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Electrolytic Decontamination of Oralloy

Throughout the Department of Energy (DOE) Complex, there are holdings of oralloy (highly enriched uranium) contaminated with plutonium (Pu) and americium (Am). Contaminated oralloy cannot be sent to Oak Ridge, the nation's oralloy disposition site, until its surface is cleaned to a swipable alpha activity level of Pu and Am below 20 disintegrations per minute (dpm)/100 cm2. Together, the NMT and Engineering Sciences and Applications (ESA) Divisions from Los Alamos National Laboratory have demonstrated the electrolytic method of decontaminating oralloy. This method results in > 99.9% reduction in the waste stream compared to previously used methods. Additionally, the electrolytic decontamination method has been used successfully to clean other metals.

In the past cleaning to this extremely low level of Pu and Am was accomplished with difficulty using a method that required large quantities of hot, concentrated nitric acid in an acid spray leach process. Rocky Flats Environmental Technology Site (RFETS), realizing that they had large holdings of contaminated oralloy, tried to develop a new method of cleaning oralloy and initiated the concept of electrolytic decontamination of oralloy. Unfortunately, before this method of cleaning could be demonstrated, operations at RFETS were halted. Thus, in the spring of 1992 RFETS and NMT began negotiations to demonstrate electrolytic decontamination at the Los Alamos Plutonium Facility. After a few months of this cooperative arrangement, NMT continued this effort on its own. Part of the interest for the Laboratory was to find a method of decontaminating oralloy for the ARIES (Automated Retirement and Integrated Extraction System) project, as well as other stockpile support activities.

Figure 3. Under electric current flow, the containment (Pu) is removed from the surface of the anode and goes into the electrolyte solution. When the current is turned off, the containment eventually precipitates out.

Electrolytic decontamination is similar to the common industrial practice of electropolishing and is accomplished by applying a low DC voltage through an electrolyte to induce a chemical reaction. Contamination is removed at the anode, the working electrode, in this case the oralloy, and goes into solution (Figure 3). The cathode, or counter electrode, can be a variety of materials, but we typically use stainless steel. Because of their capacity to handle nitrates, RFETS planned to use sodium nitrate as the electrolyte. In the presence of sodium nitrate the contaminants form a precipitate. This precipitate formation leads to easy separation of the waste from the solution; thus, the electrolyte solution can be recycled, and waste is greatly reduced compared to the acid spray leach process.

Results of early beaker experiments with contaminated oralloy coupons demonstrated the effectiveness of the electrolytic technique, reducing swipable alpha levels from > 1,000,000 dpm to < 2 dpm. (In the early phases of this project, we were unable to distinguish between U, Pu, or Am alpha; thus, we removed all of the swipable surface material.) Additionally, it was during these coupon experiments that it became evident that the precipitate would form and that the same electrolyte could be used for numerous coupon tests. Also, subsequent analysis of the electrolyte revealed no measurable amount of contamination in solution so that the electrolyte can be discarded as industrial waste.

Figure 4. The electrolytic desontamination apparatus. The process can be used on a wide variety on conductive metals including stainless steel.

These very successful experiments launched the testing of a fixture to hold hemishells for decontamination (Figure 4). The fixture was designed to follow the contour of the parts to be decontaminated so as to clean the surface uniformly and to reduce the necessary volume of electrolyte solution as well. Also, we were fortunate to get a clean glovebox to put the fixture in for testing, in which typical inside swipes are 20 dpm or less. These hemishell experiments gave similar results to the earlier coupon tests, starting with swipable values much greater than 1,000,000 dpm/100 cm2 and ending with swipable values < 20 dpm/100 cm2.

Realizing that our swipe measurements were for total alpha, we began work with the Chemical Science and Technology (CST) Division to investigate the discrimination of the different alpha particle energies. During these initial tests, we reran some of the swipes from the previous tests and obtained values as low as 8 dpm/100 cm2 Pu and Am. Distinguishing between the alpha activity levels of the oralloy and the alpha activity of the contaminants is important. It prevents overcleaning, thus it enables the cleaning technique to run for shorter times, resulting in less solid-waste product generated and lower exposures to personnel. Because of this effort, we now have an alpha spectrometer in Building PF-4, Room 106, that we use to identify the U, Pu, and Am alpha particles. Also, we have begun development on an alpha spectrometer to determine fixed (unswipable) contamination on the oralloy hemishell. This latter effort will be the development of a completely new instrument not currently available commercially.

The third-generation fixture has recently been installed in PF-4 and has been fully tested by decontaminating a number of oralloy parts. The decontaminated parts will be shipped to Oak Ridge for disposition when the Y-12 facility is ready to receive them. Moreover, other projects using electrolytic decontamination have begun, such as using in situ electrolytic decontamination of gloveboxes and working with Sandia on an automated process to clean cans to a level at which they can be removed safely from the glovebox system.

Because of the success of the electrolytic process, it is currently the base line oralloy decontamination technology for stockpile support activities. As a result of our success with electrolytic decontamination of oralloy, we have drawn national recognition‹including a request to DOE from Lawrence Livermore National Laboratory that Los Alamos National Laboratory be the ³center for oralloy decontamination² for the nation.

Contributors to the project are George Campbell, Malcolm Fowler, Mary Esther Huerta, Lorenzo Jaramillo, Lonnie Morgan, Ed Martinez, Tim Nelson (project leader), John Parker, Wilfred Romero, Wayne Smith, Nelson Stalnaker, Len Stapf, and Lee Vikdal.


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