Present Status and Future Directions of Plutonium Process Chemistry

23 Present Status and Future Directions of Plutonium Process Chemistry

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ELDON L. CHRISTENSEN—Los Alamos National Laboratory, Material Science and Technology Division, Los Alamos, NM 87545 LEONARD W. GREY—Ε. I. du Pont de Nemours & Company, Inc., Savannah River Laboratory, Aiken, SC 29808 JAMES D. NAVRATIL—Rockwell International, Rocky Flats Plant, Golden, CO 80401 WALLACE W. SCHULZ—Rockwell Hanford Operations, Richland, WA 99352

An overview is given of plutonium process chemistry used at the U. S. Department of Energy Hanford, Los Alamos National Laboratory, Rocky Flats, and Savannah River sites, with particular emphasis on solution chemistry involved in recovery, purification, and waste treatment operations. By extrapolating from the present system of processes, this paper also attempts to chart the future direction of plutonium process development and operation. Areas where a better understanding of basic plutonium chemistry will contribute to development of improved processing are indicated. Large-scale plutonium recovery/processing f a c i l i t i e s o r i g i nated at Los Alamos and Hanford as part of the Manhattan Project in 1943. Hanford Operations separated plutonium from irradiated reactor fuel, whereas Los Alamos purified plutonium, as well as recovered the plutonium from scrap and residues. In the 1950's, similar processing f a c i l i t i e s were constructed at Rocky Flats and Savannah River. A limited overview of the process chemistry used at these sites is presented. This paper will also attempt to bridge, at least partly, the gap between ongoing fundamental plutonium research and development and applied technology needs. We believe i t is important to bridge this gap, since a continuous flow of knowledge about plutonium chemistry from academic and government laboratories to the plant is necessary and beneficial 0097-615 6/ 83/0216-0349$06.00/0 © 1983 American Chemical Society

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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PLUTONIUM CHEMISTRY

in motivating and stimulating fundamental research and development studies. The research and development areas indicated in this paper are representative, we feel, of those where fundamental research and development can make a timely and substantial contribution. Our hope is that at least some of our suggestions will be pursued.


Plutonium Processing at Los Alamos Plutonium metal is prepared by two methods--direct reduction of the oxide by calcium (DOR) (1_,2_), and reduction of PuF^ by calcium in our metal preparation line (MPL)(_3) (see Figure 1). In the DOR process, the plutonium content of the reduction slag is so low that the slag can be sent to retrievable storage without further processing. Metal buttons that are produced are no purer than the oxide feed and/or the calcium chloride salt. Los Alamos purifies the buttons by electrorefining(4-,j>), yielding metal rings that are >^ 99.96 percent plutonium. The Los Alamos MPL can accept either plutonium nitrate solution or plutonium oxide as feed. If the feed is the nitrate solution, then the process steps are: precipitation of plutonium peroxide, hydrofluorination of the peroxide to PuF^, and reduction of the PuF^ to metal. If the feed is oxide, then i t is hydrofluorinated to produce PuF , which is then reduced to plutonium metal with calcium metal. This metal usually meets the specifications set by the metal fabrication personnel. This process is wel1-defined--most parameters are known. 4

Los Alamos residue processing yields Pu0 feed that can be easily hydrofluorinated. If the oxide has been prepared by calcination of the oxalate, then that oxide can be easily converted to PuF , whereas other oxides have poor fluorination characteristics. 2

4

Los Alamos has been processing unirradiated plutonium scrap since 1944.(6>) Scrap processing is presently being done at the new plutonium f a c i l i t y , using the flowsheet shown in Figure 2. Feed to this system comes from the varied research programs at Los Alamos and from other sites through the auspices of the Central Scrap Management Office at Savannah River. Plutonium Processing at Rocky Flats Chemical processing activities involve the recovery of plutonium from Rocky Flats Plant scrap, waste materials and residues, and effluent streams. The final product of this recovery and purification effort is high-purity plutonium metal for use in foundry operations.



23.

CHRiSTENSEN ET AL.

Overview of Pu Process Chemistry

Figure 1. Metal Preparation and Purification at Los Alamos


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Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983. RINATION MOVAL

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23.

CHRiSTENSEN ET AL.

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The original plutonium recovery and purification processes were adopted from Los Alamos processes in 1950. The processes at Rocky Flats are s t i l l similar today, in many respects, to the Los Alamos processes. Figure 3 shows a flowsheet for plutonium processing at Rocky Flats. Impure plutonium metal is sent through a molten salt extraction (MSE) process to remove americium. The purified plutonium metal is sent to the foundry. Plutonium metal that does not meet foundry requirements is processed further, either through an aqueous or electrorefining process. The waste chloride salt from MSE is dissolved; then the actinides are precipitated with carbonate and redissolved in 7M HN0 ; and f i n a l l y , the plutonium is recovered by an anion exchange process. 3

Impure plutonium oxide residues are dissolved in 12M HN0 0.1M_ HF under refluxing conditions, and then the plutonium is recovered and purified by anion exchange. Plutonium is leached from other residues, such as metal and glass, and is also purified by anion exchange. The purified plutonium eluate from the anion exchange process is precipitated with hydrogen peroxide. The plutonium peroxide is calcined to the oxide, and the plutonium oxide is fluorinated. The plutonium tetrafluoride is f i n a l l y reduced to the metal with calcium. 3

Acid waste streams are sent through a n i t r i c acid recovery process, and then to a secondary plutonium recovery anion exchange process. The acid waste streams are then sent to waste treatment. Direct oxide reduction (DOR) is presently being tested on production equipment. Eventually, i t is hoped to eliminate the fluoridation and bomb reduction processes and replace them with DOR. Plutonium Processing at Hanford Irradiated Fuel. A historically important and continuing mission at the Hanford site is to chemically process irradiated reactor fuel to recover and purify weapons-grade plutonium. Over the last 40 years, or so, several processes and p l a n t s Bismuth Phosphate, REDOX, and PUREX--have been operated to accomplish this mission. Presently, only the Hanford PUREX Plant is operational, and although i t has not been operated since the f a l l of 1972, i t is scheduled to start up in the early 1980's to process stored and currently produced Hanford -Reactor fuel. Of nine plutonium-production reactors built at the Hanford site, only the N-Reactor is s t i l l operating.



2

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METAL/GLASS, DISSOLUTION HEELS SAND/SLAG/CRUCIBLES

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LEACHING, RESIDUE DISSOLUTION/ FILTRATION

OXIDE DISSOLUTION/ FILTRATION

SALT DISSOLUTION CARBONATE PRECIPITATION/ REDISSOLUTION

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PEROXIDE PRECIPITATION/ CALCINATION

(Pu ELUiATE)

TO AMERICIUM RECOVERY

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(Pu EFFLUENT)

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OXIDE ION EXCHANGE

OXIDATION

ANODE HEELS

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PURIFIED Pu METAL

Figure 3. Plutonium Recovery Processes at Rocky Flats

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PURIFIED Pu METAL

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The following, albeit in very brief fashion, outlines the essentials of the technology to be employed in producing pure Pu0 from irradiated N-Reactor fuel. Following receipt of the fuel in the PUREX Plant, Zircaloy-2 cladding is chemically removed by dissolution with boiling NH F-NH N0 solution (Zirflex process). Small amounts of UF^ resulting from attack of uranium metal core by the Zirflex process reagent are metathesized to Na U 0 and then dissolved along with unattacked uranium metal in HN0 . Standard and well-known PUREX process technology is practiced to recover, separate, and purify pluto nium. New equipment has been installed in the Hanford PUREX Plant to convert, via plutonium oxalate, the purified Ρ υ ( Ν 0 ) product solution from the third plutonium cycle to Pu0 for shipment o f f s i t e . Following customary Hanford site practice, the denitrated (~0.5-l.0M HN0 ) PUREX process high-level waste (HLW) containing small amounts of plutonium and varying amounts of other actinides (Am, Np, and U) will be adjusted to > pH 9 by addition of NaOH, and stored in newly constructed underground double-shell tanks. 2

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Current plans for restart of Hanford PUREX Plant call for storage of neutralized (pH > 9) spent decladding solution in double-shell tanks. Depending upon operating procedures, from two to four such tanks (each costing about $10,000,000) will be needed to store spent decladdent. (The spent decladdent will not, according to present planning, be mixed with alkaline HLW.) There is an economic incentive to find inexpensive and safe alternative treatment/storage schemes for handling the spent decladding solution. Key to such schemes i s , as discussed subsequently, concentration of plutonium and americium, their chemical species in the decladding solution, and a simple method for removing them e f f i c i e n t l y . Plutonium Scrap Processing. In addition to recovering plutonium from irradiated reactor fuel, a Plutonium Reclamation Facility (PRF)(_7,8J is operated at the Hanford site to recover, separate, and purify kilogram amounts of plutonium from a wide range of unirradiated scrap materials. A 20 percent TBP-CC1 solution is used to extract Pu(IV) from HN0 -HF-A1(N0 ) solutions of dissolved scrap.

4

3

3

3

Plutonium Processing at Savannah River The f a c i l i t i e s at Savannah River(j)) consist of five heavywater-moderated and cooled production reactors, two chemical separations areas as a heavy water extraction plant, several test reactors, reactor fuel and target processing f a c i l i t i e s , the Savannah River Laboratory, and many other f a c i l i t i e s neces sary to support the operations. During the 1960's, two of the


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PLUTONIUM CHEMISTRY


production reactors were shut down and placed in standby condi tion; one of these is scheduled to start up again in the f a l l of 1983. Two test reactors and the heavy water extraction plant have also been shut down. Figure 4 i l l u s t r a t e s , in very brief fashion, the essentials of the flow of materials from the reactors through the separa tions area. Depleted uranium targets are processed through the F-Area canyon by well-known PUREX process technology.(10-13) Enriched uranium fuel elements are processed through the TPArea canyon by a modification of the PUREX process.(U_) Irradiated neptunium targets are processed through the Η-Area canyon by an ion exchange process.(14) Following receipt of the irradiated depleted uranium targets in the F-Area Separations Area, aluminum cladding is chemically removed by dissolution with a NaN0 -NaOH solution and sent to waste storage. The bare uranium targets are then dissolved in boiling 10M HN0 . PUREX process technology is used to separate and purify both the plutonium and the uranium. The uranium solutions are evaporated by successive evaporation steps, f i r s t to U 0 ( N 0 ) · 6 Η 0 , and then denitrated to U0 . The plutonium solutions are then concentrated by cation exchange and precipitated as PuF . The precipitate is roasted in an oxygen atmosphere to a mixture of -73 mole % PuF^ and 27 mole % Pu0 before undergoing a calciothermic reduction to plutonium metal for shipment o f f s i t e . Waste solutions from these processes which contain residual plutonium and small amounts of neptunium are processed through one of two anion exchange systems to recover both the plutonium and neptunium. The waste streams are then evaporated, acid stripped, and adjusted to pH > 13 by addi tion of NaOH before storage in underground double-shell tanks. 3

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3

2

2

3

3

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Following receipt of the burned enriched uranium fuel in the Η-Area canyon, this aluminum-clad Al-U alloy is dissolved in HN0 catalyzed with Hg(N0 ) . The dissolver solution is fre quently blended with dissolved offsite fuels, such as university research reactors and HFIR cores from Oak Ridge, before the purified uranium and neptunium are separated and purified by a modification of the PUREX process. The purified U0 (N0 ) solu tion is shipped offsite for conversion to uranium metal. This uranium metal is reprepared into fuel tubes for the SRP reac tors. Since this uranium is repeatedly recycled through the reactors, the U content has grown over the years. The puri fied neptunium, produced in the fuel tubes by the irradiation of the U , is concentrated by anion exchange, precipitated as an oxalate, and calcined to the oxide. This Np0 is then f a b r i cated into Al-clad Np0 -Al cement targets and returned to the reactors to produce Pu. 3

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CHRiSTENSEN ET AL.

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358

PLUTONIUM CHEMISTRY

The irradiated neptunium targets are dissolved in HN0 catalyzed with Hg(N0 ) . The Np and P u are then purified and partitioned from each other by two cycles of anion exchange. Both products are further purified by anion exchange, precipitated as oxalate, and calcined to oxide. The purified Pu0 is fabricated into heat sources for use in the nation's space program.

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The waste streams from the Η-Area processes are evaporated, acid-stripped, and adjusted to pH > 13 by addition of NaOH before storage in underground double-shell tanks. In addition to fuel and targets(_15,16) from SRP reactors, SRP also reprocesses a wide variety oT~ fuels from offsite research reactors and a wide range of unirradiated plutonium scrap materials.(JJ) Following customary Savannah River prac t i c e , i n i t i a l processing of each offsite material is designed to transform the actinides to a solution that is compatible with one of the solvent extraction cycles in either of the separa tions areas. A major advantage of this practice is that the uranium and plutonium isotopes can be blended over a wide range. Since the actinide can be introduced to the mainline process at several different points, a greater range of contami nants can be handled by routine operations. This simplifies many of the i n i t i a l purification steps over the steps that would be necessary i f only one entry point was available. Applied Plutonium Scrap and Waste

Technology

Needs

-

Unirradiated

Plutonium

DOR Processing of High-Grade Plutonium Scrap. Numerous 700-g P u U reductions have been performed at Los Alamos; Rocky Flats is also testing DOR in its production equipment. However, the exact chemical mechanism for the DOR reaction has not yet been fully characterized. Research and development need to be done to transform this from a batch process to a continuous process. Also, the plutonium metal product is contaminated not only with the impurities in the Pu0 feed, but also with the impurities in the CaCl reaction flux. Research and development need to be done on preparing pure CaCl or recycling the CaO-CaCl slag left after DOR. The CaO possibly could be rechlorinated in an inert atmosphere (no C0 , 0 , or H 0) and thus have a salt free of carbonate. (The presence of carbonate leads to metal with a high carbon content; the carbon makes the metal have undesirable properties.) 2

2

2

2

2

2

2

2

MSE Processing of High-Grade Plutonium Scrap. Americium is removëcl from plutoniurn in à liquid-liquid extraction process using molten salt (KC1, NaCl, MgCl ) and molten plutonium metal 2


23.

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as the immiscible liquid phases. The salt phase, containing most of the americium and some plutonium, is transferred to an aqueous process for recovery of americium and plutonium. The precipitation of Pu(111) and Am(III) carbonates from dissolved MSE residues yields actinide(III) carbonates or hydroxyl carbonates. (18^) The physiochemical properties of these compounds are needed, as well as an understanding of the soluble complexes formed in carbonate solution at high pH. Other Pyrochemical Processes. The chemistry of pyrochemical separation processes is another f e r t i l e area of research; e.g., new molten salt systems, scrub alloys, etc.; and the behavior of plutonium in these systems. Studies of liquid plutonium metal processes should also be explored, such as f i l t r a t i o n methods to remove impurities. Since Rocky Flats uses plutonium in the metal form, methods to convert plutonium compounds to metal and purify the metal directly are high-priority research projects. Plutonium Oxide Dissolution. All four sites dissolve impure PuOo residues in concentrated HN0 (10 to 14M) containing HF (

Present Status and Future Directions of Plutonium Process Chemistry

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