Ion Exchange Spectrographic Method for Determination of Impurities in Uranium and Plutonium JAMES K. BRODY, JOHN
P. FARIS,
and ROBERT F. BUCHANAN
Argonne National laboratory, lemont, 111. ,An investigation was undertaken to provide a method for determining plutonium without the hazards and the elaborate glove box facilities required for the carrier distillation procedure. The new method gives results comparable to the latter method but with sensitivities an order of magnitude better for many elements. As more plutonium becomes available for reactor and research purposes, the determination of its purity will be greatly facilitated b y the method.
P
like uranium, has a complex spectrum which completely masks the sensitive lines of a number of elements. To overconie this difficulty for uranium, Scribner and Uullin (9) developed the carrier distillation method, in n hicli a reproducible fraction of the impurities is carried out of a graphite crater, leaving the more refractory uraniuni oxide (Lv308)essentially undisturbed. This method has also been used for plutonium by hletz (6) and others ( 2 , 3 , 8 ) . Because of the toxic nature of plutonium, extensive alpha hood trains and elaborate safeguards were required during the sample preparation and arcing processes. The niaximum permissible level of soluble plutonium-239 in the body is 0.65 y . Standardization of the carrier distillation method has been hampered by the scarcity of spectroscopically pure plutonium oxide (PuO,), as a result of 11-hich uranium oxide (U308) standards have often been used as a n alternative. According to N e t z ( 7 ) , many elements behave differently in uranium oxide than in plutoniuni oxide. Furthermore, the density of the oxide must be the same for standards and samples, a condition often difficult to attain for plutonium oxide ( 7 ) . Because of the hazards involved Tvith dry, finely divided plutonium oxide used in the carrier distillation method, the separation of impurities from plutonium has received considerable attention, not only because of the greater safety in handling plutonium in solution, but also because of the need of a n independent method. X e t z (6) and others ( 1 ) have developed the cupferron technique and Van Tuyl ( I O ) has investigated solvent extraction procedures. LUTOSIUJI,
The analytical group a t Argonne Satioiial Laboratory recently became involved in the determination of impurities in plutonium. The extensive hood trains and arcing facilities required for the carrier distillation procedure, as well as standardization problems, eliminated this method as a means of providing immediate results. An anion exchange procedure using hydrochloric acid appeared t o be attractive because of its simplicity. It was possible t o predict from the n-ork of Kraus and Kelson (6),as well as the combined experience of members of this laboratory, nhich elements might be separated from uranium or plutonium. A single standard chemical hood with slight modification could be adapted for the chemical x o r k m-ith radioactive solutions. For these reasons, the ion elchange technique was investigated. PROCEDURE
I n the initial experiments, uranium was used as a stand-in for plutonium to reduce the hazards involved. The similarity of their distribution curves (11) for the Dowex-1-hydrochloric acid system allowed the inference that the same impurities could be separated from either uranium or plutonium. A 1-gram sample of uranium metal in a 250-nil. polyethylene beaker was dissolved in 15 ml. of 12-17 hydrochloric acid and a few drops of concentrated nitric acid, the latter to hasten the oxidation to uranium(V1). The solution was placed under a heat lamp until it turned yellow, then set aside to cool. The resin column (a polyethylene tube, 10 mm. in inside diameter and 200 mm. long, with a funnel 55 mni. wide sealed to the top and a constriction 2 mm. in inside diameter a t the bottom) v a s half filled with Dowex-1 resin, using quartz woo1 retaining plugs a t the top and bottom. The Dowex-1 resin was obtained in purified form from BioRad Laboratories, Berkeley, Calif., as their analytical grade resin with 10% cross linkage and 200- to 400-mesh particle size. Further purification was carried out as described below. The resin column was pretreated with concentrated hydrochloric acid, and the sample solution was poured onto the column. The impurities were eluted with two apparent column volumes of 1 2 5 hydrochloric acid followed by one column volume each of 11N, lOA-, 9N, etc., down to 5 s acid, inclusive. (The
apparent column volume is the volume of the resin plus the interstitial spaces, which is about twice the volume of the resin alone). The uranium, which formed a brown band at the top of the column started to move through the column appreciably with 5AJ acid. In order to avoid the collection of uranium in the effluent, the effluent containing most of the impurities was removed and placed under infrared lamps and the volume reduced to 1ml. The uranium was removed from the column by washing ivith 1.V hydrochloric acid. Then a second effluent was collected by washing the column with four column volumes each of 0.1-V hydrochloric acid, 0.05Y acid, and water. The volume was reduced by evaporation to a few tenths of 1 ml. Aliquots of 0.1 ml. were used for the copper spark determination. Enough of the first effluent solution was available to photograph the spectrum in several different wave length regions t o utilize the most sensitive lines. The second effluent was examined for zinc and cadmium. The copper spark method was chosen because of its high absolute sensitivity in terms of the least amount detectable. The method has already been described (4). Briefly, it consists of evaporating 0.1 ml. of a hydrochloric acid solution on a pair of newly machined copper electrodes 1/4 inch in diameter. These are sparked and the spectrum is photographed under suitable conditions. For the determination of copper, graphite electrodes were used. The sample spectra were compared visually n-ith standard spectra using a projection comparator. The resulting reproducibility of a +SO% or -307, of the amount present was sufficiently accurate in the initial experiments to survey a large number of elements and rvaluate some of the limitations of the method.
A comparison of the ion exchangecopper spark results n i t h carrier distillation results is shown in Table I. A blank run was made by using the above procedure but without the uranium sample. The spectrum was examined along n-ith the samples and the results are given in parts per million, assuming a 1-gram sample. The concentration in the blank, 17-hich is a loner limiting feature of the ion exchange procedure, is compared with the lower limits for the carrier distillation method. I n many cases the lon-er limit is better VOL. 30, NO. 12, DECEMBER 1958
1909
for the ion exchange-copper spark method. Polyethylene laboratory ware was used throughout the procedure in order to avoid the contamination of sodium, calcium, and boron from borosilicate glass ware. The resin was washed several times batchwise alternately with 6h7 hydrochloric acid and distilled Table I.
Comparison of Ion Exchange-Copper Spark with Carrier Distillation Determinations of Impurities in Uranium
(Results in parts per million) Sample 32530 Sample 32531 Ion ex.Carrier Ion ex.Carrier Cu spark dist. Cu spark dist. 15 20 7 10
