harold j. groh, w. lee poe, and john a. porter · harold j. groh, w. lee poe, and john a. porter...

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Development and Performanceof Processes and Equipment to Recover

Neptunium-237 and Plutonium-238

Harold J. Groh, W. Lee Poe, and John A. Porter

AbstractDuring the late 1950s, the Atomic Energy Commission (AEC) requested the Savannah River Site(SRS) to begin as soon as possible to produce plutonium-238 for use as a heat source for thermo-electric generators to power satellites. Thus, SRS joined the United States space race with theformer Soviet Union, as well as continuing as a key facility in the race to produce materials forthermonuclear weapons.

To accomplish this new task, a massive interdisciplinary effort was required by all of the techni-cal, engineering, maintenance, and administrative staff. New chemical and metallurgical pro-cesses were developed and installed in ingenious new equipment in the existing productionbuildings, with minimum interference with the processes operating to produce weapons mate-rials.

Neptunium-237, the precursor isotope to the production of plutonium-238, was recovered fromthe existing processes, where it had been produced as a byproduct in the reactor irradiation ofuranium. The purified neptunium-237 was fabricated into targets, which were neutron irradi-ated in SRS reactors to produce plutonium-238. The plutonium-238 was separated from nep-tunium and fission products and processed to plutonium oxide. The unconverted neptunium-237 was purified, processed to neptunium oxide, and used in additional reactor targets. Pluto-nium-238 oxide was fabricated into dense fuel forms at SRS and at other sites for use in thermo-electric generators.

Plutonium-238 produced at SRS has powered about two dozen spacecraft launched by NASA,including Transit, Viking, Voyager, Galileo, and Cassini.

Introduction

The Savannah River Site (SRS) began operationin the early 1950s to produce nuclear materialsfor weapons for the national security effort. TheUnited States was in a weapons race with theformer Soviet Union to produce thermonuclearweapons. There was great pressure on the staffof SRS to produce as rapidly as possible thetritium and weapons-grade plutonium (consist-ing predominantly of the fissionable pluto-nium-239 isotope) needed for nuclear weapons.

In 1957, the Soviet Union launched its firstsatellite, Sputnik, and began a race to spacewith the United States. In 1958, the UnitedStates launched its first satellite, Explorer I. In1960, the Soviets put the first man into space. In

1961, President Kennedy started the NationalAeronautics and Space Administration (NASA)program to put a man on the moon. In the late1950s, the Atomic Energy Commission gave SRSthe mission to produce a new reactor product,plutonium-238. This lighter isotope of pluto-nium, which had been made in only experi-mental quantities up to that time, was neededas a heat source for thermoelectric generators topower satellites. (Plutonium-238 has one lessneutron per atom than its heavier isotope,plutonium-239.) Thus, SRS became an impor-tant part of the space race as well as continuingto be a key facility in the weapons race. Thefirst plutonium-238 was made at SRS in 1961.

All of the work during these early years wasconducted under strict rules of secrecy to

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conceal technical details and production quanti-ties. In fact, in the early program, secrecy was sotight that code names were used to conceal theidentities of the isotopes neptunium-237 andplutonium-238.

Plutonium-238 has several characteristics thatmake it almost unique for fueling a radioisotopethermoelectric generator (RTG). It decays byemitting alpha particles with accompanyingevolution of heat that can be readily convertedto electricity. Gamma radiation from plutonium-238 is predominantly of low energy, requiringlittle shielding, which reduces the weightrequired for space applications. The relativelylong half-life (about 88 years) makes it suitablefor deep space missions, where sunlight is tooweak to generate much electricity from solarcells.

Plutonium-238 fueled RTGs have providedpower for about two dozen spacecraft launchedby NASA, including:

1961 The first RTG delivered 2.7 watts ofelectrical power to a Navy Transitnavigational satellite.

1969-72 Apollo astronauts left RTGs on themoon to power experiments.

1972 Pioneer spacecraft flew by Jupiterand Saturn.

1975 Viking spacecraft landed an instru-mented vehicle powered by an RTGon the surface of Mars.

1977 Voyager 1 and 2 spacecraft flew pastJupiter and Saturn (three 150-wattelectrical RTGs on each spacecraft).

1989 Galileo flyby of Jupiter (two RTGs,285 watts electrical each) waslaunched. Exploration of Jupiter andits moons is underway.

1997 Cassini probe to explore Saturn(three RTGs, 285 watts electricaleach) was launched.

1999 Galileo flew by Jupiter’s innermostmoon, Io.

Over the next 10 years, NASA is planningseveral more missions that are expected to useplutonium-238 for electric power.

Several challenges had to be faced in the devel-opment of production technology for pluto-nium-238.

• Processes and equipment had to be developedthat were compatible with the existing plantprocesses. Time was not available to constructnew production buildings. The new processeswould have to be incorporated with mini-mum interference into the facilities that werealready operating to separate weapons-gradeplutonium from uranium, and these newprocesses must not slow the production ofweapons.

• There was no available supply of the precur-sor element, neptunium-237. It would have tobe recovered and purified first before theproduction of plutonium-238 could begin.

• The radioactivity of plutonium-238 is about250 times that of plutonium-239, so radiationdamage to process materials, such as organicion exchange resins, was expected to causeproblems. In addition, there were concernsabout the adequacy of containment of pluto-nium-238 in conventional, filtered gloveboxes.

Technical Basisfor Plutonium-238 ProductionThe precursor isotope, neptunium-237, isproduced when uranium is irradiated innuclear reactors. The nuclear physics reactionsare the following (Groh 1970)

(1)

(2)

The reactions illustrate the production ofneptunium-237 as a byproduct from the irradia-tion of either natural uranium, Equation (1), oruranium enriched in the uranium-235 isotope,Equation (2). This byproduct had been produced

238U (n, 2n) 237U β6.7d

237Np

235U (n, γ) 236U (n, γ) 237U β

6.7d237Np

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at SRS since startup but had been lost to theradioactive waste streams during chemicalprocessing of the irradiated fuel elements. Itwas necessary to develop modifications to theexisting chemical separations processes toisolate the neptunium and to purify it fromuranium, plutonium and fission products. InSRS reactors, neptunium production fromenriched uranium fuels was the predominantroute, particularly after the practice of recyclingthe irradiated enriched uranium was begun toconserve the intermediate isotope, uranium-236.

Once neptunium-237 is recovered and purified,it is converted to plutonium-238 by returning itto the reactor in target elements, where thereaction shown in Equation (3) takes place:

(3)

Short exposures of the neptunium targets in thereactors limit the production of the higherisotopes of plutonium. Typical SRS plutonium-238 product had an isotopic composition ofapproximately 81% plutonium-238, 15% pluto-nium-239, 2.9% plutonium-240, and lesseramounts of plutonium-241 and plutonium-242.Unavoidably, some of the neptunium andplutonium are converted to fission productsduring the irradiation, and these must beremoved during the chemical processing.

In the late 1950s, when work was begun at SRSon processes to recover neptunium-237 andplutonium-238, a large body of informationalready existed in the literature on the chemis-try of neptunium and plutonium. But it wasnecessary quickly to augment that data in someareas and to apply it to practical separations

processes that could be operated in the SRSplants. The byproduct neptunium was recov-ered from the solvent extraction processesoperating in the two canyon buildings by acombination of modifications to the solventextraction processes to divert the neptuniumand ion exchange to recover and purify it. Ananion exchange technology was chosen toprocess the recovered neptunium after it wasre-irradiated to form plutonium-238. One of themost important chemical properties of nep-tunium and plutonium in solution is the abilityof these elements to exist in several oxidationstates or valences. This is an important tool forthe separation of these elements where theirbehavior is controlled primarily by valenceadjustment by oxidizing and reducing chemi-cals.

The varied oxidation states and ionic species ofneptunium and plutonium that can exist undernormal conditions are shown in Table 1. Ura-nium, by contrast, normally exists only in theVI oxidation state in aqueous solutions, as theUO2

2+ ion.

The solvent extraction processes are based uponthe preferential extraction from nitrate solutionof the actinide elements into tri-n-butyl phos-phate (TBP) dissolved in dodecane to separatethem from contaminating elements such asaluminum and the fission products. The orderof extraction into TBP from nitrate solutions is:U(VI) > Np(VI) Pu(IV) > Np(IV) Pu(VI) >>Np(V), Pu(III). Thus, U(VI) and Np(IV) can beseparated readily from inextractable Pu(III); andU(VI) can be separated from Np(V). Aluminumand most of the fission products areinextractable in TBP. The solvent extraction

237Np (n, γ) 238Npβ

2.1d238Pu

Table 1. Oxidation States of Neptunium and Plutonium

Oxidation State III IV V VI

Neptunium Ions Np4+ NpO2+ NpO2

2+

Plutonium Ions Pu3+ Pu4+ PuO2+ PuO2

2+

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process operated in the F-Area canyon buildingseparated weapons-grade plutonium fromnatural uranium or depleted uranium (uraniumfrom which the uranium-235 isotope had beenpartially removed). This was called the PurexProcess. The H-Area Canyon process, called theHM Process, recovered enriched uranium fromirradiated fuel elements.

The anion exchange process is based upon thefact that Np(IV) and Pu(IV) form anionic nitratecomplexes of the type Np(NO3)6

2- that arestrongly absorbed from concentrated nitratesolutions by strong-base anion exchange resin,such as Dowex 1-X4. The other oxidation statesof neptunium and plutonium are very weaklyabsorbed, as are most fission product speciesand common metallic cations. Thus Np(IV) andPu(IV) can be effectively separated from ura-nium and fission products by anion exchange,and Np(IV) can be separated from Pu(III).

Description of Processes

Recovery of Neptunium-237from Purex and HM Processes

In the Purex Process, one modification wasneeded to ensure that essentially all of theneptunium was directed to the high-level wastestreams from which it was to be recovered.Nitrite was added to the extraction section ofthe first solvent extraction contactor in whichuranium and plutonium are extracted into TBPto maintain the neptunium in the (V) oxidationstate and thus minimize loss to other wastestreams. (This contactor was originally a mixer-settler, but later was replaced by centrifugalcontactors.) The high-level waste streams weresubsequently combined and concentrated byevaporation in preparation to recover nep-tunium by anion exchange with new modularequipment installed in F Canyon. The concen-trated waste was adjusted to 8M nitric acid andtreated with ferrous sulfamate and hydrazinenitrate to produce Np(IV), which forms a stronganionic nitrate complex, and was fed to anagitated (stirred) anion exchange column toabsorb the neptunium. Most of any plutonium-

239 that was present was also absorbed as it tooforms a strong anionic nitrate complex. Theagitated resin column, which will be describedin a later section, was necessary to accommo-date solids that are typically present in concen-trated wastes. After the feed step, the columnwas washed with 8M nitric acid to removeimpurities, and then the neptunium and pluto-nium were eluted with 0.35M nitric acid.

The solution from the first bed was processedfurther to separate neptunium and plutoniumon a fixed bed of anion exchange resin asfollows. Solution acidity was adjusted to 8M,and valence adjustment was repeated usingferrous sulfamate and hydrazine stabilizer,followed by heating to 50°C to oxidize Pu(III) toPu(IV). The solution was fed to the resin bed,which absorbed the neptunium and plutonium.The bed was washed with 8M nitric acid tofurther remove impurities, and the plutoniumwas removed by washing with 5.5M nitric acidcontaining ferrous sulfamate and hydrazine toreduce plutonium to the (III) state. The recov-ered plutonium-239 was returned to the Purexsecond plutonium cycle. The neptunium waseluted with 0.35M nitric acid, processed byanother cycle of anion exchange for furtherpurification, followed by a cycle of cationexchange if necessary for thorium removal.

In the HM Process, operating conditions weremodified to isolate and purify neptuniumentirely by solvent extraction in the existingequipment. The dissolver feed was treated withferrous sulfamate to maintain the Np(IV)valence state so that neptunium was extractedalong with uranium in the 1A mixer-settler andpartitioned from uranium in the 1B mixer-settler. Further decontamination of neptuniumwas accomplished using the Purex flowsheet insecond plutonium cycle equipment that was nototherwise being used. The neptunium productsolution was concentrated by evaporation inpreparation for further processing in HB Line (alightly shielded, glovebox facility in Building221-H).

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Isolation of neptunium from the HM Processcontinued for the life of the plutonium-238program, but was discontinued from the PurexProcess after an adequate inventory was accu-mulated.

The nitric acid solutions of neptunium from thePurex and the HM Processes were transferredto the HB Line for conversion to neptuniumdioxide, NpO2, which could be fabricated intotargets for neutron irradiation to produceplutonium-238. In HB Line, neptunium wasfurther purified and concentrated, if needed, byanion exchange. After anion exchange, theconcentration of nitric acid was adjusted withinthe range of 1-2M, and the valence of theneptunium was adjusted to the (IV) state withascorbic acid and hydrazine inhibitor at about50°C. Neptunium(IV) oxalate was precipitatedat the same temperature by the addition of 1Moxalic acid solution. After digestion to aidcrystal growth, the slurry was cooled to roomtemperature and filtered and air-dried. Nep-tunium dioxide was produced from the nep-tunium oxalate by heating in air to a finaltemperature of about 550°C (Porter 1964).

Recovery of Neptunium-237and Plutonium-238 from IrradiatedTargets

Processing of irradiated targets to recoverplutonium-238 product and residual nep-tunium-237 was initially performed on a smallscale in the High-Level Caves and B-Wingfacilities of the Savannah River Laboratory. Forthe continuing larger scale operations, the veryhigh radioactivity levels associated with theneptunium and the plutonium required instal-lation of new plant equipment in the heavilyshielded separations processing areas. Thus,unique modular units called “frames” (to bedescribed later in this paper) were installed bynovel techniques in H Canyon to process theirradiated targets. HB Line was also modified toprovide facilities to convert the purified pluto-nium-238 product and the recovered nep-tunium-237 to the final oxide forms.

A schematic diagram of the neptunium targetrecovery process is shown in Figure 1. Theirradiated NpO2-aluminum targets, containingunconverted neptunium-237, product pluto-nium-238, and fission products, were cooled atleast 45 days to allow decay of short-livedfission products and then dissolved in boiling10M nitric acid containing small amounts ofmercuric ions and fluoride ions as catalysts.Dissolution was slow, requiring up to 48 hoursfor about 85% dissolution. Any undissolvedtarget material was carried over and dissolvedwith the subsequent batch of targets. Thedissolver solution contained small quantities ofsolids, primarily silica, which was removed byfiltration prior to further processing.

The plutonium-238 product and residual nep-tunium-237 were separated from aluminum,fission products, other impurities, and fromeach other by anion exchange processing. Theresin columns were normally operated as fixedbeds, but could be operated as agitated bedswhen needed for regeneration or replacementof resin. In the first cycle of anion exchange,neptunium and plutonium were both adjustedto the (IV) valence state in 8M nitric acid bytreatment with ferrous sulfamate and hydra-zine, followed by heating to 50°C. The nep-tunium and plutonium were absorbed on theresin column to separate them from aluminum,fission products, and other cationic impurities.After washing with 8M nitric acid for decon-tamination, the neptunium and plutoniumwere eluted with 0.35M nitric acid. In thesecond cycle of anion exchange, the solutionwas adjusted to 8M nitric acid, and neptuniumand plutonium were again both adjusted to the(IV) valence state and absorbed on the resin.After washing with 8M nitric acid for furtherdecontamination, the plutonium was separatedfrom the neptunium by washing the columnwith 5.5M nitric acid containing ferroussulfamate and hydrazine to reduce the pluto-nium to the (III) valence state. The neptuniumwas then eluted with 0.35M nitric acid. Theseparated neptunium and plutonium wereprocessed by one or more additional cycles of

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anion exchange as needed to obtain the desiredpurity. The purified neptunium solution wastransferred to HB Line for further processing toproduce NpO2 for recycle. The purified pluto-nium solution was further processed as de-scribed below.

Conversion of Plutonium-238 to Oxidefor Use in Heat Sources

In the early phases of the program, plutonium-238 was isolated as a dilute nitric acid solutionthat was shipped to Mound Laboratory inMiamisburg, Ohio, for additional processingand fabrication into heat sources. Later, toimprove shipping safety, the plutonium wasprocessed further in the HB Line to the solidoxide form either for shipment to another siteor for onsite fabrication of heat sources.

In the HB Line, the plutonium solution from HCanyon was processed by anion exchange, ifrequired, for decontamination and concentra-tion. Then, the nitric acid concentration wasadjusted within the range of 1-2M and pluto-nium valence was adjusted to (IV) by addingascorbic acid and hydrazine nitrate and heatingto 50°C. Plutonium(IV) oxalate was precipitatedat the same temperature by the addition of lMoxalic acid solution. After digestion to aidcrystal growth, the slurry was cooled to roomtemperature and filtered and air-dried. Pluto-nium dioxide was produced by heating theplutonium oxalate in air to a final temperatureof about 550°C.

An innovative technique was used at times inHB Line to reduce the rather high emission rateof neutrons from 238PuO2. Neutron emissionresults from spontaneous fission of plutonium-

Figure 1. 238Pu recovery from irradiated 237Np at Savannah River

Offgas

Slugs

First Second Third

Anion Anion AnionColumn Column Column

Waste Waste

Np237

Pu238

Oxalate

Precipitator Pu238

Nitrate

Filter OxalatePrecipitator

Calcining Calcining

Filter Furnace Furnace

Filler

Dissolver

Waste

Np237 Pu238 Pu238

NpO2 PuO2

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238 and also from (α,n) reactions with impuri-ties and with the small quantities of 17O and 18Oisotopes that occur in natural oxygen, which ismostly 16O. Research at Mound Laboratory andat Savannah River Laboratory showed that the17O and 18O content of 238PuO2 can be reduced tovery low values by isotopic exchange with 16O.This can be readily accomplished by heating theplutonium oxide in the presence of oxygen gasor water vapor that is highly depleted in 17Oand 18O. In HB Line, this was implemented byflowing isotopically pure 16O2 over the oxide ata temperature of a few hundred degrees centi-grade. Neutron emission rates were reduced tonear the value expected from spontaneousfission and impurities, and resulted in lowerradiation exposure of personnel.

Some of the 238PuO2 produced at SRP wasshipped to other sites for production of heatsources, but a large number of heat sources wasalso produced onsite in a new facility installedin 1978 in Building 235-F. D. T. Rankin describesthe production of heat sources at SRP in an-other paper in this symposium proceedings.

Radiation Effects of Plutonium-238

In the chemical processes, the high radiationlevel of plutonium-238 was dealt with in severalways. Perhaps the most serious problem wasradiation damage to the organic ion exchangeresins. This was controlled by choosing resins,such as Permutit SK, that were more resistantto radiation, by limiting the exposure times ofthe resin to plutonium-238, by agitating theresin beds between cycles to redistribute theresin particles, by maintaining continuous flowof process solution through resin beds loadedwith plutonium-238, and by periodic remotereplacement of the resin. In solutions, radiolysisproducts from plutonium-238 alpha particlesinterfered with the stabilization of the desiredoxidation states of plutonium and neptunium.This was controlled by the addition of reducingagents and stabilizers such as hydrazine. Acombination of ferrous sulfamate and hydrazineproved to be most effective for maintaining the

lower oxidation states of neptunium andplutonium while minimizing undesirable sideeffects such as gas formation.

Conventional gloveboxes provided adequatecontainment of plutonium-238 in the final stepsof processing. The boxes were constructed ofstainless steel and operated at subatmosphericpressure with several stages of high efficiencyfiltration for the box offgas. Local shielding forgamma and neutron radiation, including leadedgloves, protected the workers from radiationexposure.

Description of ProductionEquipment

Unitized Frame Concept

The recovery of neptunium-237 from the Purexand HM processes and the recovery of pluto-nium-238 from irradiated neptunium targetsinvolve gamma radiation levels that are equiva-lent to those encountered in processing spentnuclear fuels; therefore, these processes wereinstalled within the shielded space of theSavannah River canyons. The term canyonrefers to the heavily shielded buildings (Build-ings 221-F and 221-H) that house and supplyservices to process equipment for recoveringplutonium and uranium from spent reactorfuels.

The canyons at Savannah River provide amodule 10 feet square by 17 feet high for eachvessel having standard services (i.e., inlet andoutlet piping, steam, cooling water, electricity,sampling, and instruments). Each buildingsection has four modules, isolated by a low curbto contain liquid spills or leaks, and has a sumpand permanently installed transfer jet to movespilled liquids or leakage to a rework system.Transfers between vessels in the canyon aremade on a pipe rack in which the pipe isremotely removable. A direct jumper can maketransfers between adjacent vessels.

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As discussed above, ion exchange was selectedas a key process for recovery, separation, anddecontamination of neptunium-237 and pluto-nium-238. The ion exchange vessels and flowsof solutions to and from them are much smallerthan those used in the normal solvent extrac-tion processes for which the canyons weredesigned. Ion exchange processes had neverbefore been installed and operated in a facilitythat was remotely maintained. Since experi-mental work performed by the Savannah RiverLaboratory showed that it would be necessary,from time to time, to replace the ion exchangeresins in the canyon process vessels, a techniquewas developed to transfer them to and from thecanyon as slurry (Bebbington 1990).

The recovery of neptunium from depleteduranium required installation of four ionexchange columns in F Canyon. The recovery ofneptunium-237 and plutonium-238 fromirradiated neptunium targets required five ionexchange columns and a dissolver in H Canyon(Poe 1964). Unitized frame design was adoptedfor these units to minimize canyon spacerequirements. The frame concept means install-ing a number of small equipment pieces in asteel frame that is installed and removed by thenormal canyon crane. The specialized equip-ment was designed by adapting the small-scaleequipment to remote operation in the canyon.After building a complete prototype unit to testthe feasibility of the concept, three units wereconstructed and used many years in the can-yons. Figure 2 is a photograph of one theseframes. A fourth unit was installed in F Canyonfor initial recovery of neptunium. This unitconsisted of a single ion exchange column andits support tankage.

Placing several equipment pieces in a frame in asingle canyon module is economical of canyonspace but places a heavy burden on the servicesavailable in that module. Pipe and electricalconnections were made to the frame rather thanto individual pieces of equipment. Serviceswere piped to the equipment as part of thepermanent frame structure. This efficientlyused existing services, but additional services

were required, primarily for pneumatic liquidlevel and specific gravity instrumentation forseveral vessels in each frame. Bundles of up to 6stainless steel tubes (1/4-inch o.d.) were drawnthrough the 3-inch pipes embedded in thecanyon shielding walls. Demonstration of thisconcept had great utility in adapting existingfacilities to alternate processes. No significantproblems were encountered in installing theselines, and their useful life has been equivalentto the life of embedded piping.

Ion Exchange Columns

The ion exchange columns that were built,tested, and used in these operations were either

Figure 2. Np and 238Pu frame (Complete framewithout ion exchange columns.Frame as installed remotely withCanyon crane.)

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12- or 24-inch diameter and normally contained25 or 100 liters of resin. They normally wereused as conventional settled bed ion exchangecolumns but had the capability of use as agi-tated ion exchange bed columns. One unit wasused as an agitated bed column. All operations,including resin replacement, were performedremotely without mechanical valves. Resin wascharged and discharged as free-flowing slurryin a solution having a specific gravity close tothat of the resin. Two weirs determine flowpaths; air pressure is applied to either or both

to direct flow properly for feed, wash, elution,or resin removal. Figure 3 shows an isometricdiagram of one such ion exchange column.

Dowex 1 and Permutit SK anion exchangeresins were used. Dowex 1 resin provides higherdecontamination from fission products, butPermutit SK resin is more stable under alpharadiation. Dowex 1-X4 (4% nominal crosslinkage) was used in 40 to 60 mesh rangeparticle size. Selected use of smaller particleresins, some with cross linkages as small as 2%,

Figure 3. Ion exchange column (for installation on frame shown in Figure 2)

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was used where fast kinetic absorption wasrequired, such as in the ion exchange columnused for separation of plutonium-238 fromneptunium-237. Solution flow through the resinwas determined by the resin particle size andgas generated from alpha radiolysis.

Performance of Production Processes

The losses of either neptunium-237 or pluto-nium-238 in the H-Canyon frame operationused for irradiated neptunium targets averaged2.7% for the three neptunium processing col-umns and 4.9% for the four plutonium process-ing columns. This material was discarded toHigh-Level Waste for the first two years ofoperation, and then the frame waste wasdiverted to a new frame waste column and afurther 95 % of this was recovered. High fissionproduct decontamination was accomplished; thelimiting fission products were the radionuclideszirconium and niobium. This process gave adecontamination factor (curies in the feeddivided by curies in the product) of 100,000 forneptunium and 1,800,000 for plutonium. Thesecond ion column was used to separate theplutonium from the neptunium; this was one ofthe most crucial activities in the target process-ing because cross-contamination of one productleft in the other product would be lost to waste.Using the Dowex 1-X2 resins, removal of 97 to99% of the plutonium from the neptuniumcould be achieved. (This ~1 to ~3% of the pluto-nium lost to the neptunium is included in the4.9% waste loss described above.)

HB-Line Facilities for Conversionof Neptunium and Plutoniumto Oxides

Decontaminated neptunium-237 and pluto-nium-238 products from the H-Canyon Frameswere converted to NpO2 and PuO2 in HB Line,after an initial period of liquid shipments ofplutonium-238 nitrate to Mound Laboratory.Initially these operations were performed infacilities designed for weapons plutoniumfinishing. These facilities had been made obso-

lete when H-Canyon operations were changedfrom Purex (weapons-grade plutonium) toprocessing highly enriched uranium in the late1950s. Neptunium-237 and plutonium-238finishing operations were performed in HBLine gloveboxes. The plutonium-238 operationswere performed in a new small glovebox line,and the neptunium was finished in the old B-Line cabinets. After several years of operationin this manner, it was decided to construct anew HB Line on top of Building 221-H designedfor these operations. The new line had threesubparts: one for neptunium, one for pluto-nium-238, and the third for recovery of nep-tunium and plutonium from scrap. This newfacility was located over Sections 2 through 6 ofBuilding 221-H. It consists of the new fifth andsixth level of that building.

The new neptunium oxide line was constructedover Sections 4 and 5 of the 221-H Canyon. Theoxide line consists of two glovebox lines con-structed with adjacent operating and mainte-nance rooms to minimize spread of contamina-tion. The line decontaminated and convertedneptunium nitrate to neptunium oxide. Pro-cesses employed include ion exchange, precipi-tation, calcination to oxide, and packaging ofthe neptunium for shipment to Building 235-Ffor fabrication into targets.

The plutonium-238 oxide process was con-structed over Section 6 of the canyon. Theprocess is an improved version of the originalplutonium-238 oxide line. The plutoniumnitrate solution is converted to plutonium oxidepowder by the oxalate precipitation followed bycalcination. The cabinets associated with thisline were equipped with significant externalneutron and gamma shielding to protect theoperating staff.

The scrap recovery module consists of twoparallel lines of gloveboxes located over Sections2 and 3 of the canyon. They provide the capa-bility to introduce different types of scrapmaterials, sorting, dissolution, filtration, andtransferring solutions to the canyon for recoveryof the actinides and the associated support

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capabilities. The cabinet lines are equippedwith water jackets and lead shielding over thegloves and lead glass on the windows. Thisequipment is provided to shield the operatingstaff from the gamma radiation.

Fabrication of Neptunium Targetsin 235-F Building

Neptunium oxide from HB Line finishingoperations was blended in Building 235-F withaluminum powder and fabricated into targetsfor irradiation in SRP reactors. Target fabrica-tion was initiated in 1961 and consisted of acompacted blend of oxide and aluminum cladin an aluminum can. “Green” compacts, 3inches long and 0.86 inches in diameter, wereformed by pressing the blended powder in atool-steel die at 19.8 tons per square inch (tsi) atambient temperatures. These compacts were 90to 92% of theoretical density. A double-actingpress was used since it transmits equal force toboth ends of the compact to give more uniformcompaction. Complete densification duringgreen compact fabrication was undesirable

since some travel of the compact surface rela-tive to the can wall during hot pressing isneeded to provide a fresh metallic surface forbonding. Two compacts were loaded into animpact-extruded aluminum can, which wasthen closed by an aluminum cap. The assemblywas loaded into an Inconel-X die, which wasplaced in a vacuum furnace consisting of afloating Inconel-X back-up die inside a stainlesssteel sheath wrapped with resistance heaters.This configuration is shown in Figure 4. Theslug was heated to 600 to 620°C under avacuum of <1,000 microns of mercury and thenpressed at about 19 tsi to form a metallic bondbetween the compacts and the can wall andabout 99% of theoretical density. All of theseoperations were performed in glove boxes withcontained atmosphere to protect the workers.These target slugs were irradiated between 1961and 1965 to produce plutonium-238.

In later development, as more neptuniumbecame available for irradiation, the design ofthe targets was changed from target slugs totubular targets, which improved heat transfer

Figure 4. Hot-press-bond furnace for fabrication of Np target slugs

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and increased plutonium-238 production andpurity by allowing operation at higher neutronfluxes. Between 1966 and 1978, the design ofthese tubular elements changed three times.Billets containing the neptunium target werefabricated in Building 235-F, and the tubes wereextruded in Building 321-M. Billets consisted ofcylindrical inner and outer sleeves welded tothe bottom fitting. Compacts were placed in thespace between the two sleeves, and a top fittingwith a breather tube was welded in place. Thiscreated a contamination shield, which allowedthe billet to be removed from containmentcabinets in Building 235-F and transported toBuilding 321-M for extrusion. The initial tubulartargets were constructed with cylindricalcompacts used in the target slugs, which onextrusion yielded tubes with ribbed targetcores. Later compacts were redesigned intotrapezoidal compacts to provide tubular targetswith uniform neptunium core thickness. Thedifferent target design used differing amountsof neptunium so that the final tubular targetsvaried from 120 and 190 grams of neptuniumper foot of active tube length, and the tubediameter varied between 3 and 3.7 inches. Theinitial cladding thickness was 0.065 inch andlater was decreased to 0.040 inch as fabricationand irradiation experience showed these targetswere safe for irradiation. Fabrication of nep-tunium target was continued until the SRSreactors were shut down in the early 1990s.

References

Bebbington, W. P., 1990, History of Du Pont at theSavannah River Plant, E. I. du Pont de Nemoursand Company, Wilmington, Delaware.

Groh, H. J., and C. S. Schlea, 1970, “The Recoveryof Neptunium-237 and Plutonium-238,” Progressin Nuclear Energy, Series III, Process Chemistry,Vol. 4. Editors C. E. Stevenson, E. A. Mason andA. T. Gresky, Pergamon Press, Inc.

Poe, W. L., A. W. Joyce, and R. I. Martens, 1964,“Np-237 and Pu-238 Separation at the SavannahRiver Plant,” Industrial and Engineering Chemis-try, Process Design and Development, Vol. 3, No. 4,p. 314.

Porter, J. A., 1964, “Production of NeptuniumDioxide,” Industrial and Engineering Chemistry,Process Design and Development, Vol. 3, No. 4,p. 289.

Biographies

Harold J. Groh

Harold J. Groh has 47 years experience innuclear programs. Employed at Savannah RiverSite from 1952 through April 1990 in profes-sional and management positions, he has broadfamiliarity with DOE nuclear weapons complexand SRS capabilities, with particular technicalexpertise in nuclear fuel processing, wastemanagement, and plutonium technology. Heworked ten years as an independent consultantin numerous DOE programs including surplusweapons plutonium disposition, spent fuelmanagement, chemical safety reviews, andhigh-level waste management. Mr. Groh has aPh.D. in physical chemistry from the Universityof Rochester, and a B.S. in chemistry from St.Louis University.

W. Lee Poe, Jr.

W. Lee Poe, Jr. has more than 48 years of experi-ence in providing technical and managementsupport for large-scale nuclear projects includ-ing chemical processing, waste management,environmental protection. Employed by DuPont Company and WSRC from 1951 through1989, his experience includes operation ofchemical reprocessing of irradiated spentnuclear fuels (F and H Canyon Buildings),purification and finishing of 239Pu, 238Pu, 237Np,and U (depleted, natural, and enriched) (in FB

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and HB Lines and A Lines in both areas), targetfabrication, heavy water production, and siteand DOE complex planning. Process safety andoperational assessments, environmental protec-tion, safety and health protection, and a thor-ough understanding of the processes used in allareas of the SRS are an integral part of thisexperience. Specific responsibilities for 237Npand 238Pu processing were technical supportwithin the various plant units for design,testing, start-up, and operation of all of theprocesses described in this paper. His experi-ence also includes ten years consulting withDOE and private contractors associated withDOE nuclear programs in a wide range of areas.During this time, he has participated in publicforums on waste management activities; riskmanagement alternatives, comparisons, andranking; environmental remediation; etc.

John A. Porter

John A. Porter has 42 years experience inresearch, development, and production in thedefense nuclear industry. Employed by DuPontCompany and Westinghouse Savannah RiverCompany from May 1957 through July 1989 inprofessional and management positions at theSavannah River Site, his experience includesoperation of chemical processing plants for

irradiated fuels and targets (F and H Areasseparations plants, 239Pu metal production line,238Pu oxide production line, plutonium storagefacility, plutonium recovery facilities, tritiumfacilities); operation of waste managementfacilities and analytical control laboratoriessupporting processing plants; manager ofindustrial hygiene and radiation protectionfunctions; coordinator of environmental protec-tion programs; and conduct/management ofresearch and development activities relating tothe above. With ten years independent consult-ing in DOE nuclear programs, his specialtyareas include plutonium (and other actinides)processing, waste management, conduct ofoperations, personnel safety, and environmentalprotection. Mr. Porter has a Ph.D. in physicalchemistry from Vanderbilt University and a BSin chemistry from Clemson University.

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