where peak spreading is controlled by molecular diffusion, it is concluded that y = 0.7, while from work at high reduced velocity, where the convective process of eddy diffusion predominates, the value X = 0.5 is indicated (8, IO). These values of y and X can be substituted into Equation 1 and the resulting relationship is presented in Figure 1. It is clear that Equation 1 does not represent the data adequately; however, as pointed out by Klinkenberg (5), Equation 1 does predict the correct asymptotic behavior at high and low values of reduced velocity. Using the same values of y and X in Equation 2, a good fit is obtained with the experimental data (see Figure 1) using the value w = 0.10. However, Giddings (6) has proposed a value of w = 0.43 for nonporous, inert packing materials. The effect of adopting this value is seen in Figure 1. It would appear, therefore, that while the form of the coupling equation is accurate in describing the characteristics of the h-v data, the value of w predicted by Giddings (6) is subject t o some uncertainty.
It is worthy of note that the data presented in Figure 1 indicate n o variation with particle size in the h-v relationship. This finding is confirmed in many of the studies reviewed by Gunn (IO), who concludes that the h-v relationship is independent of particle size provided that the ratio of column diameter t o particle diameter is greater than about 12. Finally, it is clear that much of the literature on flow in packed beds, which is reported in chemical engineering journals, is directly relevant t o work on chromatography, and vice versa. However, there has been very little attempt to bring together the results in these two fields of study. M. F. EDWARDS School of Engineering Science University of Warwick Coventry, England RECEIVED for review August 19, 1968. Accepted October 7, 1968.
AIDS FOR ANALYTICAL CHEMISTS r
E. A. Mershad, M. L. Curtis, J. Y. Jarvis, and W. R . Amos Mound Laboratory, Miamisburg, Ohio 45342
To
MEET Atomic Energy Commission nuclear material accountability requirements and to improve the efficiency of 238Pu recovery operations, a procedure was developed for determining the amount of 238Puin wastes removed from glove boxes. For recovery purposes, 2asPu contaminated waste consisting of ion exchange resin, paper, rags, polyethylene, glass, and metal is segregated into various categories, such as resin, combustibles, and noncombustibles, prior to removal from glove box lines. The material is packaged in 1/2-gallon cans, 13.81 cm in diameter and 14.92 cm high, removed from the glove boxes in polyethylene bags, and packaged in 1gallon cans, 15.72 cm in diameter and 22.23 cm high. The 1-gallon cans are sealed by a special canning apparatus and enclosed in a polyethylene bag as a final precaution against alpha contamination. Each can, labeled with the appropriate waste category, may contain 238Pu ranging from negligible quantities to 10.3 grams. The 100-keV photons from 238Pu (1) were counted by a gamma pulse height analysis procedure. The method is relatively rapid, applicable to many types and densities of waste material, and accurate in the 16-mg to 10.3-gram concentration range.
EXPERIMENTAL
Apparatus. A schematic of the physical arrangement of equipment is shown in Figure 1 . Although the dimensions
(1) E. K. Hyde, I. Perlrnan, and G . T . Seaborg, “The Nuclear Properties of the Heavy Elements,” Vol. 11, Prentice-Hall, New York, N. Y . , 1964, pp 811-12. 384
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ANALYTICAL CHEMISTRY
are not critical, recalibration is required if the geometry is altered. A 1-gallon can of waste is placed on a turntable which is rotated at approximately 10 rpm. To correct for gamma absorption due to the variation in density of the waste, four calorimetered samples, each containing 5 grams of z3*Pu(as the dioxide), are used as external standards. This material is contained in stainless steel capsules 1.91 cm in diameter and 5.08 cm high. These capsules are placed in secondary aluminum capsules 2.70 cm in diameter and 6.67 cm high. The standards are mounted 0.32 cm apart in an aluminum brick to minimize scattering effects, ensuring that only those photons passing through the waste material are detected by the crystal. The detector is a 5-inch diameter by 1-inch thick NaI(T1) crystal, (Harshaw Chemical Co., Type 20MB4/A-X) with a n RIDL Model 10-17 scintillation detector preamplifier. The front and sides of the crystal are covered with 0.5-mm cadmium sheet to reduce neutron interference. When waste containing less than 1 gram of 238Puis assayed, a 0.64-cm stainless steel plate is placed between the crystal and the waste. With waste containing more than 1 gram of isotope, the 0.64-cm plate is replaced by a 1.27-cm plate. Not shown in Figure 1 is the shielding to reduce background levels. The external standards, sample, and detector system are shielded on four sides and on the bottom with 25.4 cm of polypropylene sandwiched with 0.64 cm of lead sheet. The instrumentation for measuring the gamma radiation is a 400-channel analyzer (RIDL Model 34-12B). Data printout is attained with a Friden printer (Model 44-10). The system is calibrated to 5 keV per channel, and the integrated counts under the 100-keV peak (channels 18 through 22) are used in the determinations. Background is stored in the first quadrant of the analyzer memory and is automatically subtracted from the sample data which are stored in the second quadrant.
I
1) 2) 3) 4) 5) 6) 7) 8) 9)
EXTERNAL PLUTONIUM OXIDE STANDARDS ALUMINUM BRICK 7 SAMPLE (NO. 12 CAN) TURNTABLE (10 RPM) STAINLESS STEEL P L A T E (0.64 cm for < 1 g ISOTOPE OR 1.27 cm for> 1 g ISOTOPE) DETECTOR (Nal(TI), 5 INCHES BY 1 INCH, HARSHAW 20MB4/A.X) CADMIUM SHEET (0.5 mm THICK) SCINTALLATION DETECTOR PREAMPLIFIER (RIDL MODEL 10-17) 4 0 MULTICHANNEL ANALYZER (RIDL MODEL 34-128) AND PRINTER (FRIDEN MODEL 44-10)
Figure 1. Schematic of gamma scan components First the external standards are counted. Then the external standards are counted through the rotating can of waste. (The can must be shaken prior to counting to minimize segregation of material.) Finally the waste can alone is counted. All counts are for 1 ininute live time. [An in-line system was later constructed to eliminate removing the waste material from the glove boxes for assay and returning the material to the boxes for recovery purposes. Via an in-line conveyor, the sample cans are transferred from the production areas to the gamma scanning operation, which is shown in Figure 2. The cans are placed on a turntable in a sample compartment open on two sides. The sample compartment is iowered by a cable to the bottom of a thin-walled (0.32 cm thick) stainless steel shaft, 38.10 cm by 38.10 cm by 2.93 m deep. At the bottom of the shaft are the electrical contacts for activating the turntable. Located outside the bottom of the shaft are the scintillation detector and external standards, properly aligned on opposite sides of the sample compartment. A hydraulic air cylinder moves the external standards into position and when high level cans are counted similar air cylinders move 1, 2, or 3 stainless steel plates (each 0.64 cm thick) into position between the detector and sample. The counting instrumentation and all controls are located in the laboratory with the glove box. By counting considerably below the glove box floor and by adding proper shielding around the counting area, background radiation is reduced to a minimum. After the assay is completed, the sample is again placed on the in-line conveyor and transferred to the recovery operations,] Calculations and Calibration. With the approximation that the effective absorber thickness of a can to an external standard is twice that of a sample inside the can, the calculations are performed as follows ( 2 ) :
(2) S. Glasstone, “Sourcebook on Atomic Energy,” D. Van Nostrand, New York, N. Y . , 1950, p 169.
which yield 3-
(3) where Is = intensity of the 100-keV y-radiation from the external standards through the can (counts/ min from the can alone subtracted from countsjmin from the external standard through the can) Zos = intensity (countsjmin) of the 100-keV y-radiation from the external standards with no can I d = intensity (countsjmin) of the 100-keV y-radiation from the can rod = intensity (countsjmin) of the 100-keV y-radiation from the can corrected for absorption p = 100-keV y-ray absorption coefficient (cm-1) x = absorber thickness (cm) T o enable iodto be converted to grams of isotope, 1-gallon cans were counted which contained various waste materials to which known quantities of 238Puhad been added. The known quantity of isotope was obtained either by accurately weighing 238Pu02or by taking an exact volume of a 2 3 8 P ~ ( N 0 3solution )4 in which the amount of 238Puhad been established by a-radiation counting. The waste material in the standard cans was segregated and packaged in a manner similar to that used for normal glove box waste. The following waste categories were used: (1) glass, (2) metal, (3) polyethylene, (4) various mixtures of glass, metal, and polyethylene, ( 5 ) ion exchange resin, and (6) rags and paper. For each can, loawss divided by the grams of isotope to determine F VOL. 41, NO. 2, FEBRUARY 1969
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Controls for operotion
Cable motor with limiting switch
of coble, turntable, and
hydraulic cylinders
Glove box
Stainless Steel Shaft
Counting In strumentation
Sample Compartment Cable
Nal (TI)
5.08 cm Benelex, 2.54 cm Lead, 5.08 cm Benelex
I I
12.70 cm Benelex, 12.70 em Benelex
f
7 Concrete Laboratory Floor 12.70 cm Benelex, 12.70 em Benelex Sample Compartment
5.08 cm Benelex, 1.27 cm Lead, 5.08 cm Benelex
Eloctric Contacts
External Standards
Removable Panel
Turntable
5.08 crn Benelex,
3 Steel Plates
2.54 cm Lead, 5.08 cm Benelex Hydraul ic Cy1 inder for Raising External Standards
Plates
ck)
Figure 2. In-line gamma scan operation (counts/min/g). Thus, for an unknown can, the grams ( W ) of *a8Pu could be calculated using the equation (4) In addition to segregation of waste according to the type of material, more accuracy was obtained when the waste was categorized by activity levels: (1) low level combustible waste (
