SOMETIME in 2000, far fewer loud "BOOMS" will resonate from Site 300, the Laboratory's explosives test complex. Lawrence Livermore National Laboratory's new Contained Firing Facility (CFF) will begin operation that year to provide indoor testing of high explosives, and most open-air experiments at Site 300 will be discontinued.|
The new Contained Firing Facility (Figure 1 above) will be an important adjunct to Livermore's science-based stockpile stewardship program.* Without the validation provided by underground nuclear tests, Livermore scientists must still assure the safety and reliability of our nation's nuclear stockpile as weapons age beyond their originally planned life. Computer modeling supplies a wealth of information about how the explosives and assemblies in nuclear weapons will behave, but improved hydrodynamic testing of certain components is necessary to validate the computations.
Situated in the hills between the cities of Livermore and Tracy, Site 300 has been used since 1955 to perform experiments that measure variables important to nuclear weapon safety, conventional ordnance designs, and possible accidents (such as fires) involving explosives. The CFF will drastically reduce emissions to the environment and minimize the generation of hazardous waste, noise, and blast pressures. Although emissions from open-air testing at Site 300 are well within current environmental standards, the CFF is an "insurance policy" that will allow continued high-explosives testing should environmental requirements change. Future residential development in an area less than a mile away will also benefit from the facility's environmental precautions.
The new $50-million facility is currently in the final design stage, under the leadership of Livermore's Charles F. (Joe) Baker, who is project manager for the CFF project. Holmes and Narver Inc. of Orange, California, completed the conceptual design,1 and the Parsons Infrastructure and Technology Group of Pasadena, California, started the final design in February 1996.
Construction of the new containment facilities at Building 801, scheduled to begin in April 1998, will require complete shutdown of operations at the building. According to Baker, "Even on an accelerated schedule for construction, equipment installation, final testing, and activation, downtime is estimated to be 28 months. With careful planning and early integration of acceptance testing with construction, we are working to minimize downtime and get testing at Building 801 back on line as quickly as possible."
The heart of the CFF is the firing chamber. Slightly larger than half a small gymnasium (16 by 18 meters and 10 meters high), the firing chamber will contain the blast overpressure and debris from detonations of up to 60 kilograms (kg) of cased explosive charges. The inside surfaces of the chamber will be protected from shrapnel traveling as fast as 1.5 kilometers per second with 38-millimeter-thick mild steel plates. To permit repetitive firings, all main structural elements of the firing chamber are required to remain elastic when subjected to blast. Detonations will be conducted above a 150-millimeter-thick steel firing surface (the shot anvil) embedded in the floor.|
All main structural elements of the firing chamber must be able to withstand repetitive firing as well as meet design safety standards. These criteria require the structure to withstand a 94-kg TNT blast, which is the equivalent to 60 kg of high explosives. During the testing phase of the project, "overtests" will be run using 75 kg of high explosives to assure that the building can withstand planned 60-kg detonations.
A key aspect of the new facility is that the rectangular concrete firing chamber will be made with low-cost, conventional reinforcement, as opposed to the labor-intensive, laced reinforcement commonly found in many blast-resistant structures. From a materials standpoint, a spherical chamber shape would be more blast efficient, but a slightly heavier, rectangular shape is cheaper to construct, provides easier and more desirable setup and working surfaces, and encompasses existing diagnostic systems. The thickness of the reinforced concrete walls, ceiling, and floor of the chamber will be 1.2, 1.4, and 1.8 m, respectively.
The support area, which measures about 1,500 meters2, is for preparing the nonexplosive components of an experiment and also for equipment and materials storage, personnel locker rooms, rest rooms, and decontamination showers. It also houses filters, scrubbers, and a temporary waste-accumulation area for the waste products from testing.
The diagnostic equipment area (about 600 meters2) will accommodate a multibeam Fabry-Perot velocimeter to measure velocity-time histories from as many as 20 points on an explosively driven metal surface.2 The velocimeter optical equipment will take measurements through 12 horizontal optical lines of sight into the firing chamber. There are already 11 vertical optical lines of sight from the existing camera room, which is now beneath the open-air firing table and will soon be under the new contained firing chamber.
LLNL Blast-Effects Testing
From this testing program, three important design modifications were identified:|
Close-In Shock Loading
To investigate this concern, a series of 19 experiments ranging from 25 to 200% of anticipated close-in blast loading were conducted on a one-quarter-scale section of the proposed floor design. Strain gages were embedded in the concrete and placed on the reinforcing bars, on the hold-down bolts, and under the anvil surface to measure blast-induced strains.|
During these tests, measured strains on the reinforcement, the bolts, and the anvil were all within elastic limits for steel. But tensile strains in the concrete were 10 times those allowable and would be likely to cause severe concrete cracking and pulverizing over the long term. To reduce the measured strains in the concrete to acceptable elastic levels and to prevent pulverizing, a low-cost blast attenuation system placed between the high-explosive and the anvil was developed and tested. Interestingly, of the various blast attenuation systems studied, the least expensive one, a rubber doormat-type material, proved to be the only acceptable option (Figure 5).
Total Structural Response|
Once shrapnel protection and shock loading criteria were determined, the engineering staff evaluated criteria for the entire structure of the new firing chamber. The primary design criterion was that the chamber exhibit a totally elastic response to detonations within it, meaning that the chamber must not incur any permanent changes to its size or shape over time. To evaluate the structure, Livermore staff engineered and constructed a one-quarter-scale model based on the conceptual design, and installed instruments such as strain gages, pressure transducers, and temperature gages. Sixteen scaled detonation tests were performed in the model (Figure 6), which exhibited a lightly damped vibrational response that placed the structure in alternating cycles of compression and tension. During compression, both the reinforcing steel and the concrete remained elastic. During tension, the reinforcing steel remained elastic, but the concrete elastic limit was exceeded in two areas, and the concrete cracked in both places.
Overall, the experiments demonstrated that a rectangular, conventionally reinforced, concrete structure can be used as a firing chamber. The final design will incorporate more steel reinforcing to reduce cracking.
A New Flexibility
* For more information on Livermore's stockpile stewardship program, see Science & Technology Review, August 1996, pp. 6-15.
Key Words: environment, health and safety; flash x-ray (FXR) machine; high-explosives testing; stockpile stewardship.
1. Conceptual Design Report--Site 300 Contained Firing Facilities, Holmes and Narver (April 1990). Holmes and Narver issued a revised conceptual design report in January 1995.
2. "The Multibeam Fabry-Perot Velocimeter: Efficient Measurement of High Velocities," Science & Technology Review, UCRL-52000-96-7 (July 1996), pp. 12-19.
3. Documentation includes three informal reports by J. W. Pastrnak, C. F. Baker, and L. F. Simmons: shrapnel protection, UCRL-ID-110732; close-in blast loading, UCRL-JC-116822; and design validation, UCRL-ID-119432; LLNL-Livermore, CA (1992-1995).
For further information contact Charles F. (Joe) Baker (510) 422-9536 (firstname.lastname@example.org).
CHARLES F. "JOE" BAKER received his B.S. in Civil Engineering in 1964 from the Georgia Institute of Technology. He worked for the State of California as a bridge engineer for six years before joining the Laboratory in 1970. Since then he has held a variety of positions in engineering, facilities design, construction management, and program management. Currently, he is Program Manager for the Advanced Hydrotest Facility, the Contained Firing Facility, and the Site 300 Facilities Revitalization Projects. Baker is an expert in designing buildings and structures to resist the effects of high-explosive blasts and is particularly knowledgeable about safety analyses for new facilities, investigations of accidental explosive detonation, and energetic materials testing.