[DNFSB
LETTERHEAD]
October l0, 2003
The Honorable Everet H. Beckner
Deputy Administrator for Defense Programs
National Nuclear Security Administration
Dear Dr. Beckner:
The staff of the Defense Nuclear Facilities
Safety Board (Board) has been following the Department of Energy’s (DOE)
efforts to upgrade Building 12-64 at the Pantex Plant. Upgrades
to Building 12-64 have been proposed to increase capacity in support of DOE’s
Stockpile Stewardship and Management Program and activities under the Stockpile
Life Extension Program.
In a letter, dated
The Board’s staff reviewed Revision 2 of the
Conceptual Design Report for the upgrade project that was issued in August 2003.
The Board notes that DOE is attempting
to address the structural inadequacies with the Building 12-64 bay roofs;
however, it is not clear the proposed changes will fully address the structural
weaknesses that are identified by the Board’s staff in the enclosed report.
Therefore, the Board requests that you
examine the issues in the report and provide a briefing to the Board prior to
Critical Decision 2 approval on the approach to address the identified
structural design deficiencies. Once
more detailed design proposals are available, the Board will also evaluate the
additional system upgrades and modifications necessary to allow the resumption
of nuclear explosive operations in Building 12-64. Based on this evaluation, the Board may
request that your briefing on the 12-64 structural design deficiencies address
other aspects of the Building 12-64 Production Bays Upgrade Project as well.
Sincerely,
John T. Conway
Chairman
c: Mr.
Daniel E. Glenn
Mr. Mark B. Whitaker, Jr.
Enclosure
DEFENSE NUCLEAR FACILITIES SAFETY BOARD
Staff Issue Report
MEMORANDUM
FOR: J. K. Fortenberry, Technical Director
COPIES: Board
Members
FROM: B. Jones and A. Hadjian
SUBJECT: Building 12-64 Structural Upgrade
Members of the staff of the Defense Nuclear
Facilities Safety Board (Board) reviewed the Conceptual Design Report for the
Building 12-64 Production Bays Upgrade Project, dated August 2003. This document was examined as part of the
staff’s ongoing review of the adequacy of the Building 12-64 bays to house
nuclear explosive operations. The
Board’s staff identified issues related to the structural integrity of the roof
slabs of the Building 12-64 bays. This
report documents these issues.
Background. Building 12-64 is a bermed structure, with each
bay having a minimum of 2 feet of soil cover over the roof. The bay structures were designed to release
the pressure caused by an explosion from within the bay. To this end, each roof consists of two halves separated
by a midspan shear key. Although each
half of the roof is supported by walls on three sides, only the end wall is
connected to the roof slab. These design
features support the performance objective that the roof-to-wall joint hinge,
allow the roof to rotate open and vent the internal pressure due to an internal
explosion. Bay 10 has smaller dimensions
than those of the other 16 bays and has a minimum of 4 feet of soil cover.
A letter from the Board to the Department of
Energy (DOE) dated
In an effort to utilize the facility once
again for nuclear explosive operations, DOE initiated the Building 12-64
Production Bays Upgrade Project. The
project issued Revision 2 of the Conceptual Design Report in August 2003.
Design
Deficiencies. Two significant deficiencies related to the
structural integrity of the roof slabs of the bays have been identified by the
Board’s staff.
Seismic
Loading Effects―ABS
Consulting Engineers has analyzed the roof as a simple cantilever and
determined that the roof is acceptable. However, when the roof is modeled with as-built
boundary conditions, it does not meet the evaluation criteria contained in
DOE-STD-1020, Natural Phenomena Hazards
Design and Evaluation Criteria for Department of Energy Facilities. The design of a slab with dimensions similar
to the existing bays, supported on three sides and free on the fourth side,
requires bottom reinforcing bars throughout the slab in both directions. The Building 12-64 roof slabs do not have
bottom reinforcing bars in either direction at the most likely failure zones
and are therefore inadequate to support the design basis loads. A “redesign” of the roof slabs to current
codes and standards would require a significant amount of reinforcement in
areas that currently have no reinforcement. Large cracks have already developed in the
roof slabs as a result of this design deficiency. The existing crack patterns are similar in all
bays.
Internal
Explosion Loading Effects―The existing reinforcement layout does not force yielding at the
preferred roof-to-wall joint if subjected to an internal explosion large enough
to cause yielding within the roof. Staff
calculations have shown that the as-built reinforcement layout would result in
a hinge formation away from the roof-to-end wall joint, thus leaving the roof-to-end
wall joint intact. Further, because of
the lack of bottom reinforcement, a ductile hinge formation cannot be expected.
As a result, portions of the roof could
become missile hazards if subjected to a large enough internal explosion.
The staff reviewed Technical Report SL-83-6,
An Evaluation of the Separated Bay Concept
for a Munition Assembly Complex: An
Experimental Investigation of the Department of Energy Building 12-64 Complex,
in which the results of an internal explosion test using an equivalent
300-pound high explosive charge are reported. Results from both a full-scale and half-scale
test specimen revealed that this brittle failure mode resulted in a 9-foot by
3l-foot piece of concrete flying 100 feet through the air. This behavior did not fulfill the performance objective
of the original design of a predictable uncovering of the bay to safeguard
other portions of the facility. The
report on this experiment states that the roof design must be revised, and in subsequent
bay designs this design deficiency was corrected. However, it was not corrected for Building
12-64, and the deficient design is recognized in RPT-SEI-404571, Seismic Evaluation of Building 12-64 at the
Pantex Plant, dated September 2002. Although this report claims explosive limits
have been established to eliminate the missile threat, the staff has not seen
an analysis that supports the limit established in the Conceptual Design Report.
Proposed
Upgrade. The Conceptual Design Report characterizes the
above seismic loading concern as a concrete delamination problem, and proposes
to “install shield on interior bay ceilings to catch concrete fragments
dislodged during a seismic event.” Additionally, the report proposes to “remove
two feet of earth cover over Bay 10 to reduce the facility loading during a
seismic event.” While these proposed
solutions might mitigate the effects of the problem, they would not correct the
design deficiency. Considering that the
proposed upgrade of Building 12-64 is a major construction modification
project, a permanent solution regarding the structural integrity of the roof
slabs should be considered for the long-term use of the facility.
In its current condition, each roof slab is
vulnerable to a major structural failure if subjected to either a design basis
earthquake or an internal explosion. The
roof slabs are not adequate to resist these design basis loads. The proposed shield might not attenuate the
effects of such a failure. Catching
pieces of falling concrete would not improve the response of the roof slabs,
and this approach assumes relatively small pieces of concrete would break free.
Whether a concrete member would fail
“safely” by crumbling or fail catastrophically cannot be predicted accurately
given the uncertainty of the material properties and the complex loads from
blasts and earthquakes.
Proposing to reduce the soil cover over Bay
10 to 2 feet implies that a typical roof is properly designed to carry the soil
loads. However, although Bay 10 has
twice the soil cover of the typical bays, the stresses due to dead load are
only about 15 percent larger because the roof is approximately 20 percent
smaller than the typical bays. Furthermore, the calculated deflections of Bay
10, with twice the overburden of the other bays, are slightly less than those
of the other bays because deflections are proportional to the fourth power of
member dimensions. The logic behind
reducing the soil cover of Bay 10 only is based on the unjustified assumption
that each roof behaves as a simple cantilever. However, the bays are all similarly
overstressed when the as-built boundary conditions are considered. The crack pattern in Bay 10 is similar to the
other bays.
The Conceptual Design Report states that
nuclear explosive operations in Building 12-64 will be limited to those with
explosive limits of a maximum of 230 pounds of high explosives. The report does not provide the basis for this
limit to preclude the roof detaching. A
charge of 300 pounds resulted in a 46,000-pound roof fragment flying 100 feet
during the test. It is not clear that
the 230-pound explosive limit would eliminate this missile threat.
The Conceptual Design Report notes an
alternative strengthening scheme is being considered that would include a fully
supported interior structure to support the weight of the existing roof in a
design basis earthquake. This could
resolve only the issue related to seismic loading. The use of fiber reinforced polymers to
address both seismic and internal explosions deficiencies in a cost-effective
manner should also be explored.