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Development of a Liquid Scintillator-Based Active Interrogation System for LEU Fuel Assemblies Anthony Lavietes, Senior Member, IEEE, Romano Plenteda, Nicholas Mascarenhas, Member, IEEE, L. Marie Cronholm, Michael Aspinall, Malcolm Joyce, Alice Tomanin, Paolo Peerani Abstract – The IAEA, in collaboration with the Joint Research Center (Ispra, IT) and Hybrid Instruments (Lancaster, UK), has developed a full scale, liquid scintillator-based active interrogation system to determine uranium (U) mass in fresh fuel assemblies. The system implements an array of moderate volume (~1000ml) liquid scintillator detectors, a multichannel pulse shape discrimination (PSD) system, and a high-speed data acquisition and signal processing system to assess the U content of fresh fuel assemblies. Extensive MCNPX-PoliMi modelling has been carried out to refine the system design and optimize the detector performance. These measurements, traditionally performed with 3He-based assay systems (e.g., Uranium Neutron Coincidence Collar [UNCL], Active Well Coincidence Collar [AWCC]), can now be performed with higher precision in a fraction of the acquisition time. The system uses a highflashpoint, non-hazardous scintillating fluid (EJ309) enabling their use in commercial nuclear facilities and achieves significantly enhanced performance and capabilities through the combination of extremely short gate times, adjustable energy detection threshold, real-time PSD electronics, and high-speed, FPGA-based data acquisition. Given the possible applications, this technology is also an excellent candidate for the replacement of select 3He-based systems. Comparisons to existing 3He-based active interrogation systems are presented where possible to provide a baseline performance reference. This paper will describe the laboratory experiments and associated modelling activities undertaken to develop and initially test the prototype detection system.

I.

INTRODUCTION

interrogation methods are the primary tool used by Active the International Atomic Energy Agency (IAEA) to assay uranium (U) fresh fuel assemblies to determine 235U mass. The primary detector technology implemented for this purpose relies on helium-3 (3He) gas as the detection medium. 3He gas has many attractive characteristics that make it highly suitable

Manuscript received June 7, 2013. Anthony D. Lavietes is with the International Atomic Energy Agency, Vienna, Austria (telephone: +43 (1) 260025132, e-mail: [email protected]). Romano Plenteda is with the International Atomic Energy Agency, Vienna, Austria. Nicholas Mascahrenas is with the International Atomic Energy Agency, Vienna, Austria. L. Marie Cronholm is with the International Atomic Energy Agency, Vienna, Austria. Michael Aspinall is with Hybrid Instruments, Inc., Lancaster, United Kingdom. Malcolm Joyce is with Department of Engineering, Lancaster University, Lancaster, United Kingdom. Alice Tomanin is with the University of Ghent, Belgium, and detached at the Joint Research Center Ispra. Paolo Peerani is with the Joint Research Center, Ispra, Italy.

978-1-4799-1047-2/13/$31.00 ©2013 IEEE

for use many safeguards application (e.g., non-hazardous, high thermal neutron cross-section, simple detector design, and long operational lifetime). 3He has been used for decades as the default neutron detector technology for safeguards. While 3He-based systems are compatible with many safeguards neutron detection applications, the availability and cost of the gas has become a significant obstacle. 3He gas is a by-product of tritium decay, tritium being a hydrogen isotope that is manufactured primarily for use in the production of nuclear weapons. As a result of the near cessation of nuclear weapons testing and development, the global production of tritium has essentially ended, resulting in the end of an adequate supply of 3He. What supplies remain have become exceedingly difficult to obtain and exceedingly expensive. This situation has motivated renewed interest in alternative neutron detector technologies. Aside from cost and supply issues, a number of applications currently served by 3He gas-based systems can be satisfied by implementing alternative neutron detector technologies and, in some cases, with significant benefit. A particular capability of interest to the IAEA is the rapid assay of fresh LEU fuel assemblies. The Uranium Neutron Coincidence Collar (UNCL), a 3He-based active interrogation system designed by Los Alamos National Laboratory, is currently used for this measurement. As reactor fuel designs continue to evolve, the use of neutron absorbers in fuel assemblies is increasing. Embedded neutron absorber material (typically gadolinium [Gd]) significantly and randomly alters the neutron signature from an assembly, requiring a modification in the assay methodology. The UNCL detector design has been modified to adapt to the presence of burnable poison fuel rods by inserting a layer of cadmium (Cd) to absorb thermal neutrons below about 1.3eV. This technique, referred to as “fast-mode,” avoids unwanted interference of neutron absorbers [1]. Fig 1 is a plot of the cross sections for plutonium (Pu) and U, along with the neutron capture cross sections for Cd and Gd [2]. The absorption energy of Cd, being significantly higher than that of Gd, virtually eliminates neutrons below about, and as such, removes the effects of neutron absorption of Gd from the active interrogation mass measurement method. While this technique reduces the effects of poisons, the disadvantage is a significantly reduced induced fission rate with correspondingly longer measurement times. Without the Cd layer (“thermal mode”), the UNCL measurement of a fuel assembly without Gd can be accomplished in about 6-10 minutes for a 1% uncertainty. With the addition of Gd and the

Fig 1. 235U and 239Pu fission cross-sections (yellow and red, respectively), and Cd and Gd neutron absorption cross sections (green and blue, respectively).

necessary Cd layer, the same 1% uncertainty measurement requires more than 4 hours. A larger interrogation source may be used, though the thermal detector will soon be overwhelmed by the accidentals rate due to its strong sensitivity to the interrogation source. To address the need for an assay method that would be able to rapidly perform 235U mass measurements in fresh fuel assemblies, the characteristics of liquid scintillator neutron detectors were evaluated. The benefits associated with fast (rather than thermal) neutron detection became immediately apparent. Two prominent characteristics provide significant performance benefits for active interrogation applications. The first characteristic is an exceptionally short gate time: 40ns to 100ns for liquid scintillator fast neutron detectors versus 30μs to 70μs for moderated thermal neutron detectors. The second characteristic of interest is the ability to control the neutron energy detection threshold. The short gate time results in a reduction of more than three orders of magnitude in accidental neutron coincidence, and the threshold control essentially eliminates the interrogation source neutrons from the data stream. The liquid scintillator-based system such that the

Fig 2. A liquid scintillator array comprised of twelve detectors configured to emulate the geometry of a UNCL system.

Fig 3. Sixteen channel PSD system developed by Hybrid Instruments, Ltd. (Lancaster, UK).

interrogation source neutrons can be essentially eliminated. The subsequent activities focused on the development of a full scale liquid scintillator-based UNCL prototype system is exhaustively described in an earlier publication [3]. II.

PROTOTYPE LIQUID SCINTILLATOR-BASED UNCL SYSTEM

The full scale prototype system includes an array of twelve 10cm x 10cm x 10cm liquid scintillator detectors filled with EJ-309 scintillation fluid (Fig 2), a sixteen channel pulse shape discriminator (PSD) system (Fig 3) [4, 5, 6], and a LabVIEW-based data processing system (Fig 4) that includes a National Instruments 3110 Industrial Controller and a PCI7813R data acquisition card. The acquisition card includes a 3M gate FPGA that was programmed to process the neutron signals for coincidence in real time. The system was assembled and then configured such that the individual scintillator detectors operated with a uniform gain. The neutron/gamma discrimination thresholds were set

Fig 4. A National Instruments LabVIEW-based data processing system including an industrial controller with an FPGA-based, high-speed data acquisition card.

Fig 5. Neutron/gamma scatterplots for a four detector cluster. The PSD discrimination thresholds (green) are shown between the gamma data points (red) and the neutron data points (blue). The red vertical threshold line is set to limit the effects of very high energy cosmic events.

such that the gamma/neutron ratios were also uniform, indicating similar thresholds for each detector element. Fig 5 shows the 252Cf scatter plots for a four detector cluster. The design of the liquid scintillator array was guided by the MCNPX-PoliMi [7, 8] modeling activities of the JRC Ispra. The scintillator volume, detector configuration, and array mechanical assembly was heavily influenced by this extensive effort. The user interface was designed with LabVIEW and shows total neutron and gamma counts, as well as coincidence data. The data is also processed for triples and quadruple coincidence, though the validation of the code is still in process. One of the important aspects of the data processing is the timing of the coincident events. Since the system is detecting fast neutrons and the detector spacing is on the order of 220mm separation, coincident neutron should be detected primarily within the first a single acquisition clock cycle, in this case 20ns. A small percentage of coincidences may occur on a clock edge such that each coincident neutron occurs on neighboring clock cycles. If this aspect of fast neutron detection can be shown to be valid, then it may be possible to perform neutron coincidence counting by directly counting coincidences, rather than implementing the standard shift register methodology typical for thermal neutron coincidence counting and well known in the literature [9]. To illustrate this point, Fig 6 shows the classic Rossi-alpha distribution from a

Fig 7. Anticipated liquid scintillator array reals and accidentals coincidence distribution (not to scale).

fission source that represents a neutron detection distribution for a moderated thermal neutron detector. The time distribution of neutron detection is the reason for the shift register method for coincidence counting. Fig 7 shows the anticipated fast neutron detector equivalent. In addition, the fast neutron detector does not require a pre-delay and is not subject to gate fraction considerations – both typical for thermal neutron detector systems. Fig 8 shows a preliminary distribution data set for 252Cf. While the percentage of coincident events in the second clock period is larger than expected, the concept of directly counting coincidences appears to be possible. There are also a very small number of coincidences appearing in the sixth and seventh clock cycles that is under investigation. Following the initialization of the system and baseline configuration, a number of acquisitions were performed to determine whether the system was performing as expected. All of the separated components of the system were new or significantly advanced from the previous experiments and much work is now taking place to ensure that each component is validated and calibrated with known sources prior to a more extensive set of field trials. As the focus of this development is the active interrogation of fresh LEU fuel assemblies, a series of acquisitions were performed with 252Cf and AmLi neutron sources. The intent was to show that the 252Cf can be measured in the presence of a large AmLi source without deleterious effects on the Coincidence Timing 5000

Total Coincidence

4500

4504

Acquisition Time: 300 sec

4000 3500 3000 2500 2000 1500

1068

1000 500 0

Fig 6. Rossi-alpha distribution of detection events as a function of time from an originating event [9]. R = real coincidence events, A = accidental coincidence events, P= pre-delay, G = prompt and delayed gate times, D = long delay, and τ = die-away time.

1

2

12

12

16

3

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160

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38

9

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Clock Cycle Fig 8. Relative coincident neutron timing for liquid scintillator array and a 252Cf source.

TABLE I PRELIMINARY TEST DATA Location

Source 252

HQ

Cf

252

Seibersdorf

Cf

252

Cf + AmLi

System UNCL LS UNCL LS UNCL LS

Totals 184666 110106 157095 90161 3885147 58995

R+A 18886 7437 14924 5618 1623685 1947

A 3588 5 2644 9 1610403 3

Uncertainty 0.98% 1.16% 1.08% 1.34% 13.54% 2.27%

coincidence data. The neutron detection threshold for each liquid scintillator was set to several values to characterize the effects on AmLi neutrons. A series of acquisitions were performed and, while it was apparent that the optimal threshold was not obtained, the effect on AmLi neutron detection was significant. Table I shows the preliminary test data for 600s acquisitions. About 20% of the total liquid scintillator Totals data is associated with the AmLi versus about 99% for the UNCL. Only about 2% of the liquid scintillator R+A data are due to AmLi. Both of these results are due to a combination of the short gate time and the neutron energy detection threshold. The count rates for the last series (252Cf + AmLi) were lower due to an over-conservative reseting of the neutron energy threshold and gamma/neutron discrimination levels. It is anticipated that further improvement towards the expected performance will be realized after the array is properly calibrated and configured. III.

NEXT STEPS

The system is currently under full characterization and will be configured with the proper neutron detection energy and neutron/gamma discrimination thresholds. The performance and, particularly, the AmLi neutron source immunity will be carefully assessed and optimized. Following performance validation in the laboratory, the system will begin a series of field trials to perform active interrogation of LEU fresh fuel assemblies and compare, where possible, with the 3He-based UNCL performance. CONCLUSIONS The results of the initial testing of the liquid scintillatorbased active interrogation system are in agreement with expectations. While the performance is not optimized, the data indicates that full performance with the expected capabilities will be achieved after careful characterization and calibration. Modelling activities at the JRC Ispra will continue in an effort to optimize the system configuration and mechanical design. ACKNOWLEDGMENT This work could not be accomplished without the generous and continued support of the following individuals and programmes: • Dr. James Tushingham (United Kingdom Support Programme) for the development of real-time, multichannel PSD at Hybrid Instruments, Ltd.

• Dr. Joao Goncalves (European Commission Support Programme) for supporting the novel MCNPX-PoliMi modelling activities at the JRC Ispra. • Dr. Frodo Klaassen (The Netherlands Support Programme) for the critical contributions of neutron detectors. REFERENCES [1] H. Menlove, J. Stewart, S. Qiao, T. Wenz and G. Verrecchia, "Neutron Collar Calibration and Evaluation for Assay of LWR Fuel Assemblies Containing Burnable Neutron Absorbers," Los Alamos National Laboratory, Los Alamos, NM, 1990. [2] A. LaFleur, W. Charlton, H. Menlove, M. Swinhoe and A. Lebrun, "Development of Self-Interrogation Neutron Resonance Densitometry to Improve Detection of Possible Diversions for PWR Spent Fuel Assemblies," Los Alamos National Laboratory, 2012. [3] A. D. Lavietes, R. Plenteda, L. M. Cronholm, N. Mascarenhas, M. Aspinall, M. Joyce, A. Tomanin and P. Peerani, "Liquid ScintillatorBased Neutron Detector Development," IEEE Trans. Nuc. Sci., 2013 (Publication Pending). [4] M. D. Aspinall, B. D'Mellow, R. Mackin and M. J. Joyce, "Verification of the digital discrimination of neutrons and gamma rays using pulse gradient analysis by digital measurement of time-of-flight," Nuclear Instruments and Methods, vol. A583, pp. 432-438, 2007. [5] M. D. Joyce, F. D. Aspinall, F. D. Cave, K. Georgopoulous and Z. Jarrah, "The Design, Build and Test of a Digital Analyzer for Mixed Radiation Fields," IEEE Trans. Nuc. Sci., vol. 57, no. 5, pp. 2625-2630, 2010. [6] M. J. Joyce, M. D. Aspinall, F. D. Cave and A. D. Lavietes, "Real-time digital pulse-shape discrimination in non-hazardous fast liquid scintillation detectors: prospects for safety and security," IEEE Trans. Nuc. Sci., vol. 59, no. 4, pp. 1245-1251, 2012. [7] E. Padovani, S. A. Pozzi, S. D. Clarke and E. C. Miller, MCNPX-PoliMi Users Manual, RSICC Computer Code Collection ed., Vols. CCC-791, Oak Rdge National Laboratory. [8] S. A. Pozzi, A. Enqvist, C. C. Lawrence and T. N. Massey, "Neutron light output functions measured for EJ309 liquid scintillation detectors," in Proceedings of INMM Annual Meeting, Orlando, FL, July 2012. [9] D. Reilly, N. Ensslin and H. Smith Jr., Passive Non-Destructive Assay of Nuclear Materials, U.S. Government Printing Office, 1991.