a vision for environmentally conscious plutonium processing

A Vision for EnvironmentallyConscious Plutonium Processing

Larry R. Avens and P. Gary Eller

436 Los Alamos ScienceNumber 26 2000

Number 26 2000 Los Alamos Science 437

toxic compounds by 40 percent and todevelop or update pollution preventionplans. Similar “green” regulations thatdeal explicitly with radioactive wasteare not far behind.

Recent advances in actinide scienceand technology, however, now make itpossible to drastically reduce, or eveneliminate, the problematic wastestreams from plutonium processing op-erations. Waste minimization will notonly keep actinides out of the environ-ment but also limit the amount of re-trievable fissile material that must beshipped to a nuclear-waste repository(thus reducing proliferation risks).Waste minimization will also reducethe cost of current plutonium opera-tions and ensure the viability of futureones. But most important, minimizationis the morally correct waste-manage-ment action. With it, the nation canavoid saddling future generations withcostly environmental and economic legacies from plutonium processing.

Regardless of popular or political opinions about theuses of plutonium, plutonium

processing will continue globally formany decades. In the United States,plutonium plays a central role in na-tional defense: it is routinely formedinto samples for experiments, cast ormachined into nuclear weapon pits,and extracted from retired nuclearweapons or weapon components andprepared for disposal. All these activi-ties require that plutonium be chemi-cally or mechanically processed.

An unavoidable consequence of plu-tonium processing is that it generatesradioactively contaminated gas, liquid,and solid waste streams. Although thesestreams are currently handled in a man-ner that fully complies with today’slaws, future laws will likely imposemore-stringent requirements. Tellingly,President Clinton signed on Earth Day2000 an executive order that directsfederal facilities to reduce releases of

This article discusses the processingoperations conducted at the Los AlamosPlutonium Facility and describes anumber of advanced technologies thatcould radically reduce the operations’radioactive waste streams. Incorporatingsuch technologies into an integratedwaste-management program at LosAlamos promises to provide a “90 per-cent solution” to the facility’s wasteproblem: it could reduce solid wastevolume and liquid waste radioactivityby 90 percent. This program could beimplemented within five years at Los Alamos and could also be imple-mented at other DOE sites that conductplutonium operations. In addition, theadvanced technologies could be used totreat and dispose of legacy wastes atDOE sites that must now be cleaned up,such as the Hanford Site in Washingtonand the Rocky Flats EnvironmentalTechnology Site in Colorado. Severalof these technologies are elaboratedupon in boxes that follow this article.

Environmentally Conscious Plutonium Processing

Plutonium Processes at TA-55

Virtually all plutonium operations atLos Alamos occur within the PlutoniumFacility at Technical Area 55. It is thenation’s most modern plutonium facili-ty, consisting of a 75,000-square-footbuilding that is built to withstand 200-mile-per-hour tornadic winds and anycredible seismic event. The facility hasa back-up generator to power criticalsystems in case of power outages. TA-55, as the facility is commonlycalled, opened in 1978.

The work at TA-55 supports a widerange of national programs, such asstockpile stewardship, nuclear materialsstabilization, materials disposition, andnuclear energy. Each of these programs,which are discussed briefly in the boxthat begins on this page, revolvesaround plutonium. In stockpile steward-ship, for example, pure plutonium metalis used to manufacture nuclear-weaponpits or to conduct experiments relatedto maintenance of the nation’s nuclearstockpile. The materials disposition pro-gram recovers plutonium from decom-missioned weapons and processes it foreventual disposal or for burning asmixed oxide (MOX) fuel.

Almost all the plutonium at TA-55has been recycled, having been recov-ered from previous TA-55 operations orfrom operations at other sites. But plutonium continually undergoes radioactive decay, and trace amounts ofuranium, neptunium, and americiumimpurities accumulate in aged metal.These impurities must be removed before the material can be reused. Inaddition, the plutonium used for an experiment may have been alloyed, become oxidized, or been formed intoa chemical compound; it too must bepurified before reuse.

Thus a significant portion of thework done at TA-55 involves chemicalprocessing to produce pure plutoniummetal. Nitric acid anion exchange, ornitric acid processing, and pyrochemi-cal processing are the two production-scale techniques that are used

438

Stockpile Stewardship. The primary mission at TA-55 is to service the United States’

nuclear stockpile. The largest task in the 21st century will be to fabricate nuclear

weapon pits for the stockpile.

(The pit is a weapon’s core

and contains the plutonium.)

We have already prepared a

few pits for testing, and many

operations at TA-55 are being

upgraded and modified to

build pits on a routine basis.

In addition, under the Office

of Defense Programs

sponsorship, TA-55 performs

surveillance activities on pits;

they are dissected on a

prescribed schedule to ensure that all materials and parts will perform as intended. If a

problem is identified, a remedy is developed and implemented. Furthermore, TA-55 also

prepares alloys and other actinide-containing materials for many defense-related experi-

ments. These experiments require plutonium samples that meet demanding chemical

and physical specifications.

A laser ablates the surface of a

plutonium sample. The vapor is

spectroscopically analyzed to deter-

mine the chemical purity of the

plutonium.

An induction furnace melts

plutonium, which is then cast

into pits.

A plutonium rod is being machined on

a lathe.

Major Programs at TA-55

The pressurized inert-gas metal arc

welder is used for welding pits.

Los Alamos ScienceNumber 26 2000

Materials Disposition. In 1993, President Clinton mandated the removal of 50 tonnes of

plutonium from the defense stockpile. The plutonium, some of which is in storage, comes

from retired nuclear weapons and weapon-production process-

ing streams and is never to be used again in weapons. To

support this mandate, the DOE formed the Office of Fissile

Materials Disposition, whose mission is to define the path and

final disposition of this excess plutonium.

As part of the disposition program, Los Alamos has developed

the Advanced Recovery and Integrated Extraction System

(ARIES), which will extract plutonium from pits and convert it

to either a metal puck or plutonium oxide powder. In either

form, the plutonium can be internationally monitored because

the pits’ original, classified shapes or sizes cannot be dis-

cerned. (The mass of the final product is also altered.) An

outstanding attribute of ARIES is the minimal waste that is

generated. Neither liquids nor solid chemicals are involved in

any processing steps. A full-scale pit disassembly and conver-

sion facility using ARIES technology is planned for construction and operation at the

Savannah River Site within 10 years.

Ultimately, the excess plutonium will be either incorporated into MOX fuel for use in

nuclear power reactors or immobilized in glass (vitrified) and stored in a high-level-waste

repository. TA-55 prepared the first MOX fuel made from weapons plutonium. That fuel will

be “burned” in Canada’s Chalk River Reactor, along with equivalent fuel made from

Russian weapons. Facilities to produce MOX fuel and glass waste are also planned for

the Savannah River Site.

(Clockwise from upper left): A 250-gram

sphere of 238Pu oxide with a thermal out-

put of 100 watts; a general-purpose heat

source that holds about 110 grams of238Pu and is used to fuel a radioisotope

thermoelectric generator; 238Pu supplies

energy for the Cassini satellite.

Nuclear Energy. TA-55 is also where the nation’s

plutonium-238 heat sources are fabricated. A 150-gram pellet

of plutonium-238 dioxide has a thermal output of approximate-

ly 62 watts, and radioisotope thermoelectric generators can

convert that heat energy to electrical energy. The United

States has used radioisotope power and heater units in its

space program for nearly 40 years, beginning with the Transit

navigational satellites that were launched in 1961. Since then,

plutonium-238 has provided power or heat for meteorological satellites, the Apollo Lunar

Surface Experiment Packages (ALSEP), the Viking Mars lander, the Galileo deep-space

probe to Jupiter, the Ulysses solar pole mission, and the Mars Pathfinder. A recent use of

plutonium-238 in space is to power the Cassini probe, which was launched late in 1997

and is now on its way to Saturn. Components for these power sources undergo extensive

safety testing to ensure their safe and reliable use in space.

Los Alamos is developing an aqueous recovery process to purify plutonium-238 from old

heat sources and scrap oxide that are currently stored in the DOE complex. In addition,

we are developing a molten-salt oxidation process to recover plutonium-238 from com-

bustible process residues. Implementation of these technologies will reduce waste volume

and enable us to recycle plutonium-238 for use in future space missions.

(Clockwise from upper left): The

hydride/dehydride module in ARIES; the

glove-box train for moving material be-

tween ARIES process steps; a plutonium

metal puck; a decontaminated package

for storing the plutonium.



extensively. Nitric acid processing pro-duces purified plutonium dioxide,which is then further processed by py-rochemical operations to produce verypure plutonium metal.

Nitric acid processing is the work-horse at TA-55, having been used overthe last 20 years to process literallytons of plutonium. Pyrochemical pro-cessing was used extensively at RockyFlats and is still used to purify plutoni-um at TA-55. While both processes areefficient and robust, they produce aprodigious stream of low-level andtransuranic (TRU) waste.1 There arevarious repositories around the countryfor low-level waste, including one atLos Alamos. The Waste Isolation PilotProject (WIPP) in New Mexico is thenation’s repository for defense-relatedTRU waste.

Several recent R&D successes at LosAlamos, however, have dramatically reduced the waste volume that resultsfrom these processes.

Nitric Acid Processing.Nitric acidis used both as a dissolution agent anda processing medium. Impure scrapplutonium or plutonium-contaminateditems, such as glass, graphite castingmolds, magnesium oxide crucibles, andincinerator ash, can be dissolved in orleached with nitric acid, while impureplutonium oxide can be dissolved in theacid. (Dissolving plutonium metal in nitric acid produces an unstable residuethat is pyrophoric and susceptible toshock-induced explosions. Thus forprocessing, if the plutonium is not already an oxide, it is converted to oneby burning the metal.)

During processing, the plutonium-containing nitric acid solution is passed

Los Alamos ScienceNumber 26 2000

1Low-level waste has less than 100 nanocuries ofradioactive material per gram of waste material(nCi/g). TRU waste is contaminated withtransuranic elements (elements whose atomicnumbers are greater than that of uranium) atmore than 100 nCi/g. (For 239Pu, this is equiva-lent to 1.6 µg/g.) TRU waste comes primarilyfrom defense- and space-related programs. Otherwaste categories include remotely handled TRUwaste, mixed wastes, and high-level waste. TA-55produces only low-level, TRU, and mixed wastes.

Barrels often contain a potpourri of

ordinary items, such as gloves, jars,

rags, and tape, that are contaminated

with plutonium and other actinides.

Nuclear Materials Stabilization. A significant amount

of America’s excess defense plutonium is contained in

chemical processing residues, metals, and oxides that

were left when DOE ended weapons production in 1989.

Much of this material is in a form that is inappropriate for

storage or ultimate disposition. More than 65,000 items

at 13 DOE sites—containing some 26 tonnes of plutoni-

um—have been identified as posing potential hazards to

workers and the environment.

In 1994, the Defense Nuclear Facilities Safety Board

delivered Recommendation 94-1 to the Secretary of

Energy, recommending that DOE establish a program to

resolve this problem. The DOE response included estab-

lishing a research program to develop methods to safely

stabilize and store plutonium metal, oxides, solutions,

and residues from the Rocky Flats, Hanford, and Savan-

nah River sites and from the Los Alamos and Livermore

laboratories. A major program goal has been to minimize

the creation of any further wastes.

The research program has been managed by the Labo-

ratory’s Nuclear Materials Technology Division on behalf

of the DOE complex. Much of the required stabilization

is now either completed or the required technologies are

close to being implemented. The current objective is

to ensure safe storage of the stabilized materials at

Savannah River for up to 50 years, pending their ultimate

disposition. Recent research demonstrated that up to four

times as much material as was previously allowed can

now be stored safely for 50 years.

Legacy wastes (from the top): scrap plutonium metal,

a corroded plutonium-contaminated can, pieces of a

magnesium oxide cruible once used for plutonium

processing, and impure plutonium oxide powder

inside a ceramic pestle.

Transuranic waste in interim storage at

the Idaho National Environmental

Engineering Laboratory. These barrels

are to be shipped to WIPP.


440

through a column packed with thou-sands of beads made from anion-ex-change resin (Figure 1). The resin is anorganic polymer that contains cationic(positively charged) sites. Anionic (neg-atively charged) complexes in solutionbind these cationic sites and hence sorbto the resin. As it turns out, plutoniumis one of only a handful of elementsthat can form stable anionic complexesin nitric acid; it forms plutonium hexa-nitrato Pu(NO3)6

2–. This Pu(IV) com-plex has the highest sorption coefficientof any metal ion to certain resins, par-ticularly at nitric acid concentrations ofabout 7 molar (M). Plutonium is prefer-entially sorbed, while most other ele-ments simply flow through the columnand are flushed from the system.

Once the resin has been “loaded,”the plutonium is recovered by sendingan eluting solution (0.5 M nitric acidplus a small amount of hydroxylaminenitrate) through the column. The Pu(IV)has little affinity for the resin in the di-lute nitric acid, so it desorbs and flowswith the solution into holding tanks.Pure plutonium dioxide is recoveredafter precipitation and calcination steps.

As with all plutonium processes,anion exchange produces contaminatedwaste. The resin does not remove allthe plutonium from solution, and thusthe nitric acid solutions always containlow levels of plutonium after they havepassed through the anion-exchangeprocess. The contaminated effluentsfrom the column and the holding tanksare therefore sent to an evaporator. Theevaporator bottoms, which contain mostof the residual plutonium, other ac-tinides, and impurity elements are stabi-lized in cement and disposed of as TRU waste.

The condensed vapor from the evap-orator, however, is still radioactive,with an activity of about 6 × 10–5

curies per liter (Ci/L). This distilled liq-uid is piped to holding tanks at TA-50,where it is treated with a flocculatingagent to precipitate all remaining actinides. With a miniscule activity ofless than 3 × 10–11 Ci/L, the effluent isdischarged to the environment.



��

Evaporator

Beads ofReillexTM HPQ

Pu(IV) hexanitrato anion Pu(NO3)32–7-M nitric acid

solutionCationic sites

Anion-exchange column

Nitric acid, containing plutonium and other elements, flows slowly (30 to 60 L/h) through a column packed with anion exchange resin. The plutonium sorbs to the resin, but the other elements flow through.

To recover the plutonium, an eluting solution of dilute nitric acid is flowed through the column. The Pu(IV) desorbs.

The eluting solution is diverted to a holding tank. Oxalic acid is

added so that Pu2(C2O4)3 forms and precipitates. Calcination of

the precipitate at 600°C produces pure plutonium dioxide.

Holding tank at TA-50

Holding tank

The contaminated effluents from the column and the holding tanks are treated by evaporation. The evaporator bottom is stabilized in cement and will be shipped to WIPP. The distilled liquid from the evaporator overhead is sent to TA-50.

In holding tanks at TA-50, the residual radionuclides are precipitated, removed from solution, stabilized in cement, and disposed as TRU waste. The effluent is discharged.

Purified PuO2 powder

Figure 1. Nitric Acid Processing System

For decades, Los Alamos has soughtways to improve the nitric acid processand reduce the hazardous componentsin the waste stream. For example, because some plutonium always remains sorbed to the anion-exchangeresin, ultimately the resin must also bedisposed of as TRU waste. In the late1980s, Los Alamos collaborated withReilly Industries, Inc., to developReillexTM HPQ resin. Compared withearlier resins, HPQ has an improvedsorption for plutonium. It is less proneto radiolytic or chemical degradation in the harsh, radioactive nitric acid environment. Its enhanced stability allows the resin to be used for hundredsof plutonium recovery cycles beforebeing replaced.

During the early 1990s, we began touse advanced spectroscopic techniquesto obtain a molecular-level understand-ing of plutonium in nitric acid solu-tions. That research and a fruitful col-laboration with Texas Tech Universityhave led to the development of bifunc-tional exchange resins that promise tofurther improve the plutonium anion-exchange process. This R&D story istold by Marsh et al. on page 454.

We can do more, however, to treatthe liquid-waste stream that is generatedby the nitric acid process. The dis-charged effluent from TA-50, for exam-ple, is relatively high in nitrates. Ingeneral, these act as fertilizers in theenvironment and may adversely influ-ence the local ecology. Equally disturb-ing is the fact that high nitrate concen-trations in drinking water can lead to ahealth condition, methoglobinemia, insome infants younger than six months.The nitrates will be removed from thedischarged liquid by a recycle evapora-tor, as discussed in a later section.

Pyrochemical Salt Processing.Pyrochemical processes involvingmolten calcium, potassium, and sodiumchlorides are used at TA-55 to prepareand purify plutonium metal. Two advantages of these methods are thecompactness of their equipment andtheir rapid reaction kinetics.

The plutonium dioxide from nitricacid processing is converted to plutoni-um metal in a pyrochemical processcalled direct oxide reduction (DOR). Asshown in Figure 2(a), plutonium diox-ide and calcium metal are reacted inmolten calcium chloride within a mag-nesium oxide crucible. The plutoniumdioxide is reduced to plutonium metal,while the calcium oxide byproduct remains dissolved in the calcium chloride. The calcium oxide is thenconverted back to calcium chloride insitu by bubbling chlorine gas throughthe molten salt. Once the salt is regen-erated, more plutonium dioxide and cal-cium metal can be added to the crucibleto produce more plutonium metal. TheDOR process can be run several timesbefore the plutonium metal is recoveredand the salt and crucible discarded asTRU waste.

The plutonium metal recovered fromDOR is not very pure, however, because calcium chloride always con-tains trace amounts of impurities thatare readily absorbed by the plutonium.The metal can be made very pure by electrorefining, which is described inFigure 2(b). Unfortunately, this pyro-chemical process has a low productyield. Only about 80 percent of theoriginal plutonium metal is purified.The remaining 20 percent—containingnearly all of the impurities—is fed backinto the nitric acid and pyrochemicalprocessing streams.

A third pyrochemical process,molten salt extraction (MSE), removesamericium from aged plutonium metal.No plutonium sample is isotopicallypure, but always contains a mix of isotopes, including plutonium-241. Thisisotope has a half-life of 13.2 years anddecays by beta decay to americium-241. For many years, MSE was used atRocky Flats to extract this ingrownamericium. The aged plutonium metalwas purified using liquefied sodiumchloride/potassium chloride in a magne-sium oxide crucible. The waste streamconsisted primarily of the crucible andvoluminous salt residue.

At TA-55, we have made several

improvements to the MSE process thatsubstantially reduce this waste stream.In our process, shown in Figure 2(c),the plutonium is reacted with chlorinegas at high temperature in a reusabletantalum crucible. This produces pluto-nium chloride, PuCl3, in situ, whichthen reacts with the americium in themetal to produce americium chloride,AmCl3. The AmCl3 and some PuCl3form an easily removed salt crust ontop of the purified plutonium metal.

The salt mixtures that are used inDOR and electrorefining, and that wereused in the old MSE operations, become contaminated with residual plu-tonium and other actinides. Recoveringthis plutonium by nitric acid processingwas problematic; the large concentra-tion of chloride ions would combinewith nitrate ions to form aqua regia,which is a volatile, highly corrosive liquid. Thus in the past, these residueswere discarded as TRU waste if theplutonium content was low enough.

At TA-55, even this plutonium is recovered and removed from the wastestream. Instead of nitric acid, the salt orcrucible residues are dissolved in hydrochloric acid (HCl). The plutoniumis purified by solvent extraction, andthe purified plutonium converted to plutonium dioxide powder through oxalate precipitation and calcination,similar to what is done during nitricacid processing. To treat the HCl wastesolution, the effluent is made caustic,that is, it is made basic with a pH of 10or higher. Under these conditions, theactinides precipitate and can be removedby simple filtration. At this point, thecaustic liquid waste has an activity ofabout 10–3 Ci/L and is sent to TA-50for further treatment. However, an extraction chromatography process isbeing developed to replace the causticprecipitation/filtration processes.

As a result of recent R&D work, thetechnology is now available to vacuumdistill sodium and potassium chloridesalts at elevated temperature. Distilla-tion directly separates the plutoniumresidue from the bulk salt without theuse of an aqueous process. (Given its





Ca metalCaCl2

+impure PuO2

MgO crucible

Plutonium reduction

Salt regeneration

2CaO + 2Cl2 2CaCl2 + O2

CaCl2/CaO

(a) Direct Oxide Reduction (DOR)

800°C

StirPuO2 + 2Ca Pu + 2CaO

Pu metalPu metal

(b) Electrorefining (ER)

800°C

Electrolysis

Electrolysis cell

ImpurePu metal in MgO cup

NaCl/KClmixture

Cathode

Anode

Pure Pu metal

Pu + 3Cl- PuCl3 + 3 e- Pu + 3Cl-

Anode reaction Cathode reaction

(c) Molten Salt Extraction (MSE)

Reusabletantalum crucible

Sparge tube

Impure Pu metal

AmCl3,PuCl3 salt crust

Pure Pu metal

Chlorine gas

2Pu + 3Cl2 2PuCl3PuCl3 + Am AmCl3 + Pu

Sparge at 800°C:

(2Am + 3Cl2 2AmCl3)

Figure 2. Current Pyrochemical SaltProcesses(a) DOR reduces plutonium dioxide (often

obtained from nitric acid processing) to

plutonium metal. The plutonium dioxide

is combined with calcium metal and calci-

um chloride, and the mixture is liquefied.

The plutonium dioxide is reduced to plu-

tonium metal, producing calcium oxide.

The calcium oxide is then converted back

to calcium chloride by bubbling chlorine

gas through the liquefied salt product.

This conversion step allows us to reuse

the salt and the crucible. More plutonium

dioxide and calcium metal are added to

the crucible, and the process can be run

several times before the plutonium metal

is recovered and the salt and crucible are

discarded.

(b) Metal from DOR is purified by elec-

trorefining. The impure plutonium metal

is placed in a MgO cup inside an electrol-

ysis cell, which is filled with a NaCl/KCl

salt mixture. The cell is heated to 800 °Cto melt the contents and to begin elec-

trolysis. The plutonium metal is oxidized

to plutonium chloride, PuCl 3, which

dissolves in the molten salt. The PuCl 3

migrates to the cathode, where it is

reduced to pure plutonium metal. This

drips from the cathode and collects in the

annular region outside the cup. A ring of

pure plutonium is produced within a few

days. About 20% of the original plutoni-

um stays in the cup and contains essen-

tially all of the initial impurities.

(c) MSE is used to remove americium

from aged plutonium. The plutonium

metal is liquefied in a reusable tantalum

crucible and then chlorine gas is bubbled

(sparged) through the liquid. Plutonium

chloride forms, which is reduced to pluto-

nium metal as the americium oxidizes to

americium chloride. (The americium is

also oxidized directly to americium chlo-

ride, but at a much slower rate than the

corresponding plutonium reaction.) Upon

cooling, americium chloride and some

plutonium chloride form a thin crust on

top of the plutonium metal.

low volatility, the calcium chlorideused to reduce plutonium in DOR is notamenable to distillation.) As describedin the box on “Salt Distillation” onpage 449, salt distillate with contamina-tion levels below low-level-wastethresholds has been achieved in small-scale tests. When this process is deployed for full-scale use at TA-55,the salt will be either discarded as low-level waste (at a fraction of its currentdisposal cost as TRU waste) or will berecycled into pyrochemical operations.

Waste Generation at TA-55

Nitric acid and pyrochemical pro-cessing produce radioactive waste inthe form of contaminated reagents andprocessing equipment. But routine exe-cution of the programs discussed onpages 438–440 also produces low-leveland TRU waste because nearly every-thing associated with manipulating plutonium becomes contaminated. Theair that passes through the stainless

steel glove boxes, for example, must befiltered through high-efficiency particu-late air (HEPA) filters, which then become contaminated with plutonium.Any tools used within a glove box become contaminated, as do the gloveboxes themselves and their lead-linedgloves. The towels used to wipe downequipment, the plastic containers usedto hold parts or tools, the bottles, tubing, and tape—all become contami-nated through use.

Table I lists the approximate annualquantities of solid waste generated atTA-55 during the 1990s. Over this peri-od, nearly 40,000 kilograms of solidTRU waste was generated each year.Metal items and cement (which stabi-lizes evaporator bottoms) account forall but a fraction of the waste mass, andin terms of volume, represent about 60percent of the waste. Combustibles,such as plastics, rags, rubber, and organic liquids, account for nearly 30percent of the volume. The solid TRUand low-level wastes are stored at LosAlamos’ TA-54. The low-level wastes

are buried on-site, while the TRU wastesare stored until they can be shipped toWIPP in southern New Mexico.

Table II lists the approximate annualoutput of liquid waste. While the vol-ume is dominated by industrial liquidwaste, the total amount of radioactivematerial in this category is substantiallyless than a curie. Rather, the small vol-ume of caustic liquid waste from hydrochloric processing contains about90 percent of the radionuclides in theliquid-waste stream.

The 90 Percent Solution: Integrated Waste Management

We have already developed or arecurrently investigating new technolo-gies, such as hydrothermal processing,enhanced pyrolysis, and electrochemicaldecontamination, that can drastically reduce our combustible and metallicwaste volumes. These technologies arerobust, can treat a variety of waste ma-trices, and produce minimal secondarywastes. In most cases, the equipmentused by these technologies is compactenough to fit into existing glove-boxsystems or facilities without majormodifications. We are looking to Polymer FiltrationTM and nitric acid recycle to clean up our radioactive liquid-waste streams.

By integrating these new technolo-gies into a comprehensive waste-management program, we expect toachieve three near-term goals. The firstis to decontaminate more than 90 percentof all metallic TRU waste to low-levelwaste by 2002. Second, we want to re-duce the radionuclide content of all liq-uid effluent produced at TA-55 to theindustrial waste limit by 2003. Thiswould reduce by nearly 90 percent theactivity of the liquid waste leaving thefacility. Third, by 2004, we want to re-duce the volume of combustible andnoncombustible waste by more than 90percent. In addition, we plan to vitrify,or encase in glass, the TRU-wasteevaporator bottoms rather than stabilizethem in cement. Because the glass can



Table I. Approximate Annual Solid Waste at TA-55

Category Volume Mass 55-Gallon (m3) (kg) Drums

TRU WasteCombustible 22 2600 100Metal 31 14600 84Cement 13 18000 65Filters, glass, graphite 5 1000 24Leaded gloves 3 2500 13

Low-level Waste 200 to 500

Table II. Approximate Annual Liquid Waste at TA-55

Category Volume Activity Limit Total Activity(L) (Ci/L) (Ci)

Caustic 10,000 4.5 × 10–3 < 45Acid 50,000 6 × 10–5 < 3Industrial 730,000 5 × 10–7 < 0.4

be loaded with more radionuclides, itsoverall volume and mass will be about65 percent that of the cement now used.

We believe these goals are readilyachievable. This “90 percent solution”will positively impact every experimen-tal program at TA-55 as well as theoverall waste-management program forthe facility and the Laboratory. Further-more, the costs of implementing thissolution should be recovered within afew years. Currently, the total life-cyclecost to treat TA-55’s annual TRU wasteand dispose it at WIPP is roughly $5.2 million. If the technologies described here are successfully imple-mented, the life-cycle cost to dispose ofthe reduced volume of TRU wasteshould plummet to about $0.5 million per year. Beyond the eco-nomics of these goals, however, is the

fact that their achievement is a neces-sary condition for attaining operationaland environmental excellence at TA-55.

Treatment of Combustible Wastes.The flow chart in Figure 3 summarizeshow we intend to reduce radioactive,combustible waste volumes at TA-55.A notable feature of this flow chart isthat no low-level solid wastes and onlya small volume of concentrated TRUwaste are produced. All liquids leavethe facility as industrial waste waterdestined for additional treatment at TA-50.

As indicated in Table I, combustiblewastes (for example, rags, plastics,polystyrene cubes, and organic sol-vents) constitute a significant fractionof the radioactive waste volume gener-ated at TA-55. These wastes are conta-

minated with radioactive elements andstrong oxidizers such as nitrates. Hydrothermal processing, enhanced pyrolysis, molten salt oxidation, andmediated electrochemical oxidation are currently the most promising tech-nologies for reducing combustiblewaste volumes.

Hydrothermal processing is by farthe most aggressive of the four methodsin destroying organic compounds. It isideally suited for destroying organicliquids, such as cleaning solvents, andsmall-particle-size solids, such asanion-exchange resin. It uses water as asolvent but at high enough temperaturesand pressures to render it a supercriticalfluid. Liquids and preshredded solidsare pressure-fed into the hydrothermalreactor along with 30 percent hydrogenperoxide that serves as the oxidizing



Hydrothermalprocessing

Pyrolysis

Effluent leaves facility as industrialwaste water< 5 x 10-7 Ci/L

Vent gases

Liquids

Liquids

Liquids

HEPAfilter

Salt recycle

Polymer filtration

Anion exchange resins, organic chlorocarbons,organic liquids

Polystyrene,polyethylene, cellulose,organic polymers

Polyethylene, tygon,ash from pyrolysis

Solids

Solids

Solids

Solids

Gases

GasesCatalyticconverter

TRU wasteto WIPP

Moltensalt

oxidation

Nitric acidprocessing Vitrification

Figure 3. Combustibles Flow Chart Combustible wastes will be treated by several methods. Cellulose, polystyrene, and other selected solids will be treated by pyr olysis.

Organic solvents and solids such as ion-exchange resin will be treated by hydrothermal processing. Some plastics and the residu al

ash from pyrolysis will be treated by molten salt oxidation. The solid, liquid, and gas waste streams from these treatments, ho wever,

will be handled similarly. The small volumes of solid byproducts will either be sent to nitric acid processing or be vitrified i nto glass

and shipped to WIPP. All liquid byproducts will be treated by Polymer Filtration TM to meet discharge limits for industrial waste water,

with the resulting small volume of solid residue then vitrified as TRU waste. Off gases will be filtered through high-efficiency e xhaust

systems and discharged to the atmosphere. The filters eventually enter the TRU waste stream and are sent to WIPP.

agent. Under hydrothermal conditions,reactions are extremely fast, almostcompletely destroying the organic matrices in seconds. The box on “Hydrothermal Processing” on page450 provides more information aboutthis technology.

Enhanced pyrolysis substantially decomposes solid polymeric materials(e.g., polystyrene, polyethlyene, andpolypropylene). A fully engineered sys-tem consists of a pyrolysis step, inwhich the materials are decomposedinto a variety of liquid- and gas-phaseconstituents, followed by a catalytic conversion step that treats theoff gases and liquids by oxidizing themto simple products. The integrated sys-tem provides greater than 99 percentconversion of the polystyrene to car-bon, water, carbon dioxide, and a smallamount of stabilized actinide material.Tests have shown that polyethylene,cellulose, and other organic polymersare equally amenable to this treatment.A fully engineered unit for processingsmall quantities of polystyrene and cellulose is already installed in a TA-55glove box. More information on thistechnology is given in the box on “Enhanced Pyrolysis” on page 451.

In molten salt oxidation, the contam-inated waste is oxidized by air in amolten sodium/potassium carbonate

bath. The residual actinides can be re-covered from the sodium/potassium car-bonate mixture, or the carbonate saltcontaining the plutonium can be dis-carded. At TA-55, we plan to use thistechnology to recover plutonium-238from combustible process residues. Itsimplementation will reduce waste vol-umes and enable us to recycle plutoni-um-238 for use as a heat or powersource in future space missions.

Although not shown in Figure 3, another technology being developed ismediated electrochemical oxidation,which can quickly decompose certaincombustible mixed waste, includingcation exchange resins, solvents, andplastic bottles. In contrast to hydrother-mal processing, this oxidation processoperates at low temperature and ambi-ent pressure. In the process, a powerfuloxidizing agent, such as Ag(II), is generated electrochemically at theanode of a divided electrochemical cell.When the anolyte solution comes intocontact with the waste, its radioactivecomponent is dissolved through reac-tion with the oxidizing agent. (The insoluble plutonium dioxide is oxidizedto the soluble plutonyl ion PuO2

2+.)The organic matrix components canalso be oxidized to carbon dioxide byreaction with the oxidizing agent. Withwater serving as an oxygen donor to

complete the reaction, the oxidizingagent is reduced to its original state.The solution is then recycled to theelectrochemical cell to regenerate theoxidizing agent, and the process is con-tinued. Because of the cyclic nature ofthe process, only a small amount of oxidizing agent is required to treat alarge amount of combustible waste.

Treatment of Liquid Wastes. Re-search into plutonium complexes in nitric acid has allowed us to designwater-soluble molecules that are beingdeployed in TA-55 for ultrapurifyingliquid waste streams. The polymerscapture plutonium and other actinidesand then are removed from solution byfiltration. This technology, developed atLos Alamos and licensed by PolyionicsSeparation Technologies, Inc., is knownas Polymer FiltrationTM. It is describedin the box on page 452.

All liquid that emerges from thetreatment of combustible wastes will betreated by Polymer FiltrationTM, as willthe effluent from nitric acid processing.(See Figure 4.) The aqueous liquids thatemerge from Polymer FiltrationTM willbe discharged as industrial waste water.The small volume of polymer that is fil-tered from solution will be disposed asTRU waste.

Figure 4 also shows that nitric acidrecycle technology will be added to theevaporator system at the tail end of thenitric acid processing. Essentially all ofthe nitric acid will be recycled back toplutonium-recovery processing, and nitrates will no longer leave TA-50.

Treatment of Metallic Wastes.Table I reveals that metal constitutes asubstantial fraction of the solid wastegenerated annually at TA-55. This wasteconsists primarily of discarded stainlesssteel glove boxes, but it also includesother items such as storage cans andtools. These TRU-waste items can bedecontaminated to low-level waste by aprocess known as electrochemical decontamination. This process uses elec-trolysis to remove a microscopic layerfrom metallic surfaces that contains most



Effluent fromnitric acidrecovery

EvaporatorHNO3recycle

evaporator

Evaporatoroverheads

< 6.0 × 10–5

Ci/L

Polymerfiltration

Evaporator bottoms to cement or vitrification, barrels of TRU waste to WIPP

Concentrated HNO3 recycled to nitric acid recovery

Effluent leaves thefacility as industrialwaste water< 5 x 10-7 Ci/L

Figure 4. Flow Chart for Advanced Treatment of Liquid WastesTwo technologies will be employed to clean up the liquid waste stream from nitric acid

processing. A second evaporator will recycle the nitric acid and prevent nitrates from

leaving the facility. Polymer Filtration TM will capture residual plutonium and other

actinides from the recycle evaporator’s condensed vapor, so that the radioactive con-

tent of the filtration effluent will be reduced to discharge limits for industrial waste

water before the effluent is sent to TA-50.

of the contamination. A filter then removes actinides from the electrolytesolution so that the latter can be recycledor discarded. The small volume of recovered contaminants is added to processing streams for further treatmentor discarded as TRU waste. More infor-mation about electrochemical decontami-nation is given in the box about it onpage 453.

Beyond the 90 Percent Solution

Almost all of the advanced wastetreatment and minimization technologiesdiscussed above can or will be imple-mented at TA-55 over the next three tofour years. Integration of those technolo-gies will enable the facility to reduce solid waste volumes and liquidwaste activity by 90 percent. However,we will achieve an even higher level of environmental and operational excellenceif we can reduce both solid and liquid ra-dioactive waste discharges to near zero.

We can never eliminate TRU wastetotally because the very function ofmuch of TA-55’s plutonium processingis to remove unavoidable TRU impuri-ties (such as the americium that resultsfrom the radioactive decay of plutonium-241). However, the prospectsare excellent for eliminating the largestremaining waste stream—the huge volume of industrial waste water that isdischarged following Polymer FiltrationTM

—by recycling it into plutonium recov-ery operations. All process-water discharges from plutonium operationscould be eliminated in any next-generation plutonium facility.

Obviously, the most effective strate-gy for reducing radioactive waste is toavoid its generation. Achieving such agoal will require breakthrough, not in-cremental, technologies. One possibilityis the use of room-temperature ionicliquids (RTILs), which are a new classof solvents. The unique chemical andphysical properties of these liquids can augment traditional plutonium pyrochemical processes and possibly

eliminate their waste streams.RTILs are composed of bulky, low-

valent organic cations and inorganic anions (Figure 5). Because the ions arevery large and have low charge, theyinteract only weakly and thus remainliquid at room temperature rather thancondensing into well-ordered crystallinesolids. Some desirable physical proper-ties of these ionic liquids are high conductivity, low vapor pressure, andthe ability to dissolve both organic andinorganic compounds. In addition, theionic liquids’ acidity can be adjusted.

The capabilities of RTILs, coupledwith the lack of interference from hydrolysis reactions, create a uniqueopportunity to obtain detailed informa-tion on the fundamental chemical behavior of actinide compounds underconditions far less demanding thanthose found in pyrochemical processes.Thus, their use could lead to a greaterunderstanding of actinide chemistry thatwould allow creation of next-generation

separation and purification processes. The chemistry of plutonium in RTIL

systems is currently being examined.This work has already demonstrated thestability of the Pu(III) and Pu(IV) oxidation states in chloride-rich RTILsolutions. We believe that continuedRTIL work will yield valuable thermo-dynamic data that will help improvehigh-temperature pyrochemical processes. Further, the ability to fine-tune the properties of RTIL solventsholds the promise of room-temperatureelectrochemical techniques for purify-ing and recovering plutonium.

Another promising advance lies inthe organometallic chemistry of the actinides. This area of research stemsfrom the discovery that certain actinides(An)—uranium, neptunium, and pluto-nium—dissolve in ether/iodine mix-tures. The resulting AnI3(THF) com-plexes, where THF is tetrahydrofuran,can be used to prepare myriad newcompounds and complexes.



[AlCl4]–

1-Ethyl-3-methylimidazoliumtetrachloroaluminate

1-Butylpyridinium nitrate

[NO3]–

CH3 CH2CH3

CH2CH2CH2CH3

N N

N

(a)

(b)Figure 5. Room-Temperature Ionic LiquidsRoom-temperature ionic liquids (RTILs)

are being investigated as a novel

processing medium for actinides. The

molecular structure of an RTIL consists

of a relatively large cation and ios, as

seen in (a) and (b) for two prototypical

systems. The leftmost vial in (c) contains

the RTIL diagramed in (a). The remaining

four vials contain uranium ions that were

prepared electrochemically in different

oxidation states. The colors arise be-

cause of the different oxidation states.

(c)

RTIL U(III) U(IV) U(V) U(VI)



For example, one organometalliccompound of interest is Pu(C8H8)2, because it is reported to be diamagnet-ic. If this claim is confirmed, then weshould be able to observe plutonium-239 by analyzing the compound withnuclear magnetic resonance (NMR).(To date, the plutonium-239 nucleushas not been observed by NMR.) If wecan use NMR to analyze Pu(C8H8)2,we will have a new opportunity tostudy covalent bonding in plutonium.

Another new compound—one that ispotentially more relevant for processoperations—is Pu[N(SiMe3)2]3, whereMe is a methyl group. This compoundsublimes at approximately 50°C. Because of its high vapor pressure, wecould use it to develop a chemical pro-cessing scheme based on organometal-lic chemistry. For example, we coulddissolve impure plutonium metal in aniodine/THF mixture, use the PuI3(THF)to prepare the volatile Pu[N(SiMe3)2]3complex, and then purify thePu[N(SiMe3)2]3 by sublimation. Oneadvantage of such a process is that theplutonium would be dissolved withoutreleasing potentially explosive hydro-gen gas. Another is that because theplutonium is purifed by sublimation—aphysical separation process—allreagents could be recycled or destroyed. However, the most excitinguse of Pu[N(SiMe3)2]3 would be in achemical vapor deposition (CVD)process that could deposit plutoniummetal in any shape or thickness. Depo-sition could potentially eliminate waste-intensive plutonium casting and machining operations.

While we can prepare batches ofPu[N(SiMe3)2]3, there are many chal-lenges that must be resolved before wecan use this compound in a processingscheme. Phase stability in plutoniummetal is very sensitive to the amount ofcarbon present. During the depositionprocess, carbon from the methyl groupsin Pu[N(SiMe3)2]3 could be absorbedby the plutonium metal. One of thechallenges of plutonium CVD will beto develop the process to the point thatthe final plutonium product contains lit-

tle carbon. Furthermore, plutonium formost uses is normally alloyed, andpreparing a plutonium alloy with CVDcould be difficult. But the rewards ofusing novel organometallic compoundsfor processing—safer procedures, mini-mal residual waste, and minimal needfor purification—are sufficient to acceptsuch challenges.

Summary

When integrated into a unified wasteminimization program at TA-55, theadvanced technologies we have described promise to radically reducethe radioactivity of our liquid wastestreams and reduce the volume of ourTRU solid wastes by 90 percent ormore. Our ultimate goal, however, is toexceed even this 90 percent solution byengineering a next-generation plutoniumfacility that discharges close to zero radioactive waste. Such a feat would require that we eliminate the huge volume of industrial waste-water fromleaving the building. A new plutoniumfacility may be built at Los Alamos asearly as 2008 and its constructionwould present an ideal opportunity toimplement the new technologies thatshould help us reach our near-zero-discharge goal. To reduce the TRU-solid waste from plutonium processesto near zero is a more daunting chal-lenge, but we continue to conduct theR&D that we hope will one day producethe needed breakthrough technologies. �

Further Reading

Cleveland, J. M. 1979. The Chemistry of Plutonium. La Grange Park, IL: AmericanNuclear Society.

Katz, J. J., G. T. Seaborg, and L. R. Morss, eds.1986. The Chemistry of the Actinide Elements. Vol. I and II, London: Chapmanand Hall.

Gary Eller received an B.S. in chemistry from West Virginia University and a Ph. D. ininorganic chemistry from Ohio State Universi-

ty. After a postdoc-toral appointment atGeorgia Tech, hemoved to LosAlamos and hasheld positions aspost doctoral fel-low, staff member,line manager, andproject leader.From 1994–1996 heserved as a techni-cal advisor to DOE-Hanford in the

high-level waste tanks program and since thenhas had a leadership role in the national program to stabilize and safely store excessplutonium-bearing materials for up to 50 years.His current technical interests are in the area ofactinide, environmental, and fluorine chemistryand the development of scientific informationthrough deployment to solve practical problemsin the field.

Larry Avens received his B.S. degree in chemistry from Tennessee Tech University andhis Ph.D. in inorganic chemistry from Texas

Tech University.After graduateschool he moved directly to the LosAlamos NationalLaboratory. Plutoni-um chemical pro-cessing and separa-tion science R&Dhave been Avens'areas of interest. Hehas four patents andnumerous publica-tions in the area. He

has also served on review teams associated withall areas of actinide science.

a vision for environmentally conscious plutonium processing

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