Monochromatic Wavelength Dispersive X-ray Fluorescence for Improved Pu Assay

Transcription

1 1 IAEA-CN-184/169 Monochromatic Wavelength Dispersive X-ray Fluorescence for Improved Pu Assay G. Havrilla a, M.Collins a, V. Montoya a, Z. Chen b, F. Wei b a Los Alamos National Laboratory, Los Alamos, NM USA b X-ray Optical Systems East Greenbush, NY USA havrilla@lanl.gov Abstract. Monochromatic wavelength dispersive X-ray fluorescence (MWDXRF) is a sensitive and selective method for elemental compositional analyses. The basis for this instrumental advance is the doubly curved crystal (DCC) optic. Previous work has demonstrated the feasibility of sensitive trace element detection for yttrium as a surrogate for curium in aqueous solutions. Additional measurements have demonstrated similar sensitivity in several different matrix environments which attests to the selectivity of the DCC optic as well as the capabilities of the MWDXRF concept. The objective of this effort is to develop an improved Pu characterization method for nuclear fuel reprocessing plants. The MWDXRF prototype instrument is the second step in a multi-year effort to achieve an improved Pu assay. This work will describe a prototype MWDXRF instrument designed for uranium detection and characterization. The prototype consists of an X- ray tube with a rhodium anode and a DCC excitation optic incorporated into the source. The DCC optic passes the RhKa line at kev for monochromatic excitation of the sample. The source is capable of 5 W power at 5 kv and 1. ma operation. The x-ray emission from the sample is collected by a DCC optic set at the ULa line of kev. The collection optic transmits the ULa x-rays to the silicon drift detector. The x-ray source, sample, collection optic and detector are all mounted on motion controlled stages for the critical alignment of these components. The sensitivity and selectivity of the instrument is obtained through the monochromatic excitation and the monochromatic detection. The prototype instrument performance has a demonstrated for sensitivity for uranium detection of around 2 ppm at the current state of development. Further improvement in sensitivity is expected with more detailed alignment. The main advantages of the MWDXRF approach for improved Pu assay is that it works, it is simple and it can be cost effective. 1. Introduction The measurement of Pu in reprocessing facilities is a critical function in ensuring that no material is diverted outside the controlled process. While there are several methods for determining the amount of Pu present within the facility at given sampling points, which include gamma ray, and total, coincidence and multiplicity neutron counting, none of these methods actually measures the Pu directly. We propose to improve the determination of Pu in reprocessing streams by providing a more accurate measure of the Pu content using X- ray fluorescence. The improved Pu assay will be achieved by using a new X-ray optical instrument employing doubly curved crystals (DCC) to provide a monochromatic excitation beam and to selectively detect the analyte of interest. The MWDXRF (monochromatic wavelength dispersive X-ray fluorescence) approach offers several advantages for determining the Pu content. These include the following: a) high efficiency excitation of the analyte with the monochromatic excitation, b) highly selective and sensitive detection of the analyte, c) reduced sample amount needed for analyses, d) the technique works, e) it is simple and f) the expected cost will be reasonable. While XRF has been used in the past to determine Pu:U ratios, the MWDXRF approach offers a new paradigm for radioactive materials characterization, namely direct non-destructive elemental analysis. This improvement in the Pu measurement is based on commercially available instrumentation [1]. The MWDXRF technology has been applied to online analyses

2 in petroleum refineries, laboratory scale instruments and even person-portable instruments for field analyses. The MWDXRF technology offers existing, proven technology in a new application which could change the paradigm of radioactive materials characterization. The neutron counting methods require a Cm correction because the Cm neutron emission is a factor of over 1, relative to the Pu. These neutron measurements involve the use of a Cm:Pu ratio to correct the neutron measurement in determining the Pu. The coincidence and multiplicity counting can attain a Cm:Pu ratio of around.1 (1 ppm to 1%). Since most of the Pu assay techniques rely on calculations, modelling, assumptions and corrections of measurements due to the high gamma ray and neutron emission there is a relatively high error in the Pu determination. We propose to improve the determination of Pu in reprocessing streams by providing a more accurate and direct measure of the Pu content and thereby increasing the accuracy of the Pu compositional analyses used in reprocessing plants for safeguards applications. Although neutron and gamma ray measurements can detect Pu, there is no elemental discrimination possible with neutron measurements and the gamma ray background from the fission product materials masks accurate determination of the actinide content which is necessary for applying neutron production corrections. A more accurate and direct Pu determination would have significant impact at all stages of the reprocessing facility; from Pu entering at the shear process, the accountability tank and even the leached hulls characterization stages. Another significant characterization stage is the vitrification process both prior to entering the vitrifier and after vitrification. At each step, Pu measurements are needed to insure proper safeguards are in place to prevent and detect any diversion of material. This is also important for the high level waste stream where several kg of Pu can be lost over the course of a year s worth of processing. In addition, increased accuracy will be important for pyroprocessing due to concerns over salt heterogeneity. In this work, we will discuss the MWDXRF concept and demonstrate its performance with selected actinide and actinide surrogate elements including uranium and yttrium (surrogate for curium). We will also present calculations demonstrating the anticipated analytical quantitative capabilities for the target element plutonium. 2. Concept and performance of MWDXRF The MWDXRF technology is based on the use of advanced x-ray optics called doubly curved crystals (DCC). DCC optics were first proposed in the early 5 s [2] to focus and monochromatize X-rays. Since then development has been hampered by the practical difficulty in actual fabrication of the doubly curved crystal material. In 1997 [3] and 1998 [4], new developments were achieved in fabricating a doubly curved mica crystal for CuK radiation. The DCC was demonstrated by doing monochromatic excitation of several samples and detecting the X-ray fluorescence emission. The crystal bending technology developed by Chen and Wittry provides for elastic bending of crystals into various complex shapes with precise profile control which enables custom design for selected elemental x-ray lines. 2.1 Concept for MWDXRF The MWDXRF method depends upon DCC optics for both excitation and collection. A single channel instrument consists of a point-focusing DCC excitation optic on the low power x-ray tube. The DCC excitation optic creates a high flux, monochromatic x-ray source for exciting x-ray fluorescence from samples. The excitation spot usually is around several hundred micrometers in diameter and depends upon the source spot size. The second DCC optic is used to collect the emitted X-rays of a specific energy from the specimen and focuses them onto the detector. The energy resolution of the collection optic ranges from 2 to over 1 ev depending upon the energy desired and the quality of the crystal. This arrangement creates a confocal volume within the specimen where the two focal spots must overlap to generate a signal at the detector. Since the energy resolution is determined by the collection optic, a solid state detector such as NaI or a flow proportional counter could be used. The excitation DCC optic has the following characteristics: crystal: Si<22>, geometry: Johann geometry, focusing energy: 2.2 kev(rh-k 1), source optic distance: 2.3mm, solid angle: 61 o, flux with 5W source: 4x1e8, focal spot size: 19 m x 25 m, reflection efficiency: estimated 5%.

3 The yttrium collection DCC optic has the following characteristics: crystal: Si<22>, geometry: Johann geometry, focusing energy: kev(y-k 1), sample optic distance: mm, solid angle: 4.8 o, focal spot size: 4 m x 5 m, reflection efficiency: estimated 5%. The uranium collection DCC optic has the following characteristics: Si<4>, geometry: log spiral, focusing energy: kev (U-L 1), sample optic distance: mm, solid angle: 6.4 o x 14 o, focal spot size: on detector 18 x 135 m, reflection efficiency: estimated 2%. The assembled MWDXRF prototype system consisted of an XOS X-beam Rh 5 W X-ray tube (East Greenbush, NY) operated at 5 kv and 1 ma equipped with the excitation DCC noted above as the X-ray source. The X-ray tube was controlled with a XOS PCS5 power control system connected via an Ethernet connection to the PC. The detector was a Vortex-EX Si drift detector (SII NanoTechnology Inc., Chuo-ku Tokyo, Japan) with a 5 mm 2 detector area and a 3 m thick Si layer. A schematic of a single channel of the experimental setup is shown in Figure 1. The source, source DCC optic, sample, DCC collection optic and detector were all mounted on translation stages (Newport model URS1BCC (Rotation), and 443 DCC source DCC collection X-ray tube Figure 1. Schematic diagram of a single channel MWDXRF prototype setup. Series, Newport Corporation, Irvine, CA) along with Newport actuators TRA25CC. The stages allowed precise alignment of the components. The stages were controlled with a Newport XPS 8 channel motion controller via an Ethernet interface connected to a PC. Data collection from the Vortex detector was done using the vendor supplied software via USB connection. The samples were prepared by depositing different volumes of droplets of aqueous solutions onto polypropylene films (4 m thick, Ultralene, Chemplex Industries, Palm City, FL). The known concentration of the solution along with the volume deposited, provides the total mass of the analyte in the deposit. The stock solutions were 1 mg/ml (unless otherwise noted in the text) were obtained from High Purity Standards (Charleston, SC). 2.2 Performance of MWDXRF sample detector There are several reasons for using monochromatic excitation generated by the DCC optic: 1) higher intensity due to larger collection solid angle, 2) large working distance which allows optical viewing of the sample, 3) lower background. The effects of the DCC in providing a monochromatic excitation beam are shown in Figure 2. This plot shows a comparison of monochromatic excitation with Bremsstrahlung excitation. One of the most notable features of this overlay is the significantly higher intensity of the monochromatic excitation compared to the Bremsstrahlung excitation. The Rh Ka peak at kev has an intensity of around 15, counts for monochromatic excitation while the Bremsstrahlung is only around 7 counts. The much larger Compton scatter peaks at and kev respectively are different due to the different scatter angle geometries of the two different instruments. The area of primary interest for the detection of plutonium is around kev. Even without the magnified spectrum, the background around 14 kev is substantially higher for the Bremsstrahlung excitation compared with the monochromatic excitation. Thus the combination

4 Intensity (counts) Intensity (counts) of higher excitation intensity from the Rh Ka line along with a much lower background at the detection energy gives a significantly greater signal-to-noise ratio for monochromatic excitation combined with wavelength dispersive signal collection Monochromatic Bremsstrahlung X Figure 2. Plots of monochromatic excitation (black) and Bremsstrahlung excitation (red - x 1). The spectrum of a 1 microliter, 1 ppm U solution deposit on polypropylene is shown in Figure 3. Since this is preliminary data, the detector is not yet equipped with an aperture to restrict scatter, therefore a small iron peak around 6.4 kev and the tube Compton scatter between 18-2 kev is visible. We obtained a calculated limit of detection of 1.7 ppm U for a collection time of 1 Lsec, based on the integrated peak intensity of the U L line at kev of 2,543 counts. The nominal integrated background intensity collected for 1 Lsec was only 2 counts. The 1.7 ppm detection limit exceeds our MWDXRF target of Figure Spectrum of 1 microliter deposit of 1 ppm U with 1 Lsec acquisition collected with prototype MWDXRF. ppm by a factor of 6. It should be noted that the excitation spot size is only ~2 micrometers in diameter, while the deposit measures ~6 micrometers in diameter. The total mass of U in the nominal 6 micrometer diameter deposit is 1 microgram. The difference between the excitation area and the deposit area is over a factor of 1. Hence, assuming a uniform deposition, which we know is not correct from mapping the U distribution in the deposit, a sub-ppm detection limit appears to be assured.

5 Intensity (counts) A two channel system is demonstrated in Figure 4. A deposit of 1 microliter each of 1k ppm uranium and yttrium solutions was prepared. The difference in intensity between the U and Y is an indication that the deposit is not uniform. Each spectrum was collected for 1 Lsec separately and overlaid to give the composite figure. The fact the deposit is not uniform was confirmed with elemental maps. The deposition area is larger than the 1k ppm deposit for the uranium in Figure 3. In order to demonstrate the selectivity of the MWDXRF instrument, one microliter each of 1k ppm aqueous U and Y was deposited onto a deposit of NIST SRM 274, Buffalo River Sediment. This spiked sample simulates a high mass dried solids deposit which would be expected of a reprocessing deposit. Such a deposit would exhibit a high background from the low Z elements present along with low levels of elements around the energy range of interest. Table 1, lists the elements present along with the concentrations. It is clear from Table 1. there are high concentrations of low Z elements which would generate a significant background. A comparison of the monochromatic excitation of the spiked SRM274 and the Bremsstrahlung excitation from U 1k ppm Y 1k ppm Figure 4. Composite overlay of 2 channel detection of Y and U deposits, 1 microliter each of 1k ppm, with prototype MWDXRF. a polycapillary optic is shown in Figure 5 along with the MWDXRF spectrum of uranium in the spiked sample. It is clear that the monochromatic excitation results in a significantly lower background. While the MWDXRF spectrum shows an even cleaner background, showing only the scatter (noted earlier) which would be eliminated with an aperture on the detector. The overlay of the 3 different spectra in Figure 5 clearly shows the advantage of using MWDXRF for actinide detection. The sensitivity of the detection is calculated to be in the single digit ppm level since the U deposit is not uniform. Figure 5 shows the Element Concentration (wt%) Element Concentration ( g/g) Al 6.11 Ba 414 Ca 2.6 Cr 135 C Cu 98.6 Fe 4.11 Pb 161 Mg 1.2 Mn 555 K 2. Zn 438 Si 29.8 Rb 1 Na.547 Sr 13 Ti.457 Zr 3 Table 1. Elemental concentrations of selected elements in NISTS SRM 274 Buffalo River Sediment.

6 Intensity (counts) Bremsstrahlung excitation in black with a high background. While the intensity of the Bremsstrahlung excitation spectrum is higher the high background significantly degrades the sensitivity of the elemental detection. The blue trace shows the results using monochromatic excitation. The high background is almost nonexistent. The excitation of the lighter elements is decreased which is expected since the absorption edges for these elements are significantly removed from the monochromatic excitation peak at 2.2 kev. The red trace is the MWDXRF product combining both monochromatic excitation with wavelength detection. Although there is a significant decrease in signal intensity, the gain in reduced background compensates for the loss in signal intensity. 4 3 Bremsstralung excitation Monochromatic excitation x 4 MWDXRF x Figure 5. Overlay of spectra for NIST SRM 274 spiked deposit with 1 microliter of 1k ppm aqueous U and Y solutions. (Black trace) Bremsstrahlung excitation, (blue) monochromatic excitation and (red) MWDXRF monochromatic excitation and DCC detection. The elemental maps of the SRM 274 Buffalo River Sediment spiked deposit (not shown) exhibited significant elemental heterogeneity both in the SRM solid deposit and the dried aqueous deposit. The aqueous U spike dried was almost circular, with a majority of the U on the perimeter of the deposit. The SRM solid was tear shaped and even within the solid deposit there were elemental distribution differences. This heterogeneity affects the resulting signal since not all of the U spike is present in the excitation area. Thus the resulting elemental signal observed in the red trace in Figure 5 is less than expected due to the reduced U within the excitation area. 2.2 Conclusions The MWDXRF prototype is operational for 2 channel data acquisition. The 2 channel operation is not simultaneous at this time. The 2 channels available at this time are uranium and yttrium. Both have calculated limits of detection around 2 ppm for a dried aqueous deposit of 1k ppm. This detection limit is below our target of 1 ppm. The selectivity and sensitivity have been demonstrated by spiking a NIST SRM to mimic the high mass dried solids of a reprocessing solution. Since the deposits are known to be heterogeneous we expect to achieve sub-ppm sensitivity for U with improvements in alignment and droplet deposition. We expect to achieve similar detection limits for Pu, which will allow the direct determination of Pu in reprocessing solutions and provide more accurate measurements of Pu concentrations for safeguards efforts. The next step in with this prototype instrument is the addition both thorium and plutonium collection optics to demonstrate the direct detection of plutonium using MWDXRF as well as studying potential spectral overlaps among the actinide elements.

7 1.3 Acknowledgments The authors would like to acknowledge the support of the DOE NA22 Global Safeguards program for financial support of this research effort. LA-UR Los Alamos National Laboratory is operated by the Los Alamos National Security, LLC for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-6NA References [1] Z. W. Chen, F. Wei, I. Radley, B. Beumer, Low-Level Sulfur in Fuel Determination Using Monochromatic WD XRF_ASTM D J. ASTM Intl, 2 (8), pages, 25. [2] D.W. Berreman, J.W.M. Dumond, and P.E. Marmier, New Point Focusing Monochromator, Rev. Sci Instr. 25, , [3] Z.W. Chen and D. B. Wittry, Microprobe x-ray fluorescence with the use of point-focusing diffractors, Appl. Phys. Lett. 71 (13) , [4] Z.W. Chen and D. B. Wittry, Microanalysis by monochromatic microprobe x-ray fluorescence-physical basis, properties and future prospects, J. Appl. Phys. 84 (2) , 1998.