Anal. Chem. 1982, 5 4 , 8-12
8
by the design of the instrument. The purge rate has little effect if it is maintained at approximately 30 mL/s, and fluctuations in sample flow rate should have no effect on response. The electronic components are rugged and generally available, and since there are no moving parts, the instrument is expected t o be quite maintenance free in an on-line application.
LITERATURE CITED (1) Diggins, A. A. "Determination of Low Levels of Sodium in Water with a Sodlum Response Electrode", paper presented at Symposium on Automatic Analytlcal Systems In Water Analysis, Society for Analytlcal Chemlstry, London, Nov 17, 1967. (2) Diggens, A. A. Proc.-Int. Water Conf., Eng. sot. West, pa, 1978, 83-68.
(3) Swartz, J. J. Instrum. Pulp Pap. Ind. 1975, 16. (4) Beak, R. L. "Proceedings of the 15th Annual Analytical Instrumental Symposlum, New Orleans, LA, 1969; ISA: Pittsburgh, PA, 1969. (5) Durst, R. A. "Ion-Selectlve Electrodes", U. S. Government Printing Offlce: Washington, DC, 1969; NBS Spec. Publ. (U.S.), No. 314. (8) Covington, A. K. "Ion-Selective Electrode Methodology"; CRC Press: Boca Raton, FL, 1979; pp 43-66. (7) Anders, 0. U. "Proceedlngs of the Fall Instrumentatlon-Automation Conference, Los Angeles, CA, 1961; ISA: Pittsburgh, PA, 1961. (8) Anders, 0. U., Nucleonics 1962, 20 (No. 2), 76-83. (9) Megy, J. A.; Blnney, S. E. Ind. Eng. Chern. Process Des. Dev. 1976, 15. 436-438. (10) Garber, D. I.; Kinsey, R. R. "Neutron Cross Sections"; Brookhaven National Laboratory, BNL 325, 1976.
RECEIVED for review June 10,1981. Accepted September 21, 1981.
Automated Monitoring of In-Process Plutonium Concentration T. V. Rebagay," G. A. Huff, and K. J. Hofstetter' Allied-General Nuclear Services, P.O. Box 847, Barnwell, South Carolina 198 12
An automated iow-level plutonium monitor capable of measuring total and isotopic plutonium abundances in solutions is descrlbed. To demonstrate near real-time assay of in-process plutonium, we installed a monitor on a flowing stream of a laboratory experimental facility. The stream was composed of uranium and plutonium In nltric acid at concentrations typical of a plant using a Purex fiowsheet modified to permlt coprocessing of spent nuclear fuel. The plutonium isotopic abundances were typical of those found in light water reactor grade fuel. The plutonium isotopic concentrations in the stream with the exception of 242Puwere determined by direct y-ray spectrometry. The 242Puabundance was calculated by isotope correlation techniques. Additional data were obtained on coprocessed uranium-plutonium solutions denatured with fission products (ImRu, 144Ce/'"Pr, and %P6Nb). lSsPuand 240Puconcentrations can be determined to within 2 % and 5 %, respectively, of the concentrations determined by mass spectrometry.
In the reprocessing of spent nuclear fuels, monitors capable of measuring in-process plutonium reliably a t various stages of the purification cycle are desirable. Automated on-line monitors for performing near real-time measurements of plutonium concentration in the input, product, and waste streams of reprocessing facilities are needed to provide timely inventory data of in-process plutonium. Recently, on-line and process radiation monitors have been tested at the Barnwell Nuclear Fuel Plant (1). An on-line isotopic concentration monitor capable of assaying plutonium solutions with concentrations from 150 to 575 g/L P u was developed and tested. With freshly separated light water reactor-grade plutonium nitrate solutions, accuracies between 0.2 and 0.5% in the determination of isotopic abundances were reported. The technique employed was direct y-ray spectrometry. Similar accuracies were reported for totalplutonium concentrations as measured by differential y-ray absorptimetry. However, total and isotopic plutonium assays could not be performed simultaneously by this monitor. lPresent address: P.O. Box 480, Middletown, PA 17057.
In support of uranium-plutonium coprocessing experiments, a fully automated low-level plutonium concentration monitor with sensitivity range from 0.1 to 10.0 g/L Pu was developed and installed on one of the streams of a laboratory experimental facility. Because this monitor was capable of continuously assaying the total and isotopic abundances in situ, improved process control and dynamic fissile plutonium inventory were possible. These data provide assurance that plutonium can be safe-guarded during coprocessing. EXPERIMENTAL SECTION Monitoring System. The on-line totaland isotopic plutonium concentration monitor consists of a stainless steel sample cell assembly, an intrinsic germanium low-energy photon detector, a multichannel analyzer with associated electronics, and a minicomputer to control data acquisition and processing. The sequence and timing for operating the monitor were accurately controlled by software. Figure 1 is a diagram of the detector-assay cell system. The cell is made of stainless steel and has a sample capacity of 1 mL with an effective area of 200 mm2. It is equipped with a Plexiglas window coated with a thin polycarbonate film for protection from the harsh reprocessing environment. Bubbles which may form at the edge of the cell due to radiolysis are effectively masked by a built-in tantalum collimator. Sample lines made of 0.635 cm diameter stainless steel tubing connect the sample cell to the main product stream. A 0.05 mm thick stainless steel window separates the cell from the detector as part of the containment system. The cell is then mounted in a Plexiglas secondary containment box. The low-energy photon detector has an active area of 200 mm2, a sensitive depth of 7 mm, and a 0.13 mm thick Be window. With suitable amplification, pulse pile-up rejection, and digital stabilization, the spectrometer system is capable of maintaining a full width at half-maximum (fwhm) resolution of 350 eV at 59.5 keV at a total count rate of lo4 counts/s. It has an efficiency of about 15% relative to a 3 in. X 3 in. NaT(T1) detector. The detector is coupled to the cell with a mating flange and bolted into place to ensure geometric reproducibility. A photograph of the detector-cell assembly is shown in Figure 2. Monitor Calibration. Prior to the installation of the monitor on the process stream, it was calibrated by use of simulated light water reactor-grade plutonium nitrate solution standards. The standards were processed by ion exchange to remove the americium and then carefully characterized for total plutonium content by controlled-potential coulometry and isotope dilution mass spectrometry. Isotopic plutonium distributions were determined
0003-2700/62/0354-0008$01.25/00 1981 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54. NO. 1. JANUARY 1982 * O
~lpun I. Megam of me &he isotopk concenwatlon mOnRor cel: (1) Ge detecta. (2) couple. (3) seal IOR. (4) Ta mhator, (5) P!+Xlalas wlndow. (6) thln polycarbonate fllm. and (7) cell body.
. . . . . . .
w
e 2. D e t e c t ~ - a ~ celi ~ a yasssmw: (1) secowlaary cantahnmnl box. (2) sample call, and (3) GE detector. hy mass spectrometry. A leaatsquares fitting of the data established the relatiomhip of the corrected intensities to the known plutonium concentrations. Coprocessing Feed Solutions. For the coprocessing flowsheets studied, the feed solution consisted of a W1 mixture of uranium and plutonium. A similar solution, d e n a t d with stable and radioactive cerium ("'Ce/'"Pr), ruthenium ("Ru), and zirconium (?Zr/Wb), was used in subsequent runs. AU solutions containing fission product nuclides were prepared from standard materials with well-characterizedspecific activities. The fwsion
products were formulated to produce less than 3700 MBq/g Pu [megabequerel (MBq) = 16disintegrations/s], i.e., 100 mCi/g Pu after the first U/Pu purification cycle. A t this level, their presence can be tolerated though with somewhat poorer precision in the plutonium measurements. Above this level, the increased Compton continuum from the high-energy y-rays of the fission products will degrade severely the intensities of the plutonium isotopes occurring in the 10C-200 keV region of the solution spectrum making plutonium assays meaningless. Purification of Uranium and Plutonium. A modified Purex process was employed to separate uranium and plutonium from fission products and other contaminants. Primary decontamination was effected hy contacting the aqueous feed solution with 30 vol % tributyl phosphate (TBP) in n-paraffin hydrouubon (NPH)using a solvent extraction column. Uranium and plutanium were coextracted into the organic stream while the majority of the fission products and americium remained in the aqueous stream. The organic stream was then directed to an electropulse column for valence adjustment and further purification. In the column.U(VI) was reduced to U(1V) and Pu(IV) to Pu(l11). The reduced species were stripped from the organic stream by contacting with an aqueous solution of dilute H N 0 3 and hydrazine (N,H,).This purified stream containing uranium and plutonium was used for the monitor evaluation tests. Data Acquisition and Processing. The plutonium content of the stream flowingthmugh the sample cell was assayed by direct y-ray spectrometry. Sequential acquisition of sample spectra was conducted at 400-8intervals throughout the duration of a demonstration run. The minicomputer initiated the intensity measurements. provided appropriate delay times, performed data manipulations,and provided results on a line printer. Processing of the spectra and storing of the data were automated. To gain a m to the system without interruptingthe dedicated computer, a second computer-based multichannel analyzer was linked to the detector. The secondary system permitted manual acquisition and display of analyle spectra and also provided a convenient check on the performance of the dedicated system.
RESULTS AND DISCUSSION The spectra of the background radiations before and after a demonstration run are depicted in Figure 3. As can be seen. only adventitious u'Am was present in the spectral region of interest. Representative spectra of the uranium-plutonium coproceased solutions at the s t a r t of a demonstration run and after the processing conditions had attained apparent steady state are shown in Figure 4. The general shapes of the spectra are similar. No uranium low-energy y-rays are distinguishable in the region below 60 keV. It is noted that there are no y-ray contributions from fission products to the overall spectra in A
0
1023
CHANNEL NUMBER
I
20.7
3071
B 108
10
Flpun 3. Backgound specba of me m n o r . Spackm A is me b a a g o d before me demonskaUon runs. and spedrum E Is me background aner me demonstration runs (me cell IS moroughiy flushed with "OS p h to me acqulsnbn of this spectrum).
10
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
where I , N , A, Y , 6 , and S are the measured peak intensity, the number of nuclei, the nuclear decay constant, the absolute branching intensity, the detector efficiency, and the self-absorption for the y-ray, respectively. The ratio of the peak intensities for two plutonium isotopes, i and j , occurring in the same spectral region can be related to their abundance ratio by the expression
Flgure 4. Spectra of uranium-plutonium coprocessed solution: (A) typical spectrum at start of a demonstration run; (B) spectrum at steady-state conditions: (1) (U-Pu) X-rays, (2) 239Pu(38),(3) 238Pu(43), (4) 240PU(45)1(5) 239P~(5,),(6) 241Am(80)p (7) 239pU(~2s)~ (8)2 4 1 P U ( y , (9) 23eP~(152,. The numbers in parentheses are the y-ray energies in keV.
Table I. Uranium, Fission Products, and Nitric Acid Compositions of the Coprocessing Stream of Interest
composition uranium, g/L HNO,, M Io3Ru,MBq/h "Zr, MBq/h '"Ce, MBq/h total fission products, MBqh fission products/g Pu, M W g PU flow rate of the stream, mL/min
demonstration no. 1
demonstration no. 2
12.3 2.5
0
7.2 2.25 63.6 4548.0 142.8 4754.4
0
2989.6
0 0 0
13.2
13.2
a A megabequerel (MBq) = l o 6 disintegrations/s. A millicurie (mCi) is equal to 37 MBq.
the region of interest (0 < E., < 165 keV). The fission product levels in the monitored stream are listed in Table I. Included in the table are the uranium concentrations, acidities, and the flow rates of the monitored stream for two demonstration runs. Although '"Ce possesses a y-ray of high branching intensity at 134 keV, it is not visible in the spectra (Figure 4). A combination of the low specific activity and a 400-s data acquisition time reduced the '%e interference. The solution spectra also indicate that =lAm had been significantly reduced by the purification process. The 241Amy-ray at 59.5 keV is well-separated from the neighboring plutonium y-rays. With reduced 241Amintensity, reliable assays of the plutonium isotopes could be performed by using their intense y-rays below 60 keV. For isotopic plutonium distributions, the low-energy region (E., < 165 keV) of the analyte spectrum was selected to provide relative intensity information. The y branching intensities for the P u nuclides are highest in this energy region. Data acquisition times of 400 s were sufficient for spectral analysis and permitted near real time plutonium assay. The most useful low-energy y-rays of 238Pu,239Pu,and 240Puare at 43, 51, and 45 keV, respectively, and are very prominent in freshly purified plutonium solutions. Unfortunately, 241Puhas no intense y-ray in the energy region below 60 keV. A y-ray at 148 keV, free from spectral interferences, was utilized to measure the 241Puabundance. The observed peak intensity for a plutonium isotope of interest is related t o its concentration by the equation
To calculate the ratios ( N i / N j )Le., , 23aPu/239Pu and 240Pu/ 239Pu,the intensities of the y-rays of 238Puand 240Puat 43 keV and 45 keV, respectively, were compared to that of 239Pua t 51 keV. The %lPuabundance relative to agPu was determined by comparing the intensity of the 148 keV y-ray to the nearest most intense y-ray of 239Pua t 129 keV. The technique of comparing intensities of y-rays of comparable energies minimizes the efficiency and attenuation differences in the abundance measurements. The small efficiency differences for detecting plutonium in plutonium nitrate solutions were determined during the calibration of the monitor and were utilized in the calculation of isotopic ratios. The pertinent nuclear parameters of 238Pu,239Pu,240Pu,and 241Puused in eq 2 were those reported by Gunnink, Evans, and Prindle (2). The determination of isotopic abundances by the technique of y-ray spectrometry is limited by y branching intensity. Light water reactor grade plutonium contains significant amounts of 242Pu. The y rays of this nuclide are very weak due to low emission probabilities and its long half-life. The established method for determining 242Puis mass spectrometry. In this study, the 242Puabundance was obtained from the measured abundances of 239Puand 240Puby using isotope correlation techniques ( 3 ) . The isotopic abundances were calculated from the isotopic ratios using the following equations: 240pu
241pu
-)
242pu
+-239pu + 239pu + 239pu
-1
(3) (4)
(5)
(7) where 238Pu,239Pu,240Pu,241Pu,and 242Purepresent isotopic abundances and 238Pu/239Pu, 240Pu/239Pu, and 241Pu/239Pu are isotopic ratios from eq 2. Gunnink (3) reported that plots of 242Puvs. (240Pu/ 239Pu)2(240Pu) from spent fuels of specific reactors generated a family of straight lines with slopes ranging from 0.85 to 1.25. The slope that best fits the data of the different light water reactor-grade plutonium nitrate solutions (different isotopic distributions) employed to calibrate the on-line monitor was 1.25;hence, a correlation constant of 1.25 was selected for % T u abundance calculations (eq 7). The isotopic ratios and abundances of plutonium from two demonstration runs are summarized in Tables I1 and 111. The data listed in these tables represent values obtained at the initial stage, steady state, and end of the demonstration runs
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
11
~
Table 11. Isotopic Ratios and Abundances of Plutonium from Demonstration Run No. 1 time after initial analyte flow, min
--
2'8Pu/29Pu
14.72 22.00 29.32 36.67 95.56 102.97 110.40 117.77 228.78 236.17 243.54 250.90
0.0039 0.0040 0.0035 0.0034 0.0037 0.0036 0.0035 0.0033 0.0032 0.0031 0.0036 0.0033 av (GS) 0.0035 mass spect 0.0037
isotopic ratio 2"Pu/239Pu 0.27 0.26 0.27 0.27 0.29 0.27 0.28 0.27 0.25 0.26 0.28 0.27 0.27 0.25
(0.10004) (0.10004) (0.10003) (0.10002) (O.lD002) (0.10002) (O.lD002) (O.lD002) (0.10002) (O.iDOO2) (0.10002) (0.10002) (0.10003) (0.10)
(0.03) (0.02) (0.02) (0.02) (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.02) (0.01) (0.0)
241PU/239PU 0.044 0.062 0.033 0.042 0.032 0.043 0.038 0.039 0.041 0.039 0.044 0.036 0.041 0.036
(0.01) (0.02) (0.007)i (0.008)i (0.004)i (0.0006) (0.005) (0.005) (0.005) (0.005) (0.006) (0.005) (0.008) (0.0)
238Pu 0.29 (0.03) 0.30 (0.03) 0.26 (0.02) 0.26 (0.02) 0.27 (0.02) 0.27 (0.02) 0.26 (0.02) 0.25 (0.02) 0.24 (0.02) 0.24 (0.02) 0.27 (0.02) 0.25 (0.02) 0.26 (0.02) 0.28 (0.009)
isotopic distribution, % 239Pu 2"Pu 241Pu 20 (2) 74 (2) 19 (2) 74 (2) 75 (1) 20 (2) 20 (2) 75 (1) 74 (1) 21 (1) 20 (1) 75 (1) 21 (1) 74 (1) 20 (1) 75 (1) 76 (1) 19 (1) 20 (1) 76 (1) 74 (1) 21 (1) 20 (2) 75 (1) 20 (1) 75 (1) 76 (0 03) 19 (0.02)
3.3 (0.8) 4.6 (1) 2.5 (0.5) 3.2 (0.6) 2.4 (0.3) 3.2 (0.4) 2.8 (0.4) 2.9 (0.4) 3.1 (0.4) 3.0 (0.4) 3.3 (0.4) 2.7 (0.4) 3.1 (0.6) 2.8 (0.005)
242
1.8 1.6 1.8 1.8 2.2 1.8 2.0 1.8 1.5 1.7 2.1 1.8 1.8 1.6
Pu
(0.4) (0.3) (0.3) (0.3) (0.2) (0.2) (0.2) (0.2) (0.1) (0.2) (0.2) (0.3) (0.2) (0.005)
(MS) -8 -14 +7 +1 -5 -1 1 -13 [(MSt5 GS)/ MS1100 a Plutonium is coprocessed with uranium from a feed solution consisting of mixed uranium-plutonium nitrates (U/Pu Average values measured by the on-line monitor. Average ratio, 99:l). Figures in parentheses are standard deviations. values of samples withdrawn from the stream at certain periods of the run measured by mass spectrometry. -
Table 111. Isotopic Ratios and Abundances of F'lutonium from Demonstration Run No. 2 a time after initial analyte flow, min
23PU 1239 Pu
15.34 23.53 30.88 38.62 124.22 132.08 140.01 148.02 220.77 228.97 237.19 244.94 av (GS)
0.0026 0.0028 0.0032 0.0028 0.0025 0.0028 0.0025 0.0025 0.0025 0.0024 0.0022 0.0028 0.0026 mass spect 0.0037
(MS) [(MS 1 GS)/ + 30 MS1100
isotopic ratio 2"Pu/2"Pu
(0.0002) (0.0002) (0.0002) (0.0002) (0.0001) (0.0001) (0.0001) (0.0001) (0.00008) (0.00008) (0.00007) (0.0002) (0.0003) (0.0)
241
Pu /239Pll
isotopic distribution, % 239Pu 2" Pu 241Pu
238Pu
0.21 (0.01) 0.21 (0.01) 0.24 (0.01) 0.21 (0.01) 0.23 (0.01) 0.22 (0.01) 0.23 (0.009) 0.23 (0.008) 0.23 (0.007) 0.24 (0.007) 0.25 (0.008) 0.25 (0.02) 0.23 (0.01) 0.25 (0.0)
0.041 (0.006) 0.051 (0.008) 0.050 (0.007) 0.034 (0.005) 0.036 (0.004) 0.038 (0.005) 0.037 (0.004) 0.041 (0.004) 0.041 (0.003) 0.044 (0.004) 0.035 (0.003) 0.041 (0.007) 0.041 (0.005) 0.036 ( 0 . 0 )
0.21 0.22 0.24 0.22 0.20 0.22 0.20 0.20 0.20 0.18 0.17 0.21 0.21 0.28
+8
-14
t 25
(0.02) (0.02) (0.02) (0.02) (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.01) (0.02) (0.02) (0.009)
242Pu
79 (1) 78 (1) 76 (1) 79 (1) 78 (1) 78 (1) 78 (1) 78 (1) 78 (1) 77 (1) 76 (1) 76 (1) 78 (1) 76 (0.03)
1 7 (1) 16 (1) 1 8 (1) 1 7 (1) 1 8 (1) 17 (1) 1 8 (1) 1 8 (1) 1 8 (1) 1 8 (1) 1 9 (1) 19 (2) 18 (1) 1 9 (0.02)
3.2 (0.5) 4.0 (0.6) 3.8 (0.5) 2.7 (0.4) 2.8 (0.3) 3.0 (0.4) 2.9 (0.3) 3.2 (0.3) 3.2 (0.2) 3.4 (0.3) 2.7 (0.2) 3.1 (0.5) 3.2 (0.4) 2.8 (0.005)
0.94 (0.1) 0.88 (0.1) 1.3 (0.1) 0.94 (0.1) 1.2 (0.1) 1.0 (0.1) 1.2 (0.1) 1.2 (0.1) 1.2 (0.1) 1.3 (0.1) 1.5 (0.1) 1.5 (0.3) 1.2 (0.2) 1.6 (0.005)
-3
+5
-14
+ 25
a Plutonium is coprocessed with uranium from a feed solution consisting of mixed uranium-plutonium nitrates (U/Pu ratio, 9 9 : l ) denatured with fission products. Figures in parentheses are standard deviations. Average values measured by the on-line monitoy. Average values of samples withdrawn from the stream at certain periods of the run measured by mass spectrometry,
(a demonstration run consisted of 34 sequential P u determinations). Table I1 indicates the results of uranium-plutonium solutions coprocessed from a feed solution consisting of a mixture of uranium and plutonium nitrates (U/Pu, 991) while Table III lists the data from a dmilar feed solution denatured with fission products. Data in Table I1 indicate that 23?Pucan be assayed to within 2% of the abundance (determined by mass spectrometry and for 240Puthe agreement is within 5 % . The other isotopes, 238Puand 241Pu,have deviations of 7-.11% from the mass spectrometry results. Samples were withdrawn from the stream at certain specaied periods of the run for isotopic assay by mass spectrometry. As can be seen in Table 111, the uddition of fission products to the feed solution produces only a slight increase in the deviation for 23gPu(rt3%) and 241Pu (*14%). However, the 238Puassay shows a significant loss
in accuracy (-(&30%). The large deviation of 238Puis probably due to its very low abundance (
