David H. Smith R. L. Walker J. A. Carter Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tenn. 37830
Edited by Jeanette G. Grasselli
Samplingand Analysis of RadioactiveSolutions
~
The international program on safeguards for nuclear materials is administered from Vienna by the International Atomic Energy Agency (IAEA). Scientists involved in the program are responsible for maintaining a material balance account of the radioactive materials used or generated by nuclear facilities in countries that have signed the Non-Proliferation Treaty. Radioactive materials such as uranium and plutonium are monitored, since these are the elements that can be used to manufacture nuclear weapons. The advent of the breeder reactor in many countries has necgssitated the establishment of the safeguards program, since purified uranium and plutonium are recovered from the spent fuels from these reaetors at special reprocessing facilities. The nuclear reactors currently operating in the US.are not of the breeder type, and therefore spent fuels from these reactors are not reprocessed. Thus, unauthorized use of uranium and plutonium from these solutions is not a problem. Despite this, the U.S. supports the international safeguards program both financially and by supplying the IAEA with scientists from various disciplines. Sampling solutions of spent reactor fuels is one of the most difficult problems in the safeguards program. These solutions are highly radioactive, containing many long- and short-lived fission products in addition to excess fuel and the plutonium generated in the nuclear breeding process. The conventional techniques used in the international safeguards program for analysis of these solutions require that large samples be shipped to the IAEA in Vienna for costly and time-consuming separation of the individual elements. Because shielding of plutonium is mandatory, the weight of the packing mgterial far exceeds that of 0003-2700/82/0351-827A$01.00/0 0 1982 Amican Chemical Society
r
lated and adjusted to 8 M HN03. This aliquot is divided into two fractions and enough of it taken so that there will be about 1pg of uranium per resin bead. A known quantity of a spike of high isotopic purity is added to one fraction; our spikes are >98% 233Uand 242Pu.The sample and spike are equilibrated; this is a crucial chemical step necessary for any quantitative determination by the isotopic dilution tethnique. Equilibration of uranium requires only thorough mixing of sample and spike solutions, but because of its multiplicity of oxidation states and tendency to form polymers and complexes, plutonium is a problem demanding special attention. Marsh et al. have compared the efficacy of various equilibration techniques ( 4 ) and have found that the most consistent results were obtained by reduction with Fe(I1) and sulfamic acid followed by oxidation with NaN02. This is the method we use, and we have encountered no serious problems since adopting it. Knowledge of the isotopic compositions of the spike, the unspiked sample, and the mixture of spike and sample allows calculation of the amount of uranium or plutonium present. One thousand resin beads are introduced into each solution; each resin bead is 100-200 pm in diameter and constitutes one sample for the mass spectrometer. Using 1000 beads allows us to sample about 1mg of uranium, an amount comparable to that used in several other techniques. Risk of contamination is thus no greater using our technique than it is for more conventional ones. The beads are agitated on a vortex mixer in contact with the solution for 10 min, after which the solution is removed and the beads washed to remove surface contamination.
1
the sample. It is presently extremely difficult to ship samples of this size. This difficulty, combined with recent restrictions on the amount of material that may be shipped, has required sci. entists to reexamine the currently used analytical techniques with a view toward minimizing the amount of sample required. With this goal in mind, our labura. tory recently developed a new sampleloading technique that involves the use of anion resin beads (1-3). This microsampling technique offers signif. icant advantages in three general areas. First, the quantities adsorbed are so small (1-3 ng of each element) that shipping is nu longer a problem. Second, uranium and plutonium adsorb on the heads under appropriate conditions and are thus separated from fission products and most other actinides. Finally, each bead serves as a cunvenient vehicle for hading the sample ontu a filament fur ultimate analysis by mass spectrometry.
SampIing In our technique, a known amuunt of spent-iuel dissolver solution is isu-
ANALYTICAL CHEMISTBY, VOL. 54, NO. 7. JUNE 1982
027A
10 14 0.1 1
0.1 1
5
10 14 0.1 1
5
10
14 0.1 1
5
II
10 14 0.1 1
5
10 14 0.11
5
10
IB
14 0 1 1
5
5
10 14
I-
HNOa, M
Figure 1. Plots of diffusion coefficientsvs. acid strength for elements of interest in the nuclear fuel cycle Complete separation of uranium and plutonium from fission products is achieved by this simple technique. Figure 1 presents plots of distribution coefficients vs. acid strength for several elements of concern. It is based on results originally described by Faris and Buchanan (5). Note that maximum adsorption for plutonium and uranium on strongly basic anion exchange resin (Dowex 1-X2)occurs a t The only elements about 8 M "03. adsorbed to any significant extent hesides uranium and plutonium are thorium and neptunium. Neptunium is present only as mass 237 and causes no mass interference with either element of interest. Thorium (mass 232) also causes no interference and will he of interest if a thorium-based breeder system is ever adopted. (We have already developed a technique of sequentially analyzing Pu, U, and T h from a single head.) In normal commercial operation, reactor fuels will he highly irradiated-on the order of 30 OOO MW daydton. Spent-fuel solutions from such materials have a U P u ratio of about 100, hut because the adsorption coefficient of plutonium on the head is about 100 times that of uranium, the quantities of the two elements taken up by the bead are about equal (1-3 ng uranium and plutonium Der head). This amount of olutonium eorresponds to about curies, which is'one-tenth the daily exposuse level allowed an office worker; the 828A
shielding provided by the packing material reduces exposure in handling even further. Therefore, no special shielding is necessary. Shipment of samples on heads is ewy and convenient. A number of beeds may he affixed to a glass microscope slide with collodion and shipped on the slide, or the heads may he shipped directly in the special container we have designed for this purpose. In either case, the cost of ship-
ping one sample is about $1.00 (in contrast to the $700 per sample re. quired by older techniques). Mass Spectrometry The use of anion resin heads to load samples for mass spectrometry was first suggested by Freeman et al. (6). Their primary concern was the development of microstandards, but we have adapted the technique for more general application. We use the anion
-1) Diameter
P
Ycm
Figure 2. Photograph of a rhenium filament and its support with a sketch showing a bead emplaced in it. (The bead is not drawn to scale.)
ANALYTICAL CHEMISTRY. VOL. 54. NO. 7, JUNE 1982
I
The LC disconnection. A rare peek insideyour Rheodbe sampk injector. Chances are your liquid chromatograph has a Rheodyne sample injector. And chances are you've never taken it apart. So we're offering a peek inside and a few tips on how to use it effectively and keep it working perfectly. The black disc in the center of the picture is the rotor seal. It presses against the white ceramic stator face just to the lefl of it. If you ever see a leak, you can stop it easily by increasing the force between these two parts. This is done by tightening the three black stator screws, after first loosening the adjacent set screws. Eventually you may need to replace the rotor seal -a 20-minute task that you can do yourself. Consult your instruction manual. If you've misplaced your manual, we'll send you another one-because we want you to have this helpful information on parts, service and operation. Just contact us, specifyingwhich model number you Another have.useful publication,Tech Notes 1, describes how to get the best accuracy and precision from your
injector. For example, did you know that the volume injected into the column is not always the same as the volume transferred from the syringe?To avoid error when partially filling the loop, load no more than half the loop volume. Tech Notes 2 tells how to use the Model 7125 with a short column in the sample loop. This provides a rapid clean-up technique for eliminating interferences and concentrating samples. To receive this literature address Rheodyne, Inc., PO.Box 996, Cotati, California 94928 U.S.A. Phone (707) 664-9050.
SI2 RHEODYNE THE LC CONNECTION COMPANY
CIRCLE 185 ON READER SERVICE CARD
ANALYTICAL CHEMISTRY, VOL. 54,
NO.
7, JUNE 1982
829A
p-
,'"
_.
Figure 3. Scanning electron micrograph of an unheated bead. The dark material at the interface of bead and filament is the collodion used to hold the bead in place
bead as a chemical-separating device as well as for introducing the sample onto the mass spectrometer filament. Mass spectrometric analysis of such small samples requires instruments equipped with pulse-counting detection systems that count the ions individually as they arrive at the collector; we have had several of them in operation for a number of years (7-8).Analysis of the two elements proceeds sequentially from a single rhenium filament. Plutonium, which bas a lower ionization potential than uranium, is analyzed first (at about 1450 "C). Excess plutonium is then burned off, and uranium is analyzed (at about 1700 "C). Computer programs automatically reduce the data to isotopic compositions, correcting for the interference between the two elements a t mass 238. Figure 2 is a photograph showing our canoe-shaped filament; a sketch showing a bead in the filament is also included. (The bead is not drawn to scale, being much smaller in proportion to the filament than shown.) We have analyzed several hundred samples by this technique and have observed no degradation of results; indeed, precision and accuracy are in general better. Figure 3 is a scanning electron micrograph of an unheated bead showing its nearly perfect sphericity. Figure 4 is a scanning electron micrograph of a head that has been heated for 30 min a t 1700 "C. Grain boundaries in the rhenium filament are visible. Approximately half the head has dissolved in the rhenium substrate. (See Reference 9 for a full description of this work.) The resin 830 A
3Limin at 1700 '6
bead serves as a good approximation to a point source for the ion optics of the mass spectrometer and also as an efficient reducing agent to prevent loss of sample as oxide species, thus increasing our overall collection efficiency. In addition, the bead seems to serve as a reservoir of sample, feeding it to the ionization region in a more controlled manner than solution loadings (9). Average collection efficiencies for several beads gave one ion collected for each 700 atoms of uranium loaded on the filament; for plutonium, the figure was one in 80. These represent about an order of magnitude improvement over typical results from solution loadings.
Results Results from repetitive analyses of National Bureau of Standards certified isotopic standards are given in Table I. These were analyzed over a two-month period; all results represent data taken from sequential analyses of plutonium and uranium from resin beads. The good results on the minor isotopes exemplify the ability of multistage mass spectrometers to perform precise measurements on lowabundance isotopes. Standard deviations are listed a t the confidence level of 1a. Experiments were performed to establish the range of U P u ratios over which the technique could he applied. Ratios helow 10 led to saturation of the head with plutonium and made uranium analysis extremely unreliable. Ratios greater than 1000 led to saturation with uranium. It is unlikely that reactor fuel will be burned long
ANALYTICAL CHEMISTRY. VOL. 54, NO. 7. JUNE 1982
enough to produce U P u below 10. Another experiment defined the minimum quantity of plutonium required for a reliable isotopic analysis. This was determined to be about 0.1 ng if all isotopes were measured and somewhat lower (
