Table VIII.
1 - m = fraction processing 107s each cycle Q( m ) = total conversion (infinite c\ d e s )
Optimum Conversion of NpZ3'to P U ? ~ as % Dependent on Processing Loss
Fraction Procpssin? Losr of Each .tfnfprial Each Cjclr
Ojtiniuni Eracizonnl Dfpletivn of .Ipz3i Each Exposure
(0) 0 00423 0 0192 0 0495 0 1016
0 05 0 10 0 15 0 20
(0)
n y Q -1
m =
~
YO(.
Optimum Con7 ersion
1 0 0 0 0
0 854 715 584 460
Q( m ) =
-
]
_
_
- 1)
ml 1 - mx
where
Results of calculations for a value of a of 3 are given in Table VI11 Losses owing to capture in Xpz38have been neglected Calculations Lvere made of cumulative conversion ivith successive exposures as dependent o n the consumption of Np237 a t each exposure. Results are sho\vn in Table YII. I t ivould likely be possible to add unconverted material to fresh feed to realize a large number of exposures. Processing losses Lvhich were not considered could significantly reduce recovery as indicated by the effect on infinite cycle recovery. If processing losses are large. there is a n optimum exposure for batch operation. Considrr that a n equal fractional loss of each material occurs in each stage of processing (each cycle). Equations of interest are as follo~vs: Let 1 - ,Y = optimum fraction Xp237consumption each exposure n = ratio of neutron absorption rate in PuZ3*per atom to the capture rate in NpZ3'
Acknowledgment
Contributions bvere made to this rtudy by P R . Karten,
L L Bennett, and E . H. Gift. O R N L . Literature Cited (1) Vondy. D. R., "Development of a General Method of Explicit
Solution to the Nuclide Chain Equations for Digital Machine Calculations," 1'. S. A t . E n ~ r g yComm. Rept. ORNL-TM-361 and .\ddendum. 1962. RECEIVED for review January 15, 1964 ACCEPTED A u g u s t 5. 1964
Division of Nuclear Chemistry and Technology, 146th Merting ACS. Denver, Colo., January 1964. This work was done under USAEC contract W-7405-eng-26 at Gak Ridge National Laboratory. operated by Union Carbide Corp. Nuclear Division.
NEPTUNIUM RECOVERY AND PURIFICATION A T HANFORD R . E.
ISAACSON AND B. F. JUDSON
Hanford .4tomic Produrts Operation. Genrral Electric Co
,
Richland. H'nsh
Neptunium i s routinely recovered from irradiated fuel elements at Hanford's two main separations plants. Initial development tests were started in the Purex plant in 1958, then in the Redox plant in 1959, and recently culminated in the installation o f new production systems in both plants for improved recoveries. Both recovery flowsheets employ solvent extraction techniques based on the relative extractability o f neptunium(V1). The neptunium i s coextracted with uranium and plutonium in the plant's first extraction cycles and then partiiioned and decontaminated in separate neptunium cycles. Excellent decontamination from fission products i s achieved without interfering with mainline uranium and plutonium production. Recovered neptunium i s purified b y anion exchange and shipped offsite for subsequent irradiation to plutonium-238. Over-all separation factors of uranium and fission products from neptunium are greater than 10' and 1 O"', respectively.
have recently been installed in H a n ford's Purex and Redox plants to permit continuous production-scale recovery of neptunium-237. Neptunium is formed in Hanford's nuclear reactors as a by-product of plutonium-239 production and is recovered and purified in the chemical processing plants for subsequent conversion to plutonium-238. 'The production reactor and chemical processing complex a t Hanford is operated by the General Electric C o . under prime contract to the U . S. Atomic Energy Commission. The Purex and Rcdox plants employ solvent extraction systems to separate and decontaminate plutonium from irradiated ELV PROCESS S Y S T E M S
296
l & E C PROCESS D E S I G N A N D DEVELOPMENT
uranium for subsequent use in iveapons components. The irradiated uranium is recovered and decontaminated from fission products for reuse in the V235fuel and lveapons systems. Recently. the Purex plant has also been employed for recovery of useful fission products-including strontium-90. cesium-1 37. cerium-1 44, promethium-rare earths. and technetium-99. Recovery of the transuranic neptunium-237 \vas started a t Purex in 1958 and at Redox in 1959 using main-line process equipment with special campaign operations for decontamination of accumulated material ( 7 . 3, 6). The use of the main plant system permitted rapid response to the .4EC:'s early program needs for plutonium-238 as a heat sourcp i n space
Backcycle Waste
Figure 1
.
Neptunium accumulation-Purex
pov.er units. Increaiingly higher demands for neptunium prompted a folloiu-up program to permit efficient and economical recoverv on a continuing basis. I'his paper describes the new process systems recently installed a t Hanford and summarizes program activities and process performance results to date. Formation of Neptunium
Neptunium-237 is a long-lived transuranic isotope formed by tFvo reactions in the neutron irradiation of natural and slightly
enriched uranium. L-235
(n, 7 )
,r 2 3 6
(n, Y )
,u237 '8 ) ;j d-f
Np237
hlost of the neptunium is formed by reaction of with fast neutrons. Smaller amounts are formed by slow neutron capture in L-236.since appreciable amounts of L?36are generated in the 5)-stem by neutron capture in L-'235. T h e bulk of the formed in the irradiation of natural uranium is lost to the reactor cycle hvhen uranium is processed through the gaseous diffusion plants for isotopic enrichment. Hanford is participating in an AEC program to increase neptunium production by recycling the slightly enriched uranium stream from the reactors without processing in the cascades. hlost of the U236formed in the slightly enriched uranium is thus available for neutron capture in the reactors and the over-all +eld of neptunium-237 is increased. In either case, the concentration of neptunium in uranium is very small so that recovery of gram quantities in the huge separations plants is a difficult and challenging assignment. Neptunium Recovery at Purex
T h e Purex plant at Hanford is a large radiochemical plant in ltshich irradiated uranium fuel elements are dissolved in nitric acid and processed through a series of solvent extraction and ion exchange systems T h e solvent extraction system is made u p of 13 pulse columns in which tributyl phosphate (TBP) in a kerosine-tvpe diluent is used to separate and de-
process
contaminate uranium, plutonium, and neptunium and return the associate fission products to Lvaste. T-he ion exchange units are used for final purification of the separated nitrate products before shipment to the customers. The fundamental processes used in the plant have been described by Cooper and \Valling ( 3 ) and Stoller and Richards (72)! and recent advances in technical developments were outlined by Harmon ( 6 ) . The process scheme used for the recovery of neptunium in the Hanford Purex plant is sho\vn in Figure 1. After dissolution in nitric acid. the neptunium is extracted into the 3 0 7 , TBPkerosine solvent along with uranium and plutonium in the first pulse column, the HA column. The product-containing solvent is scrubbed for fission product removal in the second column. the HS column. T h e plutonium is then partitioned into an aqueous phase in the IBX column using ferrous sulfamate to reduce the plutonium to the inextractable Pu-I11 valence state. T h e neptunium and uranium are costripped from the solvent in the I C column using dilute nitric acid. T h e I C product ( I C U ) is concentrated and uranium is then re-extracted in the 2D column. The neptunium, however. is forced into the 2D raffinate stream which is concentrated for backcycle to the HA column. Approximately one third of the backcycle stream is continuously processed through the ne\\ 3.4 column lvhere the neptunium is extracted into the organic phase. T h e neptunium is stripped in the 3B column and returned to the 3.4 column. Neptunium is thus accumulated in the 3.4 and 3B systems and is periodically decontaminated and removed to the ne\\. batch ion exchange unit (the 3X column) for final purification and loadout. The purpose of the 3'4 and 3B columns in the plant are to continuously recover the neptunium and to decontaminate without shutting do\vn plutonium production operations. Without the new columns, neptunium can be accumulated in the uranium extraction and waste backcycle systems but can be decontaminated for removal only by special campaign procesuing through the plutonium decontamination system. This requires shutdolun of essentially all normal process activities. I n addition. accumulating the neptunium inventory Luithin rhe main process system involves constant exposure to process upsets and losses of the inventory to waste streams requiring expensive and time consuming rework. The ne\v solvent extraction battery thus permits improved plant operating efficiency. VOL. 3
NO. 4
OCTOBER
1964
297
Table 1.
Neptunium Column Performance at Purex Flooding limits
L.1 (7nliinin. . . . VoI. d.. f'rrq., yo/. -hr. I cyc.sq. ,ft. 2 2 mn. ' ~
~
Cold semiworks Purrx cold trstsh Purex hot tests Acc urn illation
Ilecontamination
I" Colunirz Chi.-Z ~ P I . . ,gal.-hi. sq. / t .
~
I-
~*'irq.~ cy.-
min.
~
I
1350 2200 21 50
85 70 -8
1350 2000 1000
77,' 70"
1350 2200 1600
95
1300
75
1000
80
80
78
75
--
8 ---
Figure 2. Effect of aqueous "0.3 concentration on distribution of neptunium into 30% TBP kerosine
10
1.0
I
z
I
r
2 L C
t
1
HNOj 2 3
emp. 50 C
rf NO3 6 7 0.17 e m u 50 C
J
Ph
r a m No.1 Solvent
H Y O 3 1.05
Syrlem
Vp
Figure 3. Neptunium phase flowsheet
recovery a t
>@%
Purex-accumulation
Figure 4. Neptunium recovery a t Purex- -decontamination phase flowsheet 298
l&EC PROCESS DESIGN AND DEVELOPMENT
'l'he recovery fltncsheets are based on control shifting of the neptunium amoilg its three valence states and the relative extractability of each into the plant solvent. T h e process \\'as developed in laboratory minirnixer studies at Hanford using some of t.he valence control techniques discovered a t Savannah River 1,aboratory ( 1 : 7 7 ) . .is sho\vn in Figure 2, neptunium(\.I) is morr extractable in 30°C 'I'BP than neptunium(1V) a t low acidities while at high acidities, both are comparable. S e p t u n i u m ( \ 7 ) on the other hand is virtually inextractable. ' I he valence state of neptunium is controlled by the use of HSO, for catalytic oxidation to neptunium(V1) a n d by the use of ferrous sulfamate for reducrion to neptunium( I\-). Thus, the iirptunimn. extracted as neptunium(\-Y) in the HA column by oxiclntion ivith 0.005.Lf HNOi in the solvent. is reduced to iieptuniuni(l\.) in the IBX column and maintained as neptunium(I\.) i n the 2D and 3.4 columns by ferrous sulfamate. ' I hc rwptuniuni(l\T) is kept in the organic phase in the IBX column by virtue of the low aqueous-to-organic flow ratio but is partitioned into the aqueous phase in the 2D column by the high saturation of uranium in the solvent. Floivsheet compositions for the 3.4-3B systems are shown in l i g u r r 3 for the accumulation phase of operation. Process specifications for the t\vo columns and unique features of the equipment design are described in a separate paper (.5), Diuring the accumulation phase. the partition demands on the system are threefold separation of plutonium and fission products from the neptunium in the 3'4 column and separation of uranium from the neptunium in the 3B column. T h e 3A column feed (3.4F) is maintained a t a high nitric acid concentration to assure good extraction of neptunium(1V). T h e scrub solution (3.4s)is a high acid-ferrous sulfamate-hydrazine solution to reduce plutonium to the inextractable plutonium(111) for discharge to the raffinate ( 3 A h ' ) . T h e hydrazine serves to stabilize ferrous ion from uncontrolled nitrite reactions nt the high acidities. Feed flolv to the 3.4 column is adjusted so that the uranium concentration in the solvent phase (SAP) is l o 6 DF for uranium, > l o 8 DF for Ru-Rh-103-106, and > l o 9 DF for Zr-Nb-95. Neptunium Recovery at Redox
T h e Redox plant a t Hanford is similar in function to that of Purex but uses packed columns rather than pulse columns in the solvent extraction system a n d employs hexone rather than
20
15
25
N p Mass T u r n a r o u n d i n 3 A F
at
Purex-typical
de-
T B P as the selective solvent. T h e fundamental processes used in the plant have been described by Stoller and Richards a n d recent advances in technical developments outlined by Harmon (6, 72). T h e process schemes used for recovery of neptunium in Hanford’s Redox plant are shoivn in Figures 6 a n d 7 . In the first scheme. neptunium is extracted and accumulated in the original equipment system and periodically decontaminated for removal by special campaign processing through the plutonium decontamination system. ‘The second scheme is based on routine decontamination of neptunium through use of plutonium solvent extraction columns made available by the installation of a continuously operated prototype anion exchange contactor for plutonium decontamination. In both cases, the neptunium is extracted into the hexone solvent along with uranium and plutonium in the prec)-cle column system and is forced into the aqueous raffinate stream to separate the neptunium from uranium and plutonium in the partition column system. l‘he neptunium is accumulated by recycling bet\\-een the t\vo systems and is periodically removed to a plutonium column system for decontamination. T h e recovery floivsheets are based on aqueous-phase acid control and the relative extractability of neptunium(L.1) from acid A S S solutions as compared with acid-deficient ASS
U t O 1C 2 0 . 2E
Columns
Figure 6.
Neptunium accumulation-Redox
process VOL.
3
NO. 4
OC TOBER 1 9 6 4
299
u
to 1 c 2 0 , 2E Columns
U-Free
IO
Feed
-
10.2 t o 0.3 M
i
I
1.0 Aqueous Phase Composition IU-Free B a s i s l Evaporator 0.1
PU Tracer Np-239 0.01 Batch Product Evaporator
Loadout To Pure*
Figure 7.
Final !waste Evaporator
Underground Storage
Neptunium accumulation-modified
l&EC
PROCESS DESIGN A N D DEVELOPMENT
-1
0
1.0
2.0
Aqueous Phase p H
Figure 8. Effect of pH on Np urder Redox extraction conditions
Redox process
solutions as shown in Figure 8. T h e Redox process was converted from a n acid-deficient feed system to an acid feed system to permit neptunium recovery ( 7 ) . Ruthenium decontamination is better, however, with a n acid-deficient system so that the partition and remaining cycles remain acid deficient. ’Thus. the neptunium, which is oxidized to neptunium(L’1) in t,he feed with dichromate, is extracted in the first cycle and is not extracted in the partition cycle. I n Figure 6, the neptunium from the IA partition column is returned to the H A extraction column where it is subject to potential losses to waste as it is re-extracted for accumulation. Therefore, in Figure 7, the neptunium is returned to the IS column for accumulation, a n d the raffinate is backcycled to the H A column, thereby minimizing potential loss. After a specified amount of neptunium is accumulated in the IS-HC column system, the inventory is removed by a special water strip through the I S column scrub section a n d is transferred to the 3A and 3B columns previously used for plutonium decontamination. T h e neptunium is partitioned from residual uranium and plutonium by contact in the 3A column from a n acid-deficient feed thereby extracting the uranium a n d plutonium a n d segregating the neptunium in the raffinate. T h e neptunium is then extracted from a n acid dichromate feed in the 3A column. stripped in the 3B column and ozonated batchwise for final ruthenium decontamination. If further decontamination is required. the process steps are repeated as necessary. T h e neptunium product solution is then concentrated a n d loaded out for final purification by ion exchange a t the Purex plant. Reserving the 3A a n d 3B columns for neptunium decontamination rather than plutonium processing. minimizes the number of process operations otherwise required during campaign processing through the 2‘4-2B plutonium columns. eliminates the need for plant down-time to decontaminate the neptunium, a n d thus permits improved over-all plant operating efficiency. Performance of the system has been good with stable operation of the special purpose columns and smooth integration of the over-all process system. Over-all neptunium recovery capabilities for the Redox plant are greater than 90Yc \vith essentially no carryover to the 3EU uranium product stream. a n d losses to the HA column raffinate stream of less than 1 Oyc. Over-all separation of the uranium a n d fission products from 300
PU Product Evaporator
From Neptunium Recovery
Scrub A Mu Tanks
3BN Strip-
Chemical Make-up AMU
C o n cen trator
3XF Feed
EO,
Eluent Tank
3XF
I
P rod uct Exch.
Loadout
(Forecut step)
Scrub wastes iTo
1
Figure 9. Neptunium purification at Purex batch ion exchange flowscheme
neptuniuni. based on H A column feed concentrations are > l o 6 DF for uranium. >lo‘ DF for Ru-Rh-103. 106 and > l o 8 DF for Zr-Nb-95. Neptunium Purification at Hanford
T h e neptunium recovered in the Purex and Redox plants is purified by batch anion exchange using the flowscheme shown in Figure 9. ‘Phe neptunium product solution from the Purex solvent extraction battery (3BN) is steam-stripped and concentrated in a thermosyphon concentrator. T h e neptunium is then adjusted to neptunium(1V) bvith ferrous sulfamate. stabilized with hydrazine and loaded on a n anion exchange resin column (3X column) in a concentrated nitric acid medium. ‘I’he product from the Redox system is adjusted in feed composition and loaded on the resin column without prior concentration in the exchange cell. Once loaded on the resin. the neptunium is first Lrashed with concentrated nitric acid containing H F (3XS) to remove fission products. Residual fluoride is washed out of the column. and the neptunium is eluted with dilute nitric acid (3XE). ‘I‘he concentrated portion of the eluent (3XN) is diverted into a product tank where the neptunium is converted to the stable neptunium(\’) valence state, and the product is then bottled for shipment offsite. ~l’he purpose of the neptunium purification facility is to
provide sufficient clean-up a n d concentration of the neptunium so that shipment of the product offsite can be done safely and economically. LVithout such a system, shipment of a uniform product from the two recovery systems could not be assured a n d development of alternate recovery processes would be restricted. Dilute product concentrations and higher fission product levels would involve considerably larger a n d uneconomical, heavily shielded shipping containers. .l‘he purification flowsheet is based o n the formation of stable anionic neptunium(I\-) complexes in concentrated nitric acid solutions a n d the high loading capacities of certain anion exchange resins for such complexes. T h e basic exchange technique was developed a t Hanford a n d the process used in the new facility \vas thoroughly tested in semiworks systems (2, 70). Control of the neptunium(1V) valence in the high acid solution involves using ferrous sulfamate Lvith hydrazine as a holding reductant. T h e use of small concentrations of HF permits relatively large fission product decontamination factors b u t requires extra steps for control. Controlled product elution permits recycling ’ of dilute portions of the eluate a n d loading o u t only concenrrated product so that a final concentrator system is not required. Flowsheet compositions for the various steps in the purification process are shown in Table 111. Unique features of the equipment design are discussed in a separate paper (4). T h e neptunium(I\.) is loaded on the resin a t a temperatureof 25” C. a n d the relatively low rate of 8 m g . of neptunium per minute per square centimeter. Most of the plutonium loads simultaneously even though the prevalent plutonium valence state in the feed is P u ( I I 1 ) . Plutonium is removed from the column a t a temperature of 20’ C. by scrubbing a t 4 ml. per minute per square cetimeter. A 20-column volume fission product scrub is passed through the resin a t 4 ml. per minute per square centimeter a t 70’ C. Product elution is performed a t ambient temperature using 0.3M nitric acid. T h e first 2-column volumes are recycled to the feed so that a net neptunium concentration of 35 to 45 grams per liter c a n be maintained for final valence adjustment a n d loadout. T h e wastes from the loading a n d washing steps are treated with nitrate to destroy the hydrazine a n d with aluminum nitrate to complex the fluoride before entering the backcycle concentrator in the main process system. Performance of the system has been good; flowsheet loading a n d concentration values were readily achieved ( 8 ) . ‘The neptunium loading o n the resin has approached 70 grams per liter. Decontamination factors for uranium, plutonium, a n d fission products are summarized in T a b l e IV. Product elution a n d fission product scrubbing c a n be controlled by the use of a gamma scintillation monitor located in the resin outlet line. Bed gassing is controlled by periodically venting the system using a sonic probe to determine resin level a n d the presence of gas pockets.
Acknowledgment
K e y chemists a n d engineers in the start-up of the new process systems a n d preparation of this paper include J. S. Buckingham, J. P. Duckworth, R . \\’. Lambert, S. M. Nielson, G. A. Nicholson, H. P. Simonds, a n d R. A. Yoder.
Table 111.
Neptunium Batch Purification at Purex Flowsheet and Performance Data
Step
Composition
.Vp to Backcycle,
Volumes and Rates
c /O
Loading
6.OM “OB 0 . 1 M FeSA 0 . 1 M N,H4
8 mg. Np/min. sq. cm.
1. o
Pu scrub
6 .O M H N 0 3 0 . 1 M FeSA
15-20 col. vol. 4 ml./min. sq. cm.
1.5
15-20 col. vol. 4 ml./min. sq. cm.
2.0
0 . 1 M N2H4
FP scrub
8 .OM HNOl 0 . 0 5 M NpH4
0 . 0 1 M NaF
F scrub
8 .OM H N 0 3
1 . 5col. vol.
Forecut
0.35M H N 0 3
2 col. vol. 1 ml./min. sq. cm.
5 (to 3XF)
Product
0 . 3 5 M “OB
1 col. vol. 1 ml./min. sq. cm.
...
Table IV.
