Facile Separation of Uranium from Plutonium-238 for Isotopic Uranium Analysis Gary L. Silver
Monsanto Research Corporation, Mound Laboratory, Miamisburg, Ohio 45342 SEPARATION of uranium and plutonium has received considerable attention since macro amounts of plutonium became available years ago. A number of methods have been proposed for this separation, ion exchange being one of the more popular ( I ) . This paper describes a method of separating plutonium and uranium by homogeneously precipitating the latter in the presence of Pu(II1) with H3P02. Where 234U can be determined by other techniques, this method allows an indirect determination of 2 W . Themethod has been used to characterize ZSSPu used in heat sources.
Table I. Contribution to Gross Alpha Count by 238P11, 232U, and 234U (238U not counted) Alpha count 23sPu, 232U, 234U, ratio, 94 Oo/ 232U/234U Sample Method YO 1 Precipitation 18.7 12.6 68.6 0.18 1 Ion exchange 3.8 14.3 89.9 0.17 2 Precipitation 0.0 8.4 91.6 0.09 2 Ion exchange 0.1 89.8 0.11 10.1 3 Precipitation 17.5 8.7 73.8 0.12 4 Precipitation 19.8 6.4 73.8 0.09
EXPERIMENTAL
A solution of 2sSPu(IV) in HNO, is evaporated until denitration begins. The solution is cooled, 25 ml of concentrated HC1 is added, and the resulting solution fumed to remove nitrate. When the volume of the solution has been reduced to a few milliliters, the process is repeated. Usually two fuming operations suffice to remove most of the nitrate from the sample. When most of the HCl has been expelled by fuming, the solution is cooled and diluted to a volume of 20 ml with 1M HC1. The Pu(1V) is then reduced to the trivalent state by any standard method. We have used hydroxylamine or hydrazine in the samples cited here which varied in size from 0.5 g to 5 g. When the plutonium has been converted to the trivalent state, 1 g of UO,Cl, dissolved in 5 ml of 1M HCl is added. Smaller quantities of uranium carrier might be preferable in laboratories where semi-microanalytical techniques are practiced. The volume is now increased to 50 ml with 1M HCl and the solution is conveniently retained in a 100-ml beaker. To the solution, with stirring, is then added 40 ml of reagent grade 47% HI containing 1.5% H3P02 as a preservative. Purple fumes, a brown coloration, or a black precipitate indicates incomplete reduction of Pu(1V) to Pu(III), or failure to expel most of the “OB. Apart from a slight color change, there should be no evidence of reaction when the HI is added to the plutonium. The solution is then stirred and heated on a hot plate until inspection reveals the appearance of a precipitate. Ordinarily, between 5 and 30 ml of solution will have evaporated before a precipitate appears. When the uranium has thus been separated from the bulk of the plutonium, the solution is decanted and the precipitate washed several times with 2M HCl. It is then filtered on a small funnel with a frit of medium porosity. The precipitate is washed several times with 2M HC1 and most of it is then transferred to another funnel which can be removed from the plutonium alpha boxes. The precipitate is again washed in a fume cupboard with 2M HC1, 1M FeC13 in 1M HC1 to encourage displacement of adsorbed Pus+ with Fes+, and again with 2.44 HCl. A large portion is then transferred to a 10-ml volumetric flask (to minimize spattering). To the flask is added 5 ml of concentrated “0, and the flask is gently heated on a hot plate. When the precipitate has dissolved, the contents of the flask are transferred to a separatory funnel, diluted to 50 ml, and the acidity is adjusted to 2 M . To the funnel is then added 100 mg each of NH2S03H, FeSOa (NH&SOa 6H20, and NaF. The contents are shaken and allowed to stand for 5 min. To the funnel is then added 100 ml of 0.05MTOPO (tri-n-octyl (1) J. L. Ryan and E. J. Wheelwright, Ind. Eng. Chem., 51, 60-5, 1959. 548
ANALYTICAL CHEMISTRY
phosphine oxide) in cyclohexane, and the uranium is extracted into the organic phase. The organic phase is then transferred to a clean separatory funnel and scrubbed six times with fresh 50-ml portions of 2M “Os containing NH2S03H, FeS04 (NH4)2S04 6H20, and NaF as above (2). When scrubbing is completed, small volumes of the TOP0 (llr5Opl) are mounted on a slide for alpha counting according to the method of Kirby and Dauby (3). It is impractical to purify the uranium by reprecipitation as above because phosphate interferes. RESULTS AND DISCUSSION
We have used this technique to determine the 23W/234U ratio in several samples of plutonium. We have also used the ion exchange method to determine this ratio ( 4 ) . The advantage of the precipitation method is simplicity: the isolation of the uranium involves little more than a series of evaporations and can be done rapidly. In addition, the problem of recovering zS*Pu from irradiated and partially decomposed ion exchange resin does not arise. Compared to the ion exchange procedure, however, the method also has some disadvantages. These include the inconveniences of contaminating the 232U and 234U with carrier 238U and natural 2 W . The possible generation of dangerous amounts of explosive phosphine within the confines of an alpha box is a hazard. To prevent the accumulation of undesirable quantities of H3PO2in the alpha boxes was one reason for selecting reagent HI as the source of this acid. We have discovered interference from only those metals which can ordinarily exist as tetrapositive cations in acid solutions. Typical results obtained by this technique are listed in Table I. The difference in the z32U/234U ratio by these methods may be attributed in part to the limited capabilities of our alpha detectors at such low levels of radioactivity. By either technique, it is usually necessary to count the slides for at least 24 hr. The concentration of ZSzU in the plutonium was about 20 ppb and the 234U concentration about 700 ppm. Details of the calculations, including corrections for naturally occurring 2 W , alpha pulse counters, estimates of precision, and typical alpha pulse spectra are described elsewhere (5). (2) R. J. Baltisberger, ANAL.CHEM.,36, 2369-70 (1964). (3) H. W. Kirby and J. J. Dauby, Radiochimica Acta, 5, 133-7, 1966. (4) P. E. Figgins and R. J. Belardinelli, J . Inorg. Nucl. Chem., 28, 2193-9 (1966). (5) W. H. Smith, F. K. Tomlinson, D. W. Eppink, and G. R. Hagee, “Determination of Parts-Per-Million Quantities of Plutonium-236 in Plutonium-238,” U.S.AEC Report MLM-1486 (July, 1968).
When precipitation occurs under the conditions described above, a crystallinegreen precipitate (A) is obtained (6,7). This precipitate can be dehydrated to a cream-colored compound in a vacuum desiccator over PsOs and KOH. In air, the dehydrated compound rapidly regains its green color. Thermogravimetric analysis of (A) in air indicated that it lost weight continually to circa 100 “C and showed no further weight change to 500 “C. Elementary analysis indicated 53.5 uranium, 1 4 . 8 z phosphorus, and a weight loss upon drying of 8 . 2 z . The X-ray powder pattern of compound (A) is similar to powder pattern of the material formed by the reaction of aqueous uc14 and H3POa. Compound A thus appears to be uranium(1V) phosphite, U(HPO& 2Hz0, which contains 54.8 % uranium, 14.3 phosphorus, and 8.3 water. The use of phosphorus compounds to precipitate uranium
z
z
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z
(6) V. I. Spitsyn,V. G. Gulia, 0. G. Nemkova, and M. S . Golubkova in “Investigations in the Field of Uranium Chemistry,” V. I. Spitsyn, Ed., ANL-Trans-33, Argonne National Laboratory (November, 1964) pp 293-8. [Translated from the Russian (1963)l. (7) V. G. Gulia, 0. G. Nemkova, and E. V. Vedencheva, ibid., pp 299-304.
is not new (8, 9). To determine the zWJ/234U ratio, it is not necessary to precipitate the uranium in the plutonium sample quantitatively. Most homogeneous precipitation techniques involve the homogeneous generation of only the precipitant or the ion precipitated. In the case of uranium(1V) phosphite, this technique allows homogeneous generation of both the precipitant (phosphorus acid) and the ion precipitated, U(1V). For the precipitation step alone, the separation factor-Le., the ratio of uranium to plutonium in the precipitate when both were initially present in the same amount-is approximately 104. This factor may be difficult to realize in plutonium-238 systems where the background radiation may be as high as 109 counts/min. For the complete process-i.e., precipitation followed by solvent extraction-the separation factor is approximately 107. RECEIVED for review June 24, 1968. Accepted December 23, 1968. Mound Laboratory is operated by Monsanto Research Corporation for the U.S. Atomic Energy Commission under Contract No. AT-33-1-GEN-53. (8) G. T. Seaborg and R. A. James, US. Patent 2,917,361 (1959). (9) H. N. Ray and N. P. Bhattacharayya, Analyst, 82, 164-6 (1957).
Apparatus for Measuring Gas Release of Ceramic Nuclear Fuel Materials at High Temperatures Merrill C. Burt Battelle-Northwest, Battelle Memorial Institute, Richland, Wash. 99352 OPERATION of nuclear reactors utilizing ceramic fuels, such as UOz or UOZ-PuO2, can be abruptly terminated by fuel cladding failure caused by internal gas pressure ( I ) . The fission gases which are produced upon irradiation are not directly under the control of the fuel fabricator, but adequate expansion volume must be provided in fabrication to accommodate them (2). Other gases may also be released by desorption and by chemical reaction of impurities in the fuel at high temperatures (I). It is important for the fabricator to be able to measure the gas release of the fuel material at various stages in the fabrication process to avoid exceeding the capacity of the fuel cladding. Measurements at temperatures up to 1650 “C are presently required in this laboratory on both powdered and solid material. This paper describes a vacuum extraction apparatus designed for that purpose. Because the equipment must handle extremely toxic material such as plutonium, it was necessary to build part of the apparatus in a glove box to provide contamination control. EXPERIMENTAL
Apparatus. The main components of the apparatus constructed in this laboratory and assembled as shown in Figure 1, are as follows: FURNACE SECTION.This section, which is contained in the glove box, consists of a fused silica furnace tube, a powder loading and unloading section, a pellet loading arm, an arm with a slide magnet for use in raising and lowering the crucible, and a high speed glass mercury diffusion pump. The tungsten crucible is heated by a 21/2-kw, 450-kHz induction generator designed for use with long leads required (1) H. J. Anderson and R. J. Anicetti, AEC Report, HW-74204, Richland, Wash., July 1962. (2) M. B. Reynolds, General Electric Company, GEAP-4325, San Jose, Calif., Sept. 1963.
by the glove box application. By using a load coil of l/gW copper tubing consisting of two layers of six turns each, a heating range of 900-1700 “C, as read on an optical pyrometer, is obtained. Teflon (DuPont) spacers keep the coil turns separated from each other and the furnace. The cooling water jacket of the silica furnace tube and the diffusion pump are cooled by a closed recirculating water system to eliminate the possibility of plutonium contamination entering the building water system. Should a break occur in the closed water system, a pressure actuated switch turns off the diffusion pump, the circulating pump, and the induction heater generator. ANALYTICAL SECTION.The apparatus on the outside of the glove box includes a second mercury diffusion pump for transferring gases from the furnace section to the analytical section, the calibrated analytical section, and the main vacuum pumps for evacuation of the entire system. The main pumps are a 285-L/sec oil diffusion pump and a 5-cfm mechanical pump. The calibrated analytical section consists of a three range McLeod gauge, a Toeppler pump for transferring gas to an evacuated bulb for subsequent mass spectrometric analysis, a thermocouple gauge, an ionization gauge, and a calibrated expansion volume. The use of large diameter (28 mm 0.d.) tubing to keep the pumping speed at a maximum (3) increased the system volume and raised the detection limit. Assuming a five-gram sample and a final pressure at least five times the blank, the detection limit is approximately 0.01 ml/gram (STP). Two quartz wool plugs in the vacuum line between the two main sections of the system filter out particulate plutonium. The resistance to gas flow imposed by the quartz wool plugs is overcome by using the two mercury diffusion pumps in series. (3) Andrew Guthrie, “Vacuum Technology,” John Wiley and Sons, Inc., New York and London, 1963, pp 56-60, 414-421. VOL. 41, NO. 3, MARCH 1969
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