. The
contractor, BWXT Y- 12, is responsible for identifying the design
requirements and providing a scope of work to the architect-engineer
firm. On September 25, 2002, Parsons Engineering Gas awarded the
contract as the architect-engineer firm to perform preliminary and
detailed design work.

On March 25, 2002, the Board sent a letter to the Department of Energy
(DOE) that provided observations resulting from a January 2002 review by
the Board's staff. Those observations addressed two general areas:
building foundation alternatives, and seismic analysis and soil
structure interaction (SSI).

**Building Foundation**. A major concern regarding the building
foundation is the poor soil conditions at the proposed building site.
The soil at the site is comprised of loose heterogeneous fill material
which could cause a large amount of differential settlement. To prevent
differential settlement problems, BWXT Y-12 selected a mat foundation
coupled with the removal and replacement of existing fill with
structural fill. BWXT Y-12 has also decreased the building footprint.
The reduced footprint reduces the area of loose fill that must be
replaced with structural fill.

The proposed structural fill is a crushed limestone that is found
locally and will be used under the entire structure as a base for the
concrete foundation. The limestone fill material will be examined by a
geotechnical expert, Dr. Ken Stokoe of the University of Texas, to
determine its structural properties. The limestone fill material has a
maximum particle size of 1 to 1-1/2 inches. It is our understanding that
one of the proposed laboratory tests uses a cylinder that has a diameter
of only 3 inches. The staff believes that insufficient volume exists in
the cylinder, relative to the particle size, to accurately determine the
structural properties of the limestone fill. In this situation,
laboratory measurements of shear velocity, shear strength, and damping
will be difficult to perform with any degree of reliability and
confidence. It may be appropriate for BWXT Y-12 to consider constructing
a test fill area for field testing of the proposed fill material.

The properties derived from field test data could then be confirmed by
the laboratory testing and would provide accurate input to the detailed
soil structure interaction models needed to determine the seismic
response of the structure.

The current foundation plan requires the removal of approximately 30
feet of existing fill on the north and east sides of the foundation. The
entire foundation will have the limestone fill material placed as a base
to a depth of at least 5 feet. The foundation base of limestone fill
will therefore vary in depth from 5 feet to approximately 30 feet. The
variable thickness of the limestone material will have to be modeled
accurately during the SSI analysis to predict the seismic response of
the structure. In addition, the building site is located in an area with
unique topography-a valley that will be more difficult to model than a
flat landscape. The combined effect of a base of variable thickness and
a unique topography will necessitate a three-dimensional SSI analysis to
support the facility structural design.

Questions exist regarding the effect of the water table on the
foundation design. The water table has a maximum elevation that is
within the crushed limestone base. With the water table just below the
concrete foundation, 4 to 5 feet of the base could be submerged. During
the foundation design, the problems that could occur as a result of this
condition, as well as remedies such as waterproofing or french drains,
should be considered.

**Seismic Analysis**. The current site-specific PC-3 response spectrum
was derived by the United States Geological Survey for bedrock with a
peak ground acceleration of 0.25 g. BWXT Y-12 is updating this seismic
design spectrum in response to the Board's letter of March 25, 2002. Law
Engineering has completed the subsurface investigation and plans to
issue the geology report by December 3 1,2002. Dr. Stokoe is conducting
field crosshole geotechnical tests and dynamic laboratory tests of the
existing site material. The measured shear wave velocities will be used
to update the seismic design basis. This work is scheduled to be
completed by the end of December 2002 or early January 2003. It is
critical that the updated design spectrum be completed before extensive
structural design work begins on the building.

The proposed HEUMF site is located near two 1.5 million gallon water
storage tanks. These tanks are located approximately 600 feet north of
the site and 70 feet above the site. Neither tank is seismically
qualified. Tank failure during a seismic event would create a sufficient
flow of water to damage the building structure and the internal systems,
such as electrical and ventilation systems. Early hazard evaluations
proposed safeguards to mitigate the effect of such a catastrophic tank
failure. One of the proposed safeguards was a reinforced concrete wall
to be incorporated into the outer boundary of the new Perimeter
Intrusion and Detection Assessment System with sufficient strength to
divert the flow resulting from failure of both tanks. Another proposed
safeguard was an exterior grade sloping away from the facility to reduce
the hydraulic head of the flowing water. A calculation was completed in
April 2002 that evaluated the consequences of a failure of the tanks.
However, this analysis only evaluated the ability of the existing storm
drains to divert the water from a partial failure of both tanks, not a
total collapse. Without a detailed analysis of the performance of the
tanks during a seismic event, a conservative assumption would be that
the entire volume of the tanks is released in a short period of time.