Improving Materials Accountancy for Reprocessing using HiRX Ben Cipitia, Michael McDaniela, George Havrillab, Michael Collinsc aSandia National Laboratories Albuquerque, New Mexico, United States bLos Alamos National Laboratories Los Alamos, New Mexico, United States cKornizu Scientific Monument, Colorado, United States Abstract. The High Resolution X-Ray (HiRX) technology has the potential to replace K-Edge and Hybrid K-Edge Densitometry (HKED) for routine accountability measurements in reprocessing. This technology may significantly reduce plutonium measurement uncertainty in a simpler and less costly instrument. X-ray optics are used to generate monochromatic excitation of a sample and selectively collect emitted x-rays of the target elements. The result is a spectrum with a peak specific to one element with negligible background. Modeling was used to examine how safeguards could be improved through the use of HiRX at existing aqueous reprocessing plants. This work utilized the Separation and Safeguards Performance Model (SSPM), developed at Sandia National Laboratories, to examine how reduced measurement uncertainty decreases the overall measurement error. Material loss scenarios were also modeled to determine the effect on detection probability for protracted diversion of nuclear material. Current testing of HiRX is being used to inform the modeling effort, but a 0.1% measurement uncertainty for uranium and plutonium concentration is an optimistic goal based on laboratory results. Modeling results showed that a three-fold improvement in the overall safeguards performance of a plant if the 0.1% uncertainty goal can be achieved. The modeling results will be presented along with a discussion of the current experimental campaign results. In addition, a qualitative cost analysis will be presented to compare the use of HiRX with HKED. 1. Introduction Nuclear Material Accountancy (NMA) measurements and inspector verification of those measurements can be very challenging in a reprocessing facility due to the large number of chemical and physical material forms, and complex matrices in the process. Analytical measurements can require significant resources both in equipment costs and inspector/analyst time. Any new measurement technology being considered should reduce the uncertainty of the measurement and the cost, utilize easy-to-operate equipment, limit the burden of maintenance and calibration, and/or reduce the amount of waste generated. The High Resolution X-Ray (HiRX) technology uses an x-ray generator and doubly curved crystal optics to isolate the x-ray emission unique to each element of interest. It has the potential to reduce the measurement uncertainty considerably (compared to HKED) in a simpler and less expensive instrument. The purpose of this work was to determine whether the HiRX technology could provide an improvement in diversion detection probability as compared to current practices. Because a facility’s NMA system and the IAEA’s safeguards approach for reprocessing plants may require a large and varied number of measurements and monitoring systems, a systems-level approach was required to analyze the overall improvement of a new technology. The Separation and Safeguards Performance Model (SSPM) was used to perform this analysis. 2. HiRX Technology The HiRX technology uses a low energy x-ray tube with doubly curved optics to create a monochromatic x-ray source for excitation, and another doubly curved crystal optic for collection of 1 an x-ray emission unique to the element(s) of interest [1]. The result is a spectrum that exhibits a prominent x-ray peak specific to one element with negligible background. Multiple optics could be used on the same instrument to detect characteristic x-rays of up to four different elements. The development of HiRX was initially motivated by a desire to attain a sensitivity of 10 ppm for Pu detection, improve safety and accuracy of Pu measurements, and develop new sampling methods for reducing sample size. Experiments performed with HiRX technology at Los Alamos National Laboratory (LANL) in recent years have exceeded expectations as HiRX experiments have consistently attained better than 10 ppm detection limits and with smaller sample sizes than anticipated (100 microliters or less). From these tests, it is expected that HiRX can improve both the input and output accountancy measurements to within an error of 0.1% with a rapid turn-around-time, but this uncertainty has not yet been achieved. This would be an 8-fold improvement in measurement uncertainty for Pu accountancy in the dissolver solution and a 3-fold improvement in measurement uncertainty for the Pu nitrate product as compared to current best-practices. A bench-top prototype unit is currently being tested on actual reprocessing solutions. 3. Current Materials Accountancy in Reprocessing All currently operating reprocessing plants utilize the PUREX separation process. Reprocessing plants are divided into multiple material balance areas (MBAs) for accountancy purposes. The front end and back end of the plants process solids, and therefore rely on containment and surveillance with select neutron and gamma-ray measurements. After dissolution, the majority of separation and purification processes are typically contained in MBA 2, which is the focus of this work. Materials accountancy on MBA 2 is based on measurements of reprocessing solutions. The operator measures material flows and inventories at Key Measurement Points (KMPs) at MBA boundaries and within the process. Samples from the input accountability tank (IAT) are routinely measured using Hybrid K-Edge Densitometry (HKED). This method is a combination of K-Edge Densitometry (KED) and X-ray Fluorescence (XRF). The International Target Values (ITV) 2010 uncertainties for HKED are r=0.2%, s=0.2% for U and r=0.8%, s=0.5% for Pu in the dissolver solution [2]. The U and Pu nitrate solutions that are produced after separation are measured at the output accountability tanks (OAT) using KED. ITV uncertainties for KED are r=0.2%, s=0.2% for U and r=0.3%, s=0.3% for Pu in the product tanks [2]. Mass spectrometry can achieve better measurement uncertainties, but it is only used for approximately 10% of the samples for quality assurance checks since these measurements take much longer to complete [3]. Safeguards at the Rokkasho Reprocessing Plant are based around a yearly Physical Inventory Verification (PIV) and more frequently performed Interim Inventory Verification (IIV) inspections [4]. The PIV is designed to drain material to key accountability tanks for sampling and destructive analysis (DA) at low measurement uncertainty. Both uranium and plutonium are accounted for in the PIV, and a yearly Material Unaccounted For (MUF) value is determined. The IIV occurs monthly, with an additional Short Inventory Verification (SIV) taken every ten days. Effectively, the IIV/SIV provides a material balance once every ten days, but only plutonium is accounted for. The IIV/SIV uses the accountancy measurements from the input and output accountability tanks. The internal separations operations within MBA2 contain buffer vessels in between processes, pulsed columns where the separations occur, and evaporators for concentrating the product solutions. The existing IIV/SIV follows a random sampling pattern to draw samples from these internal vessels to verify the inventory. The pulsed columns cannot be sampled due to the variable nature of the extractions, but most of the other vessels can be. Samples are taken to a laboratory for DA, usually Isotope Dilution Mass Spectroscopy (IDMS). The data from the input and output measurements, inventories, and process monitoring data such as tank level are used to calculate the interim material balance for Pu. 2 4. Separation and Safeguards Performance Model (SSPM) Modeling was used to examine the use of HiRX in existing plants to replace traditional measurement technologies. The Separations and Safeguards Performance Model (SSPM), developed at Sandia National Laboratories, was used as the basis for this work. Built in Matlab Simulink, the SSPM was developed to analyze safeguards system design, test the performance of new measurement technologies, and perform diversion scenario analyses [5,6]. It provides the capability to generate metrics for safeguards performance. Various versions of the SSPM exist to represent different reprocessing concepts, but this work used a PUREX model to represent current reprocessing plants. The model is built upon open data, so specific plant design details may be different. The Simulink model uses signals to represent material flows between processing units. Fig. 1 shows the PUREX model. The processing units are subsystems that perform mathematical functions on the signals to represent the operations. The subsystems simulate tanks filling and emptying, and determine how the material separates into the output streams. Separations are hard assumptions that do not currently model the chemistry. The inventory of material in each processing unit is tracked. Fig. 1. PUREX SSPM The blue blocks shown in Fig. 1 are measurement blocks that simulate what is being measured, the uncertainties, and the frequency of the measurement. Additional measurement points, such as bulk material measurements, are not visible in the high-level view. The simulated measurements are used within the model to perform periodic inventory balances. 3 The red blocks shown in Fig. 1 are diversion points that can be turned on or off to represent material