Pit Manufacturing Project Presents Many Challenges

Central to the function of a thermonuclear weapon is a plutonium pit, the trigger that initiates the sustained nuclear reaction. Los Alamos' experience in making pits dates back more than 50 years, to the Manhattan Project. Since that time, the Lab has produced a small number of pits‹about a dozen a year‹for research purposes and underground testing in Nevada. By comparison, the number of pits produced yearly at the now-closed Rocky Flats defense plant near Denver was in the thousands.

In 1993, the Department of Energy (DOE) requested that Los Alamos develop the manufacturing capabilities to produce war-reserve pits as part of the stockpile stewardship program. The designation "war reserve" means that the components meet the specifications required for use in a stockpiled nuclear weapon. Los Alamos was chosen in part because it has the nation's only full-capability plutonium facility. DOE plans call for Los Alamos to develop the capability to make 20 to 50 pits a year eventually.

A casting furnace in operation.

Los Alamos established the Pit Manufacturing Project to coordinate the activities of the many divisions across the Laboratory involved in pit manufacturing. With support from many Lab divisions--Engineering Sciences and Applications (ESA), Materials Science and Technology (MST), Chemistry (C), and Decision Applications (D)--the Nuclear Materials Technology (NMT) Division is developing and implementing processes that range from casting the plutonium to the final assembly of the nuclear and nonnuclear components. Complicating matters are the changes that have had to be made in a substantial number of these processes because of the unavailability of the original equipment, the need to comply with new environmental regulations, or the ability to take advantage of improved equipment.

NMT has faced a number of challenges in setting up these capabilities. The first and probably most significant challenge has been to set up a formal manufacturing operation‹with all its controls, infrastructure, and restrictions‹in an existing research and development facility, and to do so without completely disrupting the existing programs.

To manufacture pits to the same specifications as Rocky Flats, each of the more than 100 processes involved has had to be installed, tested, and proven to function as it did at Rocky Flats. Currently, more than 98 percent of these processes have been installed and used in the development program designed to prove the effectiveness of either new procedures or significant modifications to procedures used at Rocky Flats.

With the suspension of underground testing following the advent of the Nuclear Test Ban Treaty, scientists currently have no way to prove absolutely that any production changes do not affect the pits. Because of this, each process that has undergone any significant change must go through a formal process qualification plan, which consists of a series of experiments designed to capture the possible variables of the new system. The process qualification plan is followed by an independent statistical analysis of the data by D Division before the process is fully approved.

The development program consists of 10 development pits, each of which tests one or more areas of concern to physicists or design engineers. As part of the development program, researchers will test the fabrication and assembly processes to identify and solve potential problems that may occur in the subsequent qualification and production lots. The development program also includes training personnel in nuclear manufacturing processes, providing destructive evaluation results for process feedback where this information canıt be obtained nondestructively, and establishing an assembly information database for Los Alamos manufacturing methods suitable for predicting process yields for actual production runs.

The development pits will be followed by a series of qualification pits, which will be made completely to war-reserve specifications. These pits will be used in the certification program. One of the manufacturing challenges is the selection of process materials to be used during fabrication, many of which have changed over the years. To remedy this, the War Reserve Materials Compatibility Board (WRMCB) has been established to evaluate process materials for use in war-reserve manufacturing. The WRMCB reviews historical data obtained from Rocky Flats to make a determination on whether or not a process material is compatible for use with plutonium or other war-reserve metals. If there isnıt enough historical information to make a determination, the WRMCB commissions a compatibility study to be performed by the Materials Testing Laboratory in C-ACT.

Another challenge for project researchers has been the changing environmental regulations controlling the use of organic solvents. At Rocky Flats, plutonium components were cleaned with trichloroethane, which is now banned by the Montreal Protocols. Another solvent, trichloroethylene (TCE), is currently being used, but it is a suspected carcinogen. Besides its potential health risk, TCE is extremely costly to dispose of when itıs contaminated with plutonium.

NMT Division is resolving these issues with a multilevel approach. NMT-11, in conjunction with C-PCS, has developed a hydrothermal technique for destroying TCE by reducing it to water, carbon dioxide, and hydrochloric acid. To reduce the amount of TCE produced while this system is being installed, NMT-5 and C-ACT have developed a cleaning system that reduces by 75 percent the amount of solvent used. The groups are also developing a method to recycle the TCE when it becomes too contaminated to effectively clean the components.

As a long-term solution, a new method using carbon dioxide as the cleaning agent is being developed to completely replace the solvent cleaning system. This will allow the plutonium components to be cleaned in an environmentally benign method, producing negligible waste.

In addition to a multitude of manufacturing challenges, NMT faces two challenges in the quality arena. A technical challenge is to provide a more formal method than what was used at Rocky Flats for qualifying processes and documenting the qualification.

A regulatory challenge is to institute a quality-assurance system that meets the DOEıs strenuous requirements for war-reserve components, and to do so without the substantial resources that were devoted to the quality department at Rocky Flats. To meet the regulatory challenge, small, specialized teams have been formed to qualify processes and write a documented manufacturing manual.

Despite the large number of challenges facing the pit manufacturing process, the project has had many successes and is on schedule to meet all of the major milestones for producing a certifiable pit by 2003 and a fully certified pit by 2007.

This is a significant achievement, particularly in light of last springıs Cerro Grande Fire, during which the plutonium facility was completely shut down for several weeks. The first pit produced following the fire was made on schedule just weeks after the facility reopened. A second pit was produced several weeks later to test the quality- and production-control systems.

The next stage of development is to complete the process qualification plans and implement formalized work instructions for each process. These will be completed by the April 2002 milestone, along with all of the support and quality systems necessary to satisfy war-reserve requirements for documentation and quality.n

This article was contributed by David Mann and Sean McDonald (NMT-5).

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