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Pit Disassembly and Conversion Address a 'Clear and Present Danger'

For five decades, the world's superpowers sought to increase the number of weapons in their nuclear stockpiles. With the end of the Cold War and the advent of strategic arms reduction treaties and agreements, the United States and Russia are committed to retiring thousands of weapons from their stockpiles. But peace poses a dilemma: What should be done with the hundreds of tons of weapons-grade plutonium removed from these dismantled weapons?

In 1994, the National Academy of Sciences, in the report "Management and Disposition of Excess Weapons Plutonium," called the existence of this excess material "a clear and present danger to national and international security." The safe disposal of surplus plutonium is key to the U.S./Russia arms-reduction effort.

The heart of the disposition program is pit disassembly and conversion, which is led by Los Alamos. The technology includes dismantling pits, the core of nuclear weapons; converting the plutonium from pits into a form suitable for use as an actinide ceramic materials, mixed-oxide fuel; providing technical support for the design of the Pit Disassembly and Conversion Facility (PDCF) at Savannah River, slated to open before 2010; managing the Russian Federation pit disassembly and conversion; and supporting the Russian Federation nuclear fuel activities.

A flow chart of the ARIES glove-box line.

A major part of the Laboratory's pit disassembly and conversion responsibilities is the Advanced Recovery and Integrated Extraction System (ARIES), a state-of-the art prototype glove-box line operated at the TA-55 Plutonium Facility by NMT-15.

The ARIES line converts plutonium into an unclassified form that can be stored in sealed containers and examined by international nuclear material inspectors. The technology has been demonstrated to minimize waste and reduce worker exposure. Data collected from ARIES is being used to help design the PDCF at Savannah River.

ARIES incorporates a variety of basic and applied research, and development and demonstration activities, including gas-solid kinetics, materials corrosion, glove-box and container decontamination, pit machining operations, plutonium-conversion processes, tritium removal from contaminated plutonium, actinide electrochemistry, and long-term storage packaging.

Evaluation techniques used at ARIES include nondestructive assay, classified-part sanitization, and advanced separation. The work requires expertise in a variety of disciplines, including materials science, engineering, chemistry, physics, robotics and automation, and software development.

Housed in a sequential series of glove boxes, ARIES consists of five modules, each designed to carry out a specific function. Each module incorporates advanced technologies that increase operational efficiency. The process begins when a pit is introduced into the system. The pit enters the pit bisection module, where it is bisected by a tool that works like a pipe cutter. The pit may also be disassembled using a lathe. However, unlike traditional lathes, the pit bisection tool and the advanced lathe operations produce minimal waste chips. From here, the pit undergoes a plutonium removal and conversion process.

To convert plutonium into an oxide form‹the preferred product form for mixed-oxide fuel (MOX)‹the plutonium from the pit is oxidized in the direct metal oxidation (DMO) furnace. In some pits, the plutonium may be removed by a reaction with hydrogen, which forms plutonium hydride powder. The powder is thermally treated to form a metal puck that releases the small amount of hydrogen for reaction with more plutonium. To convert plutonium metal to oxide, the puck also can be processed through the DMO furnace. Researchers currently are investigating other oxide conversion processes, including one that involves a hydride/nitride process and oxidation.

A FanucŚ robot positions a welded ARIES material can for leak testing before decontamination. This fully automated system reduces worker exposure during the packaging of plutonium oxide. The system is undergoing extensive preinstallation testing at TA-46 and should be operational within the TA-55 Plutonium Facility by the end of December.

Once the plutonium has been converted into either an oxide or a metal, it is packaged to meet Department of Energy (DOE) long-term storage criteria. This packaging involves three containers, two of which are hermetically sealed and leak-checked. Before the first hermetically sealed container can be removed from the glove-box line, researchers electrolytically clean the surface using a process similar to electropolishing.

An electrolyte that consists of a sodium sulfate and water solution uses electricity to induce a chemical reaction that removes a uniform layer of material and any contaminants on the cans. This module minimizes waste by recycling the electrolyte.

After the metal or oxide is packaged, a series of robotically operated nondestructive assay instruments confirms the quantity of plutonium in each package. These measurements are important for nuclear security and safeguards; similar techniques will be used by international inspectors to confirm the package contents without having to open them.

The pit bisector and hydride/nitride processes were collaborative efforts between Los Alamos and Lawrence Livermore National Laboratory. Currently, NMT-15 and NIS-5 are working with the Russian Federation to develop a similar nondestructive assay system for use in Russia.

ARIES has been supported since 1995 by the DOE Office of Fissile Materials Disposition, now known as NN-60 in the National Nuclear Security Administration. It was dedicated in September 1998. The ARIES line then went through a series of "hot" tests and completed its first integrated demonstration, a three-month production-type run that included seven pit types, in September 1999.

Since its first integrated demonstration, the ARIES line has been upgraded to allow for an increase in the size of the container. This change required adjustments to the nondestructive assay instruments and robot, electrolytic decontamination fixture, and inner and outer container welding station.

Other upgrades performed since 1999 include the development of a pit disassembly lathe, a process control system for the lathe, and a process-control system for the hydride/dehydride process.

ARIES is being readied for a second integrated demonstration later this year. After this demonstration, researchers will install a second set of upgrades that includes a fully automated packaging line, developed with Sandia National Laboratories; a part sanitization process; a uranium cleaning and burning glove-box line; two new direct-metal oxidation furnaces; a laser-ablation inductively coupled plasma mass spectrometer for elemental analysis; and uranium nondestructive assay equipment.

The upgrades also will include modifications to the special recovery line that removes tritium contamination from plutonium and adapting robotics for pit handling in conjunction with lathe operations.

Contributors to this project are: Timothy Nelson (NMT-15 group leader), Steven McKee (NMT-15 deputy group leader), Chris James (U.S. Pit Disassembly and Conversion project leader), Stan Zygmunt (U.S./ Russian Conversion project leader), Douglas Wedman (ARIES project leader), and David Kolman (Pit Disassembly and Conversion Research and Development project leader)


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