PROCESS CONTROL IN T H E PRODUCTION OF PU238 AND N P237 E.
K . D U K E S A N D
R . S. DORSETT
Savannah River Laboratory. E. I . du Pont de .Vemouts
Go., Aiken, S. C.
Laboratory facilities and analytical methods are described that were used to perform numerous analyses on highly radioactive samples from the P U ~ ~ ~ - process. N ~ * ~ ' Major problems were containment of high levels of alpha activity, shielding of personnel from high levels of beta-gamma radiation, and the determination of one actinide of low concentration in the presence of a high concentration of another actinide.
previously. For the first time at the laboratory. facilities were required to contain extremely high levels of alpha activity and at the same time to shield against high levels of beta-gamma radiation. Sufficient shielding had to be supplied to protect personnel against 10 to 20 mc. per ml. of beta-gamma radiation, and total containment facilities had to be provided to handle alpha activities at the curie level. I n addition, segregation of various levels of activity u a s essential for reliable analyses. 'This report describes the procedures that are used to determine Pu. Np, and other constituents, the techniques that were developed. and the laboratory facilities that were constructed to ensure minimum exposure to personnel from beta-gamma radiation and complete containment of alpha activity. Analyses Required for Process Control
T h e analyses required during various stages of the process (Figure 1) are summarized in Table I. Plutonium concentrations ranged from l o 6 to disintegrations per minute-milliliter (d, m-ml.), while N p concentrations ranged from l o 2 to l o 6 d m-ml. T h e proper handling of samples to eliminate cross-contamination is probably the most significant factor in obtaining reliable analyses. In addition to the analyses listed in the table, the final S p product and Pu product were analyzed for impurities bv emission spectrography. .Also, numerous analyses were required on cold feed solutions. but most of these analyses were performed in a separate laboratory. Laboratory Facilities
z Second Anion
Firs1 Anion Column WOIt.
1
NpZ3'
Figure 1 .
i Third A n 0 Column
Coiumn
wasla
,
Wate Np237
py23B
P"238
Separation and recovery of PuZ3*and NpZ3'
O n e standard analytical laboratory (12 X 24 feet) was equipped to perform all the analyses required to control the Pu238-Kp237process. In addition to the usual services, the laboratory contained a small shielded cell and transfer station, four gloved boxes. tbvo radiobenches. and one hood. All areas except the vented containment boxes were maintained free from contamination. T h e floor was covered with a polyvinyl sheet to protect the tile and to facilitate removal of accidental contamination. Access was restricted to one entrance. Another portal was reserved for emergency exit Shoe covers were worn and removed at a step-off pad located a t the entrance to prevent spread of contamination to other parts of the building. Radiation monitoring instruments and other safety equipment were also located a t this entrance. Other portable radiation instruments were provided. Samples. waste, and equipment were monitored before they were removed from the laboratory. VOL.
3
NO. 4
OCTOBER
1964
333
Table I.
Analyses Required in the P U * ~ ~ - NProcess ~*~' iicliuity Level
Sample Desuiplion
1. Dissolver Solution
.Vp d l m - m l . 3 x 106
Pu dlm-ml. 1
x
10'0
Gamma c/m-ml.
1
x
109
Gross a X
Analyses Required IV~ Pu X
X
~~
Other
H-. 'i. NO, ~
2. 3. 4.
Feed effluent \\'ash effluent Product eluate
5. .\djusted fepd 6. Feed effluent i . \\'ash effluent
8.
Np product eluate
x
FIRSTANIONCOLUMN 107 1 x 109 107 1 x 107 10'0 1 x 106
x 104 1 x 104 9 x 106
1
x 106 1 x 103 1 x 103 4 x 10'
2 4 1
1 x 103 1 x 103
4 x 109 1 x 106 1 x 108 8 X 10"
1
3
x 4 x 1
X
X
X
X
X
X
X
X
X
X X
X
104
X
X
X
105
X
X
X
H+. y
X
X
X
H'
SECOND ANIONCOLUMN 1 x 106 1 x 106
x 10'0 x 109 x 106 1 x 109
1
i
x
x
H',
'i
Hi, Y
X
THIRD ANIONCOLUMN
.I\djusted fred 10. Feed effluent 11. LVash effluent 12. Pu product eluate 9.
1 1
x x
102 102
1 x 106 1 1 1
x 106 x 104 x 104
Filtrate and wash 14. NpZ3'oxide ( d l m - m g . )
1 x 105 1.4 X l o 6
1 1
x
x
NpZ3'PRECIPITATION 106 1 x 103 10' . 1 x 104
Adjusted feed Filtrate and wash PuZsanitrate
1 x 102 1 x 102 1 x 103
4 1 3
x x x
P u PRECIPITATION ~ ~ ~ 10" 1 x 104 109 1 x 103 1012 1 x 105
13.
15. 16. 17.
Solutions that contained high levels of beta-gamma activity were transported to the laboratory in shielded containers. These solutions were transferred to a small shielded cell in Ivhich appropriate dilutions were made. T h e shielded cell provided sufficient shielding to reduce radiation levels by a factor of one thousand. Manipulations \vere performed in the cell by remotely operated tongs. T h e cell \vas equipped with a high level drain. a vacuum-operated apparatus for disposal of liquid tvaste, and 2 Gilmont microburets. All equipment was constructed so that it could be easily disassembled with tongs and removed without breaking the sealed liner of the cell. T h e liner was constructed of ply~voodand could be removed from the cell with no contamination problem. Use of the linrr minimized contamination of the inside of the cell, permitted disposal of contaminated equipment when equipment requirements changed, and allowed the cell to be used for other programs when the Pu238-;Yp23iprogram was complete. Sample Nos. 1 to 3 were placed in the shielded cell, and appropriate dilutions were made. All analyses were then performed, in the adjoining facilities. on these dilutions. A transfer station. attached to the shielded cell, was used to package and unpackage samples and equipment going to and from the shielded faci!ity. Sample dilutions were passed through the transfer station to the adjoining gloved box for further processing. T h e transfer station was maintained a t low contamination levels to ensure safe transfer of materials into the laboratory. Solutions that did not constitute radiation hazards were processed in a gloved box. radiobench: or hood. Each gloved box \vas connected by a transfer port to a hood or radiobench. T h e transfer port permitted direct transfer of equipment or material from gloved boxes to hoods or radiobenches nithour danger of contaminating the laboratory. Each of four gloved boxes was used for a specific range of concentrations to minimize the possibility of cross-contamination. Each gloved box had connections to a high level drain 334
l&EC
PROCESS DESIGN A N D DEVELOPMENT
X
X
X
X
X
X
X
X
X
X
H-.
X
" I
y
H+
X X X
X
X X
X
H-. y
and to other services such as air, vacuum. and nitrogen. Solid waste and other material were removed through a bag port. An electric sealer was used to seal the plastic bags to give airtight containment of high levels of alpha contamination. A large gloved box was used for solutions that contained less than 108 d:'m-ml. alpha activity. This low level facility \vas equipped to perform dilutions and N p extractions. Sample No. 11. and the dilution on sample S o . 1 \\'ere handled in this facility. N p analyses were performed by T T A extraction and additional dilutions were made and passed into a radiobench for gross alpha and gross gamma mounts. A gloved box connected to the transfer station of the shielded cell was used for samples containing between 108 and 10'0 d,'m-ml. total alpha. No ?Jp extraction apparatus was installed in this facility since the samples that entered this facility were high enough in N p concentration that the analysis was performed on dilutions in the radiobenches. Sample dilutions and acid analyses were the two operations carried out in this facility. Sormally sample Nos. 6, 8, 9, and 16 were brought directly into this facility for analysis. A stainless steel box was used to process samples greater than 10'0 d/m-ml. in Pu alpha. T h e box contained a heat lamp, stirrer for N p extraction. and an ion exchange column for Pu separation. Acid analyses were also performed in this facility. Sample Nos. 4, 5. 12: 15. and 1' were brought directly into this gloved box for analysis. Neptunium oxide samples were handled separately in a gloved box and hood. l ' h e gloved box was equipped with a microbalance. After samples were weighed they were transferred to the hood for dissolution and analysis. Sample No. 1 4 \vas al\vays analyzed in this gloved box. T h e single hood was equipped with a heat lamp. a Model B spectrophotometer, and a high level drain. Radiobenches \ v u e equipped with magnetic stirrers or vortex mixers for theno)-ltrifluoroacetone ( T T A ) extraction. a heat lamp for drying aqueous mounts, a heater for drying organic mounts. a meeker burner. and a high level drain. Beta-gamma and alpha con-
tamination on equipment in hoods and radiobenches was kept under 1000 counts per minute (c!m) per 100 sq. cm. as determined by conventional smear techniques. T h e ledges of these facilities were kept free of contamination. Sample Nos. 7 , 10. and 13 could be handled directly in these facilities. Analytical Procedures
Determination of Pu235. Plutonium was determined by I’’1.4 exti.action and alpha counting ( 8 , 7 7 , 73). T h e relative standard deviation for the method was =k2yG. Ilydroxylamine hydrochloride and ferrous ion were added to reduce the Pu to the I11 oxidation state. T h e Pu(II1) was selectively oxidized to Pu(1V) with sodium nitrite. T h e solution of Pu(I\*j was adjusted to 1 M in acid and equilibrated \\ith ‘I‘TA4-xylene. An aliquot of the TTA-xylene was mounted and counted for alpha activity Iron, neptunium. zirconium, sulfate. phosphate. fluoride, and oxalate interfere \vith the TT.4 extraction. Iron was not present in sufficient quantities in process solutions to cause difficulty. Neptunium interference occurred only in analyses for Pu in the final N p product samples in which the ratio by Iveight of Np to Pu was >100. Some of the N p remained in thc I\7 oxidation state and \vas extracted with the Pu(I\’). ‘l‘his interference was eliminated by heating the samples to 70’ C:. for 5 minutes after adding the sodium nitrite. This treatment completely oxidizes S p ( 1 V ) to the unextractable L7 and V I oxidation states. For samples of dissolved target material that \cere high in fission product activity Zr was removed by first oxidizing Pu to the unextractable V I oxidation state u.ith potassium dichromate and heat. and then extracting Zr into T T A . Sulfate. phosphate, fluoride. and oxalate interferences \cere eliminated by complexing with aluminum nitrate. Initially. some sampling problems were created by gassing in concentrated solutions of P u * ~ ~When . samples were drawn u p slo\vly into micropipets. gassing in the pipet was minimized and sampling was accurate. Isotopic Analysis o f P u . T h e distribution of the isotopes of Pu was determined in Pu product solutions with a surface ionization mass spectrometer (3.4. 15, 76). Owing to the high specific activity of Pu:13a sample size was limited to 0.01 pg, to prevent gross contamination of the mass spectrometer. T h e accuracy of these determinations was 1 1 % for the major isotopes PuZ38 and P ~ 7 3 ~ . Determination of Np2”. Neptunium was determined by T T A extraction, alpha counting, and alpha pulse height analysis ( 7 . 9 ) . T h e relative standard deviation for the analyses \vas =2Yc; however, precision decreased when the ratio of Pu alpha activity to Kp alpha activity was high, and several extraction cycles were required to obtain adequate separation of N p from Pu. T h e relative standard deviation was approximately + 370 for two extraction cycles and approximately 3 ~ 4 for 7 ~three extraction cycles. Ferrous sulfamate was added to reduce Np(V) or Np(V1) to Np(1V) ; Np(1V) was extracted from a 1 .OM nitric acid solution into TTA-xylene, a n aliquot of the TTA-xylene extract was counted for alpha activity, and N p alpha activity was determined by pulse height analysis. Sulfate, phosphates, fluoride, and oxalate were complexed with aluminum nitrate before extraction to prevent interference, but Fe(III), Zr, and Pa extract into TTA-xylene. Iron interferes by partially absorbing the alpha activity on the counting plate, while the beta activity of Zr and Pa cause high resl:!:.; when this activity is above the beta threshold of the alpha proportional counter. These interferences were rliminated by stripping the N p from the TTA-xylene into
8M nitric acid; the Fe, Zr, and Pa remain in the T T A xylene (70). T h e N p aqueous phase was then diluted to a nitric acid concentration of 1 . O M , ferrous sulfamate was added. and the TTA extraction \vas repeated. Plutonium, uranium, curium, and americium were only slightly extracted into ’ITA-xylene under conditions of this method. These alpha emitters interfere with pulse height analysis when they represent a larger percentaqe of the alpha activity than does Np. Plutonium alpha interference was the ma,jor problem in S p analyses. This interference was overcome by repetitive extractions of the Np and alpha pulse height analysis of the alpha activity. LVhen the alpha activity of Pu was less than 100 times the alpha activity of the N p a single TTA-xylene extraction was adequate. Two extraction cycles \cere used \vhen the Pu alpha was IO2 to l o 5 times the N p alpha and three extraction cycles were used when the Pu alpha v a s greater than 1O 5 times the N p alpha. Determination of Cationic Impurities in N e p t u n i u m Oxide a n d Plutonium Nitrate. Emission spectrography was used to determine Fe: Cr: Ni, M n . ‘41: C a , and M g in neptunium oxide (6). T h e same cations plus Cu and Zr were determined in plutonium nitrate solutions. Neptunium and plutonium \vere removed from samples prior to spectrographic analysis. Neptunium oxide was dissolved in 8 M nitric acid. treated Lvith 0.1M hydrazine and 0.1M sulfamic acid to adjust N p to the I\’ oxidation state, and the anionic nitrate complex of Np(I\’) was absorbed on Dowex 1-X4 anion exchange resin (5). Anion exchange was also used to remove Pu from the plutonium nitrate product. Not all of the metal cationic impurities’were recovered in effluents from the anion exchange step. A standard solution of impurities was treated in a manner identical with that of each sample to determine the recovery factor for that sample. Determination of Gross Alpha a n d Gross Gamma. Gross alpha and gross gamma activitl- of solutions was determined by mounting a n aliquot on a stainless steel plate and counting the activity. Alpha activity was counted with a n alpha proportional counter. Gamma activity was counted with a gamma scintillation counter. Aliquots of samples or dilutions were chosen to give approximately l o 4 gamma counts per minute or lo4 alpha disintegrations per minute. Each mount was counted for 10 minutes to minimize the counting error. Dilutions were made in 1hl nitric acid to prevent hydrolvsis of the actinides. Dried mounts were coated with collodian prior to counting to fix the activity to the plate. In addition, a strip of tape \vas placed over gamma mounts to seal in the activity. Determination of Free Acid. Free acid was determined in solutions containing hydrolyzable ions such as AI, Pu, and N p by complexing the hydrolyzable ions with sodium fluoride and titrating to the phenolphthalein end point with sodium hydroxide (73). Titrations were performed in gloved boxes or hoods. Localized shielding and small aliquots of sample were used to reduce beta-gamma radiation to acceptable levels. Determination of Nitrate. Nitrate ion was determined by measuring the absorbancy of the ferrous nitrosyl sulfate (FeSOd.NO) a t 530 mp (74). T h e relative standard deviation was +2yo using a Beckman Model B spectrophotometer. Conclusions
T h e facilities and procedures described in this report were used for approximately 2 years to provide analytical services for the pilot-scale PuZ3a-Np2a7 process. T h e success of this operation demonstrated that the analytical procedures and facilities were highly satisfactory. Consequently. the VOL. 3
NO. 4
OCTOBER 1964
335
experience gained in the operation was available as a guide for analytical control of the large-scale Pu-Np process in the Savannah River plant.
literature Cited (1) Burney. G. .A,, I N D .E s c . C I I E \ , .PROCESS I ~ E S I G IDEVELOP. ; 3,
328 (1964). (2) Coogler, A. L., Craft, K. C . , 'l'ctz1;iff. l i . N.. "A Facility for the Production of P U * ~ ~Proc. , ' ' Conf. on Hot 1,aborntories and Equipment, 11th. New York. November 1963. (3) Inghram, M. G., Chupkn, LV. .\.. Rw. Sci. Instr. 24, 518-20 (July 1953). (4) Inghram. M. G.. Hayden. I
