ination o
PIutoniurn
WILLIAM J. MAECK, GLENN L. BOOMAN, MAXINE ELLIOTT KWSSY, and JAMES E. REIN Atomic Energy Division, Phillips Petroleum Co., lduho Falls, Idaho
b A two-cycle extraction procedure is described for the separation and determination of plutonium in uraniumfission product mixturss. Plutonium is oxidized to +6 with permanganate and quantitatively extracted as a tetieolkylamrnonium trinitrate complex into methyl isobutyl ketone from an acid-defkient aluminum ni:rate salting from solution. ~ ~ u ~ o ~is~ stripped urn the organic phase and reduced to 4-3 with a I?rdr.nxylam~ne-iron(ll) mixture, oxidized to -4-4 with nitrite, then quantifativdj extracted into fhenoyltrifluoroacetene-xyl~n~~ Qver-all Ftssion product decontamination is greater than 1 X i o 4 and u r a n k carryOver-all through is !ess than O,OS;%. recovery of pbterium is 88B%70a dispensing with the need for a yield defemination, have described the separation and cieterminazion of uranium (1, 2) and neptunium (3) based 03 extraction af the trinitrate complexes with the tetrapropylamm0niu.n ion, [HaN]+[i\IOz(NO,),]-, from an aciddeficient aluminum nitrate salting media into methyl isobutyl ketone. I n this paper3 the extension of this system f 3 plutonium is described. The high degree of separation from fission products xdres this system especially abtractive as an initial step for separation of the +S actinides from samples containing high levels of fission product activity. The exiraction is simple t o perform and easily adapted without special a p p a d a s in remote handlmg facilities. The separated organic phase is sufficiently low in activity level to permit further handling in a conventional laboratory. The high decontamination ievel achieved prevents fission product beta-gamma interference ia the final alpha pulse height analysis (6). I n the proposed method, after the three actinides are quantitatively extracted into methyl isobutyl ketone, plutonium (and neptunium) is quantitatively stripped by contact with an aqueous phase containing a reductant. The oxidation state is adjusted to +4 with nitrite and a final extraction is made into thenoyltrifluoroacetoneREVIOUS I’APERS
74
0
ANALYTICAL CHEMllSTRY
xylene patterned after the methods of Moore (4,6) and Murray (6). The ratio of neptunium-237 to plutonium activity produced in irradiation of uranium fuels is less than 0.01. Although neptunium carries through with plutonium in the proposed method, its effect is insignificant in gross acounting. In addition, the activities of neptunium and plutonium are resolvable by alpha pulse height analysis. EXPERIMENTAL
Apparatus. The apparatus was the same as described previously (S), except that thenoyltrifluoroacetone extractions were made in 50-ml. centrifuge tubes and the sample plates were 1-inch stainless steel planchets. Reagents. The source of chemicals and the preparation of the aluminum nitrate salting solution were described previously (8). The 0.2M ferrous sulfate and 0.22N sodium nitrite solutions should be prepared fresh daily. The 1.25X hydroxylamine hydrochloride and 0.05X potassium permanganate solutions are stable for a month or longer. The potassium permanganate solution is stored in a dark bottle. Unless otherwise stated, the plutonium levels in extractions were approximately 100,000 disintegrations per minute (d.p.m.). The plutonium stock solution was prepared from high purity Hanford metal dissolved in 6N hydrochloric acid. The isotopic composition of the metal was 95.40% P U * ~4.31% ~, Pua40,0.28% and 0.01% P u ~ * ~ . Procedure. Add 6 ml. of salting solution to a test tube containing 0.1 ml. of 0.05-I[ potassium permanganate. Pipet 1 ml. or less of sample into the tube. Add 3 ml. of methyl isobutyl ketone, stopper the tube, and extract for 5 minutes on the extraction wheel. Centrifuge to facilitate phase separation. Pipet 3 ml. of 3.125M nitric acid, 2 ml. of 1.25X hydroxylamine hydrochloride, and 2 ml. of the above organic phase into another 15 X 125 mm. test tube. Stopper the tube and strip for 10 minutes, and then centrifuge. Carefully transfer 2 mP. of the aqueous strip phase to a 50-ml. centrifuge tube containing 0.1 ml. of freshly prepared 0.2144 ferrous sulfate and allow the mixture to stand for 5 minutes. Add 3 ml. of freshly prepared 0.22M sodium nitrite and let stand until gas evolution ceases. Add 5 ml. of 0.5.M thenoyltrifluoroacetone-xylene and stir vigor-
ously for 20 minutes. Remove an aliquot of the organic phase and dry on a planchet under a heat lamp. Ignite and count. DISCUSSION
For the reasons stated in the paper on neptunium (8), potassium permanganate was selected for oxidation of phtonium to the sexivalent state, which is extractable into methyl isobutyl ketone. Under the conditions of the proposed procedure, extraction of plutonium was 90% complete in 2 minutes and 99% or more in 5 minutes. These values were obtained by a-counting the organic phases and are considered reliable to &2%. Several factors influenced the choice of varia.bles studied in the stripping step. Hydroxylamine was chosen over iron(I1) as the reductant because the large amounts of iron(I1I) formed by oxidation during stripping would be extracted into the thenoyltrifluoroacetone using up the plutonium extraction reagent. High acidity was thought desirable to repress plutonium hydrolysis and polymer formation. If a sufficiently high nitrate concentration could be used, uranium stripping could be suppressed. Stripping data as a function of hydroxylamine and nitric acid concentrations under conditions of the proposed procedure are summarized in Table I. The intermediate nihric acid and the higher hydroxylamine level (condition D) was chosen for the procedure. A series of statistical T and P tests indicated no differences in plutonium recovery for conditions A, B, and D, and that condition D gave higher recovery (at the 5% level of significance) as well as better precision (at the 1% level of significance) than condition C. At the highest nitric acid level (conditions E and F), the higher salting strength prevented complete stripping of the plutonium. Examination of Figure 1 shows the high level of decontamination obtained in this initial extraction and strip. However, a substantial portion of uranium is stripped with the plutonium requiring further separation. For %his the thenoyltrifluoroacetone-xylene extraction system, selective for 4-4 ions,
IO'
I o7
Ce
IO(
SAMPLE
HEXONE EXTRACTION STRIP PHASE
I.
Figure
Distri-
bution of fission product activify in
HEXONE EXTRACTION
method
Id
Leik. ?-month c ~ o l s d Right. 1.5-yeat cooled
STdlP PHASE
SO
10
IO
\ I
ENERGY
was chosen. Attempts were made to extract the plutonium from strips of the initial methyl isobutyl ketone extraction according to the conditions specified by Moore and Hudgens (6), both with and without added nitrite. Recoveries were always incomplete. Schneider (7') used both iron(I1) and hydroxylamine plus heat for reduction of plutonium to +3 followed by oxidation to +4 with nitrite. Careful control of acidity is required for quantitative extraction of plutonium(1V) by thenoyltrifluoroacetone. Eased on this report and others of similar nature, a six-variable complete factorial experiment was designed to establish a set of conditions that would give quantitative extraction. Each variable was set at, two levels requiring 26 or 64 experiments. The variables and levels (under the volume and time conditions of the recommended procedure) were: Acidity of strip: 1 and 1.9iM HiYOa Hydroxylamine concn. of strip: 0.1,
-
0.1 htEV
FINAL TTA PHASE
-
Ru
0 2 03
04
05
droxylamine were selected based on two postulated reactions: KOz-
+ 3NHzOH + B f F2: 2x2 + 5Hz0
3x02-
(1)
+ NHzOH + 3H+ F2: 4 N 0
4- 3Hz0 (2) so that a twofold excess was added for both cases for the oxidation of plutonium. The experiments were performed in a randomized order. The resulting data indicated that maximum recoveries without either heat or added iron(I1) were 70%, that heat without iron(I1) increased recoveries to near quantitative, that added iron(I1) gave quantitative recoveries with or without heat, that recovery was independent of the nitrite concentration, and that
Table 1.
0.5M
Heat: none, 5 minutes in a 80" C. water bath Acidity of TTA extraction: 0.75 and 1.2SM "01 Kitrite/hydroxylamine moIe ratio: 0.67, 6.0
The mole ratios of nitrite to hy-
Condition -4 B C D
E F
07 0 8
0.9
10
the lower acid level. for the thenoyltrifluoroacetone exhi action gsve slightlv higher, but not statistically significant recoveries. The reasons for chooshg the acid strip and hydroxylanline levels have been stated earlier. Eased on these res-dts, the addition of iron(I1) without heat, ana the lower nitrite and acid levels were seicctiii; the former wacs selected for corvenience and the latter to minimize t b tofa:! ionic or salting stre,igth arid hence improve decontamination f:xn thow fission products carried through to t5is stage oi the procedure. The recovery of ti-je over-all procedure was establish( d 5y psosessiag m s u n ples containing 1-moctb-cooled fission products from irradiated 90% 'Pienriched uranium and an added 1 X IO6 d.p.m. aliauot of the plutonium stock
Stripping of Plutonium from Methyl l s ~ b ~ t Ketone yl Pbuse as a Fvm+nn of Hydroxylamine and Nitric Acid ConcerPtraliarre
Iron(I1): none, 0.1 ml. of 0.2M ferrous sulfate
06
"Os 1 .OO 1 .OO 1.88 1.88 3.00 3.00
Molarity NHlOH .HC1 0.1
KO.
0.5
8 8 8 8
0.1 0.5
I 1
0.5
0.1
of
Multiplicates
PU
Stripped, yo $9.9 99.9
99.4 99.8 85.19 93.4.
Coeff. of Variation
,-.,a,
