Determination of plutonium concentration and plutonium isotopic composition in mixed oxide fuels

86

Journal

of Nuclear Materials I53 (I 988) 86-90 North-Holland. Amsterdam

DETERMINATION OF PLUTONIUM CONCENTRATION AND PLUTONIUM ISOTOPIC COMPOSITION IN MIXED OXIDE FUELS W. DAMS, R. KRAUSE and K.-H. NELGES AMEM GmbH, Postfaeh I I 00 69, D-6450 Hanau 11, Fed. Rep. Germany

In manufacturing mixed oxide fuels, quality specifications about the plutonium content and the plutonium isotopic composition are to be respected. Fast and accurate measuring procedures with low amounts of wastes and small sample volumes are required. Current analyzing methods are the photometric determination of the Putoiaiand Pu( VI) concentration and a combined cr-spectrometric Pu isotopic determination. A single beam bi-frequency photometer permits a rather broad range of application (0.1-Z mglml Pu). The preparation of samples could be carried out nearly full automatically by employing a dispenser system. Applying a combined cu-spectrometric and mass spectrometric Pu isotopic determination, only one filament has to be prepared for both measurements. The filaments are mounted on a sample changer and I3 samples can be measured successively.

1. Introduction Mixed oxide fuels are produced by two different fabrication methods, namely the OCOM (optimized co-milling) process and the co-precipitation process. For the coprecipitation, the belonging plutonium concentration, especially the plutonium( VI) concentration, is an important process parameter which must be controlled permanently. Therefore an elementary measuring method has been developed, which must meet the following conditions - process oriented arrangement, - low technical expenditure, - simple determination, - final results directly indicated.

2. Supervision of mixed oxide fabrication by applying a photometer for plutonium concen~ation off-line measurement 2.1. Posing the problems Since the A(U/Pu)C (ammonia uranyl plutonyl carbonate) process bases on simultaneous precipitation of U and Pu as mixed crystal, tetravalent plutonium must be converted into the hexavalent form by oxidation. For supervising the fabrication process a fast Pu,,,,, and Pu( VI)/Pu,,,,, measurement is required. Accordingly, a process photometer needing only a short calibration time, is a suitable measuring inst~ment.

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Fig. I. Measuring principle of the process photometer.

The process photometer operates with a single beam bifrequency principle. The emitted light is measured after passing the cuvette and a rotating disc which is equipped with two optical filters. A precision filter selects the spectral region of the Pu(VI) peak at 830 nm. The reference filter is fixed at 860 nm to prevent absorptions of the measuring solution. This measuring principle, as it is demonstrated in fig. 1, leads to background compensation. Unspecific absorptions and variations in intensity can mostly be neglected. The test assembly of the process photometer is outlined in fig. 2. The process photometer is arranged modularwise, consisting of emitter and cell integrated in a glove box; gauge head and electronic package is situated outside. For the transmission of the measuring beam a special “window flange” is fit in the side wail of the glove box. The gauge head is fixed outside on the flange, the cuvette holder with cuvette and emitter is screwed on inside. For measurement a commercial 2’mm

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are diluted with 2N HN@ onto the same volume to achieve an optimum measuring range. Both solutions are pumped through the 2 mm flow cell. Extinction is measured after 5 ml have passed through. The electronic package calculates the plutonium concentration, Pu( VI) content, and Putofal content by aid of a stored linearized calibration curve. Both values are quoted to obtain the Pu(VI)/Pu,,,,, ratio. After measurement the cell is cleaned with 2N HNO,. 2.3. Resufts

Fig. 2. Test assembly of the process photometer. flow cell is used through which the measuring solution is transported by pump. As the dosage pumps are separated from the modular unit (outside the glove box) a dispenser for the preparation of the sample is applied. The analyzing procedure with its different steps is shown in fig. 3. For the determination of the Pu( VI)/PU,,,,~ ratio in the measuring solution, a sample is pre-diluted 1:lO and then equal volumes are pipetted into two vessels with dispenser pump. Concerning the Putotal analysis, a determinated Ce(IV) solution is added to one of the samples for converting plutonium completely into the hexavalent form. After half an hour reaction period, both samples

Some advantages of the process photometer are that no specially constructed glove box is needed, merely a “window flange” must be installed; the samples can be prepared nearly full automatically with dispenser pumps; a small volume is needed: 1 ml of 1: 10 diluted U, Pu-nitrate solution in IO ml measuring solution; no wave length calibration, no reference solution is needed; the plutonium concentration can be measured directly due to a programmed calibration curve (calibrating with standards only); the broad application range shows plutonium concentrations between 0.1 and 2 mglml as it is shown in fig. 4; the accuracy of the Pu(VI)/Pu~~~=, dete~ination is about l%, the accuracy of direct measurement of the plutonium concentration is about 5%.

SAMPLING U,Pu FEED SOLUTION I ALIOUOTATION

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Fig. 3. Process flow sheet of the photometric measurement.

3. Combined mass-spectrometric and a-spectrometric determination of plutonium isotopic composition 3.1. Posing the problems Since mass-spectromet~ can hardly differentiate the masses of 238U and 238Pu in a measuring range resulting from plutonium contents between 2 and 5% in mixed oxide fuels, cu-spectrometric 238Puanalysis is preferred. Some boundary conditions for accurate plutonium and uranium isotopic composition analyses must be observed: U and Pu must be separated by ion exchanger (see fig. 6). Impurities have to be removed in order not to influence the measurement. 241Am must be isolated completely because it interferes with the plutonium determination. The 238U/238Pumass ratio should range between 2 x 1O4and 5 x 104.To achieve an accuracy less than 1Wrelative with mass-spectrometric measurement a separating factor less than 2 x 1O-6 would be necessary. Since such a separating factor can hardly be realized in routine measurement a combined cY-spectrometric and mass-spectrometric plutonium isotopic determination is employed. This method as it is represented in fig. 5 (flow sheet) is now DIN and

W. Dams et al./Determination

88

ofPu concentration

and Pu isotopic composition

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Differing from classical methods, a varied measuring method has been developed, shown in fig. 5, at which the samples for cr-spectrometry need no longer be prepared separately; mass-spectometric filaments can be used for both analyses. Main advantages are a reduced amount of wastes, a lower radiation rate, a shorter operating time, and reduced costs. At first the Pu/U separation is carried out as usual, then the filaments are prepared for mass-spectrometric measurement; the sample volume amounts to 1 ~1, and the plutonium concentration is 50 ng/Gl. Now the filaments are conditioned and incandescented. After 13 filaments are fixed on a sample changer and the sample changer is placed into a specially constructed vacuum chamber, the a-spectra are measured on all filaments with one travel. Finally the results are put into a laboratory data system to be calculated and printed out. 3.3. Results

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FINAL Pu ISOTOPIC COMPOSITION

Fig. 5. Flow sheet of the Pu isotopic composition determination.

3.3.1. Pu, U ion exchange To demonstrate the Pu/U ion exchange procedure, a corresponding elution curve is illustrated in fig. 6. 3.3.2. Preparation offilaments The preparation technique of filaments is a main process criterion for both the cr-spectrometry and mass-spectrometry measurement. Therefore a reproducible sample

89

W. Dams et al/Determination of Pu concentration and Pu isotopic composition

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Fig. 6. Elution curve of U/Pu separation.

preparation concerning the time and the temperature, constant sample valumes (111)) defined sample area, and defined sample surfaces are required. This is shown in the

a

SEM (Scanning Electron Microscope) photograph of a typical plutonium routine sample, fig. 7. 3.3.3. Test assembly The development and technical achievement is outlined in fig. 8. The test assembly shows the following characteristics. The detector/filament position is defined exactly, the sample changer is shielded in order to avoid obstructions and at last the installation of a specially constructed vacuum chamber, where 13 samples can be measured successively, offers the possibility of further automation. 3.3.4 Measurement of &spectra The specially constructed vacuum chamber guarantees an optimum arrangement of detector and sample. Consequently, a rather high resolution is achievable with the measuring parameters, as the use of a Si surface barrier detector with an active area of 150 mm2 and an a-resolution of 18 keV fwhm (full width at half maximum). The distance between sample and detector is approximately 15 mm, and the sample area on the Re filament amounts to approximately 0.7 x 3 mm. The plutonium amount on the filaments is 50 ng. The measuring period lasts about

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Fig. 7. SEM photograph of a rhenium Clement prepared for (Yspectrometry. Magnification: (a) 69 fold, (b) 690 fold.

Fig. 8. Measuring device for cx-spectrometq on rhenium filaments.

W.Dams et al./Determination

90

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and Pu isotopic composition

4. Conclusion

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Measuring periods within 0.5-l h, amounts of samples of 50 ng Pu, and pulse depths about lo5 pulses are regarded to be sufficient to obtain exact analyzing results with less expenditure of energy. Consequently the application of a process photometer for determination of the plutonium concentration and the combined cu-spectrometric plutonium isotopic determination leads to reduced wastes and to a respectable rationalization.

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References 0. S- 1 h (depending on 238Pu abundance.) The a-spectra determinations are carried out by a multi-channel analyzer (MCA) with 2096 channels. This test assembly effects resolutions from 18 to 25 keV (fwhm) at pulse depth of 1O5counts/channel (see fig. 9). 3.3.5. Accuracy The measuring method as it is described in the last paragraph, reaches an error rate of about 1% (Is), mainly caused by the counting statistics. Other factors influencing the error rate are the evaluation method and the offgrade of calibration standards (respectively their material).

[ I] DIN Nr. 25489: Determination of uranium and plutohium content and isotopic composition, mass-spectroscopic method (1986). [ 21 R. Berg, private communication: The determination of plutonium in liquid wastes from the WAK reprocessing plant (1987). [ 3 ] E. Kuhn, W. Ochsenfeld and H. Schmieder, KfK Report No. 1306 (1979). [4 ] W. Beyrich and G. Spannagel,

(1979).

private communication