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ELECTROLYTIC DECONTAMINATION OF TH E 3013 INNER CAN

D. E. Wedman, T. 0.Nelson, Y. RiveraPO Box 1663, MS E510Los Alamos National LaboratoryLos Alamos, NM 87545

ABSTRACT

Disposition of plutonium recovered from nuclear weaponsor production residues must be stored in a manner thatensures safety. The criteria that has been established toassure the safety of stored materials for a minimum of 50years is DOE-STD-3013. This standard specifies both therequirements for containment and furthermore specifies thatthe inner container be decontaminated to a level of s20dpd100 cm2 swipable and 2500 dpd 10 0 cm2 direct alphasuch that a fa i l ~ ef the outer containment barrier will havea lower probability of resulting in a spread ofcontamination.Los Alamos National Laboratory (LANL) has designed acontainment package in accordance with the DOE standard.The package consists of an optional convenience (foodpack) can, a welded type 304L stainless steel inner(primary) can, and a welded type 304L stainless steel outer(secondary) can. With or without the food pack can, thematerial is placed inside the primary can and welded shutunder a helium atmosphere. This activity takes place totallywithin the confinement of the glove box line. Following thewelding process, the can is checked for leaks and then sentdown the line for decontamination. Once decontaminated,the sealed primary can may be removed from the glove boxline. Welding of the secondary container takes placeoutside the glove box line.The highly automated decontamination process 'that hasbeen developed to support the packaging of Special NuclearMaterials is based on an electrolytic process similar to thewide spread industrial technique of electropolishing. Thecan is placed within a specially designed stainless steelfmture built within a partition of a glove box. This fixtureis then filled with a flowing electrolyte solution. A low DCelectric current is made to flow between the can, acting asthe anode, and the fixture, acting as the cathode.The passage of current through this electrolytic cell resultsin a uniform anodic dissolution of the surface metal layersof the can. Iron is oxidized to the ferric ion, nickel to the

K. Weisbrod, H. E. Martinez, S. LimbackPO Box 1663, MS 5576Los Alamos National LaboratoryLos Alamos, NM 87545

nickelous ion, and chromium to the hexavalent form. Underthe alkaline conditions of the electrolyte solution, the ironand nickel components precipitate as hydroxides while thechromium remains soluble in solution as the chromate ion.The nature of the precipitate is such that the actinidecontaminants, which are freed from the surface during theelectro-dissolution of the underlying stainless steel, areinsoluble and become entrained in the precipitate. Theapplication of ultrafiltration technology results in aseparation of the actinide activity from the flowing solution.This process results in a rapid decontamination of the can.The electrolyte is fully recyclable, and the separation of thechromium from the actinides results in a compact, nonRCRA secondary waste product.Following the decontamination, the system provides a flowof rinse water through the fixture to rinse the can ofremaining salt residues. The system then carries out adrying cycle. Finally, the fixture is opened from theopposite side of the partition and the can surface monitoreddirectly and through surface smears to assure thatdecontamination is adequate.

1. INTRODUCTION

In order to facilitate the disassembly and conversion ofnuclear materials from nuclear weapons to a storable form,it is necessary to provide a means for packaging and safelong term (greater than fifty years) storage of the materials.To this end, the United States Department of Energy (DOE)has published a storage standard for these materials (DOE-STD-3013). As part of the Advanced Retirement andIntegrated Extraction System (ARIES) demonstration takingplace at Los Alamos National Laboratory (LANL), materialis being packaged to this standard. (The ARIES process is apilot line for a planned United States Pit Disassembly andConversion Facility (PDCF) to be built at a location not yetdetermined.)In accordance with the DOE standard, special nuclearmaterials returned from the stockpile and converted to either

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This repon was prepand as an account of work sponsored by an agency of tbcUnited States Government Neither the Uoited States Govcnxmt nor my agencytbcnof, nor an y of their tmployets makes any wuraaty, arprcp or implied, orassumes any legal liabiiity or responsibility for the acmmcy, completeness, or use-fulness of any information, apparatus, product, or process disclosed, or m n t sthat its use would not infringe privately owaed rigins. Rcfcratcc hatin to any spc-cific commercial product, process, or service by trade name, trademark, inanufac-turcr. or otherwire does not necessarily constitute or impiy iuendonement,mmn-mend;rtion. or favoring by the Uoited States Goyernmmt or my agency thereof.The views and opurions of authors exprrued b#dn do not neassviiy state orreflect thosc of the United States Government or any rgency thaeof.

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DISCLAIMERPortions of this docum ent m ay b e illegiblein electronic image products. Images areproduced from the best available originaldocument.

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.oxide powder or metal pucks are to be stored in a doubly results in the oxidation of water to protons and oxygen gas.contained hermetically sealed storage vessel, the inner The cathodic process observed is the reduction of water tovessel of which must be free of contamination to the levels hydroxide and hydrogen gas.specified in the Code of Federal Regulations, 10-CFR-835,that is, levels 520 dpmA00 cm2 of alpha for swipable Once dissolved from the surface, the solvated iron andcontamination and 1500 dpd100 cm2 for direct alpha nickel components precipitate as Fe(OH), and Ni(OH),measurements. respectively. This precipitation reaction is key to thesuccess of the technique in that the precipitation of theseFor the ARIES process, type 304L stainless steel canisters metals effectively entrains the actinide contamination,are utilized as the primary inner container. These cans are restricting contamination to the precipitating solid sludge.introduced into the glove box line where they are loadedwith a nominal 4.4 kg of nuclear material (either oxide ormetal) and welded closed. A secondary canister thenBecause the primary canister is introduced into and handledwithin the contaminated glove box system within which theweapons components, plutonium metal, and plutoniumoxide are handled, the can must be decontaminated to reachthe specified contamination limits. Furthermore, secondarywelding occurs outside the primary glove box line making itimperative that the inner can be shown to bedecontaminated prior to its removal from the glove box line.

-044H++0,F e ( O H ) , c 3 0 H -+ Fe3+ FeN i ( 0 H ) p - 20H-+ Ni2+ Ni

CrH 20+croa-+SOH- + C++20H -+H,encloses the primary, and this canister also welded closed. c2HzoFigure 1 Chemistry of the electrolytic decontaminationprocess on stainless steel.The chromium component of the metal hydrolyzes to thechromate ion. Periodically, (on the order of once every fiftyThe electrolytic decontamination of 304L stainless steel in cans processed) the precipitate is allowed to settle from thealkaline electrolyte has been well demonstrated at LANL,'-4. electrolyte solution where it is decanted from the bottom ofThis method has been successfully applied to the the electrolyte reservoir and filtered. The chromium maydecontamination of stainless steel glove boxes, uranium then be reduced to chromium(3+) and precipitated out as theweapon components, and other items at LANL. hydroxide. This procedure allows for a separation betweenthe actinide and chromium components.The electrolytic decontamination process is similar indesign to the wide-spread industrial practice of In order to maintain pH balance in the solution, sodiumelectropolishing. The electrolytic decontamination hydroxide is dosed by a pH control subsystem. Lefttechnology utilizes an electric current to dissolve surface unadjusted, the pH of the solution will fall, due to thelayers of the metal and, in the process, cleans away surface hydrolysis reaction of the chromium component. Thiscontamination. n e major differences between this reduction in PH has results in an increased solubility of theapplication and commercial electropolish applications are metals. This increase in solubility has a deleterious effectthe voltage-current requirements and reduction in on the decontamination efficiency because of the increasedelectrolyte concentration necessary to produce a desired potential for recontamination of the surfaces by solubilizedfinish. The finish observed in the electrolytic actinides.decontamination process is more of a uniform electrolyticetch, than a polish. The electrolytic decontamination process has beenextensively tested on stainless steel. The results of testingm e chemistry ofthe process is depicted in Figure 1. When on primary storage Cans indicates that initial surfacecurrent is made to flow between two shinless steel surfaces contamination levels achieved through nominal handling ofthough an alkaline solution of 0.2 molar sodium sulfate, the can reach as high as millions to tens of millions ofdissolution of the anode occurs resulting in the generation disintegrations per minute (dpm) of swipable transuranicof hon(3+), nickel(2+) and chromi~m(6-t) ions at the contamination. This contamination level was readilyanode-solution interface. At current densities on the order reduced to non detectable swipable contamination levelsof 40 mA/cm2as are of interest in the current study, on the within just 10 minutes of electrolysis time at a Currentorder of 88% of the current passing through the anode

2. THEORETICALBACKGROUND

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density of 40 mA/cmz. Non-swipable contamination wasreduced to just a few hundred dpm.The majority of the non-swipable contamination remainingafter decontamination was observed to migrate about thecan upon successive decontamination cycles. Theconclusion reached was that this is a recontamination effect:actinide materials were being re-deposited on the cansurface, probably along grain boundaries where it was notsusceptible to removal by swiping. During these studies, nofiltration was employed and the precipitate generated in theprocess was recycled through the system. It was thisobservation which lead to the inclusion of ultrafiltrationwithin the decontamination system.

bottom of the fixture to make the electrical contact with thestorage can. During the decontamination cycle, electrolytesolution flows through the annulus and the can is madeanodic, the fixture made cathodic.

Because neither sulfate nor sodium ions are involved in theelectrode or solution reactions, there is no degradation ofthe electrolyte solution. Continuous reuse of the electrolytesolution is permitted, providing waters lost to electrolysisand evaporation are replenished. This is accomplished byperiodically transferring enough of the rinse water solutionto the electrolyte reservoir to replenish the lost water. Thisprocedure minimizes the waste from this process to acompact metallic oxide powder. The balance on the waterusage has not yet been determined, but aqueous waste isexpected to be extremely low (less than ten liters per year ofoperation).2A. Design of the ARIES Decontamination SystemThe ARIES decontamination process was specificallyengineered around the described chemistry to enable theefficient and routine decontamination of the primary canwhile generating as minimal a waste stream as possible. Tothat end, a decontamination system similar to that shown inFigure 2 was constructed.The decontamination system for the ARIES lineincorporates a high degree of automation. The system islaid out within a multiple compartment glove box with thedecontamination fixture built into one of the partition wallswhich separates a section of high contamination from one ofessentiallyno contamination.

The decontamination fixture is a clam-shell housingconstructed of stainless steel. The fmture may be openedfrom either side of the partition to enable the can to beplaced into the fixture from the contaminated side of thepartition, and removed from the non-contaminated side.

?gure 2: Photograph of an automated primary nuclear m aterialitorage can decontamination system. The dec ontamination fixtures built into the glove boxpartition at the far end of the glove boxn the center photo. The bottom image is the LabViewTM peratornterface computer display.

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the fixture, displacing evolved gasses and electrolytethrough the top of the fxture. The flow then returns to theelectrolyte reservoir.The system is operated via a computer running LabViewTMsoftware. The computer controls the actuation of valves,the activation and deactivation of pumps, heaters, and theelectrolysis power supply. The computer continuouslymonitors air and solution flow rates, valve positions, flowpressures, temperatures, electrolyte pH, the concentration ofhydrogen gas, both within the glove box and within thehead space above the electrolyte reservoir where off-gassingoccurs. If an off-normal situation is sensed, the computer isprogrammed to place the system into a safe condition.Additionally, intermittent operator interaction is required toassure the system is continuously attended during operation.The full process run can be likened to that of an elaboratedish washer with electrolysis (wash), rinse, and dr y cycles.A full run takes roughly an hour to complete, dependingslightly on the input electrolysis time. The process beginswith the operator placing a can (welded and leak tested) intothe fixture from the contaminated side of the glove boxpartition and closing the door. The operator then actuatesthe software which takes the operator through a short check-list of items to verify that the system is ready to operate.The system next establishes a flow of compressed air whichaids in the dissipation of the evolving hydrogen gas fromthe head space of the electrolyte reservoir. The main pumpis then activated to establish a flow of electrolyte throughthe ultrafiltration module. Once this flow is established, thepermeate flow (ca. one gal per minute) is made to passthrough the decontamination fxture. At this point, theelectrolysis current is activated for the time specified(nominally 10 to 20 minutes).Following the electrolysis stage of the process, theelectrolysis current is shut off. The flow of electrolytethrough the fixture continues for several minutes to allowdissipation of the hydrogen gas. At the conclusion of thistime, the electrolyte is purged from the fixture bycompressed air to prepare for rinsing.

The can is rinsed by activation of a secondary pump flowingdeionized water through the fixture. This procedure reducesthe amount of salt residue left on the can following thedrying cycle.Because the thermal conductivity of the stainless steelfixture is low, rinse water is left within the furture duringthe start of the drying cycle. Heaters at the bottom of thefurture are activated and transfer heat across the entirefixture via the heated water. Once the fixture and contents

are sufficiently heated, the rinse water is purged from thefixture by a flow of compressed air.After purging the rinse water from the fixture, the heaterscontinue to apply heat and a stream of compressed aircontinues to pass through the fixture. A few minutes later,vacuum is applied to reduce the boiling point of the water,all the while, continuing to flow air.After a prescribed time, the heating cycle ends and the cansurface is cooled by the flow of air. Once the temperatureof the fxture drops below 80C, the air flow isdiscontinued, and the process is completed. At this point,the operator opens the door to the chamber on theuncontaminated side of the partition, verifies that the cansurface is dry, and performs both an alpha surface surveyand an alpha smear survey to verify decontamination.2B. TestResultsIn test runs of the system at an applied current density of 40mA/cm2, he rate of metal removal was found to be uniformover 1000 minutes of electrolysis, achieving a rate ofapproximately 8 monolayers per second of electrolysis time(see Figure 3). This indicates an expected material removalon the order of 96000 monolayers in a nominal 20 minutedecontamination run. Metal removal was observed to beuniform across the surface of the can with no preferentialattack at the weld or laser engraved markings. The effect onthe surface appeared to be that of a uniform electrolytic etch(see Figure 4). The laser engraved markings remainedvisible throughout, although the depth of the markings wasreduced and the discoloration (presumed oxide) left behindfrom the laser-marking process was removed, reducing thevisual contrast of the markings.

50 ,45 i /

0 100 200 300 400 500 600 700 BOO 900 1000 1Ruming lime (inn)

Figure 3: Observed loss of mass to the can as a function ofelectrolysis time.

00

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A mass balance has been carried out for the data collectedduring these tests and indicates that current efficiency formetal removal process is on the order of 12.5%, theremainder of the current being consumed by the competingwater oxidation reaction. No electrolysis reactionsinvolving sodium or sulfate ions were observed.

(left) and same surface after 6 grams of m etal have been removedby the electrodecontamination echnique (right).Hydrogen generation during the process can be calculatedthrough Faraday's Law with the assumption that the onlyprocess occurring at the cathode is the reduction of water.The mass balance supports this assumption. The resultinggeneration of hydrogen is then calculated to be 0.30 l/min(0.01 1 ch ) . Oxygen generation at the anode is calculatedat 0.14 Vmin (0.0048 c h ) .2C. ConclusionsThe ARIES decontamination process has been fullyinstalled within the LANL Plutonium Facility. It is beingevaluated for effectiveness and waste generation as part ofthe ARIES demonstration project for eventual inclusion inthe PDCF. The electrolytic decontamination technology hasa proven track record at LANL, and is expected to performwell in this application. Initial testing of a prototype of theprocess was successful, though recontamination of the cansurface by the precipitate was found to be an issue.Ultrafiltration was added to the process system to minimizethis effect and has been tested successfully in similarapplications.The process system includes a high level of automation inorder to minimize operator error, decrease worker radiationexposure, and to increase productivity. The system hasbeen extensively tested in an uncontaminated state and hasbeen found to perform reliably. The process system isexpected to be fully operational and begin processing cansin July of 1998.

NOMENTCLATURE

ARIES:CFR:cm :DOE:dpm:LANL:mA:PDCF:RCRA:SNM:

Advanced Retirement and Integrated ExtractionSystemCode of Federal RegulationscentimetersDepartment of Energydisintegrationsper minuteLosAlamos National LaboratorymilliamperesPit Dismantelment and Conversion FacilityResource Conservation and Recovery ActSpecial Nuclear Materials

ACKNOWLEDGEMENTS

We would like to acknowledge the Department of EnergyOffice of Material Disposition for the funding of thisproject.

REFERENCES

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Wedman, D. E.; Lugo, J. L.; Ford, D. K.; Nelson, T. 0.;Trujillo, V. L.; and Martinez,H. E., "ElectrochemicalDecontamination System for Actinide ProcessingGloveboxes," WM'98 Proceedings: HLW, LLW, MixedWastes and Environmental Restoration -- WorkingTowards a Cleaner Environment, WMS, Tucson, 1998.Wedman, D. E.; Martinez, H.E., Nelson, T.O.,"Electrolytic Decontaminationof Conductive Materials foHazardous Waste Management", 1995 Special SymposiumBook: Emerging Technologies in Hazardous WasteManagement VII, Plenum,NY, 998Wedman, D. E.; Nelson, T. 0.;Martinez, H. E.,"Electrolytic Decontaminationof Stainless SteelMaterials in a Sodium Nitrate Electrolyte for HazardousWaste Management," WM'96 Proceedings: HLW,LLW, Mixed Wastes and Environmental Restoration --Working Towards a Cleaner Environment, WMS,Tucson., 1996.Wedman, D.E.; Martinez, H.E. and Nelson, T.O.,"Electrolyze Radioactive Contamination Away,"Chemtech, 26(4) (1 996) 26-29